Discovery, utilization and perspective of Honglian cytoplasmic male sterile rice

Abstract

Hybrid rice has been researched and utilized in China for 50 years. Three mainly cytoplasmic male sterile rice are wild abortive (WA), boro-II (BT), and Honglian (HL). Honglian (HL) type hybrid rice has made important contributions to the development of hybrid rice by combining scientific research, application research and industrial utilization. We identified Honglian cytoplasmic male sterile (HL-CMS) gene, orfH79, which located downstream of atp6 in mitochondrial genome. CMS RNA, atp6-orfH79, were translated into a cytotoxic protein, ORFH79. Data showed ORFH79 interacts with P61, which is a subunit of complex III of mitochondrial electron transfer chain. This interaction may impair energy production and leads to the accumulation of reactive oxygen species (ROS) in mitochondria, consequently resulting in abnormal pollen development. Furthermore, we cloned two restorer of fertility (Rf) genes, Rf5 and Rf6, both of which are PPR genes. Neither RF5 nor RF6 binds to CMS RNA, atp6-orfH79, directly. RF5 interacts with glycine rich protein 162 (GRP162) and RFC3 (restoration of fertility complex subunit 3) thus forming a restoration of fertility complex (RFC) to process CMS RNA. GRP162 can form homodimer and bind to CMS RNA with two binding sites directly. RCF3 is transported into the mitochondria as a membrane protein and functions as a scaffold protein for the assembly of the RFC complex. The RFC complex was detected with a size ranging from 400 to 500 kD. Moreover, Rf6 encodes a novel pentatricopeptide repeat (PPR) family protein with a characteristic duplication of PPR motifs 3 to 5. Results show RF6 is also targeted to mitochondria, where it physically associates with hexokinase 6 (OsHXK6) and promotes the processing of the aberrant CMS RNA, atp6-orfH79, to restore the fertility of CMS line. To create more available germplasm in hybrid rice, we developed a molecular marker of orfH79 to screen wild rice and bred some novel CMS lines. We also developed molecular markers of Rf5 and Rf6 to breed novel stronger restore lines which contain two Rf genes. Consequently, we bred several combinations such as Honglian you 6, Yueyou 938, Luoyou 8 and Luoyou 9348 with high yield, good quality, multiple resistance and wide adaptation. HL hybrid rice has been widely planted in Yangzte river areas and South China with the yield of 10 to 12 tons per hectare. HL hybrid rice combinations also have been planted in Southeast Asian countries, such as Vietnam, Indonesia, Malaysia, Bangladesh and Phillippine. Most combinations exhibited 140% or more of yield compared with the local rice cultivars in Southeast Asian countries. HL hybrid rice even more is becoming the leading hybrid rice and priority choice in most of Asian countries and other Africa countries.