Abstract
A novel hybrid weakness gene, DTE9, associated with a dark tip embryo (DTE) trait, was observed in CR6078, an introgression line derived from a cross between the Oryza sativa spp. japonica “Hwayeong” (HY) and the wild relative Oryza rufipogon. CR6078 seeds exhibit protruding embryos and flowers have altered inner floral organs. DTE9 was also associated with several hybrid weakness symptoms including decreased grain weight. Map-based cloning and transgenic approaches revealed that DTE9 is an allele of OsMADS8, a MADS-domain transcription factor. Genetic analysis indicated that two recessive complementary genes were responsible for the expression of the DTE trait. No sequence differences were observed between the two parental lines in the OsMADS8 coding region; however, numerous single nucleotide polymorphisms were detected in the promoter and intronic regions. We generated overexpression (OX) and RNA interference (RNAi) transgenic lines of OsMADS8 in HY and CR6078, respectively. The OsMADS8-OX lines showed the dark tip embryo phenotype, whereas OsMADS8-RNAi recovered the normal embryo phenotype. Changes in gene expression, including of ABCDE floral homeotic genes, were observed in the OsMADS8-OX and OsMADS8-RNAi lines. Overexpression of OsMADS8 led to decreased expression of OsEMF2b and ABA signaling-related genes including OsVP1/ABI3. HY seeds showed higher ABA content than CR6078 seeds, consistent with OsMADS8/DTE9 regulating the expression of genes related ABA catabolism in CR6078. Our results suggest that OsMADS8 is critical for floral organ determination and seed germination and that these effects are the result of regulation of the expression of OsEMF2b and its role in ABA signaling and catabolism.
Highlights
Hybrid weakness (HW) or hybrid breakdown is a postembryonic barrier in the F2 or later generations of interspecific or intraspecific crosses (Stebbins, 1950)
Hybrid weakness has been reported in rice at both the interspecific or intraspecific crosses, and most studies have reported that this phenomenon is mainly controlled by dominant or recessive complementary interactions of unlinked loci (Fukuoka et al, 1998, 2005; Kubo and Yoshimura, 2002; Matsubara et al, 2007; Yamamoto et al, 2007; Jiang et al, 2008)
Characterization of the genetic basis of hybrid weakness contributes to our knowledge of the underlying mechanisms of reproductive isolation and has practical significance with regard to the maximal utilization of invaluable genes from the interspecific or intraspecific crosses in plant breeding (Chen et al, 2014)
Summary
Hybrid weakness (HW) or hybrid breakdown is a postembryonic barrier in the F2 or later generations of interspecific or intraspecific crosses (Stebbins, 1950). A form of hybrid incompatibility during the postembryonic stage of plant development, is frequently observed in many other plant species including Arabidopsis thaliana (Bomblies et al, 2007). Hybrid weakness manifests in rice through characteristic dwarfing, chlorotic phenotype, stunted growth, and poor seed setting (Ichitani et al, 2011; Chen et al, 2013). Three causal genes for hybrid weakness at two loci, Hwi and Hwi, have been cloned in rice (Chen et al, 2014). A locus with two leucine-rich repeat receptor-like kinase (LRRRLK) genes, induces hybrid weakness while Hwi encodes a secreted putative subtilisin-like protease. Hybrid weakness is associated with abnormal growth and development, there have been no reports indicating a role of plant hormones in regulating this disorder in rice
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