Cyclic nucleotide-gated channel 18 is an essential Ca ²⁺ channel in pollen tube tips for pollen tube guidance to ovules in Arabidopsis

Abstract

In flowering plants, pollen tubes are guided into ovules by multiple attractants from female gametophytes to release paired sperm cells for double fertilization. It has been well-established that Ca2+ gradients in the pollen tube tips are essential for pollen tube guidance and that plasma membrane Ca2+ channels in pollen tube tips are core components that regulate Ca2+ gradients by mediating and regulating external Ca2+ influx. Therefore, Ca2+ channels are the core components for pollen tube guidance. However, there is still no genetic evidence for the identification of the putative Ca2+ channels essential for pollen tube guidance. Here, we report that the point mutations R491Q or R578K in cyclic nucleotide-gated channel 18 (CNGC18) resulted in abnormal Ca2+ gradients and strong pollen tube guidance defects by impairing the activation of CNGC18 in Arabidopsis. The pollen tube guidance defects of cngc18-17 (R491Q) and of the transfer DNA (T-DNA) insertion mutant cngc18-1 (+/−) were completely rescued by CNGC18. Furthermore, domain-swapping experiments showed that CNGC18’s transmembrane domains are indispensable for pollen tube guidance. Additionally, we found that, among eight Ca2+ channels (including six CNGCs and two glutamate receptor-like channels), CNGC18 was the only one essential for pollen tube guidance. Thus, CNGC18 is the long-sought essential Ca2+ channel for pollen tube guidance in Arabidopsis.

Tip‐localized Ca2+‐permeable channels control pollen tube growth via kinase‐dependent R‐ and S‐type anion channel regulation

“…The nature of the activated Ca 2+ ‐permeable channels in our experimental system is as yet unknown, but Ca 2+ ‐permeable channels of the CNGC and GLR families, namely GLR1.2, GLR3.7, and CNGC18, seem to participate in generating the tip‐focused Ca 2+ gradient in PTs (Frietsch et al ., ; Michard et al ., ; Gao et al ., ), as pointed out in our model (Fig. ).…”

Calcium transport across plant membranes: mechanisms and functions

“…The electrophysiological properties of CNGCs have been tested using heterologous expression systems (Table ). These studies revealed that CNGCs that harbour the GQN, GQG and GQS triplets are selective for Ca 2+ and promote Ca 2+ uptake (Gao et al ., , , ; Wang et al ., ). Some heterologously expressed CNGCs are activated when coexpressed with a Ca 2+ ‐dependent protein kinase, CPK32, implying regulation of their activity by phosphorylation (Zhou et al ., ).…”

“…Gao et al . () recently reported that up to eight different subunits of putative Ca 2+ ‐permeable channels can be expressed at the same time in the PM of an individual plant cell.…”

Signaling with Ions: The Keystone for Apical Cell Growth and Morphogenesis in Pollen Tubes

“…The disruption of AtCNGC18, localized in the subapical membrane, induces male sterility due to defective PT growth (Frietsch et al, 2007). AtCNGC18 was recently shown to drive inward cationic currents activated by cAMP/cGMP in an animal heterologous system (Gao et al, 2016). AtCNGC7, which localizes at the flank of the growing tip, and AtCNGC8 have weaker phenotypes, but the double knockout is male sterile (Tunc-Ozdemir et al, 2013a).…”