Regulation of Grain Chalkiness and Starch Metabolism by FLO2 Interaction Factor 3, a bHLH Transcription Factor in Oryza sativa

Chalkiness is a key determinant that directly affects the appearance and cooking quality of rice grains. Previously, Floury endosperm 2 (FLO2) was reported to be involved in the formation of rice chalkiness; however, its regulation mechanism is still unclear. Here, FLO2 interaction factor 3 (OsFIF3), a bHLH transcription factor, was identified and analyzed in Oryza sativa. A significant increase in chalkiness was observed in OsFIF3-overexpressed grains, coupled with a round, hollow filling of starch granules and reduced grain weight. OsFIF3 is evolutionarily conserved in monocotyledons, but variable in dicotyledons. Subcellular localization revealed the predominant localization of OsFIF3 in the nucleus. The DAP-seq (DNA affinity purification sequencing) results showed that OsFIF3 could affect the transcriptional accumulation of β-amylase 1, α-amylase isozyme 2A-like, pectinesterase 11, β-glucosidase 28 like, pectinesterase, sucrose transport protein 1 (SUT1), and FLO2 through the binding of the CACGTG motif on their promoters. Moreover, FLO2 and SUT1 with abundant OsFIF3 binding signals showed significant expression reduction in OsFIF3 overexpression lines, further confirming OsFIF3's role in starch metabolism regulation and energy material allocation. Taken together, these findings show that the overexpression of OsFIF3 inhibits the expression of FLO2 and SUT1, thereby increasing grain chalkiness and affecting grain weight.