Genetic Mechanism of Dominant Earliness in Kefeng A, a New Rice Cytoplasmic Male Sterile Line

Rice Science, 2009, 16(4): 267–273 Copyright © 2009, China National Rice Research Institute. Published by Elsevier BV. All rights reserved DOI: 10.1016/S1672-6308(08)60090-6

Genetic Mechanism of Dominant Earliness in Kefeng A, a New Rice Cytoplasmic Male Sterile Line XIAO Yu-long1, YU Chuan-yuan1, LEI Jian-guo1, LI Ma-zhong1, JIANG Lin2, WAN Jian-min2 (1Rice Research Institute, Jiangxi Academy of Agricultural Sciences, Nanchang 330200, China; 2State Key Laboratory for Crop Genetics and Germplasm Enhancement, Nanjing Agricultural University, Nanjing 210095, China)

Abstract: Kefeng A is an early maturing indica cytoplasmic male sterile (CMS) line of rice. Combinations derived from Kefeng A and late maturing indica restorer lines showed dominant earliness to various extents. To understand the genetic basis of dominant earliness, the genotype of photoperiod-sensitive genes in Kefeng A was analyzed using a complete set of heading b

b

time near isogenic lines (NILs) EG0 to EG7, ER, LR, T65, T65E , T65E m, T65m, NIL(Hd1) and NIL(Hd4). Results indicated u

that Kefeng A contained two dominant photoperiod-sensitive alleles E1 and Se-1 on E1 and Se-1 loci, respectively, and the u

u

genotype of photoperiod-sensitivity genes for heading time in Kefeng A was E1E1e2e2E3E3Se-1 Se-1 Ef-1Ef-1. Based on the detected heading time genotype, in combination with the heading time of Kefeng A and the early maturing phenomenon in its derived F1 hybrids, it is speculated that Kefeng A might carry a dominant inhibitor gene Su-E1 for the dominant photoperiodsensitive gene E1, and a recessive inhibitor gene i-Se-1 for another dominant photoperiod-sensitive gene Se-1. The reason why F1 hybrids from Kefeng A exhibited early maturing was hereby analyzed and the breeding value of dominant earliness related genes in Kefeng A was discussed. Key words: dominant earliness; heading time; genotype; genetic mechanism; cytoplasmic male sterile line; rice

Growth duration is an important determinant for regional rice cropping systems [1]. Rice growth duration is codetermined by its vegetative and reproductive growth duration. For the majority of rice varieties, the vegetative growth duration varies in a wide range whereas the reproductive growth duration is relatively stable. Therefore, heading time, i.e. days from sowing to heading, is commonly used to reflect the growth duration. Heading time of a rice variety is determined by its photoperiod-sensitivity, thermal-sensitivity and basic vegetative growth duration [2]. However, thermalsensitivity generally exists in rice and its heredity is stable, and the growth duration of a rice variety is thus mainly determined by its basic vegetative growth duration and photoperiod-sensitivity. A number of studies have been done on the genetic effect of heading time. Among the reported rice photoperiod-sensitive genes and their modification genes, E1 and Se-1 are the two main photoperiodsensitive genes and the main factors that affect the Received: 20 May 2009; Accepted: 27 August 2009 Corresponding author: YU Chuan-yuan ([email protected]) This is an English version of the paper published in Chinese in Chinese Journal of Rice Science, Vol. 23, No. 3, 2009, Pages 271–276.

heading time of rice. E2 and E3 genes have minor effects, but the presence of E2 and/or E3 can enhance the photoperiod-sensitivity of E1, resulting in a significantly delayed heading time [3-5]. A recessive photoperiod-sensitivity inhibitor gene i-Se-1, which widely exists in photoperiod-insensitive rice varieties, can repress the photoperiod-sensitivity Se-1 gene, so early indica rice, though carrying photoperiod-sensitive genes, would not express photoperiod-sensitivity [6-7]. Besides, among the photoperiod-insensitive genes and modification genes that affect heading time, Ef-1 is a dominant gene conferring early maturity, which could shorten the basic vegetative growth duration of rice [8]. Early maturity and high yield have long been the contradiction in hybrid rice breeding. One solution to this problem is exploitation of dominant earliness genes: introducing dominant earliness gene(s) into hybrid rice parents and breeding cytoplasmic male sterile (CMS) lines or restorer lines with these genes [9]. 0 Kefeng A, a new early indica CMS line, was bred by hybridization of Butuolong, a local early indica rice with 5608, a sister line of Jiangnongzao IIB, followed by ten consecutive backcrossing with Jiangnongzao IIA. This new CMS line possesses a lot of desirable

