Figure 1.

A schematic model of a positive feedback regulation at the tip of growing pollen tubes. Two NADPH oxidases, RbohH and RbohJ, located at the plasma membrane of the tip,² producing ROS in the cell wall. Plasma membrane Ca²⁺-permeable channel(s) may be activated by ROS to induce Ca²⁺ influx into the cytosol as shown in root hairs.⁵ In turn, the ROS-producing activities of Rbohs are synergistically activated by direct binding of Ca²⁺ to their cytosolic EF-hand motifs and their phosphorylation by Ca²⁺-dependent protein kinases² as also shown for other Rboh proteins.^6-8 Phosphorylation of RbohH and/or RbohJ may also be mediated by ANXUR receptor-like kinases.³