Genes for calcium-permeable channels in the plasma membrane of plant root cells

Abstract

In plant cells, Ca 2+ is required for both structural and biophysical roles. In addition, changes in cytosolic Ca 2+ concentration ([Ca 2+] cyt) orchestrate responses to developmental and environmental signals. In many instances, [Ca 2+] cyt is increased by Ca 2+ influx across the plasma membrane through ion channels. Although the electrophysiological and biochemical characteristics of Ca 2+-permeable channels in the plasma membrane of plant cells are well known, genes encoding putative Ca 2+-permeable channels have only recently been identified. By comparing the tissue expression patterns and electrophysiology of Ca 2+-permeable channels in the plasma membrane of root cells with those of genes encoding candidate plasma membrane Ca 2+ channels, the genetic counterparts of specific Ca 2+-permeable channels can be deduced. Sequence homologies and the physiology of transgenic antisense plants suggest that the Arabidopsis AtTPC1 gene encodes a depolarisation-activated Ca 2+ channel. Members of the annexin gene family are likely to encode hyperpolarisation-activated Ca 2+ channels, based on their corresponding occurrence in secretory or elongating root cells, their inhibition by La 3+ and nifedipine, and their increased activity as [Ca 2+] cyt is raised. Based on their electrophysiology and tissue expression patterns, AtSKOR encodes a depolarisation-activated outward-rectifying (Ca 2+-permeable) K + channel (KORC) in stelar cells and AtGORK is likely to encode a KORC in the plasma membrane of other Arabidopsis root cells. Two candidate gene families, of cyclic-nucleotide gated channels ( CNGC) and ionotropic glutamate receptor ( GLR) homologues, are proposed as the genetic correlates of voltage-independent cation (VIC) channels.