Cytoplasmic Male Sterility of Rice with Boro II Cytoplasm Is Caused by a Cytotoxic Peptide and Is Restored by Two Related PPR Motif Genes via Distinct Modes of mRNA Silencing

Abstract

Cytoplasmic male sterility (CMS) and nucleus-controlled fertility restoration are widespread plant reproductive features that provide useful tools to exploit heterosis in crops. However, the molecular mechanism underlying this kind of cytoplasmicnuclear interaction remains unclear. Here, we show in rice (Oryza sativa) with Boro II cytoplasm that an abnormal mitochondrial open reading frame, orf79, is cotranscribed with a duplicated atp6 (B-atp6) gene and encodes a cytotoxic peptide. Expression of orf79 in CMS lines and transgenic rice plants caused gametophytic male sterility. Immunoblot analysis showed that the ORF79 protein accumulates specifically in microspores. Two fertility restorer genes, Rf1a and Rf1b, were identified at the classical locus Rf-1 as members of a multigene cluster that encode pentatricopeptide repeat proteins. RF1A and RF1B are both targeted to mitochondria and can restore male fertility by blocking ORF79 production via endonucleolytic cleavage (RF1A) or degradation (RF1B) of dicistronic B-atp6/orf79 mRNA. In the presence of both restorers, RF1A was epistatic over RF1B in the mRNA processing. We have also shown that RF1A plays an additional role in promoting the editing of atp6 mRNAs, independent of its cleavage function.

Effects of Rf-1, Rf-3 and Rf-6(t) Genes on Fertility Restoration in Rice (Oryza sativa L.) with WA- and BT-type Cytoplasmic Male Sterility

“…I observed that the Rf-1 gene was more or less effective on both cytoplasm types: the Rf-1 derived from IR36 restored pollen fertility in a gametophytic manner in BT-type CMS (Table 1), and partially restored pollen fertility in WA-type CMS (Table 2). These results are consistent with those obtained in previous studies using indica and japonica varieties (Young and Virmani 1984, Raj and Virmani 1988, Bharaj et al 1991, 1995, Teng and Shen 1994, Yao et al 1997, Akagi et al 2004, Komori et al 2004, Wang et al 2006. At present, it is remains to be determined whether the partial restoration of pollen fertility in WA-type CMS is a pleiotropic effect of Rf1a or Rf1b (Wang et al 2006), or is due to the clustering of multiple Rf genes of the Rf-1 locus.…”

“…These results are consistent with those obtained in previous studies using indica and japonica varieties (Young and Virmani 1984, Raj and Virmani 1988, Bharaj et al 1991, 1995, Teng and Shen 1994, Yao et al 1997, Akagi et al 2004, Komori et al 2004, Wang et al 2006. At present, it is remains to be determined whether the partial restoration of pollen fertility in WA-type CMS is a pleiotropic effect of Rf1a or Rf1b (Wang et al 2006), or is due to the clustering of multiple Rf genes of the Rf-1 locus. Nucleotide sequencing of the Rf-1 locus revealed that it consists of multiple PPR protein genes, a gene family to which most of the plant Rf genes belong (Bentolila et al 2002, Koizuka et al 2003, Brown et al 2003.…”

“…Although the Rf-1 locus consists of two genes, Rf1a and Rf1b (Wang et al 2006), the genes were tightly linked and co-segregated in the NILs, CSSLs and F 1 plants used in the present study. Therefore, we treated these Rf-1 genes as a single locus, Rf-1, in this report, unless otherwise mentioned.…”

“…1). The single nucleotide polymorphic marker 01-45 was used to detect the Rf1b sequence according to the method of Wang et al (2006). The Rf-1A (Rf1a) gene was detected by PCR using the specific primers Rf1-3F (5′-tgatgatcgaggaggaggta-3′) and Rf1-4R (5′-aagagtattgtaggtaatgac-3′), which were designated according to the reported Rf-1A sequence (Komori et al 2004).…”

“…In rice (Oryza sativa L.), Rf-1, a gene that restores Chinsurah Boro-or BT-type CMS, has also been cloned (Akagi et al 2004, Komori et al 2004, Kazama and Toriyama 2003. Wang et al (2006) demonstrated that the locus consists of a cluster of duplicated genes. BT-type CMS is used for seed production of japonica hybrid varieties .…”