PETAL LOSS gene regulates initiation and orientation of second whorl organs in the Arabidopsis flower

PETAL LOSS is a new class of flower development gene whose mutant phenotype is confined mostly to the second whorl, Two properties are disrupted, orga n initiation and organ orientation. Initiation is frequently blocked, espec ially in later-formed flowers, or variably delayed. The few petals that ari se occupy a wider zone of the flower primordium than normal. Also, a minori ty of petals are trumpet-shaped, thread-like or stamenoid. Studies of ptl c ombined with homeotic mutants have revealed that the mutant effect is speci fic to the second whorl. not to organs with a petal identity. We propose th at the PTL gene normally promotes the induction of organ primordia in speci fic regions of the second floral whorl, In prl mutants, these regions are e nlarged and organ induction is variably reduced, often falling below a thre shold. A dominant genetic modifier of the ptl mutant phenotype was found in the Landsberg erecta strain that significantly boosts the mean number of p etals per flower, perhaps by reinforcing induction so that the threshold is now more often reached, The second major disruption in prl mutants relates to the orientation adopted by second whorl organs from early in their deve lopment. In single mutants the full range of orientations is seen, but when B function (controlling organ identity) is also removed, most second whorl organs now face outwards rather than inwards. Orientation is unaffected in B function single mutants. Thus petals apparently perceive their orientati on within the flower primordium by a mechanism requiring PTL function suppo rted redundantly by that of B class genes.