Pectin methylesterases and pectin dynamics in pollen tubes.

Pectic polysaccharides are an essential component of the primary plant cell wall. They are particularly prominent in pollen tubes, where they control the structure and yielding characteristics of the cell wall at the growing apex of these rapidly expanding cells. The properties of pectin meshworks, to a considerable degree, are regulated by postsecretory changes resulting from the activity of cell wall–associated enzymes. For example, the homogalacturonan (HGA) fraction, a major component of pectins, is typically deposited into the apoplasm in highly methylesterified condition. Pectin methylesterases (PMEs; EC 3.1.1.11) in the apoplasm catalyze the demethylesterification of these HGAs, exposing carboxyl residues, which can be cross-linked by calcium. These changes markedly affect the rheological properties of the cell wall as well as its porosity and ionic status. It follows that the tight control of PME activity, both spatially and temporally, occupies a pivotal position in the control of cell wall growth and development. The pollen tube represents an excellent model system for studying PMEs since the wall of the apical growth region consists almost exclusively of a pectinaceous network. This essay focuses on recent findings that underline the significance of PMEs for pollen tube growth and discusses the complex array of mechanisms that exist to regulate PME activity. Although representing a particular system, data obtained for PMEs in pollen tubes are likely to enhance our understanding of the importance of these enzymes in other cell types.