Flavonoids are indispensable for complete male fertility in rice

Abstract

Flavonoids are essential for male fertility in some but not all plant species. In rice, the chalcone synthase mutant oschs1 produces flavonoid-depleted pollens and is male sterile. The mutant pollens are viable with normal structure but they displayed reduced germination rate and pollen tube length. Analysis of pollens of oschs1/+ heterozygous lines showed that pollen flavonoid deposition is a paternal effect and fertility is independent from the haploid genotypes (OsCHS1 or oschs1). To understand which classes of flavonoids are involved in male fertility, we further analyzed rice mutants for branch-point enzymes of the downstream flavonoid pathways, including flavanone 3-hydroxylase (OsF3H; flavonol pathway entry enzyme), flavone synthase II (CYP93G1; flavone pathway entry enzyme) and flavanone 2-hydroxylase (CYP93G2; flavone C-glycoside pathway entry enzyme). Rice osf3h and cyp93g1 cyp93g2 CRISPR/Cas9 mutants, and cyp93g1 and cyp93g2 T-DNA insertional mutants showed altered flavonoid profiles in anthers but only osf3h and cyp93g1 cyp93g2 mutants displayed reduction in seed yield. Our findings indicated that flavonoids are essential for complete male fertility in rice and a combination of different classes (flavanones, flavonols, flavones, and flavone C-glycosides) are likely to be important, as opposed to the essential role played primarily by flavonols as reported previously in several plant species.