A conserved function for Arabidopsis SUPERMAN in regulating floral-whorl cell proliferation in rice, a monocotyledonous plant

Studies of floral organ development in two dicotyledonous plants, Arabidops is thaliana and Antirrhinum majus, have shown that three sets of genes (A, B and C) can pattern sepals, petals, stamens and carpels [1,2]. Mechanisms that define boundaries between these floral whorls are unclear, however. Th e Arabidopsis gene SUPERMAN (SUP), which encodes a putative transcription f actor, maintains the boundary between stamens and carpels [3-5], possibly b y regulating coil proliferation. By overexpressing SUP cDNA in rice, we exa mined whether its effects on whorl boundaries are conserved in a divergent monocotyledonous species, High-level ectopic SUP expression in transgenic r ice resulted in juvenile death or dwarf plants with decreased axillary grow th. Plants with lower levels of SUP RNA were vegetatively normal, but the f lowers showed ubiquitous ventral carpel expansion. This was often coupled w ith reduced stamen number, or occurrence of third-whorl stamen-carpel mosai c organs. Additionally, proliferation of second-whorl ventral cells produce d adventitious lodicules, and flowers lost the asymmetry that is normally i nherent to this whorl. We predict that SUP is a conserved regulator of flor al whorl boundaries and that it affects cell proliferation.