Regulation of stress-induced phosholipid hydroperoxide glutathione peroxidase expression in citrus

Recent findings in our laboratory showed that in citrus cells, salt treatme nt induced the accumulation of mRNA and a protein corresponding to phosphol ipid hydroperoxide glutathione peroxidase (PHGPX), an enzyme active in the cellular antioxidant system. The protein and its encoding gene, csa, were i solated and characterized, and the expected enzymatic activity was demonstr ated (G. Ben-Hayyim et al., 1993, Plant Sci. 88: 129-140; D. Holland et al. , 1993, Plant Mel. Biol. 21. 923-927; D. Holland et al., 1994, FEES Lett. 3 37: 52-55; T. Beeor-Tzahar et al., 1995, FEES Lett. 366: 151-155). In an at tempt to find out how salt induces the expression of an antioxidant enzyme, the regulation of PHGPX in citrus cells was studied at both the mRNA trans cript and the protein levels. A high and transient response at the csa. mRN A level was observed after 4-7 h of exposing salt-sensitive cells to NaCl, or abscisic acid, whereas no response could be detected in the salt-toleran t cells under the same conditions, tert-Butylhydroperoxide, a substrate of PHGPX, induced csa mRNA transcripts after only 2 h, and abolished the diffe rential response between salt-sensitive and salt-tolerant cells. On the bas is of these results and those obtained under heat and cold stresses, it is suggested that csa is directly induced by the substrate of its encoded enzy me PHGPX, and that salt induction occurs mainly via the production of react ive oxygen species and hydroperoxides.