Complex networks of regulatory genes control morphogenesis but how are these networks translated into the local changes in tissue growth that shape multicellular organisms? Jérôme Pelloux and co-workers () have been investigating the modulation of phyllotaxis (the arrangement of leaves and flowers along plant stems) in Arabidopsis by the transcription factor BELLRINGER (BLR). In plants, the formation of new lateral organs depends on demethylesterification of homogalacturonan (HG), a major component of plant cell wall pectins. The researchers show that ectopic primordia form in the floral meristem of Arabidopsis blr mutants because of ectopic expression of the pectin methylesterase PME5, which changes the demethylesterification state of HG. Thus, BLR normally represses PME5 expression in the meristem, thereby influencing the establishment of the phyllotactic pattern. However, in the elongating stem, the researchers report, BLR activates PME5 expression to maintain phyllotaxis. These results identify BLR as an important component of the regulatory network that controls HG demethylesterification and, in turn, phyllotaxis in Arabidopsis.