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characteristics such as stable sterility, good agronomic characters, early maturity, fine grain quality and high out-crossing rate. Many testing experiments indicated that medium and late season hybrids derived from this CMS line showed significant earliness. To better understand the genetic basis of dominant earliness of this CMS line, we used a complete set of near isogenic lines (NILs) for heading time to test the genotype of heading time genes in Kefeng A and analyzed the mechanisms underlying the dominant earliness in F1s from this CMS line. Besides, breeding value and prospects of the dominant earliness genes of Kefeng A were further discussed.

MATERIALS AND METHODS Experimental rice materials Rice materials included five indica rice CMS lines: Xieqingzao A, Zhong 9A, Jin 23A, Xinxiang A and II-32A; four early indica restorer lines: R036, R1236, R1320 and R402; six medium and late indica restorer lines: R1083 (photoperiod-sensitive), F10, R135 (photoperiod-sensitive), R752 (photoperiod-sensitive), Minghui 63 and R838 (all above materials were preserved by author’s research group); 14 NILs for heading time: EG0, EG1, EG2, EG3, EG4, EG5, EG6, EG7 (from Kyoto University, Japan), ER, LR, T65, T65Eb, T65Ebm and T65m (from Breeding Research Division, Agricultural Department, National Chung Hsing University, Taiwan, China); Nipponbare and its heading time QTL NILs Hd1 and Hd4 (from Biological Resource Research Institute, Ministry of Agriculture, Forest and Fishery, Japan), and Kefeng B, the maintainer line of Kefeng A. All the F1s were made between the year of 2003 and 2005. Field experiment Testing experiments for earliness observation All the early-season combinations were sown on 25 March, 2005, and single-season and late-season ones were sown on 20 May, 2004 and 20 June, 2005, respectively. After 30 days, seedlings were transplanted into the experimental fields at Rice Research Institute, Jiangxi Academy of Agricultural Sciences, Nanchang, China (28º41'N) in three rows with 10 seedlings per

row at a planting density of 13.3 cm × 26.7 cm. The initial heading dates of all the combinations were recorded. Short-day treatments The responses of the experimental materials including the heading time NILs, Kefeng B and their F1s to short-day were analyzed under the natural short-day condition in winter in Linshui County, Hainan Province (18°29ƍ N), China. Seeds of the experimental materials were sown on 15 December, 2005 and the seedlings were transplanted on 16 January, 2006. Seedlings were planted in two rows with 10 seedlings per row at a planting density of 13.3 cm × 26.7 cm. The sowing dates and initial heading dates were recorded. Long-day treatments Using the natural long-day conditions in summer in Nanchang, Jiangxi Province, China (28°41ƍ N), the heading time NILs, Kefeng B and their F1s were planted to check the responses to long-day treatment. All the materials were sown on 22 May, 2006 and transplanted on 21 June, 2006, in the same pattern with the short-day treatment. The sowing dates and initial heading dates were recorded. Field management was the same as rice production. Heading date recording and analyses Initial heading dates were recorded when the main panicle exerted 1 cm out from the flag leaf sheath. All the experimental materials were investigated with a 2-day interval. Heading time was then calculated as the mean of the heading times of all ten plants of each experimental material. Over mid-parent heterosis (OMPH) of F1s was calculated as follows: OMPH = (MF í Mp)/Mp ×100% Here, MF is the mean heading time of F1, and Mp is the mean heading time of both parents.

