Regulation of osmotic stress-responsive gene expression by
the LOS6/ABA1 locus in Arabidopsis.
Xiong L, Lee H, Ishitani M, Zhu JK.
Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721, USA.
Drought and high salinity induce the expression of many plant genes. To
understand the signal transduction mechanisms underlying the activation of these
genes, we carried out a genetic screen to isolate Arabidopsis mutants defective
in osmotic stress-regulated gene induction. Here we report the isolation,
characterization, and cloning of a mutation, los6, which diminished osmotic
stress activation of a reporter gene. RNA blot analysis indicates that under
osmotic stress the transcript levels for stress-responsive genes such as RD29A,
COR15A, KIN1, COR47, RD19, and ADH are lower in los6 plants than in wild type
plants. los6 plants were found to have reduced phytohormone abscisic acid (ABA)
accumulation and to be allelic to the ABA-deficient mutant, aba1. LOS6/ABA1
encodes a zeaxanthin epoxidase that functions in ABA biosynthesis. Its
expression is enhanced by osmotic stress. Furthermore, we found that there
exists a positive feedback regulation by ABA on the expression of LOS6/ABA1,
which may underscore a quick adaptation strategy for plants under osmotic
stress. Similar positive regulation by ABA also exists for other ABA
biosynthesis genes AAO3 and LOS5/ABA3 and in certain genetic backgrounds, NCED3.
This feedback regulation by ABA is impaired in the ABA-insensitive mutant abi1
but not in abi2. Moreover, the up-regulation of LOS6/ABA1, LOS5/ABA3, AAO3, and
NCED3 by osmotic stress is reduced substantially in ABA-deficient mutants.
Transgenic plants overexpressing LOS6/ABA1 showed an increased RD29A-LUC
expression under osmotic stress. These results suggest that the level of gene
induction by osmotic stress is dependent on the dosage of the zeaxanthin
epoxidase enzyme.