Silencing of the tobacco pollen pectin methylesterase NtPPME1 results in retarded in vivo pollen tube growth

Sperm delivery in flowering plants requires extensive pollen tube growth through the female sporophytic tissues of the pistil. The apical cell wall emerges as a central player in the control of pollen tube growth, since it provides strength to withstand the internal turgor pressure, while imparting sufficient plasticity to allow cell wall extension through the incorporation of new membrane and wall material. Within this scenario, pectin methylesterases (PMEs; EC 3.1.1.11) emerge as crucial regulators in determining the mechanical properties of pectins, the major component of the apical pollen tube wall. We previously identified NtPPME1, a pollen specific PME from Nicotiana tabacian. Here we show that silencing of NtPPME1 results in a mild but significant decrease of in vivo pollen tube growth while the overall PME activity In pollen is not significantly affected. Although the precise mechanisms responsible for the observed phenotype are not known, it seems likely that the cell must maintain a closely regulated level of PME activity in order to maintain the equilibrium between strength and plasticity in the apical cell wall. A relatively minor disturbance of this equilibrium, as caused by NtPPME1 silencing, compromises pollen tube growth.