RESULTS Performance of dominant earliness of Kefeng A The growth duration of Kefeng A in Nanchang, Jiangxi Province, China was approximate to those of

XIAo Yu-long, et al. Genetic Mechanism of Dominant Earliness in Kefeng A, a New Rice CMS Line

Jin 23A and Zhong 9A. Early rice hybrids derived from Kefeng A and early indica restorer lines showed earliness, but it is not significant compared to the restorer lines. However, when Kefeng A was crossed with medium and/or late indica restorer lines such as Minghui 63, R838 and R752, and the F1s were planted as mid-season or late season hybrid rice accordingly, their heading times were significantly shortened compared with those derived from the same restorer lines with other commonly used CMS lines, showing a significant difference of incomplete dominant earliness (Table 1). Early indica rice normally carrying recessive repressive allele at Se-1 locus, which is the reason why early indica rice does not show photoperiodsensitivity [6-7]. The medium and late indica hybrids derived from Kefeng A generally mature early under long-day conditions. Based on the heading time of Kefeng A and the derivative F1s of Kefeng A with photoperiod-sensitive late indica restorer lines such as R135 and R752, it is speculated that Kefeng A might also carrying other dominant earliness genes, in addition to the recessive repressive allele at Se-1 locus. Genotypic analyses of photoperiod-sensitive genes for heading time Allele analyses at E1, E2 and E3 loci All the EG NILs carry a main dominant photoperiod-sensitive allele at Se locus, and they

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differed at E1, E2 and E3 loci (Table 2). By investigating the heading time of the EG NILs different at E1 locus and F1s of EG NILs and Kefeng B, we noted that there were the differences in the heading time among EG NILs and F1s of EG NILs and Kefeng B under the long-day conditions. EG0 (e1e1e2e2e3e3) headed 16 days earlier than EG1 (E1E1e2e2e3e3), and the F1 of EG0 ×Kefeng B headed only 1.2 days earlier than the F1 of EG1×Kefeng B; EG2 (e1e1E2E2e3e3) headed 16 days earlier than EG4 (E1E1E2E2e3e3), whereas the F1 of EG2×Kefeng B headed 1 day later than the F1 of EG4×Kefeng B; EG3 (e1e1e2e2E3E3) headed 27 days earlier than EG6 (E1E1e2e2E3E3), and the F1 of EG3 ×Kefeng B headed 7 days earlier than the F1 of EG6 ×Kefeng B; EG5 (e1e1E2E2E3E3) headed 25 days earlier than EG7 (E1E1E2E2E3E3), and the F1 of EG5 ×Kefeng B headed only 5 days earlier than the F1 of EG7×Kefeng B. Because the photoperiod-sensitive allele E1 is dominant to its photoperiod-insensitive allele e1, if Kefeng B carried a photoperiod-insensitive allele, F1s from the e1-carrying NILs and Kefeng B should have significantly earlier heading times than F1s from the E1-carrying NILs and Kefeng B. Therefore, we speculated that Kefeng B might carry a dominant photoperiod-sensitive allele E1 at this locus. Comparing the heading time of EG NILs which differ only at E2 locus, we found that the EG NILs carrying the recessive e2 allele headed earlier than those carrying the E2 allele under the long-day

Table 1. Performance of heading time of combinations derived from Kefeng A in 2004 and 2005. Late-season rice in 2004 Parental line and combination Kefeng A(B) R1083 Kefeng A/1083 Zhong 9A/1083 Xinxiang A/1083 Minghui 63 Kefeng A/Minghui 63 Xinxiang A/Minghui 63 Zhong 9A/Minghui 63 F10 Kefeng A/F10 Zhong 9A/F10 II-32A/F10 R135 Kefeng A/R135 Zhong 9A/R135 II-32A/R135 Xinxiang A/R135

Heading time (d) 58 85 71 85 87 96 73 93 91 80 68 81 90 95 74 87 90 87

Early-season rice in 2005 Parental line and combination Kefeng A(B) R036 Kefeng A/R036 Jin 23A/R036 Zhong 9A/R036 R1236 Kefeng A/R1236 Jin 23A/R1236 Zhong 9A/R1236 Xinxiang A/R1236 R1320 Kefeng A/R1320 Jin 23A/R1320 Zhong 9A/R1320 R402 Kefeng A/R402 Jin 23A/R402

