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Chemical basis of resistance in pulses to Callosobruchus maculatus (F.) (Coleoptera: Bruchidae)
Journal of Stored Products Research 36 (2000) 89±99
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Chemical basis of resistance in pulses to Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) S. Ignacimuthu*, S. Janarthanan, B. Balachandran Entomology Research Institute, Loyola College, Chennai-600 034, India Accepted 1 July 1999
Abstract Seeds of various wild and cultivated pulses express varied degrees of resistance to damage by Callosobruchus maculatus, a major pest of stored pulses. Seeds were analysed for trypsin/chymotrypsin inhibitors (qualitatively and quantitatively) and protein pro®les (SDS-polyacrylamide gel electrophoresis) to study the chemical basis of resistance to bruchid infestation. Seeds of the wild pulses Dunberia ferruginea, Neonotonia wightii, Tephrosia cararensis, Cajanus albicans and Vigna bourneae dier from other wild germplasms as well as cultivars in having characteristically higher levels of trypsin/ chymotrypsin inhibitors. SDS-PAGE analyses of seed proteins of Dolichos lablab, Lablab purpureus and Rhyncocea cana reveal the presence of high levels of non-nutrient chemicals such as vicilins, a-amylase inhibitors, and lectins in quantities that may impart resistance to C. maculatus. The importance of these protease inhibitors and other non-nutrient chemicals are discussed with reference to resistance to C. maculatus infestation. # 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: Pulses; Trypsin inhibitor; Chymotrypsin inhibitor; SDS-PAGE; Bruchid; Resistance
1. Introduction Pulses are a good source of dietary proteins and other essential nutrients. However, postharvest insect infestation severely aects quality and storability of produce (Singh, 1978; Steele et al., 1985). Pulses are known to contain a variety of non-nutrient compounds such as lectins, proteinase inhibitors, a-amylase inhibitors and other compounds which could be part of an array of constitutive defences against attack by stored grain pests, particularly those belonging * Corresponding author. Tel.: +91-44-826-5542; fax: +91-44-826-5544. E-mail address: [email protected] (S. Ignacimuthu) 0022-474X/99/$ - see front matter # 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 2 - 4 7 4 X ( 9 9 ) 0 0 0 3 1 - 4
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to the family Bruchidae (Janzen et al., 1986; Gatehouse et al., 1990). While the exact nature of resistance in certain pulses to Callosobruchus maculatus (F.) remains largely unknown, some recent studies emphasize the presence of certain non-nutrient (storage) proteins in seeds of resistant varieties as the basis of resistance (Macedo et al., 1993). In addition, trypsin and chymotrypsin inhibitors, besides their well-known function as storage proteins, are also important in imparting resistance (Liener and Kakade, 1980; Ryan, 1990). The ecacy of any secondary (non-nutrient) compound is indicated by biological assay against the insect pest (Agarwal et al., 1988). Several studies suggest the importance of biological assay to assess the potential of such compounds to impart resistance (Gatehouse et al., 1989; Cardona et al., 1990; Xavier-Filho, 1991; Pusztai et al., 1993; Macedo et al., 1995; Fory et al., 1996). In view of the variation observed in damage due to feeding by C. maculatus on various wild as well as cultivated pulses, the present study was planned to identify chemical factors that are responsible for imparting resistance.
2. Materials and methods 2.1. Seeds Seeds of the wild pulses were collected from the Palney Hills of the Western Ghats, Tamilnadu, India (latitude 108; longitude 788; altitude 2010 m from MSL), whereas some cultivated seeds (Cajanus cajan (L.) Millspaugh ICP14770 and ICP14990; Cicer arietinum (L.) COG29 and CO3; Vigna unguiculata (L.) Walp. CO4 and V. mungo (L.) Hepper CO5) were obtained from the Department of Plant Breeding, Tamil Nadu Agricultural University, Coimbatore, India. The other cultivated seeds namely, V. unguiculata VCP8; V. mungo VBN1; V. radiata (L.) Wilczek K851 and UGG4 were obtained from National Pulses Research Centre, Vamban, Tamil Nadu, India. 2.2. Chemical analysis Trypsin and chymotrypsin inhibitors were estimated following the methods described by della Gatta et al. (1989) and Erlanger and Frances Edel Cooper (1996). They were expressed in terms of units/g seed materials (a trypsin or a-chymotrypsin unit (TU or CU) is de®ned as the amount of trypsin or a-chymotrypsin that catalyses the hydrolysis of 1 mol of substrate per min. A trypsin or chymotrypsin inhibitor unit (TIU or CIU) is the reduction in activity of the respective enzymes by one unit, i.e. TU or CU respectively. 2.3. Electrophoretic studies on trypsin and chymotrypsin inhibitors Seeds were milled to a ®ne powder (after removal of the seed coat) and protease inhibitors were extracted by incubating overnight in Tris±HCl±CaCl2 buer, pH 8.1 (at a ratio of 1:10 w/ v) at 48C. The homogenate (the ®ne powder with the extraction buer) was subsequently centrifuged at 12,000 rpm at 48C for 15 min. The supernatant collected was used for electrophoretic analysis with polyacrylamide gel electrophoresis (Native PAGE) as described by
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Kollipara et al. (1991). At the end of the run, the stacking gel was removed and the separating gel stained for trypsin and chymotrypsin inhibitor activities based on Uriel and Bergas (1968) with some modi®cations as described by Kollipara and Hymowitz (1992). 2.4. Seed protein pro®le To analyse proteins of both wild and cultivar pulses, SDS-polyacrylamide gel electrophoresis (SDS-PAGE) was performed according to the method of Laemmli (1970). Proteins were extracted from powdered pulses with 0.1 M Tris±HCl buer, pH 8.0 (at a ratio of 1:10 w/v) using sterilised glass powder in a mortar and pestle for 15 min at 0±58C (on ice). Extracts were centrifuged at 15,000 g for 10 min at 58C. Aliquots of supernatant were used to quantify the proteins using the Bradford method with BSA as standard (Bradford, 1976). An equal quantity of each of the protein samples (100 mg) was applied to the gels. Standard molecular marker proteins (Sigma) were simultaneously run for comparison. Table 1 Trypsin and chymotrypsin inhibitor contents (units/g) in wild and cultivated germplasms1 Varieties
Trypsin inhibitors
Chymotrypsin inhibitors
29602 05.7a 27742 04.2b 27002 09.0b 18862 07.2c 10352 04.1d 8902 14.7e 5232 09.0f 4402 04.8g 3382 06.5h 3002 12.2i 1342 03.2j
4122 34.9a 4252 16.3a 3852 13.1b 2692 14.8c 1482 06.5d 1252 04.9e 1362 11.4de 62 2 14.7f 46 2 07.3gf 43 2 05.7g 22 2 07.3h
12382 02.5a 12332 36.2a 10752 33.6b 7002 25.3c 5752 16.3d 4182 01.2e 4082 10.7e 3352 01.8f 3332 18.0f 2892 09.8g
1532 6.0a 1772 5.6b 1522 3.4a 60 2 7.3c 82 2 2.5d 60 2 2.5ec 58 2 5.8ec 48 2 4.9f 0 2 0.0 41 2 6.5f
Wild pulses Cajanus albicans Dunberia ferruginea Neonotonia wightii Vigna bourneae Tephrosia cararensis Lablab purpureus(brown) Rhyncocea rufescens Rhyncocea cana Dolichos lablab Lablab purpureus (black) Canavalia virosa Cultivars Cicer arietinum CO3 Cicer arietinum COG 29 Vigna unguiculata CO4 Vigna unguiculata VCP8 Cajanus cajanICP 14770 Vigna radiataUGG4 Cajanus cajan ICP 14990 Vigna mungo VBN1 Vigna radiata K851 Vigna mungo CO5
1 Values represent mean 2SE of four replications. Data analysed using Student's t-test (P < 0.01). Values in the same column with the same superscript letters are not signi®cantly dierent.
