Genetic Studies On Panicle Enclosure In Rice (Oryza Sativa L.)

The successful application of hybrid rice has made great contribution to solving the food problem in China. However, the phenomenon of panicle enclosure exists in all the male sterile (MS) lines used in hybrid rice production. This seriously affects the yields of MS seed reproduction and hybrid seed production. Panicle enclosure in rice is mainly caused by the shortening of uppermost internode. Elucidating the molecular genetic mechanism of panicle enclosure will be helpful for solving the problem of panicle enclosure in MS lines and creating new germplasms in rice. In this study, we investigated the genetic basis of panicle enclosure in rice from the aspects of natural variation and mutants, respectively. The main research contents and results are as follows:1. Mapping of QTLs underlying panicle enclosure-related traits: A recombinant inbred line (RIL) population derived from a cross between a tropical japonica rice cultivar DZ60 and an indica rice cultivar H359 was grown under two environments and phenotyped for uppermost internode length (UIL), flag leaf sheath length (FLSL) and UIL-to-FLSL ratio (UFR). Using a molecular marker linkage map constructed previously based on this population, QTL mapping was performed for these three panicle enclosure-related traits. A total of 10 QTLs were detected, of which 3 were for UIL, located on chromosomes 1, 4 and 12, explaining 3.20%-14.59% of the phenotypic variance; 4 were for FLSL, located on chromosomes 1, 6, 8 and 12, explaining 2.59%-20.70% of the phenotypic variance; and 3 were for UFR, located on chromosomes 4, 6 and 8, explaining 2.89%-6.00% of the phenotypic variance. qUIL-1 and qFLSL-1 were mapped at the same position and displayed the largest effects expressed steadily across the environments, suggesting that they are a major QTL underlying UIL and FLSL simultaneously. Searching of the rice genome sequence indicated that the interval of this QTL covers the semidwarf gene sd1. Hence, we guess that the effect of this QTL might come from the sd1 gene. 2. Genetic analysis of mutant esp1: A panicle enclosure mutant was obtained from the progeny of an indica rice restorer line Zao-R974 after irradiation mutagenesis. The mutant displayed the characteristics of reduced plant height, delayed heading, shortened uppermost internode, partial enclosure of panicle, shorter panicle, 50% fewer grains per panicle, decreased number of secondary branches and degeneration of partial spikelets. We named the mutant as esp1 (enclosed shorter panicle 1). Anatomical observation indicated that the average longitudinal length of parenchyma cells in esp1 uppermost internode (41.29μm) was only 69.85% of that of the wild-type (59.11μm), suggesting that the extension of cells in esp1 uppermost internode was cumbered, leading to less elongated uppermost internode and thus causing panicle enclosure. Genetic analysis showed that the mutant phenotype was controlled by a recessive gene, which was regularly inherited and unaffected by genetic background. GA3 spraying experiment indicated that esp1 is insensitive to gibberellin. Using the F2 and BC1 populations of a cross between esp1 and the japonica cultivar Nipponbare, we fine mapped ESP1 to a region between SSR markers RM26281 and GRM40 on chromosome 11, with genetic distances of 0.29 cM and 0.048 cM to the two markers, respectively. According to the physical map of the region covering ESP1, it was suggested that there was a 260-kb deletion between markers RM26281 and GRM40 in the mutant genome. According to the rice genome sequence annotation, there are 52 genes in this region. These results will facilitate the cloning of ESP1 gene.3. Genetic analysis of mutant esp2: A mutant of panicle enclosure was acquired from the tissue culture progeny of indica rice cultivar Minghui-86. In the mutant, panicles were entirely enclosed by flag leaf sheaths and the uppermost internode was almost completely degenerated, but the other internodes did not have obvious changes in length. We named the mutant as esp2 (enclosed shorter panicle 2). Anatomical observation revealed that in the uppermost internode of esp2 the number of cells was dramatically reduced, the growth and differentiation of the cells was stopped, and the development of stem pith cavity and vascular bundles was arrested. Genetic analysis indicated that the mutant phenotype was controlled by a recessive gene, which could be steadily inherited and was not affected by genetic background. Apparently, ESP2 is a key gene for the development of uppermost internode in rice. Gene interaction analysis showed that in regard to the trait of uppermost internode length, ESP2 has recessive epistasis over ESP1 and two uppermost internode elongation genes (EUI1 and EUI2). The double mutants of esp1esp2, eui1esp2 and eui2esp2 all hardly have visible uppermost internode. GA3 spraying experiment indicated that esp2 is also insensitive to gibberellin like esp1. Using an F2 population of a cross between esp2 and a japonica rice cultivar Xiushui-13, we fine mapped ESP2 to a 14-kb region on the end of the short arm of chromosome 1. According to the rice genome sequence annotation, only one intact gene exists in this region, namely, a putative phosphatidylserine synthase gene. Sequencing analysis on the mutant and the wild type indicated that this gene was inserted by a 5287-bp retrotransposon sequence. Hence, we took this gene as the candidate of ESP2. Quantitative RT-PCR analysis indicated that the ESP2 candidate gene is expressed in various growth stages and tissues in the wild-type Minhui-86, and the expression is enhanced by exogenous GA3. The results of this study will facilitate functional analysis of ESP2 gene.