Downregulation of the Petunia hybrida α-Expansin Gene PhEXP1 Reduces the Amount of Crystalline Cellulose in Cell Walls and Leads to Phenotypic Changes in Petal Limbs

The expansins comprise a family of proteins that appear to be involved in the disruption of the noncovalent bonds between cellulose microfibrils and cross-linking glycans, thereby promoting wall creep. To understand better the expansion process in Petunia hybrida (petunia) flowers, we isolated a cDNA corresponding to the PhEXP1 alpha-expansin gene of P. hybrida. Evaluation of the tissue specificity and temporal expression pattern demonstrated that PhEXP1 is preferentially expressed in petal limbs during development. To determine the function of PhEXP1, we used a transgenic antisense approach, which was found to cause a decrease in petal limb size, a reduction in the epidermal cell area, and alterations in cell wall morphology and composition. The diminished cell wall thickness accompanied by a reduction in crystalline cellulose indicates that the activity of PhEXP1 is associated with cellulose metabolism. Our results suggest that expansins play a role in the assembly of the cell wall by affecting either cellulose synthesis or deposition.