Abstract
Ascorbate and glutathione are major antioxidants and redox buffers in plant cells but also play key functions in growth, development, and stress responses. We have studied the regulation of ascorbate and homoglutathione biosynthesis in common bean (Phaseolus vulgaris) nodules under stress conditions and during aging. The expression of five genes of the major ascorbate biosynthetic pathway was analyzed in nodules, and evidence was found that L-galactono-1,4-lactone dehydrogenase, the last committed step of the pathway, is posttranscriptionally regulated. Also, in nodules under stress conditions, gamma-glutamylcysteine synthetase was translationally regulated, but homoglutathione synthetase (mRNA and activity) and homoglutathione (content and redox state) were not affected. Most interestingly, in nodules exposed to jasmonic acid, dehydroascorbate reductase activity was posttranslationally suppressed, ascorbate oxidase showed strong transcriptional up-regulation, and dehydroascorbate content increased moderately. These changes were not due to a direct effect of jasmonic acid on the enzyme activities but might be part of the signaling pathway in the response of nodules to stress. We determined ascorbate, homoglutathione, and ascorbate-glutathione pathway enzyme activities in two senescing stages of nodules undergoing oxidative stress. When all parameters were expressed on a nodule fresh weight basis, we found that in the first stage ascorbate decreased by 60% and homoglutathione and antioxidant activities remained fairly constant, whereas in the second stage ascorbate and homoglutathione, their redox states, and their associated enzyme activities significantly decreased. The coexistence in the same plants of nodules at different senescence stages, with different ascorbate concentrations and redox states, indicates that the life span of nodules is in part controlled by endogenous factors and points to ascorbate as one of the key players.
Highlights
Ascorbate and glutathione are major antioxidants and redox buffers in plant cells and play key functions in growth, development, and stress responses
These include L-galactono-1,4-lactone dehydrogenase (GalLDH), which catalyzes the last step of ascorbate synthesis in the mitochondria; g-glutamylcysteine synthetase, which catalyzes the synthesis of g-glutamylcysteine in the plastids; and glutathione synthetase (GSHS) and homoglutathione synthetase, which catalyze, respectively, the synthesis of GSH and hGSH in the plastids and cytosol
Despite the Cdinduced decreases in the expression of GalLDH and three other genes of ascorbate biosynthesis (Fig. 3), the GalLDH activity and the total ascorbate content of nodules
Summary
Ascorbate and glutathione are major antioxidants and redox buffers in plant cells and play key functions in growth, development, and stress responses. Ascorbate and glutathione (GSH; gGluCys-Gly) are major cellular antioxidants and redox buffers and play important functions in growth, development, and stress responses (Smirnoff, 2000; Pastori et al, 2003) Another thiol tripeptide, homoglutathione (hGSH; gGlu-Cys-bAla), is present in nodules and other organs of some legume species in addition to or in place of GSH (Matamoros et al, 1999) and may share with it some antioxidative and regulatory properties (Zopes et al, 1993; Frendo et al, 2005). We have studied the regulation of ascorbate and hGSH biosynthesis in common bean (Phaseolus vulgaris) nodules under stress conditions and during natural senescence. The soluble protein content declined by 36% in the NaCl-treated nodules
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