Radial axis differentiation in a globular embryo is marked by HAZ1, a PHD-finger homeobox gene of rice.

Homeobox genes that encode transcription factors play important roles in development and differentiation of both plant and animal systems. From a cDNA library of 3-day after-pollination (DAP) rice embryos we cloned a HAZ1 cDNA that encodes a protein with a PHD-finger domain and a homeodomain. A database search showed that HAZ1 was most similar in its entire amino acid sequence to Zmhox1a (52% identity) and Zmhox1b (50%), PHD-finger family homeodomain proteins of maize. Differing from Zmhox1, overexpression of HAZ1 brought no morphological change either in tobacco or in rice. In situ hybridization showed that HAZ1 was expressed at a higher level in the outer layers of a developing embryo than in the inner parts of the embryo at 3 DAP. At 4 and 5 DAPs, the expression of HAZ1 was concentrated at the ventral part of an embryo. These results indicate that HAZ1 marks outer layer cells of a globular embryo before any morphological differentiation is discerned in it. Radial axis differentiation marked by HAZ1 is then collapsed dynamically along with embryo morphogenesis, and HAZ1 later marks the ventral surface of the embryo.

FRIZZY PANICLE is required to prevent the formation of axillary meristems and to establish floral meristem identity in rice spikelets.

Inflorescences of grass species have a distinct morphology in which florets are grouped in compact branches called spikelets. Although many studies have shown that the molecular and genetic mechanisms that control floret organ formation are conserved between monocots and dicots, little is known about the genetic pathway leading to spikelet formation. In the frizzy panicle (fzp) mutant of rice, the formation of florets is replaced by sequential rounds of branching. Detailed analyses revealed that several rudimentary glumes are formed in each ectopic branch, indicating that meristems acquire spikelet identity. However, instead of proceeding to floret formation, axillary meristems are formed in the axils of rudimentary glumes and they either arrest or develop into branches of higher order. The fzp mutant phenotype suggests that FZP is required to prevent the formation of axillary meristems within the spikelet meristem and permit the subsequent establishment of floral meristem identity. The FZP gene was isolated by transposon tagging. FZP encodes an ERF transcription factor and is the rice ortholog of the maize BD1 gene. Consistent with observations from phenotypic analyses, FZP expression was found to be restricted to the time of rudimentary glumes differentiation in a half-ring domain at the base of which the rudimentary glume primordium emerged.

Partial conservation of LFY function between rice and Arabidopsis.

The LFY/FLO genes encode plant-specific transcription factors and play major roles in the reproductive transition as well as floral development. In this study, we reconstructed the phylogenetic tree of the 49 LFY/FLO homologs from various plant species. The tree clearly shows that the LFY/FLO genes from the eudicots and monocots formed the two monophyletic clusters with very high bootstrap probabilities, respectively. Furthermore, grass LFY/FLO genes have experienced significant acceleration of amino acid replacement rate compared with the eudicot homolog. To test whether grass LFY/FLO genes have a conserved function with those of eudicots, we introduced RFL, a rice LFY homolog, into the Arabidopsis lfy mutant. The RFL gene driven by LFY promoter partially rescued the lfy mutation, suggesting that the functions of LFY and RFL partly overlap. Interestingly, the RFL but not LFY, strongly activated the expression of AP1 and AG, the downstream targets of LFY, even in the vegetative tissues. The LFY::RFL transgenic Arabidopsis plants exhibited abnormal patterns of development such as leaf curling, bushy appearance and the transformation of ovules into carpels. All of the results indicate that both the partial conservation and divergence of LFY function between rice and Arabidopsis.