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DEVELOPMENT AND HORMONE ACTION

Expression Level of ABERRANT PANICLE ORGANIZATION1 Determines Rice Inflorescence Form through Control of Cell Proliferation in the Meristem^1,[W]

Research Institute for Bioresources, Okayama University, Kurashiki, Okayama 710–0046, Japan (K.I.-K., M.M.); Graduate School of Agriculture and Life Sciences, University of Tokyo, Yayoi, Bunkyo, Tokyo 113–0032, Japan (N.Y., T.O., Y.N., J.K.); and National Institute for Basic Biology, Okazaki 444–8585, Japan (S.I.)

Two types of branches, rachis branches (i.e. nonfloral) and spikelets (i.e. floral), are produced during rice (Oryza sativa) inflorescence development. We previously reported that the ABERRANT PANICLE ORGANIZATION1 (APO1) gene, encoding an F-box-containing protein orthologous to Arabidopsis (Arabidopsis thaliana) UNUSUAL FLORAL ORGANS, suppresses precocious conversion of rachis branch meristems to spikelets to ensure generation of certain number of spikelets. Here, we identified four dominant mutants producing an increased number of spikelets and found that they are gain-of-function alleles of APO1. The APO1 expression levels are elevated in all four mutants, suggesting that an increase of APO1 activity caused the delay in the program shift to spikelet formation. In agreement with this result, ectopic overexpression of APO1 accentuated the APO1 gain-of-function phenotypes. In the apo1-D dominant alleles, the inflorescence meristem starts to increase in size more vigorously than the wild type when switching to the reproductive development phase. This alteration in growth rate is opposite to what is observed with the apo1 mutants that have a smaller inflorescence meristem. The difference in meristem size is caused by different rates of cell proliferation. Collectively, these results suggest that the level of APO1 activity regulates the inflorescence form through control of cell proliferation in the meristem.

² These authors contributed equally to the article.

³ Present address: School of Pharmaceutical Sciences, University of Shizuoka, 52–1, Yada, Shizuoka 422–8526, Japan.

The author responsible for the distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Junko Kyozuka (akyozuka{at}mail.ecc.u-tokyo.ac.jp).

^[W] The online version of this article contains Web-only data.

www.plantphysiol.org/cgi/doi/10.1104/pp.109.136739

^* Corresponding author; e-mail akyozuka{at}mail.ecc.u-tokyo.ac.jp.

Received February 5, 2009; accepted April 14, 2009; published April 22, 2009.