Ascorbate and Homoglutathione Metabolism in Common Bean Nodules Under Stress Conditions and During Natural Senescence

Jorge Loscos , Manuel A. Matamoros , and Manuel Becana ^*

Departamento de Nutricion Vegetal, Estacion Experimental de Aula Dei, Consejo Superior de Investigaciones Cientificas, Apartado 202, 50080 Zaragoza, Spain

^* Corresponding author; email: becana{at}eead.csic.es.

Ascorbate and glutathione are major antioxidants and redox buffersin plant cells but also play key functions in growth, development,and stress responses. We have studied the regulation of ascorbateand homoglutathione biosynthesis in common bean (Phaseolus vulgaris)nodules under stress conditions and during aging. The expressionof five genes of the major ascorbate biosynthetic pathway wasanalyzed in nodules and evidence was found that L-galactono-1,4-lactonedehydrogenase, the last committed step of the pathway, is posttranscriptionallyregulated. Also, in nodules under stress conditions, ${gamma}$ -glutamylcysteinesynthetase was translationally regulated but homoglutathionesynthetase (mRNA and activity) and homoglutathione (contentand redox state) were not affected. Most interestingly, in nodulesexposed to jasmonic acid, dehydroascorbate reductase activitywas posttranslationally suppressed, ascorbate oxidase showedstrong transcriptional up-regulation, and dehydroascorbate contentincreased moderately. These changes were not due to a directeffect of jasmonic acid on the enzyme activities but might bepart of the signaling pathway in the response of nodules tostress. We determined ascorbate, homoglutathione, and ascorbate-glutathionepathway enzyme activities in two senescing stages of nodulesundergoing oxidative stress. When all parameters were expressedon a nodule fresh weight basis, we found that in the first stageascorbate decreased by 60% and homoglutathione and antioxidantactivities remained fairly constant, whereas in the second stageascorbate and homoglutathione, their redox states, and theirassociated enzyme activities significantly decreased. The coexistenceof nodules at different senescence stages in the same plants,with different ascorbate concentrations and redox states, indicatesthat the life span of nodules is in part controlled by endogenousfactors and points to ascorbate as one of the key players.