벼의 탈립성과 lignin 생합성을 조절하는 homeobox 유전자들의 기능 연구

Abstract

Seed shattering is an important determinant of grain yields. In rice, a major quantitative trait locus (QTL) that controls this process is qSH1. Although the degree of shattering is correlated with the level of qSH1 expression, some qSH1-defective cultivars display moderate shattering while others show a non-shattering phenotype. Os05g38120 (SH5) on Chromosome (Chr) 5 is highly homologous to qSH1. Although this study demonstrated that SH5 transcripts were detected in various organs, this gene was most highly expressed at the abscission zone (AZ) in the pedicels. When expression of this gene was suppressed in easy-shattering indica rice cultivar ‘Kasalath’, AZ development was reduced and seed losses were thereby diminished. By contrast, the extent of shattering, as well as AZ development, was greatly enhanced in moderate-shattering ‘Dongjin’ japonica rice when SH5 was over-expressed. Likewise, overexpression of SH5 in the usually non-shattering ‘Ilpum’ japonica rice led to an increase in seed shattering because lignin levels were decreased at the base of the spikelet in the absence of AZ development. This investigation also revealed that SH5 induced the expression of two shattering-related genes, SHAT1 and Sh4, that are necessary for proper AZ formation. Therefore, it is likely that SH5 modulates seed shattering by enhancing AZ development and inhibiting lignin biosynthesis. The BELL homeobox protein involve SH5 and qSH1 and can bind to KNOX proteins as a three-amino acid-loop-extension (TALE) complex. Among these KNOX proteins, OSH15 binds to SH5 and qSH1. Both OSH15 knockdown and RNAi plants display defects in their abscission zones as well as ectopic accumulations of lignin in their internode regions. Although OSH15 was universally expressed from the seedling stage to mature spikelets, it was preferentially expressed at the abscission zone during spikelet development. Loss-of-function OSH15 mutants exhibited a dwarf phenotype due to a shortening of the internodes. Examination of osh15-1 mutants indicated that ectopic lignin was deposited between the small vascular tissues when lignin biosynthesis genes were highly expressed in the intercalary meristem (IM) region. Knockout and activation lines of lignin biosynthesis genes were produced to determine the role of lignification in seed shattering and internodal elongation. Here, the cad2 null mutant displayed an easy-shattering phenotype because lignin deposition was inhibited. In addition, several activation lines for PAL1, CCR1, and CAD2 showed dwarf phenotypes due to ectopic lignin accumulations. These findings demonstrated that the SH5–OSH15 TALE complex is required for repressing lignin deposition, which then leads to seed shattering and internodal elongation in the IM.