The Arabidopsis salt overly sensitive 4 mutants uncover a
critical role for vitamin B6 in plant salt tolerance.
Shi H, Xiong L, Stevenson B, Lu T, Zhu JK.
Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721, USA.
Salt stress is a major environmental factor influencing plant growth and
development. To identify salt tolerance determinants, a genetic screen for salt
overly sensitive (sos) mutants was performed in Arabidopsis. We present here the
characterization of sos4 mutants and the positional cloning of the SOS4 gene.
sos4 mutant plants are hypersensitive to Na(+), K(+), and Li(+) ions. Under NaCl
stress, sos4 plants accumulate more Na(+) and retain less K(+) compared with
wild-type plants. SOS4 encodes a pyridoxal kinase that is involved in the
biosynthesis of pyridoxal-5-phosphate, an active form of vitamin B6. The
expression of SOS4 cDNAs complements an Escherichia coli mutant defective in
pyridoxal kinase. Supplementation of pyridoxine but not pyridoxal in the growth
medium can partially rescue the sos4 defect in salt tolerance. SOS4 is expressed
ubiquitously in all plant tissues. As a result of alternative splicing, two
transcripts are derived from the SOS4 gene, the relative abundance of which is
modulated by development and environmental stresses. Besides being essential
cofactors for numerous enzymes, as shown by pharmacological studies in animal
cells, pyridoxal-5-phosphate and its derivatives are also ligands for P2X
receptor ion channels. Our results demonstrate that pyridoxal kinase is a novel
salt tolerance determinant important for the regulation of Na(+) and K(+)
homeostasis in plants. We propose that pyridoxal-5-phosphate regulates Na(+) and
K(+) homeostasis by modulating the activities of ion transporters.