Ions and Pollen Tube Growth
The Pollen Tube
Ions play a crucial role in the control of pollen tube growth. In this review we focus on four that seem especially important: calcium (Ca 2+ ), protons (H + ), potassium (K + ), and chloride (Cl -). Ca 2+ in the extracellular medium is essential for growth; it forms a steep intracellular tip-focused gradient, and exhibits a prominent extracellular tip-directed Ca 2+ influx. pH is also essential for growth. H + form an intracellular gradient consisting of a slightly acidic domain at the extreme
... apex and an alkaline band located along the clear zone. H + also exhibit an apical influx, but in contrast to Ca 2+ show an efflux along the clear zone, in the region occupied by the intracellular alkaline band. K + and anions (possibly Cl -) appear to participate in the growth process, as evidenced by the striking extracellular fluxes that are associated with tube elongation. K + exhibits an apical influx, while an anion displays an apical efflux. An exciting finding has been the discovery that pollen tube growth oscillates in rate, as do all the ionic expressions noted above. While the ionic activities and fluxes show the same period as growth, they usually do not show the same phase. The exploration of phase relationships, using cross-correlation analysis, reveals that most ion expressions lag growth. Thus, intracellular Ca 2+ activity follows growth rate by 1-4 s, whereas extracellular Ca 2+ influx follows growth rate by 12-15 s (130 • ). These observations suggest that Ca 2+ is a follower rather than a leader in growth. Despite the knowledge that has been gained, several aspects of ionic expression and function remain to be determined. Their elucidation will contribute greatly to our overall understanding of the control of pollen tube growth. 48 P.K. Hepler et al.