Minisymposium 22: Development

Abs # 36003: The YABBY gene DROOPING LEAF regulates carpel identity and midrib formation in rice.

Presenter:	Yamaguchi, Takahiro , aa07013@mail.ecc.u-tokyo.ac.jp
Authors	Yamaguchi, Takahiro  (A)   An, Gynheung  (B)   Miyao, Akio  (C)   Hirochika, Hirohiko  (C)   Hirano, Hiro-Yuki  (A)
Affiliations:	(A): Graduate School of Agricultural and Life Sciences, University of Tokyo (B): Pohang University of Science and Technology (C): National Institute of Agrobiological Resources

Genetic and molecular studies in two model eudicots, Arabidopsis thaliana and Antirrhinum majus, have led to the establishment of the ABC model for floral organ identity. Although a number of studies of ABC orthologs suggest that this model is highly conserved among eudicots, elucidation of the mechanisms of floral organ determination is incomplete in monocots. We have been studying floral organ specification in rice to reveal the mechanisms of flower development in monocots with interest in evolutionally developmental studies. As a first approach, we have isolated the DROOPING LEAF (DL) gene in rice. Mutations in dl locus result in homeotic transformation of the carpel into stamens, which is not typical of any ABC mutants. Molecular isolation revealed that DL encodes a transcription factor of YABBY family protein, which is closely related to CRABS CLAW (CRC) in Arabidopsis, and is expressed specifically in carpel primordium. These data indicate that carpel identity is regulated by DL in rice. As a next step, we studied genetic relationships of DL with ABC-MADS box genes by analyzing expression patterns of these genes in floral homeotic mutants defective in DL and the class B gene SUPERWOMAN1 (SPW1) and transgenic plants constitutively expressing SPW1. These studies indicated that DL is essential for carpel specification and interacts antagonistically with the class B genes. It is also suggested that DL functions independent or downstream of the class C genes and may be negatively regulated by putative A-function genes. We suppose that genetic relationships of DL/CRC with the ABC genes are fundamentally conserved between rice and Arabidopsis, but DL plays more essential roles in carpel specification in rice. It is of interest to reveal the roles of the class C genes in reproductive organ specification in rice and the genetic relationships with DL, so approaches to isolate knock-outs of the class C genes are in progress. In addition to the roles in flower development, DL is necessary for leaf development. Loss-of-functions of DL show midribless phenotype and it was expressed in the central region of developing leaves. Ectopic expression of DL thickened the lateral region of the leaf like midrib region. These results suggest that DL promotes midrib formation. Together with the functions and expressions in flower development, DL may not be involved in abaxial specification such as YABBY genes in Arabidopsis.