The dual role of PhEXPA1, a Petunia hybrida expansin, in cell wall metabolism and in plant architecture specification.

Expansins are wall-loosening proteins inducing wall stress relaxation and irreversible wall extension in a pH-dependent manner. Despite a substantial body of work has been carried out on the characterization of many expansins genes in different plant species, the knowledge about their precise biological roles during plant development remains scarce. To yield insights into the expansion process in Petunia hybrida, PhEXPA1, an expansin gene preferentially expressed in petal limb, has been characterized. The constitutive overexpression of PhEXPA1 significantly increased expansin activity, cells size and organ dimensions. Moreover, 35S::PhEXPA1 transgenic plants exhibited an altered cell wall polymer composition and a precocious timing of axillary meristem development compared to wild-type plants1. Furthermore, a PhEXPA1 promoter insertional mutant have been generated and analyzed for altered petal morphology and plant branching pattern. These findings supported a previous hypothesis that expansins are not merely structural proteins involved in plant cell wall metabolism but they also take part in many plant development processes.