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Systemic acquired resistance induced by tobacco mosaic virus in Nicotiana tabacum cv. Xanthi-nc does not disappear at 32°C
Physiological
Plant Pathology
(1984)
25, 307-312
Systemic acquired resistance induced by tobacco mosaic virus in Nicotiana tabacum cv. Xanthi-nc does not disappear at 32°C EMMA A. WESTSTEIJN Willie
Commelin
:Acceptedfor
Scholten Phytopathological
publication
July
Laboratory,
Jaualaan
20, 3742 CP Burn,
The Netherlands
1984)
Systemic acquired resistance to tobacco mosaic virus (TMV) was induced in Nicotiana tabacum cv. Xanthi-nc plants by incubating them at 22 “C for 9 days after inoculating them with TMV. After induction of resistance the plants were incubated for varying periods of time at either 22 C or 32 “C before challenge inoculation with TMV on the live leaves above the primary inoculated ones. They were then incubated at 22°C. The level of acquired resistance, as measured by the reduction in lesion size relative to the controls, in plants that had been incubated at 32 “C for up to 8 days, was similar to the level in plants that had been incubated continuously at 22°C. Thus, systemic acquired resistance is maintained for at least 8 days in plants incubated at 32 “C.
INTRODUCTION
Acquired resistance against virus infection can be induced in plants by a previous infection with various viruses [7, 1.5, 27, 18, 24-261, fungi [13, 221 or bacteria [I, 311, as well as by treatment with a number of chemical compounds including polyacrylic acid [3, 5, 10, 11, 161, aspirin [3, 331, salicylic acid [19, 331, benzoic acid [33], ethephon [19] and several polyanions [30]. Acquired resistance is expressed either as a reduction in lesion size or as a reduction in lesion number or both. The appearance of new proteins, so-called pathogenesis-related proteins (PRs) [2], is a phenomenon associated with pathogen-induced resistance [I, 3, 7, 15, 17, 211. These proteins are believed to be causally involved in this resistance. As the same PRs appear in chemically induced acquired resistance [3, 11, 16, 19, 331, this type of resistance and pathogen-induced resistance are considered to be due to a similar mechanism. Most of the investigations on acquired resistance have been carried out with tobacco and tobacco mosaic virus (TMV). Depending on the genetic constitution of the host, this virus induces a systemic mosaic or a local hypersensitive reaction. The hypersensitive reaction however occurs only at low temperatures, while at temperatures above 28 “C no local lesions develop, the virus spreads systemically and induces a systemic mosaic [27]. Both virus and chemically induced resistance in tobacco have been reported to Abbreviations 0048-4059/84/060307
used in text:
PRs,
+ 06 $03.00/O
pathogenesis-related
proteins; 0
TMV,
1984 Academic
tobacco
mosaic
Press Inc.
virus. (London)
Limited
308
E. A. Weststeijn
break down at 3‘2 “C [14, 281. In leaves in which resistance was induced by treatment [15], the with polyacrylic acid [IO, II] or aspirin [33] or by systemic virus infections reduction in lesion number compared to non-induced plants was lower in plants incubated at 32 “C for 2 days before challenge inoculation than in plants incubated at lower temperatures. However, information on the effect of incubating plants in which acquired resistance has been induced by TMV for a period at 32 “C, on the subsequent development of lesions at 22°C from a challenge inoculum is lacking. This acquired resistance is primarily characterized by a reduction in the size of lesions [ 25,26]. In a previous study [32] evidence was obtained which indicated that the resistance operating locally around primary lesions and in systemically resistant leaves upon challenge inoculation is expressed as a reduction in virus spread at 32 ‘C. In this paper, the effect ofincubating plants with systemic