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2.5. Bioassay A stock culture of C. maculatus was reared on green gram (V. radiata K851 cultivar) seeds at 282 18C in glass jars covered with muslin cloth. For feeding bioassays, seeds were conditioned at 282 18C and 70% r.h. for a period of 10 days before use. Seeds of each cultivar/wild germplasm were kept in glass vials covered with muslin cloth and a single pair of newly emerged adults of C. maculatus from the stock culture was introduced into each of six vials for oviposition. Six replicates were maintained for each variety. Vials were maintained at 282 18C and 70% r.h. After 5 days, the numbers of eggs laid on the seeds of each variety were counted. Such seeds were observed for a maximum duration of 60 days and the numbers of emerging adults were counted for a duration of 8 days with adults being isolated to prevent further breeding. Developmental period, percentage adult emergence and percentage loss in weight [(initialÿ®nal)/initial seed weight 100] were calculated to evaluate developmental performance of C. maculatus on cultivars and wild germplasm of pulses. 2.6. Statistical analysis All data were subjected to Student's t-test and the level of signi®cance was tested at P < 0.01. Each value was statistically compared with other values of the same parameter. 3. Results Seeds of wild germplasms generally were found to contain higher amounts of trypsin and chymotrypsin inhibitors than cultivated seeds (Table 1). Maximum levels of trypsin inhibitors
Fig. 1. Electrophoretogram showing trypsin inhibitor bands in the wild germplasms. Note strong reaction at the region of Rf 0.60±0.80. Wild pulses: (a) Rhyncocea rufescens, (b) Rhyncocea cana, (c) Canavalia virosa, (d) Lablab purpureus (black), (e) Lablab purpureus (brown), (f) Tephrosia cararensis, (g) Dolichos lablab.
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(TI) were observed in Cajanus albicans (Wight and Arnott) Maesen with 2960 units/g followed by seeds of Dunberia ferruginea Wight and Arnott and Neonotonia wightii (Wight and Arnott) Lackey which exhibited 2774 units/g and 2700 units/g, respectively. The seeds of a brown variety of Lablab purpureus (L.) Sweet were found to contain a signi®cantly (P < 0.01) higher amount of TI than the black variety. A signi®cant (P < 0.01) dierence in TI content was noticed at intraspeci®c levels between cultivars (Table 1) except in Cicer arietinum where no signi®cant dierences were observed. The amount of TI was always greater than the amount of chymotrypsin inhibitor. Among wild germplasms the amount of chymotrypsin inhibitors (CI) were higher in D. ferruginea (425 units/g), C. albicans (412 units/g) and N. wightii (385 units/g) (Table 1). Interestingly, CI was absent from the V. radiata K851 cultivar. Although the range of inhibitors in both the cultivated and wild accessions overlapped, overall the levels were signi®cantly higher in the wild accessions. Electrophoretograms (Fig. 1) showed prominent bands of TI in the seeds of L. purpureus (black) at Rf values 0.75 and 0.80, whereas L. purpureus (brown) showed two successive bands at the same region with Rf values 0.75 and 0.77, respectively. Although the other wild genotypes such as Rhyncocea rufescens (Willdenow) de Candolle and D. ferruginea reveal a clear TI band at Rf value 0.60, the seeds of V. bourneae Gamble, on the other hand, exhibited three bands at Rf values 0.50, 0.60 and 0.70. Among the various cultivars, gel electrophoresis revealed a TI band at Rf value 0.70 in the cultivated seeds of V. radiata (K851 and UGG4) and C. arietinum (COG 29 and CO3) (Fig. 2). Interestingly, the other cultivated seeds did not reveal any inhibitor bands.
Fig. 2. Electrophoretogram showing trypsin inhibitor bands. Note the absence of reaction in cultivated germplasms. Wild pulses: (a) Cajanus albicans, (b) Dunberia ferruginea, (c) Vigna unguiculata, (d) Neonotonia wightii. Cultivars: (e) Vigna radiata K851, (f) Vigna radiata UGG4, (g) Cicer arietinum COG29, (h) Cicer arietinum CO3.
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Four intense CI bands at Rf values 0.65, 0.75, 0.77 and 0.80 were observed in L. purpureus (black) while the brown variety of L. purpureus showed three intense bands with Rf values 0.75, 0.77 and 0.80 (Fig. 3). The seeds of Canavalia virosa (Roxburgh) Wight and Arnott had two intense bands at Rf values 0.45 and 0.75, whereas Rhyncocea rufescens had one prominent band at Rf value 0.60 and two more bands at Rf values 0.77 and 0.80. Two bands at Rf values 0.77 and 0.80 were observed in the seeds of R. cana (Willdenow) de Candolle while only one band at Rf value 0.60 was observed in the seeds of Dolichos lablab L. The electrophoretogram showed no CI bands in the cultivated seeds. In the present investigation, trypsin and chymotrypsin inhibitors varied in both number and migration patterns among various wild germplasms of pulses. There was also variation in intensity of these bands and some seeds showed no bands (Figs. 1±3). This could be due to lower concentration of an inhibitor protein or lower anity (weaker inhibition) for bovine trypsin or a-chymotrypsin. Inhibitor contents relative to wild germplasms and cultivar pulses are presented in Table 1. A broad variability of activities among samples was observed for all tested inhibitors. Wild varieties of pulses, D. ferruginea, N. wightii, C. albicans, R. rufescens and V. bourneae used in this study were characterised by higher values of both TI and CI (Table 1). Among them, C. albicans, D. ferruginea, R. rufescens were found to be infested by the bruchid, C. maculatus (see results of bioassay studies, Table 2) or, in other words, the susceptible varieties (reported above) showed higher inhibitor activity against trypsin and chymotrypsin. The other wild germplasms exhibiting lower inhibitor activity against trypsin and chymotrypsin were found to be susceptible (except the genus Lablab, in which the gels showed more intense TI and CI bands). SDS-PAGE seed protein pro®les showed dierences in banding patterns among various wild germplasm lines (Fig. 4). Major polypeptide components at 94, 56 and 52 kD regions were identi®ed in wild accessions of L. purpureus (brown), R. cana and D. lablab. All the other wild
Fig. 3. Electrophoretogram showing chymotrypsin inhibitor bands in the wild germplasm. Note strong reaction at the region of Rf 0.45±0.80. Wild pulses: (a) Rhyncocea rufescens, (b) Rhyncocea cana, (c) Canavalia virosa, (d) Lablab purpureus (black), (e) Lablab purpureus (brown), (f) Tephrosia cararensis, (g) Dolichos lablab and (h) Cajanus albicans.
