Circadian- and light-regulated oscillatory expression of CSA in rice leaves is required for pollen fertility.

The oscillatory expression of CSA in rice leaves is regulated by the circadian clock and red/far-red light signals, mediated through DOF5 and PIL11, and is required for normal pollen fertility. Photoperiod-sensitive male-sterile lines represent a pivotal innovation in the development of hybrid rice. However, the underlying mechanisms governing photoperiod-sensitive male reproductive development remain poorly understood. Our previous studies demonstrated that the carbon starved anther (csa) mutant exhibits male sterility under short-day (SD) conditions but partial fertility under long-day (LD) conditions. In this study, we report that CSA expression follows an oscillatory rhythm in rice leaves under both SD and LD conditions, a pattern regulated by both circadian clock and light signals. Tissue-specific RNA interference knockdown of CSA in leaves was associated with reduced pollen viability, suggesting that CSA expression in leaves contributes to normal male fertility. Promoter truncation assay results indicate that distinct regions of the CSA promoter contribute differentially to the regulation of CSA expression in leaves versus anthers, and that both the CSA expression level in anthers and the rhythmic expression pattern of CSA in leaves are associated with the restoration of male fertility. Using dual-luciferase, yeast one-hybrid, and electrophoretic mobility shift assays, we identified two proteins, PIL11 and DOF5, which directly bind to specific motifs (an E-box and T/AAAAG motif) within the CSA promoter truncation, thereby regulating its transcription. These findings elucidate novel mechanisms linking light sensing to the expression of circadian-controlled genes, thus connecting photoperiod with male reproductive development in rice.