Molecular Basis Underlying the S5-Dependent Reproductive Isolation and Compatibility of indica/japonica Rice Hybrids

Qing Ji,Hongmei Wang,Qing Chao,Mingliang Xu,Chunxia Liu,Minghong Gu,Meijing Zhang,Jufei Lu,Bing Lin,Qiaoquan Liu,Yan Zhang

doi:10.1104/pp.111.189571

Abstract

The S5 locus regulates spikelet fertility of indica/japonica hybrid rice (Oryza sativa). There are three alleles at the S5 locus, including an indica allele (S5i), a japonica allele (S5j), and a wide-compatibility allele (S5n). This study analyzed the molecular basis for S5-dependent reproductive isolation and compatibility of indica/japonica rice hybrids. Three S5 alleles were expressed at extremely low levels, and only in the ovary. S5n was more similar to S5i in both RNA and protein expression profiles. The S5 locus was not essential for embryo sac development, although deleterious interactions between S5i and S5j resulted in reduced rates of spikelet fertility. The yeast two-hybrid system was used to test direct interactions between S5-encoded proteins. The results indicated that the S5i- and S5j-encoded eukaryotic aspartyl proteases formed both homodimers and heterodimers, whereas the S5n-encoded aspartyl protease was incapable of dimerization. Site-directed mutagenesis revealed that a single amino acid difference between S5i- and S5j-encoded aspartyl proteases (phenylalanine/leucine at residue 273) was primarily responsible for embryo sac abortion. The S5 locus may have promoted the subspeciation of indica and japonica, but it also enables gene flow between them.

Highlights

The S5 locus regulates spikelet fertility of indica/japonica hybrid rice (Oryza sativa)
The results revealed that various genetic elements presented and affected spikelet fertility, including genetic background effects, allelic interactions, and the interactions between the S5 locus and genetic backgrounds (Table I)
The spikelet fertilities of S5jS5j-Balilla, S5jS5n-Balilla, and S5nS5n-Balilla were essentially equivalent. This implied that S5n did not negatively impact spikelet fertility when introduced into the indica or japonica background

Summary

Introduction

The S5 locus regulates spikelet fertility of indica/japonica hybrid rice (Oryza sativa). WCVs can overcome reproductive barriers and produce fertile hybrids when crossed to indica or japonica subspecies To explain this phenomenon, Ikehashi and Araki (1986) proposed a “one-locus spore-gametophytic interaction” genetic model (i.e. the S5 locus is responsible for the fertility of intersubspecific hybrids), and one of three alleles, namely S5i, S5j, and S5n, is present. The S5 locus, plays a important role in embryo sac sterility, and S5n from a WCV can greatly enhance embryo sac fertility (Ikehashi and Araki, 1986; Liu et al, 1992, 1997; Yanagihara et al, 1995; Wang et al, 1998, 2005; Yan et al, 2000; Liang et al, 2001; Yi et al, 2001) It is critical, to understand the genetic mechanisms that underlie S5-dependent fertility of intersubspecific rice hybrids. Analysis of the Sa locus, which controls male sterility of indica/japonica hybrids (Long et al, 2008), revealed a similar triallelic system, suggesting that such systems are widespread within rice (Ouyang et al, 2010)

Methods

Results

Discussion

Conclusion