acquired resistance at 32 “C beforc challenge inoculation at 22 C was investigated. EXPERIMENTAL AND RESULTS Systemic acquired resistance was induced in Nicotiana tabacum cv. Xanthi-nc plants by inoculating them on three lower leaves with a suspension of purified TMV [12] which gave rise to 100-200 lesions per leaf. Control plants were left uninoculated. .After 9 days in the greenhouse at 22-24-C, the inoculated leaves were cut off to prevent systemic invasion of the rest of the plant by the virus during incubation at 32 C. At the same time the corresponding leaves were removed from the control plants. Some of the systemically resistant and control plants were challenge inoculated with TMV on the live leaves immediately above the ones removed and then incubated in the greenhouse at 22224 “C. Other plants were incubated at 22 “C in the greenhouse, or at 32 “C in a growth chamber, for a further 4 or 8 days before challenge inoculation with TMV on the live leaves as described above. After challenge inoculation the plants were incubated in the greenhouse. To assess the level of induced resistance, lesion size was determined 7 days after challenge inoculation, by measuring two diameters at right angles per lesion, with the aid of a magnilier with a micrometer. Lesion numbers were also recorded. Figure 1 shows that lesions developing from a challenge inoculation in systemically resistant leaves were always smaller than those in corresponding control leaves, irrespective of whether the plants had been incubated at 22 “C or at 32 “C after induction of resistance. The level of acquired resistance, as measured by the degree of’ reduction in lesion size over that on control plants was similar on both 22”C- and 32 “C-treated plants, but varied according to leaf age, being greater in the younger leaves than in older ones. Significantly larger lesions were formed in leaves on systemically resistant plants and on control plants which had been incubated at 32 “C for 4 or 8 days before the challenge inoculation than in leaves on plants which had been incubated at 22 “C. Thus the resistance mechanism had been weakened to a similar extent in both systemically resistant and in control leaves by the 32 “C treatment. No consistent differences were found in lesion numbers between systemically resistant and control plants, or between plants incubated at 32 “C and 22 “C before challenge inoculation.
Acquired
resistance
to TMV in Nicotiana
309
1 (a)
3
0
(e)
Cd)
3-
0-
I
I234 Leaf
5
position
FIG. 1. Size of TMV lesions in systemically resistant and control leaves of Nicotiana tabacum cv. Xanthi-nc, 7 days after challenge inoculation. Systemic acquired resistance was induced by incubating the plants at 22 “C for 9 days after inoculation with TMV. Challenge inoculations were carried out immediately (a) ; after a further incubation for 4 days at 22 “C (b) or at 32 ‘C (c) ; or after a further incubation for 8 days at 22 “C (d) or at 32 “C (e). All plants were incubated at 22°C after challenge inoculation. 0, Systemically resistant; 0, control. Each point is the mean from eight leaves, 10 lesions measured per leaf. Leaf 1 is the leaf immediately above the primary inoculated ones or the corresponding leaf on control plants. Lesion sizes were compared using Wilcoxon’s two sample test. Lesion sizes in systemically resistant leaves were significantly different from those in control leaves [a = 0.005 for all pairs except for leaf 1 (a ~~ 0.05) and leaf 2 (a = 0.025) both in Fig. 1 (e)]. Bars represent standard deviations.
DISCUSSION The results described in this report show that the extent of reduction in lesion size in plants due to acquired resistance was similar for plants incubated for up to 8 days at 22 “C and at 32 “C before challenge inoculation. Thus, in contrast to statements in two reviews [14, ,281 systemic acquired resistance does not disappear at 32 “C.
