CNGCs in plants, like animal homologs, are non-selective cation channels that are Ca2+ permeable (Talke et al. 2003). The first null mutation reported for a plant CNGC (cngc2) resulted in a dwarf plant with a lesion mimic/pathogen response phenotype, as well as poor fertility (Clough et al. 2000). While a specific role of CNGC2 in pollen has not been investigated, this gene is highly expressed at early stages of pollen development. In contrast to cngc2's weak fertility phenotype, a complete male sterile phenotype results from a null mutation of isoform CNGC18 (Frietsch S, Schroeder J, Harper JF, unpublished). Two independent T-DNA insertions in CNGC18 were found to completely block pollen transmission of the mutant allele. A cytolog-ical analysis indicated that the primary defect was a failure of cngc18 pollen to undergo directional growth into the transmitting tract of the pistil. Evidence from a GFP tagging strategy suggests that CNGC18 has a tip-focused plasma membrane location. While the ion conductance properties of CNGC18 have not been directly tested, CNGCs are thought to be Ca2+ permeable, nonspecific cation channels (Leng 1999; Talke et al. 2003) that are activated by cyclic nucleotides and inhibited by the binding of Ca2+/calmodulin (Talke et al. 2003; Hua et al. 2003). The apparent tip-focused location of CNGC18 suggests that this channel may function in regulating tip-localized Ca2+ signals that coordinate the directional growth machinery (Fig. 1B).
The cngc18 null phenotype indicates that CNGC18 provides an essential function that is not compensated by any of the other 6 or 7 pollen-expressed CNGC isoforms (Fig. 2B). In mature pollen, CNGC18 and CNGC8 are the two most highly expressed CNGCs, with other isoforms showing less than half of their expression levels. Since some CNGCs are thought to form hetero-multimers, multiple isoforms expressed in plant pollen may form a highly diverse set of channels with different ion permeation or gating properties. Nevertheless, a function for these additional pollen isoforms has not yet been established.
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