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

SUMMARY 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‐YB 9 in rice seed development.