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Table 2 Biological performance of Callosobruchus maculatus on dierent species of wild and cultivated pulses1 Varieties
Fecundity (No. of eggs/female)
Adult emergence (%)
Developmental period (days)
Seed damage (%)
45 2 3.6a Nil2 Nil2 Nil2 Nil2 Nil2 41 2 5.8a 20 2 5.2b 57 2 7.2c Nil2 ±
37 2 1.6a ± ± ± ± ± 32 2 1.4b 34 2 3.0ab 38 2 0.7a ± ±
19 2 1.7a ± ± ± ± ± 14 2 2.8b 4 2 1.4c 15 2 3.4a ± ±
39 2 1.4a 47 2 2.6b Nil2 2 Nil 69 2 5.9c 63 2 4.2cd 75 2 6.8ce 70 2 5.3cf 77 2 1.5e 72 2 4.2ef
27 2 2.2a 25 2 1.6a ± ± 28 2 4.1a 27 2 3.5a 24 2 1.5a 24 2 1.3a 25 2 2.5a 26 2 0.5a
39 2 2.5a 24 2 3.1b ± ± 43 2 3.5ac 37 2 1.6a 38 2 1.4a 37 2 2.0a 44 2 4.5ac 42 2 1.7c
Wild pulses Cajanus albicans Dolichos lablab Vigna bourneae Lablab purpureus (brown) Canavalia virosa Lablab purpureus (black) Dunberia ferruginea Rhyncocea cana Rhyncocea rufescens Neonotonia wightii Tephrosia cararensis
96 2 6.3a 76 2 5.4b 71 2 5.6bc 70 2 0.7cd 68 2 4.0cde 59 2 9.0e 34 2 8.3f 27 2 6.4fg 25 2 4.6g 14 2 2.3h ±
Cultivars Cajanus cajan ICP14770 Cajanus cajan ICP14990 Cicer arietinum COG29 Cicer arietinum CO3 Vigna unguiculata CO4 Vigna unguiculata VCP8 Vigna mungo CO5 Vigna mungo VBN1 Vigna radiata K851 Vigna radiata UGG4
1202 8.8a 1132 9.5a 27 2 2.1b 19 2 3.6c 85 2 5.8d 97 2 4.6e 58 2 6.2f 49 2 3.4g 76 2 3.5h 88 2 6.5id
1
Values represent mean 2 SE of six replications. Data analysed by Student's t-test (P < 0.01); ±, not tested. Values in the same column with the same superscript letters are not signi®cantly dierent. 2 No adult emergence was observed.
germplasms showed variations in polypeptides of low molecular weights in the range of 20±45 kD. All cultivar accessions show similar banding patterns except Cicer arietinum (Fig. 5). In general, polypeptide components of 94, 66, 56, 52 and 30 kD were identi®ed in all accessions of cultivars together with some minor bands. Developmental performance of C. maculatus on various varieties of wild germplasms has been summarized in Table 2. Maximum numbers of eggs (96) were laid on the seeds of C. albicans, whereas eggs were not laid on the seeds of Tephrosia cararensis de Candolle. Variation in percentage adult emergence was observed in both cultivated and wild germplasms with observed higher values in wild germplasms. Percentage adult emergence was found to be 41, 45, 57 and 20 in the wild seeds of Dunberia ferruginea, C. albicans, R. rufescens and R. cana, respectively (Table 2). Adult emergence was not observed from other wild germplasms. Accordingly, they supported lower levels of infestation by the bruchid as is re¯ected by
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Fig. 4. SDS-PAGE pro®le of wild germplasms. (a) Lablab purpureus (brown), (b) Lablab purpureus (black), (c) Canavalia virosa, (d) Cajanus albicans, (e) Dunberia ferruginea, (f) Tephrosia cararensis, (g) Vigna unguiculata, (h) Rhyncocea rufescens, (i) Rhyncocea cana, (j) Neonotonia wightii and (k) Dolichos lablab.
percentage seed damage. Minimum percentage of seed damage was recorded in R. cana (4%) and maximum percentage of seed damage was recorded in C. albicans (19%). The dierence in percentage seed damage was statistically signi®cant (P < 0.01) among infested seeds of wild germplasms (Table 2). Oviposition, percentage adult emergence and percentage seed damage from cultivated seeds showed interspeci®c as well as intraspeci®c variation. Callosobruchus maculatus laid the maximum number of eggs on the seeds of C. cajan ICP 14770 (120), while the minimum number of eggs (27, COG29 and 19, CO3) was observed on the seeds of C. arietinum (Table 2). However, in contrast to the wild germplasms, successful adult emergence was observed from all the cultivated seeds except for C. arietinum (COG 29 and CO3) (Table 2). Likewise, percentage adult emergence was higher in various species of genus Vigna than Cajanus, with the maximum being observed from the seeds of V. radiata K851 (77%). Nevertheless, intraspeci®c variations were also observed. Developmental period of C. maculatus in cultivated seeds was shorter than in wild seeds within the same genus (Table 2). Seed damage or loss in seed weight in insect infestation studies (bioassay studies) was also carried out. Among cultivars, seeds of C. cajan ICP 14990 showed lesser loss in grain weight (24%) when compared to other cultivar varieties (Table 2).
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Fig. 5. SDS-PAGE pro®le of cultivated seeds. (a) Vigna unguiculata VCP8, (b) Vigna unguiculata CO4, (c) Cicer arietinum COG29, (d) Cicer arietinum CO3, (e) Vigna mungo CO5, (f) Vigna mungo VBN1, (g) Vigna radiata K851, (h) Vigna radiata UGG4, (i) Vigna radiata CO3, (j) Vigna radiata CO4, (k) Cajanus cajan ICP 14770 and (l) Cajanus cajanICP 14990.