Heading time (d) 66 89 80 82 87 82 80 81 82 83 87 82 84 87 87 80 87

Single-season rice in 2005 Parental line and combination Kefeng A(B) R752 Kefeng A/R752 Jin 23A/R752 Zhong 9A/R752 Xieqingzao A/R752 Minghui 63 Kefeng A/Minghui 63 Jin 23A/Minghui 63 Zhong 9A/Minghui 63 Xieqingzao A/Minghui 63 R838 Kefeng A/R838 Jin 23A/R838 Zhong 9A/R838 Xieqingzao A/R838 II-32A/R838

Heading time (d) 62 89 90 115 121 118 94 79 84 84 87 85 76 84 84 85 88

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Table 2. Genotype of near isogenic lines (NILs) for heading time and heading times of NILs, Kefeng B and F1 plants from crosses of NILs× Kefeng B. Heading time (d) NIL EG0 EG1 EG2 EG3 EG4 EG5 EG6 EG7 ER LR T65 T65m T65Eb T65Ebm Kefeng B

Genotype e1e1e2e2e3e3Se-1nSe-1nEf-1Ef-1 E1E1e2e2e3e3Se-1nSe-1nEf-1Ef-1 e1e1E2E2e3e3Se-1nSe-1nEf-1Ef-1 e1e1e2e2E3E3Se-1nSe-1nEf-1Ef-1 E1E1E2E2e3e3Se-1nSe-1nEf-1Ef-1 e1e1E2E2E3E3Se-1nSe-1nEf-1Ef-1 E1E1e2e2E3E3Se-1nSe-1nEf-1Ef-1 E1E1E2E2E3E3Se-1nSe-1nEf-1Ef-1 E1E1e2e2E3E3Se-1eSe-1eEf-1Ef-1 E1E1e2e2E3E3Se-1uSe-1uEf-1Ef-1 E1E1E2E2E3E3Se-1eSe-1eef-1ef-1 e1e1E2E2E3E3Se-1eSe-1eef-1ef-1 E1E1E2E2E3E3Se-1eSe-1eEf-1Ef-1 e1e1E2E2E3E3Se-1eSe-1eEf-1Ef-1 

conditions (Table 2). EG0 (e1e1e2e2e3e3) headed 1 day earlier than EG2 (e1e1E2E2e3e3), and the F1 of EG0 ×Kefeng B headed 2.6 days earlier than the F1 of EG2 ×Kefeng B; EG1 (E1E1e2e2e3e3) headed 1 day earlier than EG4 (E1E1E2E2e3e3), and the F1 of EG1×Kefeng B headed 0.4 days earlier than the F1 of EG4 ×Kefeng B; EG3 (e1e1e2e2E3E3) headed 3 days earlier than EG5 (e1e1E2E2E3E3), and the F1 of EG3×Kefeng B had approximate heading time to the F1 of EG5×Kefeng B; EG6 (E1E1e2e2E3E3) headed 1 day earlier than EG7 (E1E1E2E2E3E3), whereas the F1 of EG6×Kefeng B headed 2 days later than the F1 of EG7×Kefeng B. These results suggested a recessive allele e2 at E2 locus in Kefeng B. Comparing the heading time of EG NILs which differ only at E3 locus, we found that the NILs carrying the recessive e3 allele headed earlier than those with the dominant E3 allele under the long-day conditions (Table 2). EG0 (e1e1e2e2e3e3) headed 9 days earlier than EG3 (e1e1 e2e2E3E3), and the F1 of EG0× Kefeng B headed only 1.6 days earlier than the F1 of EG3×Kefeng B; EG1 (E1E1e2e2e3e3) headed 20 days earlier than EG6 (E1E1e2e2E3E3), whereas the F1 of EG1× Kefeng B headed only 7.4 days earlier than the F1 of EG6×Kefeng B; EG2 (e1e1E2E2e3e3) headed 11 days earlier than EG5 (e1e1E2E2E3E3), whereas the F1 of EG2×Kefeng B headed one day later than the F1 of EG5×Kefeng B; EG4 (E1E1E2E2e3e3) headed 20 days earlier than EG7 (E1E1E2E2E3E3), whereas the F1 of EG4×Kefeng B headed only 5 days earlier than the F1 of EG7×Kefeng B, which indicated that Kefeng B

