SEUSS and LEUNIG regulate cell proliferation, vascular development and organ polarity in Arabidopsis petals.

Unlike in animals where cell migrations and programmed cell death play key roles in organ shape determination, in plants organ shape is largely a result of coordinated cellular growth (cell divisions and cell elongations). We have investigated the role of the SEUSS and LEUNIG genes in Arabidopsis thaliana (L.) Heynh. petal development to better understand the molecular mechanisms through which cellular growth and organ shape are coordinated in plants. SEUSS and LEUNIG encode components of a putative transcriptional regulatory complex that controls organ identity specification through the repression of the floral organ identity gene AGAMOUS. SEUSS and LEUNIG also regulate petal shape through AGAMOUS-independent mechanisms; however, the molecular and cellular actions of SEUSS and LEUNIG during petal development are unknown. Here we show that SEUSS and LEUNIG control blade cell number and vasculature development within the petal. Furthermore, SEUSS and LEUNIG regulate petal polarity along the adaxial/abaxial axis. We present a model where SEUSS and LEUNIG are required to potentiate the key polarity genes PHABULOSA and FILAMENTOUS FLOWER/YABBY1 and thus influence cellular growth within the developing petal blade.