4. Discussion Identi®cation and analysis of sources of resistance in seeds provide us with opportunities to identify and breed pulse varieties that can naturally resist bruchid attacks. The production of grain legumes is devastated by C. maculatus and, with current limitations on chemical control of stored grain pests, the probability of identifying the chemical basis of resistance in pulses, speci®cally the enzyme inhibitors and some deterrent proteins, prompted us to pursue this study. Protease inhibitors are proteins or polypeptides which bind proteolytic enzymes and, in plants which produce them, are thought to provide a form of natural defence against herbivorous insects (Green and Ryan, 1972; Ryan, 1979; Broadway et al., 1986; Xavier-Filho and Campos, 1989; Burgass et al., 1994). In the present investigation in cultivars, both quantitative and qualitative analyses on TI and CI showed lower activities when compared to those from their wild counterparts, except C. arietinum, and all were susceptible to C. maculatus. It has been explained that, since a threshold value to discriminate between resistant and susceptible lines cannot be established for any tested inhibitors, evaluating only one or two antimetabolites cannot provide an eective way of checking the degree of resistance towards storage pests (Piergiovanni et al., 1994). This conclusion agreed with previous studies where several antimetabolites in a larger number of germplasms of pulses had been analysed (Baker et al., 1989; Xavier-Filho et al., 1989; Cardona et al., 1990; Piergiovanni et al., 1991; Fory et
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al., 1996). It seems more likely, in the light of these observations, that more antimetabolites could be involved in the mechanism of bruchid beetle resistance in the case of the present pulse varieties. Furthermore, antinutrients, ``like the vicilins (55 kD polypeptides), lectins (30 kD polypeptides), and a-amylase inhibitors (13±17 kD polypeptides) which are reported to have a role in imparting resistance to bruchids'' (Gatehouse et al., 1989; Pratt et al., 1990; Pereira De Sales et al., 1992; Macedo et al., 1993, 1995; Fory et al., 1996), may also be involved in imparting resistance in the present investigation. The present work on protein pro®les enumerates the presence of bands of 56 and 52 kD in wild accessions of Dolichos lablab, R. cana and L. purpureus under denaturing conditions. In addition, our results displayed several minor bands of higher mobilities on SDS-PAGE gels which match well with the protein antimetabolites reported in earlier ®ndings (Liener and Kakade, 1980; Ryan, 1990; Macedo et al., 1993, 1995). Acknowledgements The authors gratefully acknowledge the ®nancial assistance and support provided by a grant to Fr S. Ignacimuthu from the Department of Biotechnology, Government of India, New Delhi. We also thank Dr Alok Sen, Entomology Research Institute, Loyola College, Chennai, for his fruitful and critical suggestions. References Agarwal, A., Sone, Lal, Gupta, K.C., 1988. Natural products as protectants of pulses against pulse beetles. Bulletin of Grain Technology 26, 154±164. Baker, T.A., Nielsen, S., Shade, R.E., Singh, B.B., 1989. Physical and chemical attributes of cowpea lines resistant and susceptible to Callosobruchus maculatus (F.) (Coleoptera: Bruchidae). Journal of Stored Products Research 25, 1±8. Bradford, M.M., 1976. A rapid and sensitive method for the quanti®cation of microgram quantities of protein utilising the principle of dye binding. Analytical Biochemistry 72, 248±254. Broadway, R.M., Duey, S.S., Pearce, G., Ryan, C.A., 1986. Plant proteinase inhibitors: a defence against herbivorous insects? Entomologia Experimentalis et Applicata 41, 33±38. Burgass, E.P.J., Main, C.A., Stevens, P.S., Christeller, J.T., Gatehouse, A.M.R., Laing, W.A., 1994. Eects of protease inhibitor concentration and combinations on the survival, growth and gut enzyme activities of the black ®eld cricket, Teleogryllus commodus. Journal of Insect Physiology 40, 803±811. Cardona, C., Kornegay, J., Posso, C.E., Morales, F., Ramirez, H., 1990. Comparative value of four arcelin variants in the development of dry bean lines resistant to the Mexican bean weevil. Entomologia Experimentalis et Applicata 56, 197±206. della Gatta, C., Piergiovanni, A.R., Ng, N.Q., Carnovale, E., Perrino, P., 1989. Trypsin inhibitor levels in raw and cooked cowpea (Vigna unguiculata ) seeds. Lebensmittel-Wissenschaft und Technologie 22, 78±80. Erlanger, B.F., Frances Edel Cooper, A.G., 1996. The action of chymotrypsin on two new chromogenic substrates. Archives of Biochemistry and Biophysics 115, 206. Fory, L.F., Finhardi-Filho, F., Quintero, C.M., Osborn, T.C., Cardona, C., Chrispeels, M.J., Mayer, J., 1996. Alpha amylase inhibitors in resistance to common beans to the Mexican bean weevil and the bean weevil (Coleoptera: Bruchidae). Journal of Economic Entomology 89, 204±210. Gatehouse, A.M.R., Dewey, F.M., Dove, J., Fenton, K.A., Pusztai, A., 1989. Eect of seed lectins from Phaseolus vulgaris on the development of larvae of Callosobruchus maculatus. Mechanism of toxicity. Journal of the Science of Food and Agriculture 35, 373±380.
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Gatehouse, A.M.R., Minney, H., Dobie, P., Hilder, V.A., 1990. Biochemical resistance to bruchid attack in legumes; investigation and exploitation. In: Fujii, K., Gatehouse, A.M.R., Johnson, C.D., Mitchell, R., Yoshida, T. (Eds.), Bruchids and Legumes: Economics, Ecology and Coevolution. Kluwer Academic Publishers, Dordrecht, pp. 241± 256. Green, T.R., Ryan, C.A., 1972. Wound induced proteinase inhibitors in plant leaves: a possible defence mechanism against insects. Science 175, 776±777. Janzen, D.H., Ryan, C.A., Liener, I.E., Pearse, G., 1986. Potentially defensive proteins in mature seeds of 59 species of tropical Leguminosae. Journal of Chemical Ecology 12, 1469±1480. Kollipara, K.P., Domagalsjki, J.M., Hymowitz, I.A., 1991. Quick method of resolving soybean Kunitz trypsin inhibitors using polyacrylamide gel electrophoresis. Soybean Genetics Newsletter 18, 234±236. Kollipara, K.P., Hymowitz, T., 1992. Characterization of trypsin and chymotrypsin inhibitors in the wild perennial Glycine species. Journal of Agriculture and Food Chemistry 40, 2356±2383. Laemmli, U.K., 1970. Cleavage of structural proteins during assembly of the head of bacteriophage T4. Nature 227, 680±685. Liener, I.E., Kakade, M.L., 1980. Protease inhibitors. In: Liener, I.E. (Ed.), Toxic Constituents of Plant Foodstus. Academic Press, New