Homoglutathione synthetase and glutathione synthetase in drought-stressed cowpea leaves: Expression patterns and accumulation of low-molecular-weight thiols

Glutathione (GSH) is an abundant metabolite and a major antioxidant in plant cells. However, in the Leguminosae, homoglutathione (hGSH) may replace glutathione (GSH) partially or completely. To date, cowpea (Vigna unguiculata) has been considered a non-hGSH-producing species, and no hGSHS cDNA has been isolated. Here we report on the cloning of a full-length cDNA coding for a hGSHS (EC 6.3.2.23) and the cloning of a partial cDNA coding for a putative glutathione synthetase (GSHS; EC 6.3.2.3) in cowpea leaf extracts. These cDNAs possess, respectively, the leucine/proline hGSHS signature and the alanine/alanine GSHS signature at the 3′ end. Expression analysis showed a significant up-regulation of hGSHS during progressive drought stress that could be directly related to the drought tolerance of the cowpea cultivar used, while GSHS was mainly constitutively expressed. Nevertheless, quantification of low-molecular-weight thiols confirmed the previous findings that cowpea is essentially a GSH producing plant, as no hGSH was detected in the leaves. These findings raise new questions regarding the function, activity and substrate specificity of the cloned hGSHS cDNA. These questions are discussed.