310
E. A. Weststeijn
In studying the effects of temperature on acquired resistance it is important to distinguish between conditions affecting the induction of resistance and those affecting its expression. It is also important to realize that there are two parameters which can be used to measure acquired resistance, namely lesion size and lesion number. This discussion will, unless otherwise stated, be confined to results obtained with AL tabacum and TMV, since most investigations on acquired resistance have been carried out with this host-virus system. Lesion size and lesion number are independent characteristics of the reaction of a leaf to inoculation with virus. For example, with increasing leaf age larger numbers of smaller lesions develop [18], with increasing temperature after inoculation fewer but larger lesions develop [9] and treatment with abscisic acid for 14 days before inoculation leads to fewer but smaller lesions to develop [7]. Pathogen-induced systemic resistance is primarily characterized by a reduction in lesion size [25,X], but with increasing levels of resistance a reduction in lesion number is found as well [I, 6, 7, 25, 261. H owever, TMV-induced resistance in ;L’icotania glutinosa is characterized by a reduction in lesion size but an increase in lesion numbers [8J. This latter case is a further example of the independency between lesion number and size. Chemically induced acquired resistance leads to reduction in both lesion number and size [5, 11, 19, 331. Treating plants with polyacrylic acid at 32 “C does not induce resistance [IO, I/]. In contrast, treatments with salicylic acid and aspirin at 32 “C do induce resistance. \;t’hen lesion size is used as a parameter, this resistance appears to be expressed at 32’ C to the same extent as at 22 “C [19]; with lesion numbers as a parameter, resistance is found at 22 “C but not at 32 “C [19, 331. The resistance induced b>. treatments with polyanions at 20 “C is expressed at 32 “C as a reduction in lesion size, but not in lesion number [30]. The same holds for TMV-induced resistance [4, 321. Incubating leaves for several days at 32 “C, after the induction of resistance with polyacrylic acid [IO, II], aspirin [33] or systemic virus infections [25] at 20 “C, caused them all to be less resistant in terms of the numbers of lesions formed but in the case of polyacrylic acid treatments the lesions were small [II]. The results of the current study show that incubation for up to 8 days at 32 “C, after induction of sy-stem& acquired resistance with TMV at 22 “C, did not affect the level of resistance as regards lesion size. Taken altogether these results indicate that when acquired resistance is considered only in relation to its effects on number of lesions then it appears to break down after treatments at 32 “C, but if it is considered only in relation to lesion size then it appears unaffected by treatments at 32 “C. Some of the confusion in the literature about tht effects of treatments at 32 “C on the breakdown of acquired resistance has arisen because some workers have measured changes in lesion number only and havpc ignored or overlooked changes in lesion size. Nevertheless, when acquired resistance is considered in relation to its effects on lesion size, temperature conditions during induction and expression of resistance appear to be important. Thus, higher levels of systemic acquired resistance are induced by TMV in plants which are incubated at 29 “C rather than 19 ‘C after the primary- inoculation [25], but when resistance is induced at 2 1 “C, the level of resistance
Acquired
resistance
to TMV in Nicotiana
311
expressed against challenge inoculations at 2O”C, 26°C and 29°C reduces as the temperature increases [2.5]. Resistance induced at 22 “C with TMV is also expressed to some extent at 32 “C [32], although TMV infection at that temperature does not lead to necrosis or virus localization. This implies that induced resistance involves inhibition of virus multiplication, or spread, or both. The observation that tobacco necrosis virus induces resistance to TMV in Samsun tobacco plants, a systemic host for TMV [20], supports this conclusion. As the processes of the hypersensitive reaction occur earlier and faster in leaves with systemic acquired resistance than in those without [23, 291, it is conceivable that acquired resistance consists of both a reduction in virus multiplication [29] and a stimulation ofthe processes involved in virus localization. With increasing temperature the localization mechanism becomes less efficient [9], until at 32 “C it no longer operates. However, once acquired resistance has been induced at a lower temperature, the increased capacity of the leaves to react with an early hypersensitive reaction is not lost at 32 “C but is expressed again when the plants are returned to a lower temperature. REFERENCES I. AH., P., BENJAMA, A., SAMSON, R. & GIANINAZZI, S. (1981). Induction chez le tabac par Pseudomonas syringae dr nouvelles prottines (protCines ‘b’) associ&s au d&elopement d’une rt.sistance nonsptcilique ?I une deuxieme infection. Phytopathologixhe Zeit~hrift 102, 201-212. 