The Roles of the Cation Transporters CHX21 and CHX23 in the Development of Arabidopsis Thaliana

The Arabidopsis thaliana genome encodes a family of 28 proteins whose members have been associated with the
transport of monovalent cations across membranes. Experiments have been performed to elucidate the biochemical
function and the role in plant development of two closely related members of this CHX family. A genotype carrying
a knockout of the AtCHX23 gene (At1g05580) showed no phenotype when grown in glasshouse conditions. In
particular, it did not exhibit the reduced root growth phenotype observed for a knockout of its homologue AtCHX21
when exposed to elevated sodium concentration. However, it was not possible to produce plants that were
homozygous knockout for both AtCHX21 and AtCHX23. Reverse transcription-PCR (RT-PCR) experiments revealed
that both genes are highly expressed in flower buds, flowers, and pollen. However, examination of pollen grain
viability and pollen tube growth through excised styles did not reveal a phenotypic difference between
the chx21–chx23– condition and other haplotypes. Crosses between selected mutants and wild-type plants in which
the chx21–chx23– haplotype was produced by either the male or female parent demonstrated unequivocally that the
chx21–chx23– haplotype could not pass through the female line. This suggests that the genes share a critical
function in the development and/or function of the female gametophyte and that this function cannot be provided by
other members of the AtCHX gene family. Experiments were carried out using the heterologous expression of
AtCHX23 in Saccharomyces cerevisiae genotypes carrying combinations of deletions of genes involved in the
transport of sodium or potassium across membranes. The results show that CHX23 would only complement the
poor colony growth phenotype associated with the deletion of the yeast gene kha1. The conclusion is that both
AtCHX21 and AtCHX23 act in potassium homeostasis within the female gametophyte and this is discussed in terms
of the diversification of gene sequence and function within the CHX gene family.