Effect of extracellular calcium, pH and borate on growth oscillations in Lilium formosanum pollen tubes

Calcium ions (Ca2+), protons (H+), and borate (B(OH)(4)(-)) are essential ions in the control of tip growth of pollen tubes. All three ions may interact with pectins, a major component of the expanding pollen tube cell wall. Ca2+ is thought to bind acidic residues, and cross-link adjacent pectin chains, thereby strengthening the cell wall. Protons are loosening agents; in pollen tube walls they may act through the enzyme pectin methylesterase (PME), and either reduce demethylation or stimulate hydrolysis of pectin. Finally, borate cross-links monomers of rhamnogalacturonan II (RG-II), and thus stiffens the cell wall. It is demonstrated here that changing the extracellular concentrations of Ca2+, H+ and borate affect not only the average growth rate of lily pollen tubes, but also influence the period of growth rate oscillations. The most dramatic effects are observed with increasing concentrations of Ca2+ and borate, both of which markedly reduce the rate of growth of oscillating pollen tubes. Protons. are less active, except at pH 7.0 where growth is inhibited. It is noteworthy, especially with borate, that the faster growing tubes exhibit the shorter periods of oscillation. The results are consistent with the idea that binding of Ca2+ and borate to the cell wall may act at a similar level to alter the mechanical properties of the apical cell wall, with optimal concentrations being high enough to impart sufficient rigidity to the wall so as to prevent bursting in the face of cell turgor, but low enough to allow the wall to stretch quickly during periods of accelerating growth.