The rice LEC1-like transcription factor OsNF-YB9 interacts with SPK, an endosperm-specific sucrose synthase protein kinase, and functions in seed development.

LEAFY COTYLEDON1 (LEC1), a NUCLEAR FACTOR-Y (NF-Y) family member, plays a critical role in embryogenesis and seed development in Arabidopsis. Previous studies have shown that rice OsNF-YB9 and OsNF-YB7 are homologous to Arabidopsis LEC1. However, the functions of LEC1-like genes in rice remain unclear. Here we report that OsNF-YB9 and OsNF-YB7 display sub-functionalization in rice. We demonstrate that OsNF-YB7 is expressed mainly in the embryo, whereas OsNF-YB9 is preferentially expressed in the developing endosperm. Heterologous expression of either OsNF-YB9 or OsNF-YB7 in Arabidopsis lec1-1 was able to complement the lec1-1 defects. We failed to generate osnf-yb7 homozygous mutants due to lethality caused by OsNF-YB7 defects. Loss of OsNF-YB9 function caused abnormal seed development: seeds were longer, narrower and thinner and exhibited a higher chalkiness ratio. Furthermore, the expression of genes related to starch synthesis was deregulated in osnf-yb9. OsNF-YB9 could interact with SPK, a sucrose synthase protein kinase that is predominantly expressed in rice endosperm. Knockout of SPK resulted in chalky seeds similar to those observed in the osnf-yb9 mutants. Ectopic expression of OsNF-YB9 in both rice and Arabidopsis resulted in unhealthy plants with small seeds. Taken together, these results suggest a critical role for OsNF-YB9 in rice seed development.