A LEAFY COREGULATOR ENCODED BY UNUSUAL FLORAL ORGANS

Background: Development of petals and stamens in Arabidopsis flowers r equires the function of the organ-identity gene APETALA3 (AP3), whose RNA is expressed specifically in petal and stamen primordia. AP3 expre ssion is positively regulated by the meristem-identity gene LEAFY (LFY ), which is expressed ubiquitously in young flowers, It is unknown how the transition from ubiquitous expression of LFY to region-specific e xpression of AP3 is made. It has previously been proposed far Antirrhi num that another gene, FIMBRIA TA (FIM), mediates between the LFY and AP3 orthologs, with the three genes acting in a simple regulatory hier archy. FIM is activated later than the LFY ortholog, and its expressio n is more restricted than that of the LN ortholog. Results: We have te sted whether the model proposed for Antirrhinum applies to Arabidopsis , by creating transgenic plants in which the NM ortholog UNUSUAL FLORA L ORGANS (UFO) was expressed constitutively from the promoter of the c auliflower mosaic virus 35S gene. In 35S::UFO flowers, AP3 was express ed precociously and ectopically, confirming that UFO is an upstream re gulator of AP3. However, 35S::UFO could not restore petal and stamen d evelopment in lfy mutants, indicating that UFO can only function in th e presence of LFY activity, The failure of 35S::UFO to rescue lfy muta nts is consistent with our observation that UFO expression levels are not markedly changed in lfy mutants. Conclusions: We conclude that UFO is not a simple mediator between meristem- and organ-identity genes, but is likely to be a partially dispensable co-regulator that acts tog ether with LFY. The interplay between LFY and UFO provides a paradigm for how a global regulator such as LFY activates selected target genes only in restricted regions within its expression domain.