Circadian clock- and phytochrome-regulated Dof-like gene, Rdd1, is associated with grain size in rice

We report here on the characterization of a putative Dof transcription factor gene in rice (Oryza sativa) - rice Dof daily fluctuations 1 (Rdd1). Daily oscillations in Rdd1 expression were retained after transferring to continuous dark (DD) or light (LL) conditions, indicating circadian regulation. However, Rdd1 showed arrhythmic expression in etiolated coleoptiles. Experiments revealed that the Rdd1 transcript accumulated up to 1 h after transferring from DD to LL conditions and decreased thereafter. We examined Rdd1 expression using phytochrome (phy)-deficient mutants, and the results showed that phyA and most likely phyB contributed to the regulation of Rdd1 expression. To further examine the role of Rdd1, transgenic rice plants were produced that carried Rdd1 in either a sense (RDD1-S) or antisense (RDD1-AS) orientation, driven by a constitutive promoter. The expression of endogenous Rdd1 in response to far-red light was found to be modified in RDD1-AS plants compared with wild-type (WT) or RDD1-S plants. In addition, RDD1-AS plants were smaller and flowered later than WT or RDD1-S plants; decreases in grain length, width and 1000-grain weight were also recorded. This study demonstrates that Rdd1 is a circadian clock and phy-regulated gene, which is associated with grain size in rice.