An interconnection between tip-focused Ca and anion homeostasis controls pollen tube growth.

Plant reproduction is the basis for economically relevant food production. It relies on pollen tube (PTs) growth into the female flower organs for successful fertilization. The high cytosolic Ca concentration ([Ca]) at the PT tip is sensed by Ca-dependent protein kinases (CPKs) that in turn activate R- and S-type anion channels to control polar growth. Lanthanum, a blocker for plant Ca-permeable channels was used here to demonstrate a strict dependency for anion channel activation through high PT tip [Ca]. We visualized this relationship by live-cell anion imaging and concurrent triggering of Ca-elevations with the two-electrode voltage-clamp (TEVC) technique. The anion efflux provoked by a TEVC-triggered [Ca] increase was abolished by Lanthanum and was followed by an overall rise in the cytosolic anion concentration. An interrelation between Ca and anion homeostasis occurred also on the transcript level of CPKs and anion channels. qRT-PCR analysis demonstrated a co-regulation of anion channels and CPKs in media with different Cl and NO compositions. Our data provides strong evidence for the importance of a Ca-dependent anion channel regulation and point to a synchronized adjustment of CPK and anion channel transcript levels to fine-tune anion efflux at the PT tip.