NIL Jiangxi 73.0 89.0 74.0 82.0 90.0 85.0 109.0 110.0 78.0 92.0 99.0 98.0 92.0 75.0 56.0

F1 of NIL×Kefeng B Hainan 62.0 63.6 63.2 65.8 65.0 62.0 66.2 65.0 63.0 64.2 70.2 65.0 66.0 66.4 53.6

Jiangxi 71.4 72.6 74.0 73.0 73.0 73.0 80.0 78.0 78.0 79.0 81.2 82.6 80.4 78.0

Hainan 66.0 63.6 64.0 66.0 65.0 64.0 64.0 65.0 63.0 62.4 65.0 68.0 63.6 65.0

might carry an E3 dominant photoperiod-sensitive allele at E3 locus. Combining above analyses, we conclude that Kefeng B carries dominant photoperiod-sensitive allele E1, photoperiod-insensitive allele e2 and dominant photoperiod-sensitivity enhancing allele E3. Allele analysis at Se locus The only difference between the genotype of NILs ER and LR is that the former carries a photoperiodinsensitive allele Se-1e while the latter carries a photoperiod-sensitive allele Se-1u. Related studies indicated that the photoperiod-sensitive alleles Se-1n and Se-1u were dominant to photoperiod-insensitive allele Se-1e [10-11]. LR headed 14 days later than ER (Table 2), due to the photoperiod-sensitive Se-1u allele in LR and the photoperiod-insensitive allele Se-1e in ER. The difference of heading time of F1s from these two NILs with Kefeng B was only 1 day, indicating that Kefeng B might carry the dominant Se-1u allele at this locus. Allele analysis at Ef locus T65Eb headed 7 days earlier than T65 and T65Ebm headed 13 days earlier than T65m, due to T65Eb and T65Ebm both carrying a dominant earliness allele Ef-1. As we know, if Kefeng B did not carry the dominant earliness allele Ef-1, the heading time of F1s of T65Eb and T65Eb with Kefeng B should vary significantly from that of F1s of T65 or T65m with Kefeng B. However, the experimental results showed no significant difference

XIAo Yu-long, et al. Genetic Mechanism of Dominant Earliness in Kefeng A, a New Rice CMS Line

among the four F1s. Hence, Kefeng B might carry the dominant earliness allele Ef-1 at this locus. Performance of heading time of F1s from QTL near isogenic lines and Kefeng B QTL NIL(Hd1) for heading time does not contain a dominant photoperiod-sensitive allele Hd1 (Se-1) as Nipponbare (E1E1e2e2e3e3Se-1Se-1) does, but contains a photoperiod-sensitive allele Hd4 (E1). QTL NIL(Hd4) does not contain the photoperiod-sensitive allele Hd4 (E1) as Nipponbare does, but contains the photoperiodsensitive allele Hd1 (Se-1) [12-14]. The heading time of F1s from the two NILs with Kefeng B was identical and close to that of F1s from Nipponbare×Kefeng B (Table 3), we extrapolate that the F1s of these three combinations had identical heading time genotype, and Kefeng B might carry the photoperiod-sensitive alleles Hd4 (E1) and Hd1 (Se-1), which further testified that the genotype of heading time photoperiod-sensitive alleles of Kefeng B is E1E1e2e2E3E3Se-1uSe-1u. Genetic basis of earliness of F1s from Kefeng A The difference in heading time between EG7 (E1E1E2E2E3E3Se-1nSe-1nEf-1Ef-1) and EG0 (e1e1e2e2 e3e3Se-1nSe-1nEf-1Ef-1) was 37 days, 11 days longer than the sum of differences between EG1, EG2, and EG3 with EG0 (26 days) (Table 2), indicating that there were certain interactions among photoperiodsensitive genes other than simple additive effects. Besides, there existed relatively strong interactions between alleles at E1 or E3 loci and between alleles at E1 and E3 loci and the photoperiod-sensitive allele at Se-1 locus. The enhancing photoperiod-sensitivity allele at E2 locus neither has any significant effect by itself, nor interacts with alleles at Se-1 locus, but the existence of E2 gene could enhance the photoperiodsensitivity of E1 and E3 genes [5-7]. Additionally, comparing the heading time of LR and EG6, we could notice that E1 and E3 genes had stronger interactions with Se-1n than with Se-1u. Although Kefeng B carries two main photoperiodsensitive genes E1 and Se-1, the heading times under the long-day and short-day conditions were nearly identical and were both shorter than EG0 (Table 2), indicating that neither of the two main photoperiodsensitive genes expressed. Hence, there might be inhibitor

