Analysis of the determination of floral meristem identity in Arabidopsis thaliana

At the onset of flowering, the Arabidopsis primary inflorescence meristem starts to produce flower meristems on its flank.Determination of floral fate is associated with changes in the growth pattern and expression of meristem identity genes, and suppression of a subtending leaf called bract.During the transition from vegetative to reproductive phase, the primary inflorescence produces lateral meristems that develop into either branches or flowers.The conversion of meristem identity from branch to flower is largely dependent on floral meristem identity genes LEAFY (LFY) and APETALA1 (AP1), both encoding transcription factors.Several studies, however, suggest that the attainment of the high level of LFY expression is a key step to confer correct floral identity to lateral meristems.Although many factors have been shown to promote LFY expression, none are expressed specifically to lateral meristems, raising the question of how local activation of LFY is regulated.In an attempt to find out new factor involved in the determination of floral meristem identity, I analyzed the function of the Arabidopsis PUCHI gene, a putative transcription factor of the AP2/EREBP family which has previously been shown to play roles in lateral root morphogenesis.I characterized two recessive puchi alleles, puchi-1 and puchi-2, grown under continuous-light and short-day conditions and showed that both mutants influenced inflorescence architecture in two ways.First, the number of branches is increased due the conversion of early arising flowers into branches.Second, puchi mutant flowers are subtended by rudimentary bracts, exhibiting a partial feature of branches.These results indicate that PUCHI is required for floral meristem identity and bract suppression.PUCHI is transiently expressed in the adaxial side of early flower primordia.Moreover, expression of GFP-PUCHI, in the same domain and at the same time, is sufficient to suppress the puchi phenotype.The iv expression domain of PUCHI does not overlap the cryptic bract, indicating that PUCHI acts to suppress bract formation non-cell autonomously.Next, I examined the interaction of PUCHI with BLADE-ON-PETIOLE1 (BOP1) and BOP2, which encode a pair of redundant regulatory proteins involved in various developmental processes including bract suppression.In addition to the ectopic bract formation that has previously been reported, I found that the bop1 bop2 double mutant displayed partial conversion of flowers to branches.Furthermore, the puchi bop1 bop2 triple mutant showed synergistic enhancement of defects in the determination of floral meristem identity and ectopic bract formation.The BOP1 and BOP2 expression did not rely on PUCHI and vice versa.I also demonstrated that the defect in floral meristem specification in the triple mutant associated with a drastic reduction of the LFY and AP1 expression.The present study thus suggests that PUCHI is required for proper conversion of secondary inflorescences to flowers.Functions of PUCHI in the floral fate determination and bract suppression overlap with that of BOP1 and BOP2, and that PUCHI acts together with the BOP genes to promote expression of LFY and AP1, two central regulators of floral meristem identity.Expression patterns of the PUCHI and BOP genes point to a role in spatial control of flower-specific activation of these meristem identity genes.