Pentatricopeptide-repeat family protein RF6 functions with hexokinase 6 to rescue rice cytoplasmic male sterility

Abstract

Cytoplasmic male sterility (CMS) has been extensively used for hybrid seed production in many major crops. Honglian CMS (HL-CMS) is one of the three major types of CMS in rice and has contributed greatly to food security worldwide. The HL-CMS trait is associated with an aberrant chimeric mitochondrial transcript, atp6-orfH79, which causes pollen sterility and can be rescued by two nonallelic restorer-offertility (Rf) genes, Rf5 or Rf6. Here, we report the identification of Rf6, which encodes a novel pentatricopeptide repeat (PPR) family protein with a characteristic duplication of PPR motifs 3-5. RF6 is targeted to mitochondria, where it physically associates with hexokinase 6 (OsHXK6) and promotes the processing of the aberrant CMSassociated transcript atp6-orfH79 at nucleotide 1238, which ensures normal pollen development and restores fertility. The duplicated motif 3 of RF6 is essential for RF6-OsHXK6 interactions, processing of the aberrant transcript, and restoration of fertility. Furthermore, reductions in the level of OsHXK6 result in atp6-orfH79 transcript accumulation and male sterility. Together these results reveal a novel mechanism for CMS restoration by which RF6 functions with OsHXK6 to restore HL-CMS fertility. The present study also provides insight into the function of hexokinase 6 in regulating mitochondrial RNA metabolism and may facilitate further exploitation of heterosis in rice.cytoplasmic male sterility | pentatricopeptide repeat | restorer-of-fertility | RF6 | hexokinase 6 P lant cytoplasmic male sterility (CMS) is a trait characterized by a lack of functional pollen grains in plants, although female gametes are still viable (1). CMS has been widely used to produce hybrid seeds to improve plant yield, improve resistance to diseases or stresses, or enhance taste (2). Plant CMS results from genomic incompatibility between mitochondria and nuclei and is typically associated with an aberrant chimeric gene in the mitochondria (3-6). A specific set of nuclear genes called restorer-of-fertility (Rf) genes, which primarily belong to the pentatricopeptide repeat (PPR) family, repress CMS through the promotion of the cleavage, degradation, or editing of CMS-associated aberrant transcripts or the inhibition of the translation of these genes (7-9). PPR proteins regulate RNA metabolism at multiple levels, from RNA editing, stability, processing, and splicing to translation (10, 11). PPR proteins can be classified into PLS and P families based on the structure of PPR motifs (11). PLS family members generally exhibit direct RNA binding (12)(13)(14) or editing activity (15-17), whereas P-family proteins often lack sites for binding to RNA, particularly mitochondrial CMS-related RNA (18,19). Most of the known PPR proteins implicated in restoration of fertility (RF) are P-family members. Thus, RF-related PPR proteins may require additional partners, such as RNA-binding or splicing factors to process CMSrelated transcripts.Rice CMS is categorized into three major types: Wild Abortive (WA), originating from...

Ability of Rf5 and Rf6 to Restore Fertility of Chinsurah Boro II-type Cytoplasmic Male Sterile Oryza Sativa (ssp. Japonica) Lines

“…Our mapping study essentially produced the same results. Furthermore, the cloning of Rf6 indicated that Rf6 is the ORF of Os08g0110200 and is also capable of restoring the fertility of BT-type CMS plants (Huang et al 2015). Therefore, we concluded that qSF8-1 is Rf6 .…”

“…Rf5 and Rf6, the two non-allelic fertility restorer genes in ‘93-11’ associated with HL-type CMS, have been mapped and cloned (Huang et al 2012, 2015) . Rf5 and Rf1a are the same gene, and Rf6 is also capable of restoring the fertility of BT-type CMS plants (Huang et al 2015). We mapped qSF10-1 to a region on chromosome 10 containing Rf5 ( Rf1a ) and mapped qSF8-1 to a region on chromosome 8 containing Rf6 .…”

“…Rf3 and Rf4 are two major fertility restorer genes for WA-type CMS, and have been mapped on chromosomes 1 and 10, respectively (Ahmadikhah and Karlov 2006; Tang et al 2014; Zhang et al 1997). Rf5 and Rf6 , which have been cloned and mapped on chromosomes 10 and 8, respectively, are the two major fertility restorer genes for HL-type CMS (Hu et al 2012; Huang et al 2000, 2012, 2015). Rf17 (chromosome 4) and Rf2 (chromosome 2) are the fertility restorer genes for Chinese wild rice-type CMS (Fujii and Toriyama 2009) and Lead rice-type CMS (Itabashi et al 2011), respectively.…”

Plant Mitochondrial Genome Evolution and Cytoplasmic Male Sterility

“…Among these is the question of how mitochondrial gene and genome evolution influence the interactions of the nuclear genome and the origin of cytoplasmic male sterility (CMS). CMS is a maternally inherited trait conferred by the mitochondrial genome that results in a failure to produce functional pollen and/or male reproductive organs (Pruitt and Hanson, 1991;Budar and Pelletier, 2001;Chase, 2007;Kubo et al, 2011;Suzuki et al, 2013), except in the presence of restorer-of-fertility genes (Chase, 2007;Cho et al, 2012;Lee et al, 2014;Huang et al, 2015). CMS is also important in specifying gynodioecy in natural populations (Hanson and Bentolila, 2004;Miller and Bruns, 2016), a phylogenetically widespread reproductive strategy in flowering plants.…”

Mapping QTLs for Fertility Restoration of Different Cytoplasmic Male Sterility Types in Rice Using Two Oryza sativa ×O. rufipogon Backcross Inbred Line Populations

“…To date, more than 17 Rf genes or loci that rescue different CMS types have been identified, distributed across all rice chromosomes apart from chromosome 9 (Gramene, http://www.gramene.org). Of these, seven Rf genes have been identified and functionally characterized: Rf1 ( Rf1a and Rf1b ) for BT-type CMS [20, 21], Rf2 for LD-type CMS [22], Rf4 for WA-type CMS [23, 24], Rf5 and Rf6 for HL-CMS [25, 26], and Rf17 for CW-CMS [27]. Molecular cloning and characterization of these Rf genes not only further described the cytoplasmic-nuclear interactions, but also provided powerful molecular tools to assist in hybrid breeding by accelerating the development of novel restorer lines.…”