| A screen to identify mutations in Arabidopsis that lead to oxidative stress tolerance has been developed. The screen is based on the performance of seedlings growing in a chemically-induced oxidative stress environment. The extent of root growth within agar-solidified medium has been shown to be a reliable indicator of whole-plant growth under oxidative stress. The stress conditions developed employed aminotriazole (AT), a catalase inhibitor, and buthionine sulfoximine (BSO), an inhibitor of glutathione synthesis, at concentrations shown to induce a clear but non-lethal degree of stress. Several mutant lines that display enhanced tolerance to oxidative stress including oxt1 and oxt2 were isolated out of ∼90,000 seeds from mutagenized populations.;The oxt1 mutant, the most dramatic stress-tolerant line, contains a dominant, gain-of-function mutation that confers tolerance to the stress conditions used in the screen, as well as oxidative stress induced by hydrogen peroxide, the herbicides acifluorfen and methyl viologen. oxt1 also shows tolerance to environmental stresses such as high temperature and drought. Genomic mapping revealed that the oxt1 gene is located within a 0.455-cM interval on chromosome I, a region encoding no known antioxidant defense genes. Characterization by enzymological and molecular analyses reveals somewhat subtle differences between the oxt1 mutant and the wild type, suggesting an alteration that affects signaling or responses to stress. For example, the expression of specific antioxidant defense genes and the corresponding enzymes are altered in the oxt1 mutant.;The stress hormone, abscisic acid, and members of its key biosynthetic genes, 9-cis epoxycarotenoid dioxygenase (NCED), were examined in oxt1 revealing clear alterations in regulation. Together with the fact that oxt1 also exhibits tolerance to drought, these observations demonstrate that there is a link between oxidative signaling and the regulation of NCED expression, thereby affecting abscisic acid synthesis. Although some of the altered responses in oxt1 gene expression are subtle, they collectively result in pronounced differences in the ability of oxt1 plants to tolerate stress.;Another oxidative stress mutant, oxt2, shows tolerance especially under a combination of high light and high temperature conditions. The T-DNA in this line inserted into the promoter region of the Arabidopsis gene, AT4G39070 (a zinc-finger protein or salt tolerance protein), on chromosome IV. Expression of this gene is strongly diminished and its knock down appears to confer some tolerance to oxidative stress but a slight sensitivity to high salt conditions. |