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Table 3. Mean heading time of NILs and their F1s and mid-parent heterosis of F1s from NILs and Kefeng B. Parent and F1 Nipponbare NIL(Hd1) NIL(Hd4) Kefeng B Nipponbare/Kefeng B NIL(Hd1)/Kefeng B NIL(Hd4)/Kefeng B

Mean heading time (d) Hainan 72.0 75.0 72.4 53.6 62.4 62.0 63.0

Jiangxi 88.2 92.8 86.4 56.0 66.0 68.0 68.8

Mid-parent heterosis (%)

-8.18 -8.60 -4.58

genes in Kefeng B which epistatically repressed the two main photoperiod-sensitive genes. Checking the performance of heading time of F1s from NILs of EG0 to EG5 with Kefeng B, if Kefeng B carried two dominant photoperiod-sensitivity inhibitor genes, the heading time of F1s should be identical with Kefeng B, however, the data in Table 2 showed a 15- to 18-day longer heading time between the F1s and Kefeng B, but identical to EG0; Likewise, if Kefeng B carried two recessive photoperiod-sensitivity inhibitor genes, the heading time of F1s should be identical to the isogenic lines. Therefore, it is impossible that Kefeng B carries either two dominant or two recessive photoperiod-sensitivity inhibitor genes. The heading times of EG1 and EG4 carrying the E1 gene were significantly longer than that of EG0, whereas their F1s had nearly identical heading times. Consequently, it is extrapolated that Kefeng A carries a dominant gene repressive to E1 and a recessive gene repressive to Se-1. Thus, in F1s of EG0 to EG5 with Kefeng A, only the Se-1 gene was expressed, and the expression of E1 was repressed. The heading time genotypes were the same in the four F1s derived from EG6, EG7, ER and LR with Kefeng B, in which E3 and Se-1 genes were expressed. In addition, the heading times of them were longer than F1s from EG0 to EG5 with Kefeng B, in which E1 was repressed. EG0 and the F1 of EG0 with Kefeng B had the same heading date, indicating that the photoperiod-sensitive gene Se-1 was expressed while E1 was repressed in the F1, which was the same with ER near isogenic line. Accordingly, Kefeng B might carry a dominant photoperiod-sensitivity gene repressive to E1 and a recessive photoperiod-sensitivity gene repressive to Se-1. Here, we temporarily designated the dominant photoperiod-sensitivity gene repressive to E1 as Su-E1 gene.

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The heading time genotype of late indica rice restorer line Minghui 63 is E1E1e2e2E3E3Se-1eSe-1e ef-1ef-1, with a recessive photoperiod-sensitivity allele repressive to Se [16]. The heading time of Minghui 63 was 94 days when planted as medium rice, significantly longer than that of the F1 from Minghui 63×Kefeng B (79 days), indicating that the E1 and Se-1 photoperiodsensitive genes in Minghui 63 were repressed. The heading time genotype of late indica restorer line R752 is E1E1E2E2E3E3Se-1eSe-1eEf-1bEf-1b [16], with a photoperiod-insensitive allele Se-1e at Se-1 locus, which could induce relatively longer basic vegetative growth duration and weak photoperiod-sensitivity [10]. R752 shows photoperiod-sensitivity due to the presence of E1 allele at E1 locus. R752 had a heading time of 89 days, identical with the F1 of R752×Kefeng B, when planted as medium rice, indicating that the E1 gene was repressed, whereas the allele at Se-1 locus was expressed in the F1. When R752 was crossed with Jin 23A, Zhong 9A and Xieqingzao A, respectively, all the F1s expressed over-parent late maturing, due to interactions between the two photoperiod-sensitive alleles at E1 and Se-1 loci. Because of the co-existence of dominant earliness Ef-1 gene, which can significantly shorten the basic vegetative growth duration, and the two inhibitor genes Su-E1 and i-Se-1 epistatically act on the two dominant photoperiod-sensitive genes E1 and Se-1, respectively, Kefeng A shows early maturing. Moreover, the hybrids derived from Kefeng A with medium or late indica restorer lines shows earliness to various extents.