2. AWTONIW, J. F., RITTER, C. E., PIERPOINT, W. S. & LOON, L. C. VAN (1980). Comparison of three pathogenesis-related proteins from plants of two cultivars of tobacco infected with TMV. Journal of General Virology 47, 79-87. 3. ANTONIW, J. F. & WHITE, R. F. (1980). The effects of aspirin and polyacrylic acid on solublr leaf proteins and resistance to virus infection in live cultivars of tobacco. Phytopathologische Zeit~chriJt 98,331-341. 4. BAI&S, E., SZIR~~KI, I. & KIRALY, Z. (1977). The r8le of cytokinins in the systemic acquired resistance of tobacco hypersensitive to tobacco mosaic virus. Physiological Plant Pathology 11,29%37. 5. CASSELLS, A. C., BARNETT, A. & BARLASS, M. (1978). The effect of polyacrylic acid treatment on the susceptibility of Nicotiana tabacum cv. Xanthi nc to tobacco mosaic virus. Physiological Plant Pathology 13, 13-21. 6. FRASER, R. S. S. (1979). Systemic consequences of the local lesion reaction to tobacco mosaic virus in a tobacco variety lacking the N-gene for hypersensitivity. Physiological Plant Pathology 14, 383. 394. 7. FRASER, R. S. S. (1982). Are “pathogenesis-related” proteins involved in acquired systemic resistance of tobacco plants to tobacco mosaic virus? Journal of&era1 Virology 58, 305-313. 8. FRASER, R. S. S., LOUGHLIN, S. A. R. & WHENHAM, R. J. (1979). Acquired systemic susceptibility to infection by tobacco mosaic virus in Nicotiana glutinosa L. Journal of General Virology 43, 131-141. 9. GABORJANYI, R. & ELHAMMADY, M. (1969). Effect of temperature on size of local lesions induced by TMV in tobacco plants. Acta Phytopathologica Academiae Scientiarum Hungaricae 4, 125-l 29. 10. GIANINAZZI, S. (1982). Anti-viral agents and inducers of virus rrsistance: analogies with interferon. In A&e Defence MechaniJms in Plants, Ed. by R. K. S. Wood, pp. 275~.298. Plenum Prrss, New York & London. inducrd in plants by polyacrylic acid. Journal I I. GIANINAZZI, S. & KASSANIS, B. (1974). V’ lrus rrsistance oJ’Genera1 Virology 23, 1-9. 12. GOUUING, G. \‘. & HEBERT, T. T. (1967). A simple technique for purification of tobacco mosaic virus in large quantities. Phytopathology 57, 1285. 13. HE.C~IT, E. I. & BATEMAN, D. F. (1964). N on-specific acquired resistance to pathogens resulting from inltiction by Thielaviopsir b&cola or viruses in tobacco. Phytopathology 54, 523-530. I+. KASSAUIS, B. (1981). Some sprculations on thr nature of the natural defencr mechanism of plants against virus infection. Phytopathologixhe Zeitschrzift 102, 272~ 291. 15. K.xs:w~s. B.. GIANINAZZI. S. & WHITE, R. F. (1974j. A possible explanation of the resistancr of xilus-i&wed tobacco plants to second infection. Journal ofGenera Virology 23, I l-16. 16. I
312
E. A. Weststeijn L. C. VAN (1975). Polyacrylamide disc electrophoresis of the soluble leaf proteins from Nicotiana var. “Samsun” and “Samsun NN”. IV. Similarity of qualitative changes of specilic proteins after infection with different viruses and their relationship to acquired resistance. Virology
17. LOON,
tabacum
67,566575. 18. LOON, L. C. VAN (1976). Systemic acquired resistance, peroxidase activity and lesion size in tobacco reacting hypersensitively to tobacco mosaic virus. Physiological Plant Pathology 8, 231-242. 19. LOON, L. C. VAN & ANTONIW, J. F. (1982). Comparison of the effects of salicylic acid and cthephon with virus-induced hypersensitivity and acquired resistance in tobacco. Netherlands Journal of Plant
Pathology 88, 237-256. 20. LOON, L. C. VAN & DIJKSTRA, tobacco
mosaic
Netherlands 21.
22.
virus-
Journal
and
J. (1976). Virus-specific expression of systemic acquired resistance in tobacco necrosis virus-infected “Samsun NN” and “Samsun” tobacco.
of Plant Pathology
82, 231-237.
LOON, L. C. VAN & KAMMEN, A. VAN (1970). Polyacrylamide disc electrophoresis of the soluble leaf proteins from Nicotiana tabacum var. “Samsun” and “Samsun NN”. II. Changes in protein constitution after infection with tobacco mosaic virus. Virology 40, 199-211. MANDRYK, M. (1963). Acquired systemic resistance to tobacco mosaic virus in Nicotiana tabacum evoked by stem infection with Peronospora tab&m Adan. Australian Journal of Agricultural Research
14, 315-318. 23.
PRITCHARD, D. W. & Ross, A. F. (1975). The virus lesions in hypersensitive responding
24.