York. Macedo, M.L.R., Andrade, L.B.S., Morales, R.A., Xavier-Filho, J., 1993. Vicilin variants and the resistance of cowpea (Vigna unguiculata ) seeds to the cowpea weevil (Callosobruchus maculatus ). Comparative Biochemistry and Physiology 105C, 89±94. Macedo, M.L.R., Fernandes, K.V.S., Sales, M.P., Xavier-Filho, J., 1995. Puri®cation and properties of storage proteins (vicilins) from cowpea (Vigna unguiculata ) seed which are susceptible or resistant to the bruchid beetle Callosobruchus maculatus. Brazilian Journal of Medical Biology Research 28, 183±190. Pereira De Sales, M., Macedo, M.L.R., Xavier-Filho, J., 1992. Digestibility of cowpea (Vigna unguiculata ) vicilins by pepsin, papain and bruchid (insect) midgut proteinases. Comparative Biochemistry and Physiology 1033, 945± 950. Piergiovanni, A.R., Sonnante, G., della Gatta, C., Perrino, P., 1991. Digestive enzyme inhibitors and storage pest resistance in cowpea (Vigna unguiculata ) seeds. Euphytica 54, 191±194. Piergiovanni, A.R., della Gatta, C., Sergio, L., Perrino, P., 1994. High antinutrient levels and bruchid resistance of cowpea (Vigna unguiculata ) seeds. Euphytica 80, 59±62. Pratt, R.C., Singh, N.K., Shade, R.E., Murdock, L.L., Bressan, R.A., 1990. Isolation and partial characterization of a seed lectin from tepary bean that delays bruchid beetle development. Plant Physiology 93, 1453±1459. Pusztai, A., Even, S.W.B., Grant, G., Brown, D.S., Stewart, J.C., Peumans, W.J., Van Damme, E.J.M., Bardocz, S., 1993. Antinutritive eects of wheat germ agglutinin and other N-acetylglucosamine speci®c lectins. Brazilian Journal of Nutrition 70, 313±321. Ryan, C.A., 1979. Proteinase inhibitors. In: Rosenthal, G.A., Janzen, D.H. (Eds.), Herbivores: Their Interaction with Secondary Plant Metabolites. Academic Press, New York, pp. 599±618. Ryan, C.A., 1990. Protease inhibitors: genes for improving defenses against insects and pathogens. Annual Review of Phytopathology 28, 425±449. Singh, S.R., 1978. Pests of grain legumes and their control in Nigeria. In: Singh, S.R., Van Emden, H.F., Taylor, T.A. (Eds.), Resistance to Pests of Cowpeas in Nigeria. Academic Press, New York, pp. 267±297. Steele, W.M., Allen, D.J., Summer®eld, R.J., 1985. Cowpea (Vigna unguiculata ) (L.) (Walp.). In: Summer®eld, R.J., Roberts, E.H. (Eds.), Grain Legume Crops. Collins, London, pp. 537±544. Uriel, J., Bergas, J., 1968. Characterization of natural inhibitors of trypsin and chymotrypsin by electrophoresis in acrylamide agarose gels. Nature 218, 578±580. Xavier-Filho, J., Campos, F.A.P., 1989. Proteinase inhibitors. In: Cheeke, P.R. (Ed.), Toxicants of Plant Origin, vol. III. CRC Press. Xavier-Filho, J., Campos, F.A.P., Ary, M.B., Silva, C.P., Carvalho, M.M.M., Macedo, M.L.R., Lemos, F.J.A., Grant, G., 1989. Poor correlation between the levels of proteinase inhibitors found in seeds of dierent cultivars of cowpea (Vigna unguiculata ) and the resistance/susceptibility to predation by Callosobruchus maculatus. Journal of Agriculture and Food Chemistry 37, 1139±1143. Xavier-Filho, J., 1991. The resistance of seeds of cowpea (Vigna unguiculata ) to the cowpea weevil (Callosobruchus maculatus ). Memoris Institute of Oswaldo 86, 75±77.
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Chemical basis of resistance in pulses to Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) S. Ignacimuthu*, S. Janarthanan, B. Balachandran Entomology Research Institute, Loyola College, Chennai-600 034, India Accepted 1 July 1999
Abstract Seeds of various wild and cultivated pulses express varied degrees of resistance to damage by Callosobruchus maculatus, a major pest of stored pulses. Seeds were analysed for trypsin/chymotrypsin inhibitors (qualitatively and quantitatively) and protein pro®les (SDS-polyacrylamide gel electrophoresis) to study the chemical basis of resistance to bruchid infestation. Seeds of the wild pulses Dunberia ferruginea, Neonotonia wightii, Tephrosia cararensis, Cajanus albicans and Vigna bourneae dier from other wild germplasms as well as cultivars in having characteristically higher levels of trypsin/ chymotrypsin inhibitors. SDS-PAGE analyses of seed proteins of Dolichos lablab, Lablab purpureus and Rhyncocea cana reveal the presence of high levels of non-nutrient chemicals such as vicilins, a-amylase inhibitors, and lectins in quantities that may impart resistance to C. maculatus. The importance of these protease inhibitors and other non-nutrient chemicals are discussed with reference to resistance to C. maculatus infestation. # 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: Pulses; Trypsin inhibitor; Chymotrypsin inhibitor; SDS-PAGE; Bruchid; Resistance
1. Introduction Pulses are a good source of dietary proteins and other essential nutrients. However, postharvest insect infestation severely aects quality and storability of produce (Singh, 1978; Steele et al., 1985). Pulses are known to contain a variety of non-nutrient compounds such as lectins, proteinase inhibitors, a-amylase inhibitors and other compounds which could be part of an array of constitutive defences against attack by stored grain pests, particularly those belonging * Corresponding author. Tel.: +91-44-826-5542; fax: +91-44-826-5544. E-mail address: [email protected] (S. Ignacimuthu) 0022-474X/99/$ - see front matter # 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 2 - 4 7 4 X ( 9 9 ) 0 0 0 3 1 - 4
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to the family Bruchidae (Janzen et al., 1986; Gatehouse et al., 1990). While the exact nature of resistance in certain pulses to Callosobruchus maculatus (F.) remains largely unknown, some recent studies emphasize the presence of certain non-nutrient (storage) proteins in seeds of resistant varieties as the basis of resistance (Macedo et al., 1993). In addition, trypsin and chymotrypsin inhibitors, besides their well-known function as storage proteins, are also important in imparting resistance (Liener and Kakade, 1980; Ryan, 1990). The ecacy of any secondary (non-nutrient) compound is indicated by biological assay against the insect pest (Agarwal et al., 1988). Several studies suggest the importance of biological assay to assess the potential of such compounds to impart resistance (Gatehouse et al., 1989; Cardona et al., 1990; Xavier-Filho, 1991; Pusztai et al., 1993; Macedo et al., 1995; Fory et al., 1996). In view of the variation observed in damage due to feeding by C. maculatus on various wild as well as cultivated pulses, the present study was planned to identify chemical factors that are responsible for imparting resistance.
