Dual roles of the bZIP transcription factor PERIANTHIA in the control of floral architecture and homeotic gene expression.

Flowers develop from floral meristems, which harbor stem cells that support the growth of floral organs. The MADS domain transcription factor AGAMOUS (AG) plays a central role in floral patterning and is required not only for the specification of the two reproductive organ types, but also for termination of stem cell fate. Using a highly conserved cis-regulatory motif as bait, we identified the bZIP transcription factor PERIANTHIA (PAN) as a direct regulator of AG in Arabidopsis. PAN and AG expression domains overlap, and mutations in either the PAN-binding site or PAN itself abolish the activity of a reporter devoid of redundant elements. Whereas under long-day conditions pan mutants have merely altered floral organ number, they display in addition typical AG loss-of-function phenotypes when grown under short days. Consistently, we found reduced AG RNA levels in these flowers. Finally, we show that PAN expression persists in ag mutant flowers, suggesting that PAN and AG are engaged in a negative-feedback loop, which might be mediated by the stem-cell-inducing transcription factor WUSCHEL (WUS).