ROBERTS, D. A. (1982). Systemic acquired resistance localized viral infections. Virology 122, 207-209. Ross, A. F. (1961). Systemic acquired resistance
relationship of ethylene to formation tobacco leaves with and without
of tobacco mosaic induced resistance.
Virology 64, 295-307.
25.
Virology
in hypersensitive
plants virus
by non-necrotic,
induced
by localized
infections
in
plants
formation. Publ. methods
In Vzruses of Plants, Ed. by A. B. R. Co., Amsterdam. with plant viruses, and on local lesions.
14, 340-358.
26. Ross, 27.
induced
A. F. (1966). Systemic effects of local lesion Beemster & J. Dijkstra, pp. 1277150. North-Holland SAMUEL, G. (1931). Some experiments on inoculating
Annals of Applied Biology 18, 494-507. 28. SHIMOMURA, T. & OHASHI, Y. (1980). Necrotic lesion formation virus-infected plants. Reuiew of Plant Protection Research 13, 74-96.
and
localization
of infection
in
29.
SIMONS, T. J. & Ross, A. F. (1971). Metabolic changes associated with systemic induced resistance to tobacco mosaic virus in Samsun NN tobacco. Phytopathology 61,293~~300. 10. STEIN, A., LOEBENSTEIN, G. & SPIEGEI., S. (1979). Further studies of induced interference by a synthetic polyanion of infection by tobacco mosaic virus. Physiologzcal Plant Pathology 15, 241-255. il. SZIR,&I, I., BAL~ZS, E. & KIR~LY, 2. (1980) Role of different stresses in inducing systemic acquired resistance to TMV and increasing cytokinin level in tobacco. Physiological Plant Pathology 16,
277-284. 12. WESTSTEIJN, Xanthi-nc
E. A. after
(1981). Lesion growth and virus inoculation with tobacco mosaic
32 “C. Physiological ‘$3. WHITE, R. tobacco.
Plant Pathology
F. (1979).
Virology
Acetylsalicylic 99, 410412.
localization virus and
in leaves of ,Gcotiana tabacum cv. incubation alternately at 22 “C and
18, 357-368. acid
(aspirin)
induces
resistance
to
tobacco
mosaic
virus
in
Plant Pathology
(1984)
25, 307-312
Systemic acquired resistance induced by tobacco mosaic virus in Nicotiana tabacum cv. Xanthi-nc does not disappear at 32°C EMMA A. WESTSTEIJN Willie
Commelin
:Acceptedfor
Scholten Phytopathological
publication
July
Laboratory,
Jaualaan
20, 3742 CP Burn,
The Netherlands
1984)
Systemic acquired resistance to tobacco mosaic virus (TMV) was induced in Nicotiana tabacum cv. Xanthi-nc plants by incubating them at 22 “C for 9 days after inoculating them with TMV. After induction of resistance the plants were incubated for varying periods of time at either 22 C or 32 “C before challenge inoculation with TMV on the live leaves above the primary inoculated ones. They were then incubated at 22°C. The level of acquired resistance, as measured by the reduction in lesion size relative to the controls, in plants that had been incubated at 32 “C for up to 8 days, was similar to the level in plants that had been incubated continuously at 22°C. Thus, systemic acquired resistance is maintained for at least 8 days in plants incubated at 32 “C.
INTRODUCTION
Acquired resistance against virus infection can be induced in plants by a previous infection with various viruses [7, 1.5, 27, 18, 24-261, fungi [13, 221 or bacteria [I, 311, as well as by treatment with a number of chemical compounds including polyacrylic acid [3, 5, 10, 11, 161, aspirin [3, 331, salicylic acid [19, 331, benzoic acid [33], ethephon [19] and several polyanions [30]. Acquired resistance is expressed either as a reduction in lesion size or as a reduction in lesion number or both. The appearance of new proteins, so-called pathogenesis-related proteins (PRs) [2], is a phenomenon associated with pathogen-induced resistance [I, 3, 7, 15, 17, 211. These proteins are believed to be causally involved in this resistance. As the same PRs appear in chemically induced acquired resistance [3, 11, 16, 19, 331, this type of resistance and pathogen-induced resistance are considered to be due to a similar mechanism. Most of the investigations on acquired resistance have been carried out with tobacco and tobacco mosaic virus (TMV). Depending on the genetic constitution of the host, this virus induces a systemic mosaic or a local hypersensitive reaction. The hypersensitive reaction however occurs only at low temperatures, while at temperatures above 28 “C no local lesions develop, the virus spreads systemically and induces a systemic mosaic [27]. Both virus and chemically induced resistance in tobacco have been reported to Abbreviations 0048-4059/84/060307
used in text:
PRs,
+ 06 $03.00/O
pathogenesis-related
proteins; 0
TMV,
1984 Academic
tobacco
mosaic
Press Inc.
