MIL2 (MICROSPORELESS2) regulates early cell differentiation in the rice anther

The formation of diverse, appropriately patterned cell types is critical in the development of all complex multicellular organisms. In flowering plants, anther patterning is a complex process essential for successful sexual reproduction. However, few genes regulating this process have been characterized to date. We report here that the gene MICROSPORELESS2 (MIL2) regulates early anther cell differentiation in rice (Oryza sativa). The anthers of mil2 mutants were characterized using molecular markers and cytological examination. The MIL2 gene was cloned and its expression pattern was analyzed through RNA in situ hybridization. The localization of the MIL2 protein was observed by immunostaining. MIL2 encodes the rice homolog of the Arabidopsis TAPETUM DETERMINANT1 (TPD1) protein. However, mil2 anthers display phenotypes different from those of tpd1 mutants, with only two layers of anther wall cells formed. MIL2 has an expression pattern distinct from that of TPD1. Its transcripts and proteins predominate in inner parietal cells, but show little accumulation in reproductive cells. Our results demonstrate that MIL2 is responsible for the differentiation of primary parietal cells into secondary parietal cells in rice anthers, and suggest that rice and Arabidopsis anthers might share similar regulators in anther patterning, but divergent mechanisms are employed in these processes.