DISCUSSION Earliness of rice variety is determined by its photoperiod-insensitive genes, photoperiod-sensitivity inhibitor genes, relative early maturing basic vegetative growth genes and relative modification genes. Ling et al [17] studied the earliness of a Russian rice variety USSR5 and found that earliness was mainly determined by e1, Ef-1 and i-Se-1 genes. At present, though breeders have found domestically several dominant earliness germplasms [18-19] and mapped an earliness gene in a rice variety 6442S, the genetic basis of earliness have not been well understood in hybrid rice breeding [19-20]. In this study, we found that

Kefeng A carries a dominant earliness gene Ef-1 which can shorten the basic vegetative growth duration, and Su-E1 and i-Se-1 inhibitor genes which epistatically repress the dominant photoperiod-sensitive genes E1 and Se-1, respectively. Therefore, the F1s of this CMS line with medium or late indica rice restorer lines show earliness to various extents. The early maturing gene Ef-1 and the recessive photoperiod-sensitivity inhibitor gene i-Se-1 exist widely in early indica rice and photoperiod-insensitive late indica rice [6-7, 17, 21-23]. Since no report yet has documented the dominant repressive function on photoperiod-sensitive gene E1, the discovery of the dominant inhibitor gene Su-E1 is of great importance and meaningfulness in solving the contradiction of early maturity and high yield. Whatever the genes for earliness are recessive or dominant, they are able to enhance panicle heading and shorten growth duration in pure rice lines. However, to make the hybrid F1 early mature, it requires either of the two parents carrying dominant earliness genes or both of the parents carrying recessive photoperiodsensitivity inhibitor genes in hybrid rice breeding. Transgression for growth duration in hybrid rice is determined by the existence of main photoperiodsensitive genes and photoperiod-sensitivity inhibitor genes in both parents, as well as the complementarity of photoperiod-sensitive genes. Utilization of heterosis between indica and japonica inter-subspecies is considered as the most effective way in high-yielding rice breeding [25]. Nevertheless, the derived F1 hybrids, due to interactions among different main photoperiod- sensitive genes, generally show over-parent late maturity [25-26], which in turn constrains the utilization of inter-subspecific heterosis. The dominant photoperiod-sensitivity inhibitor gene Su-E1 found in this study and the recessive photoperiod-sensitivity inhibitor gene i-Se-1 which exists widely in photoperiod-insensitive indica rice could effectively eliminate the interactions among dominant photoperiod-sensitive genes in indica- japonica hybrids, overcome the obstacle of the transgression for growth duration, thus being of great use in breeding intersubspecific hybrid rice with desirable growth duration.

ACKNOWLEDGEMENTS This study was supported by the Main Scientific

XIAo Yu-long, et al. Genetic Mechanism of Dominant Earliness in Kefeng A, a New Rice CMS Line

and Technical Leaders Training Program Funded Project of Jiangxi Province, China (Grant No. 060005) and the Major Scientific and Technological Innovation Project of Jiangxi Province, China (Grant No. 20061 G0100300). We extend our sincere acknowledgements to Professor T. TANISAKA and Associate Professor Y. OKUMOTO at the Kyoto University, Japan for providing EG0 to EG7 heading time near isogenic lines, Professor Kuo-Hai TSAI at the Breeding Research Division, Agricultural Department, National Chung Hsing University, Taiwan, China for providing ER, LR, and T65 to T65m near isogenic lines, and Dr. M YANO from Biological Resource Research Institute, Ministry of Agriculture, Forest and Fishery, Japan for providing heading time QTL near isogenic lines Hd1 and Hd4.

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