virus. (London)
Limited
308
E. A. Weststeijn
break down at 3‘2 “C [14, 281. In leaves in which resistance was induced by treatment [15], the with polyacrylic acid [IO, II] or aspirin [33] or by systemic virus infections reduction in lesion number compared to non-induced plants was lower in plants incubated at 32 “C for 2 days before challenge inoculation than in plants incubated at lower temperatures. However, information on the effect of incubating plants in which acquired resistance has been induced by TMV for a period at 32 “C, on the subsequent development of lesions at 22°C from a challenge inoculum is lacking. This acquired resistance is primarily characterized by a reduction in the size of lesions [ 25,26]. In a previous study [32] evidence was obtained which indicated that the resistance operating locally around primary lesions and in systemically resistant leaves upon challenge inoculation is expressed as a reduction in virus spread at 32 ‘C. In this paper, the effect ofincubating plants with systemic acquired resistance at 32 “C beforc challenge inoculation at 22 C was investigated. EXPERIMENTAL AND RESULTS Systemic acquired resistance was induced in Nicotiana tabacum cv. Xanthi-nc plants by inoculating them on three lower leaves with a suspension of purified TMV [12] which gave rise to 100-200 lesions per leaf. Control plants were left uninoculated. .After 9 days in the greenhouse at 22-24-C, the inoculated leaves were cut off to prevent systemic invasion of the rest of the plant by the virus during incubation at 32 C. At the same time the corresponding leaves were removed from the control plants. Some of the systemically resistant and control plants were challenge inoculated with TMV on the live leaves immediately above the ones removed and then incubated in the greenhouse at 22224 “C. Other plants were incubated at 22 “C in the greenhouse, or at 32 “C in a growth chamber, for a further 4 or 8 days before challenge inoculation with TMV on the live leaves as described above. After challenge inoculation the plants were incubated in the greenhouse. To assess the level of induced resistance, lesion size was determined 7 days after challenge inoculation, by measuring two diameters at right angles per lesion, with the aid of a magnilier with a micrometer. Lesion numbers were also recorded. Figure 1 shows that lesions developing from a challenge inoculation in systemically resistant leaves were always smaller than those in corresponding control leaves, irrespective of whether the plants had been incubated at 22 “C or at 32 “C after induction of resistance. The level of acquired resistance, as measured by the degree of’ reduction in lesion size over that on control plants was similar on both 22”C- and 32 “C-treated plants, but varied according to leaf age, being greater in the younger leaves than in older ones. Significantly larger lesions were formed in leaves on systemically resistant plants and on control plants which had been incubated at 32 “C for 4 or 8 days before the challenge inoculation than in leaves on plants which had been incubated at 22 “C. Thus the resistance mechanism had been weakened to a similar extent in both systemically resistant and in control leaves by the 32 “C treatment. No consistent differences were found in lesion numbers between systemically resistant and control plants, or between plants incubated at 32 “C and 22 “C before challenge inoculation.
Acquired
resistance
to TMV in Nicotiana
309
1 (a)
3
0
(e)
Cd)
3-
0-
I
I234 Leaf
5
position
FIG. 1. Size of TMV lesions in systemically resistant and control leaves of Nicotiana tabacum cv. Xanthi-nc, 7 days after challenge inoculation. Systemic acquired resistance was induced by incubating the plants at 22 “C for 9 days after inoculation with TMV. Challenge inoculations were carried out immediately (a) ; after a further incubation for 4 days at 22 “C (b) or at 32 ‘C (c) ; or after a further incubation for 8 days at 22 “C (d) or at 32 “C (e). All plants were incubated at 22°C after challenge inoculation. 0, Systemically resistant; 0, control. Each point is the mean from eight leaves, 10 lesions measured per leaf. Leaf 1 is the leaf immediately above the primary inoculated ones or the corresponding leaf on control plants. Lesion sizes were compared using Wilcoxon’s two sample test. Lesion sizes in systemically resistant leaves were significantly different from those in control leaves [a = 0.005 for all pairs except for leaf 1 (a ~~ 0.05) and leaf 2 (a = 0.025) both in Fig. 1 (e)]. Bars represent standard deviations.