2. Materials and methods 2.1. Seeds Seeds of the wild pulses were collected from the Palney Hills of the Western Ghats, Tamilnadu, India (latitude 108; longitude 788; altitude 2010 m from MSL), whereas some cultivated seeds (Cajanus cajan (L.) Millspaugh ICP14770 and ICP14990; Cicer arietinum (L.) COG29 and CO3; Vigna unguiculata (L.) Walp. CO4 and V. mungo (L.) Hepper CO5) were obtained from the Department of Plant Breeding, Tamil Nadu Agricultural University, Coimbatore, India. The other cultivated seeds namely, V. unguiculata VCP8; V. mungo VBN1; V. radiata (L.) Wilczek K851 and UGG4 were obtained from National Pulses Research Centre, Vamban, Tamil Nadu, India. 2.2. Chemical analysis Trypsin and chymotrypsin inhibitors were estimated following the methods described by della Gatta et al. (1989) and Erlanger and Frances Edel Cooper (1996). They were expressed in terms of units/g seed materials (a trypsin or a-chymotrypsin unit (TU or CU) is de®ned as the amount of trypsin or a-chymotrypsin that catalyses the hydrolysis of 1 mol of substrate per min. A trypsin or chymotrypsin inhibitor unit (TIU or CIU) is the reduction in activity of the respective enzymes by one unit, i.e. TU or CU respectively. 2.3. Electrophoretic studies on trypsin and chymotrypsin inhibitors Seeds were milled to a ®ne powder (after removal of the seed coat) and protease inhibitors were extracted by incubating overnight in Tris±HCl±CaCl2 buer, pH 8.1 (at a ratio of 1:10 w/ v) at 48C. The homogenate (the ®ne powder with the extraction buer) was subsequently centrifuged at 12,000 rpm at 48C for 15 min. The supernatant collected was used for electrophoretic analysis with polyacrylamide gel electrophoresis (Native PAGE) as described by
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Kollipara et al. (1991). At the end of the run, the stacking gel was removed and the separating gel stained for trypsin and chymotrypsin inhibitor activities based on Uriel and Bergas (1968) with some modi®cations as described by Kollipara and Hymowitz (1992). 2.4. Seed protein pro®le To analyse proteins of both wild and cultivar pulses, SDS-polyacrylamide gel electrophoresis (SDS-PAGE) was performed according to the method of Laemmli (1970). Proteins were extracted from powdered pulses with 0.1 M Tris±HCl buer, pH 8.0 (at a ratio of 1:10 w/v) using sterilised glass powder in a mortar and pestle for 15 min at 0±58C (on ice). Extracts were centrifuged at 15,000 g for 10 min at 58C. Aliquots of supernatant were used to quantify the proteins using the Bradford method with BSA as standard (Bradford, 1976). An equal quantity of each of the protein samples (100 mg) was applied to the gels. Standard molecular marker proteins (Sigma) were simultaneously run for comparison. Table 1 Trypsin and chymotrypsin inhibitor contents (units/g) in wild and cultivated germplasms1 Varieties
Trypsin inhibitors
Chymotrypsin inhibitors
29602 05.7a 27742 04.2b 27002 09.0b 18862 07.2c 10352 04.1d 8902 14.7e 5232 09.0f 4402 04.8g 3382 06.5h 3002 12.2i 1342 03.2j
4122 34.9a 4252 16.3a 3852 13.1b 2692 14.8c 1482 06.5d 1252 04.9e 1362 11.4de 62 2 14.7f 46 2 07.3gf 43 2 05.7g 22 2 07.3h
12382 02.5a 12332 36.2a 10752 33.6b 7002 25.3c 5752 16.3d 4182 01.2e 4082 10.7e 3352 01.8f 3332 18.0f 2892 09.8g
1532 6.0a 1772 5.6b 1522 3.4a 60 2 7.3c 82 2 2.5d 60 2 2.5ec 58 2 5.8ec 48 2 4.9f 0 2 0.0 41 2 6.5f
Wild pulses Cajanus albicans Dunberia ferruginea Neonotonia wightii Vigna bourneae Tephrosia cararensis Lablab purpureus(brown) Rhyncocea rufescens Rhyncocea cana Dolichos lablab Lablab purpureus (black) Canavalia virosa Cultivars Cicer arietinum CO3 Cicer arietinum COG 29 Vigna unguiculata CO4 Vigna unguiculata VCP8 Cajanus cajanICP 14770 Vigna radiataUGG4 Cajanus cajan ICP 14990 Vigna mungo VBN1 Vigna radiata K851 Vigna mungo CO5
1 Values represent mean 2SE of four replications. Data analysed using Student's t-test (P < 0.01). Values in the same column with the same superscript letters are not signi®cantly dierent.
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2.5. Bioassay A stock culture of C. maculatus was reared on green gram (V. radiata K851 cultivar) seeds at 282 18C in glass jars covered with muslin cloth. For feeding bioassays, seeds were conditioned at 282 18C and 70% r.h. for a period of 10 days before use. Seeds of each cultivar/wild germplasm were kept in glass vials covered with muslin cloth and a single pair of newly emerged adults of C. maculatus from the stock culture was introduced into each of six vials for oviposition. Six replicates were maintained for each variety. Vials were maintained at 282 18C and 70% r.h. After 5 days, the numbers of eggs laid on the seeds of each variety were counted. Such seeds were observed for a maximum duration of 60 days and the numbers of emerging adults were counted for a duration of 8 days with adults being isolated to prevent further breeding. Developmental period, percentage adult emergence and percentage loss in weight [(initialÿ®nal)/initial seed weight 100] were calculated to evaluate developmental performance of C. maculatus on cultivars and wild germplasm of pulses. 2.6. Statistical analysis All data were subjected to Student's t-test and the level of signi®cance was tested at P < 0.01. Each value was statistically compared with other values of the same parameter. 3. Results Seeds of wild germplasms generally were found to contain higher amounts of trypsin and chymotrypsin inhibitors than cultivated seeds (Table 1). Maximum levels of trypsin inhibitors
Fig. 1. Electrophoretogram showing trypsin inhibitor bands in the wild germplasms. Note strong reaction at the region of Rf 0.60±0.80. Wild pulses: (a) Rhyncocea rufescens, (b) Rhyncocea cana, (c) Canavalia virosa, (d) Lablab purpureus (black), (e) Lablab purpureus (brown), (f) Tephrosia cararensis, (g) Dolichos lablab.
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(TI) were observed in Cajanus albicans (Wight and Arnott) Maesen with 2960 units/g followed by seeds of Dunberia ferruginea Wight and Arnott and Neonotonia wightii (Wight and Arnott) Lackey which exhibited 2774 units/g and 2700 units/g, respectively. The seeds of a brown variety of Lablab purpureus (L.) Sweet were found to contain a signi®cantly (P < 0.01) higher amount of TI than the black variety. A signi®cant (P < 0.01) dierence in TI content was noticed at intraspeci®c levels between cultivars (Table 1) except in Cicer arietinum where no signi®cant dierences were observed. The amount of TI was always greater than the amount of chymotrypsin inhibitor. Among wild germplasms the amount of chymotrypsin inhibitors (CI) were higher in D. ferruginea (425 units/g), C. albicans (412 units/g) and N. wightii (385 units/g) (Table 1). Interestingly, CI was absent from the V. radiata K851 cultivar. Although the range of inhibitors in both the cultivated and wild accessions overlapped, overall the levels were signi®cantly higher in the wild accessions. Electrophoretograms (Fig. 1) showed prominent bands of TI in the seeds of L. purpureus (black) at Rf values 0.75 and 0.80, whereas L. purpureus (brown) showed two successive bands at the same region with Rf values 0.75 and 0.77, respectively. Although the other wild genotypes such as Rhyncocea rufescens (Willdenow) de Candolle and D. ferruginea reveal a clear TI band at Rf value 0.60, the seeds of V. bourneae Gamble, on the other hand, exhibited three bands at Rf values 0.50, 0.60 and 0.70. Among the various cultivars, gel electrophoresis revealed a TI band at Rf value 0.70 in the cultivated seeds of V. radiata (K851 and UGG4) and C. arietinum (COG 29 and CO3) (Fig. 2). Interestingly, the other cultivated seeds did not reveal any inhibitor bands.
