Floral Patterning in Lotus japonicus

Floral patterning in Papilionoideae plants, such as pea (Pisum sativum) and Medicago truncatula, is unique in terms of floral organ number, arrangement, and initiation timing as compared to other well-studied eudicots. To investigate the molecular mechanisms involved in the floral patterning in legumes, we have analyzed two mutants, proliferating floral meristem and proliferating floral organ-2 (pfo-2), obtained by ethyl methanesulfonate mutagenesis of Lotus japonicus. These two mutants showed similar phenotypes, with indeterminate floral structures and altered floral organ identities. We have demonstrated that loss of function of LjLFY and LjUFO/Pfo is likely to be responsible for these mutant phenotypes, respectively. To dissect the regulatory network controlling the floral patterning, we cloned homologs of the ABC function genes, which control floral organ identity in Arabidopsis (Arabidopsis thaliana). We found that some of the B and C function genes were duplicated. RNA in situ hybridization showed that the C function genes were expressed transiently in the carpel, continuously in stamens, and showed complementarity with the A function genes in the heterogeneous whorl. In proliferating floral meristem and pfo-2 mutants, all B function genes were down-regulated and the expression patterns of the A and C function genes were drastically altered. We conclude that LjLFY and LjUFO/Pfo are required for the activation of B function genes and function together in the recruitment and determination of petals and stamens. Our findings suggest that gene duplication, change in expression pattern, gain or loss of functional domains, and alteration of key gene functions all contribute to the divergence of floral patterning in L. japonicus.