DISCUSSION The results described in this report show that the extent of reduction in lesion size in plants due to acquired resistance was similar for plants incubated for up to 8 days at 22 “C and at 32 “C before challenge inoculation. Thus, in contrast to statements in two reviews [14, ,281 systemic acquired resistance does not disappear at 32 “C.
310
E. A. Weststeijn
In studying the effects of temperature on acquired resistance it is important to distinguish between conditions affecting the induction of resistance and those affecting its expression. It is also important to realize that there are two parameters which can be used to measure acquired resistance, namely lesion size and lesion number. This discussion will, unless otherwise stated, be confined to results obtained with AL tabacum and TMV, since most investigations on acquired resistance have been carried out with this host-virus system. Lesion size and lesion number are independent characteristics of the reaction of a leaf to inoculation with virus. For example, with increasing leaf age larger numbers of smaller lesions develop [18], with increasing temperature after inoculation fewer but larger lesions develop [9] and treatment with abscisic acid for 14 days before inoculation leads to fewer but smaller lesions to develop [7]. Pathogen-induced systemic resistance is primarily characterized by a reduction in lesion size [25,X], but with increasing levels of resistance a reduction in lesion number is found as well [I, 6, 7, 25, 261. H owever, TMV-induced resistance in ;L’icotania glutinosa is characterized by a reduction in lesion size but an increase in lesion numbers [8J. This latter case is a further example of the independency between lesion number and size. Chemically induced acquired resistance leads to reduction in both lesion number and size [5, 11, 19, 331. Treating plants with polyacrylic acid at 32 “C does not induce resistance [IO, I/]. In contrast, treatments with salicylic acid and aspirin at 32 “C do induce resistance. \;t’hen lesion size is used as a parameter, this resistance appears to be expressed at 32’ C to the same extent as at 22 “C [19]; with lesion numbers as a parameter, resistance is found at 22 “C but not at 32 “C [19, 331. The resistance induced b>. treatments with polyanions at 20 “C is expressed at 32 “C as a reduction in lesion size, but not in lesion number [30]. The same holds for TMV-induced resistance [4, 321. Incubating leaves for several days at 32 “C, after the induction of resistance with polyacrylic acid [IO, II], aspirin [33] or systemic virus infections [25] at 20 “C, caused them all to be less resistant in terms of the numbers of lesions formed but in the case of polyacrylic acid treatments the lesions were small [II]. The results of the current study show that incubation for up to 8 days at 32 “C, after induction of sy-stem& acquired resistance with TMV at 22 “C, did not affect the level of resistance as regards lesion size. Taken altogether these results indicate that when acquired resistance is considered only in relation to its effects on number of lesions then it appears to break down after treatments at 32 “C, but if it is considered only in relation to lesion size then it appears unaffected by treatments at 32 “C. Some of the confusion in the literature about tht effects of treatments at 32 “C on the breakdown of acquired resistance has arisen because some workers have measured changes in lesion number only and havpc ignored or overlooked changes in lesion size. Nevertheless, when acquired resistance is considered in relation to its effects on lesion size, temperature conditions during induction and expression of resistance appear to be important. Thus, higher levels of systemic acquired resistance are induced by TMV in plants which are incubated at 29 “C rather than 19 ‘C after the primary- inoculation [25], but when resistance is induced at 2 1 “C, the level of resistance
Acquired
resistance
to TMV in Nicotiana
311