Fig. 2. Electrophoretogram showing trypsin inhibitor bands. Note the absence of reaction in cultivated germplasms. Wild pulses: (a) Cajanus albicans, (b) Dunberia ferruginea, (c) Vigna unguiculata, (d) Neonotonia wightii. Cultivars: (e) Vigna radiata K851, (f) Vigna radiata UGG4, (g) Cicer arietinum COG29, (h) Cicer arietinum CO3.
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Four intense CI bands at Rf values 0.65, 0.75, 0.77 and 0.80 were observed in L. purpureus (black) while the brown variety of L. purpureus showed three intense bands with Rf values 0.75, 0.77 and 0.80 (Fig. 3). The seeds of Canavalia virosa (Roxburgh) Wight and Arnott had two intense bands at Rf values 0.45 and 0.75, whereas Rhyncocea rufescens had one prominent band at Rf value 0.60 and two more bands at Rf values 0.77 and 0.80. Two bands at Rf values 0.77 and 0.80 were observed in the seeds of R. cana (Willdenow) de Candolle while only one band at Rf value 0.60 was observed in the seeds of Dolichos lablab L. The electrophoretogram showed no CI bands in the cultivated seeds. In the present investigation, trypsin and chymotrypsin inhibitors varied in both number and migration patterns among various wild germplasms of pulses. There was also variation in intensity of these bands and some seeds showed no bands (Figs. 1±3). This could be due to lower concentration of an inhibitor protein or lower anity (weaker inhibition) for bovine trypsin or a-chymotrypsin. Inhibitor contents relative to wild germplasms and cultivar pulses are presented in Table 1. A broad variability of activities among samples was observed for all tested inhibitors. Wild varieties of pulses, D. ferruginea, N. wightii, C. albicans, R. rufescens and V. bourneae used in this study were characterised by higher values of both TI and CI (Table 1). Among them, C. albicans, D. ferruginea, R. rufescens were found to be infested by the bruchid, C. maculatus (see results of bioassay studies, Table 2) or, in other words, the susceptible varieties (reported above) showed higher inhibitor activity against trypsin and chymotrypsin. The other wild germplasms exhibiting lower inhibitor activity against trypsin and chymotrypsin were found to be susceptible (except the genus Lablab, in which the gels showed more intense TI and CI bands). SDS-PAGE seed protein pro®les showed dierences in banding patterns among various wild germplasm lines (Fig. 4). Major polypeptide components at 94, 56 and 52 kD regions were identi®ed in wild accessions of L. purpureus (brown), R. cana and D. lablab. All the other wild
Fig. 3. Electrophoretogram showing chymotrypsin inhibitor bands in the wild germplasm. Note strong reaction at the region of Rf 0.45±0.80. Wild pulses: (a) Rhyncocea rufescens, (b) Rhyncocea cana, (c) Canavalia virosa, (d) Lablab purpureus (black), (e) Lablab purpureus (brown), (f) Tephrosia cararensis, (g) Dolichos lablab and (h) Cajanus albicans.
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Table 2 Biological performance of Callosobruchus maculatus on dierent species of wild and cultivated pulses1 Varieties
Fecundity (No. of eggs/female)
Adult emergence (%)
Developmental period (days)
Seed damage (%)
45 2 3.6a Nil2 Nil2 Nil2 Nil2 Nil2 41 2 5.8a 20 2 5.2b 57 2 7.2c Nil2 ±
37 2 1.6a ± ± ± ± ± 32 2 1.4b 34 2 3.0ab 38 2 0.7a ± ±
19 2 1.7a ± ± ± ± ± 14 2 2.8b 4 2 1.4c 15 2 3.4a ± ±
39 2 1.4a 47 2 2.6b Nil2 2 Nil 69 2 5.9c 63 2 4.2cd 75 2 6.8ce 70 2 5.3cf 77 2 1.5e 72 2 4.2ef
27 2 2.2a 25 2 1.6a ± ± 28 2 4.1a 27 2 3.5a 24 2 1.5a 24 2 1.3a 25 2 2.5a 26 2 0.5a
39 2 2.5a 24 2 3.1b ± ± 43 2 3.5ac 37 2 1.6a 38 2 1.4a 37 2 2.0a 44 2 4.5ac 42 2 1.7c
Wild pulses Cajanus albicans Dolichos lablab Vigna bourneae Lablab purpureus (brown) Canavalia virosa Lablab purpureus (black) Dunberia ferruginea Rhyncocea cana Rhyncocea rufescens Neonotonia wightii Tephrosia cararensis
96 2 6.3a 76 2 5.4b 71 2 5.6bc 70 2 0.7cd 68 2 4.0cde 59 2 9.0e 34 2 8.3f 27 2 6.4fg 25 2 4.6g 14 2 2.3h ±
Cultivars Cajanus cajan ICP14770 Cajanus cajan ICP14990 Cicer arietinum COG29 Cicer arietinum CO3 Vigna unguiculata CO4 Vigna unguiculata VCP8 Vigna mungo CO5 Vigna mungo VBN1 Vigna radiata K851 Vigna radiata UGG4
1202 8.8a 1132 9.5a 27 2 2.1b 19 2 3.6c 85 2 5.8d 97 2 4.6e 58 2 6.2f 49 2 3.4g 76 2 3.5h 88 2 6.5id
1
Values represent mean 2 SE of six replications. Data analysed by Student's t-test (P < 0.01); ±, not tested. Values in the same column with the same superscript letters are not signi®cantly dierent. 2 No adult emergence was observed.
germplasms showed variations in polypeptides of low molecular weights in the range of 20±45 kD. All cultivar accessions show similar banding patterns except Cicer arietinum (Fig. 5). In general, polypeptide components of 94, 66, 56, 52 and 30 kD were identi®ed in all accessions of cultivars together with some minor bands. Developmental performance of C. maculatus on various varieties of wild germplasms has been summarized in Table 2. Maximum numbers of eggs (96) were laid on the seeds of C. albicans, whereas eggs were not laid on the seeds of Tephrosia cararensis de Candolle. Variation in percentage adult emergence was observed in both cultivated and wild germplasms with observed higher values in wild germplasms. Percentage adult emergence was found to be 41, 45, 57 and 20 in the wild seeds of Dunberia ferruginea, C. albicans, R. rufescens and R. cana, respectively (Table 2). Adult emergence was not observed from other wild germplasms. Accordingly, they supported lower levels of infestation by the bruchid as is re¯ected by
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Fig. 4. SDS-PAGE pro®le of wild germplasms. (a) Lablab purpureus (brown), (b) Lablab purpureus (black), (c) Canavalia virosa, (d) Cajanus albicans, (e) Dunberia ferruginea, (f) Tephrosia cararensis, (g) Vigna unguiculata, (h) Rhyncocea rufescens, (i) Rhyncocea cana, (j) Neonotonia wightii and (k) Dolichos lablab.
