Establishment of polarity in lateral organs of plants

Background: Asymmetric development of plant lateral organs initiates by par titioning of organ primordia into distinct domains along their adaxial/abax ial axis. A recent model proposes that a meristem-bom signal, acting in a c oncentration-dependent manner, differentially activates PHABULOSA-like gene s, which in turn suppress abaxial-promoting factors. As yet, no abaxial fac tors have been identified that when compromised give rise to adaxialized or gans. Results: Single mutants in either of the closely related genes KANADI1 (KAN 1) or KANADI2 (KAN2) have little or no effect on plant morphology. However, in kan1 kan2 double mutant plants, there is a replacement of abaxial cell types by adaxial ones in most lateral organs. The alterations in polarity e stablishment are associated with expansion in the expression domain of the PHB-like genes and reduction in the expression of the previously described abaxial-promoting YABBY genes. Ectopic expression of either of the KANADI g enes throughout leaf primordia results in dramatic transformation of adaxia l cell types into abaxial ones, failure of lateral blade expansion, and vas cular tissue formation. Conclusion: The phenotypes of KANADI loss- and gain-of-function alleles sug gest that fine regulation of these genes is at the core of polarity establi shment. As such, they are likely to be targets of the PHB-mediated meristem -born signaling that patterns lateral organ primordia, PHB-like genes and t he abaxial-promoting KANADI and YABBY genes appear to be expressed througho ut primordia anlagen before becoming confined to their corresponding domain s as primordia arise. This suggests that the establishment of polarity in p lant lateral organs occurs via mutual repression interactions between ab/ad factors after primordium emergence, consistent with the results of classic al dissection experiments.