expressed against challenge inoculations at 2O”C, 26°C and 29°C reduces as the temperature increases [2.5]. Resistance induced at 22 “C with TMV is also expressed to some extent at 32 “C [32], although TMV infection at that temperature does not lead to necrosis or virus localization. This implies that induced resistance involves inhibition of virus multiplication, or spread, or both. The observation that tobacco necrosis virus induces resistance to TMV in Samsun tobacco plants, a systemic host for TMV [20], supports this conclusion. As the processes of the hypersensitive reaction occur earlier and faster in leaves with systemic acquired resistance than in those without [23, 291, it is conceivable that acquired resistance consists of both a reduction in virus multiplication [29] and a stimulation ofthe processes involved in virus localization. With increasing temperature the localization mechanism becomes less efficient [9], until at 32 “C it no longer operates. However, once acquired resistance has been induced at a lower temperature, the increased capacity of the leaves to react with an early hypersensitive reaction is not lost at 32 “C but is expressed again when the plants are returned to a lower temperature. REFERENCES I. AH., P., BENJAMA, A., SAMSON, R. & GIANINAZZI, S. (1981). Induction chez le tabac par Pseudomonas syringae dr nouvelles prottines (protCines ‘b’) associ&s au d&elopement d’une rt.sistance nonsptcilique ?I une deuxieme infection. Phytopathologixhe Zeit~hrift 102, 201-212. 2. AWTONIW, J. F., RITTER, C. E., PIERPOINT, W. S. & LOON, L. C. VAN (1980). Comparison of three pathogenesis-related proteins from plants of two cultivars of tobacco infected with TMV. Journal of General Virology 47, 79-87. 3. ANTONIW, J. F. & WHITE, R. F. (1980). The effects of aspirin and polyacrylic acid on solublr leaf proteins and resistance to virus infection in live cultivars of tobacco. Phytopathologische Zeit~chriJt 98,331-341. 4. BAI&S, E., SZIR~~KI, I. & KIRALY, Z. (1977). The r8le of cytokinins in the systemic acquired resistance of tobacco hypersensitive to tobacco mosaic virus. Physiological Plant Pathology 11,29%37. 5. CASSELLS, A. C., BARNETT, A. & BARLASS, M. (1978). The effect of polyacrylic acid treatment on the susceptibility of Nicotiana tabacum cv. Xanthi nc to tobacco mosaic virus. Physiological Plant Pathology 13, 13-21. 6. FRASER, R. S. S. (1979). Systemic consequences of the local lesion reaction to tobacco mosaic virus in a tobacco variety lacking the N-gene for hypersensitivity. Physiological Plant Pathology 14, 383. 394. 7. FRASER, R. S. S. (1982). Are “pathogenesis-related” proteins involved in acquired systemic resistance of tobacco plants to tobacco mosaic virus? Journal of&era1 Virology 58, 305-313. 8. FRASER, R. S. S., LOUGHLIN, S. A. R. & WHENHAM, R. J. (1979). Acquired systemic susceptibility to infection by tobacco mosaic virus in Nicotiana glutinosa L. Journal of General Virology 43, 131-141. 9. GABORJANYI, R. & ELHAMMADY, M. (1969). Effect of temperature on size of local lesions induced by TMV in tobacco plants. Acta Phytopathologica Academiae Scientiarum Hungaricae 4, 125-l 29. 10. GIANINAZZI, S. (1982). Anti-viral agents and inducers of virus rrsistance: analogies with interferon. In A&e Defence MechaniJms in Plants, Ed. by R. K. S. Wood, pp. 275~.298. Plenum Prrss, New York & London. inducrd in plants by polyacrylic acid. Journal I I. GIANINAZZI, S. & KASSANIS, B. (1974). V’ lrus rrsistance oJ’Genera1 Virology 23, 1-9. 12. GOUUING, G. \‘. & HEBERT, T. T. (1967). A simple technique for purification of tobacco mosaic virus in large quantities. Phytopathology 57, 1285. 13. HE.C~IT, E. I. & BATEMAN, D. F. (1964). N on-specific acquired resistance to pathogens resulting from inltiction by Thielaviopsir b&cola or viruses in tobacco. Phytopathology 54, 523-530. I+. KASSAUIS, B. (1981). Some sprculations on thr nature of the natural defencr mechanism of plants against virus infection. Phytopathologixhe Zeitschrzift 102, 272~ 291. 15. K.xs:w~s. B.. GIANINAZZI. S. & WHITE, R. F. (1974j. A possible explanation of the resistancr of xilus-i&wed tobacco plants to second infection. Journal ofGenera Virology 23, I l-16. 16. I
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