In rice, the flower consisting of lodicules, stamens and carpels is enclosed by the lemma and palea to form the floret, which together with sterile lemmas and rudimentary glumes constitutes the spikelet. Thus, the flower and the inflorescence units of rice are distinct from those of eudicots. The ABC model, which explains the genetic mechanism underlying floral organ specification in eudicots, is largely applicable to the specialized flowers of rice. For instance, the function of class B genes is conserved to specify the lodicule (a petal homologue) and the stamen. Two class C genes are functionally diversified in rice: one specifies stamen identity together with class B genes, whereas the other is mainly responsible for the determinacy of the flower meristem. By contrast, carpel specification in rice is regulated by a YABBY gene, DROOPING LEAF (DL). Homeotic transformation of the stamen or carpel in loss-of-function mutants of class B genes or DL reveals a mutual repression mode of action for these genes. Additional genes responsible for the development of spikelet organs, such as the lemma, palea, sterile lemma and rudimentary glume, have been identified in rice. Mutations in some of these genes affect the development of only spikelet organs, whereas mutations in others affect the development of both flower and non-floral spikelet organs. In this review, we describe the genetic mechanism underlying flower and spikelet development in rice, and discuss the regulation of maintenance and fate of reproductive meristems, the activity of which is closely associated with flower and spikelet development.