percentage seed damage. Minimum percentage of seed damage was recorded in R. cana (4%) and maximum percentage of seed damage was recorded in C. albicans (19%). The dierence in percentage seed damage was statistically signi®cant (P < 0.01) among infested seeds of wild germplasms (Table 2). Oviposition, percentage adult emergence and percentage seed damage from cultivated seeds showed interspeci®c as well as intraspeci®c variation. Callosobruchus maculatus laid the maximum number of eggs on the seeds of C. cajan ICP 14770 (120), while the minimum number of eggs (27, COG29 and 19, CO3) was observed on the seeds of C. arietinum (Table 2). However, in contrast to the wild germplasms, successful adult emergence was observed from all the cultivated seeds except for C. arietinum (COG 29 and CO3) (Table 2). Likewise, percentage adult emergence was higher in various species of genus Vigna than Cajanus, with the maximum being observed from the seeds of V. radiata K851 (77%). Nevertheless, intraspeci®c variations were also observed. Developmental period of C. maculatus in cultivated seeds was shorter than in wild seeds within the same genus (Table 2). Seed damage or loss in seed weight in insect infestation studies (bioassay studies) was also carried out. Among cultivars, seeds of C. cajan ICP 14990 showed lesser loss in grain weight (24%) when compared to other cultivar varieties (Table 2).
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Fig. 5. SDS-PAGE pro®le of cultivated seeds. (a) Vigna unguiculata VCP8, (b) Vigna unguiculata CO4, (c) Cicer arietinum COG29, (d) Cicer arietinum CO3, (e) Vigna mungo CO5, (f) Vigna mungo VBN1, (g) Vigna radiata K851, (h) Vigna radiata UGG4, (i) Vigna radiata CO3, (j) Vigna radiata CO4, (k) Cajanus cajan ICP 14770 and (l) Cajanus cajanICP 14990.
4. Discussion Identi®cation and analysis of sources of resistance in seeds provide us with opportunities to identify and breed pulse varieties that can naturally resist bruchid attacks. The production of grain legumes is devastated by C. maculatus and, with current limitations on chemical control of stored grain pests, the probability of identifying the chemical basis of resistance in pulses, speci®cally the enzyme inhibitors and some deterrent proteins, prompted us to pursue this study. Protease inhibitors are proteins or polypeptides which bind proteolytic enzymes and, in plants which produce them, are thought to provide a form of natural defence against herbivorous insects (Green and Ryan, 1972; Ryan, 1979; Broadway et al., 1986; Xavier-Filho and Campos, 1989; Burgass et al., 1994). In the present investigation in cultivars, both quantitative and qualitative analyses on TI and CI showed lower activities when compared to those from their wild counterparts, except C. arietinum, and all were susceptible to C. maculatus. It has been explained that, since a threshold value to discriminate between resistant and susceptible lines cannot be established for any tested inhibitors, evaluating only one or two antimetabolites cannot provide an eective way of checking the degree of resistance towards storage pests (Piergiovanni et al., 1994). This conclusion agreed with previous studies where several antimetabolites in a larger number of germplasms of pulses had been analysed (Baker et al., 1989; Xavier-Filho et al., 1989; Cardona et al., 1990; Piergiovanni et al., 1991; Fory et
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al., 1996). It seems more likely, in the light of these observations, that more antimetabolites could be involved in the mechanism of bruchid beetle resistance in the case of the present pulse varieties. Furthermore, antinutrients, ``like the vicilins (55 kD polypeptides), lectins (30 kD polypeptides), and a-amylase inhibitors (13±17 kD polypeptides) which are reported to have a role in imparting resistance to bruchids'' (Gatehouse et al., 1989; Pratt et al., 1990; Pereira De Sales et al., 1992; Macedo et al., 1993, 1995; Fory et al., 1996), may also be involved in imparting resistance in the present investigation. The present work on protein pro®les enumerates the presence of bands of 56 and 52 kD in wild accessions of Dolichos lablab, R. cana and L. purpureus under denaturing conditions. In addition, our results displayed several minor bands of higher mobilities on SDS-PAGE gels which match well with the protein antimetabolites reported in earlier ®ndings (Liener and Kakade, 1980; Ryan, 1990; Macedo et al., 1993, 1995). Acknowledgements The authors gratefully acknowledge the ®nancial assistance and support provided by a grant to Fr S. Ignacimuthu from the Department of Biotechnology, Government of India, New Delhi. We also thank Dr Alok Sen, Entomology Research Institute, Loyola College, Chennai, for his fruitful and critical suggestions. References Agarwal, A., Sone, Lal, Gupta, K.C., 1988. Natural products as protectants of pulses against pulse beetles. Bulletin of Grain Technology 26, 154±164. Baker, T.A., Nielsen, S., Shade, R.E., Singh, B.B., 1989. Physical and chemical attributes of cowpea lines resistant and susceptible to Callosobruchus maculatus (F.) (Coleoptera: Bruchidae). Journal of Stored Products Research 25, 1±8. Bradford, M.M., 1976. A rapid and sensitive method for the quanti®cation of microgram quantities of protein utilising the principle of dye binding. Analytical Biochemistry 72, 248±254. Broadway, R.M., Duey, S.S., Pearce, G., Ryan, C.A., 1986. Plant proteinase inhibitors: a defence against herbivorous insects? Entomologia Experimentalis et Applicata 41, 33±38. Burgass, E.P.J., Main, C.A., Stevens, P.S., Christeller, J.T., Gatehouse, A.M.R., Laing, W.A., 1994. Eects of protease inhibitor concentration and combinations on the survival, growth and gut enzyme activities of the black ®eld cricket, Teleogryllus commodus. Journal of Insect Physiology 40, 803±811. Cardona, C., Kornegay, J., Posso, C.E., Morales, F., Ramirez, H., 1990. Comparative value of four arcelin variants in the development of dry bean lines resistant to the Mexican bean weevil. Entomologia Experimentalis et Applicata 56, 197±206. della Gatta, C., Piergiovanni, A.R., Ng, N.Q., Carnovale, E., Perrino, P., 1989. Trypsin inhibitor levels in raw and cooked cowpea (Vigna unguiculata ) seeds. Lebensmittel-Wissenschaft und Technologie 22, 78±80. Erlanger, B.F., Frances Edel Cooper, A.G., 1996. The action of chymotrypsin on two new chromogenic substrates. Archives of Biochemistry and Biophysics 115, 206. Fory, L.F., Finhardi-Filho, F., Quintero, C.M., Osborn, T.C., Cardona, C., Chrispeels, M.J., Mayer, J., 1996. Alpha amylase inhibitors in resistance to common beans to the Mexican bean weevil and the bean weevil (Coleoptera: Bruchidae). Journal of Economic Entomology 89, 204±210. Gatehouse, A.M.R., Dewey, F.M., Dove, J., Fenton, K.A., Pusztai, A., 1989. Eect of seed lectins from Phaseolus vulgaris on the development of larvae of Callosobruchus maculatus. Mechanism of toxicity. Journal of the Science of Food and Agriculture 35, 373±380.
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