Targeting redox metabolism of the maize-Azospirillum brasilense interaction exposed to arsenic-affected groundwater

Arsenic in groundwater constitutes an agronomic problem due to its potential accumulation in the food chain. Among the agro-sustainable tools to reduce metal(oid)s toxicity, the use of plant growth-promoting bacteria (PGPB) becomes important. For that, and based on previous results in which significant differences of As translocation were observed when inoculating maize plants with Az39 or CD Azospirillum strains, we decided to decipher the redox metabolism changes and the antioxidant system response of maize plants inoculated when exposed to a realistic arsenate (As-V) dose. Results showed that As-V caused morphological changes in the root exodermis. Photosynthetic pigments decreased only in CD inoculated plants, while oxidative stress evidence was detected throughout the plant, regardless of the assayed strain. The antioxidant response was strain-differential since only CD inoculated plants showed an increase in superoxide dismutase, glutathione S-transferase (GST), and glutathione reductase (GR) activities while other enzymes showed the same behavior irrespective of the inoculated strain. Gene expression assays reported that only GST23 transcript level was upregulated by arsenate, regardless of the inoculated strain. As-V diminished the glutathione (GSH) content of roots inoculated with the Az39 strain, and CD inoculated plants showed a decrease of oxidized GSH (GSSG) levels. We suggest a model in which the antioxidant response of the maize-diazotrophs system is modulated by the strain and that GSH plays a central role acting mainly as a substrate for GST. These findings generate knowledge for a suitable PGPB selection, and its scaling to an effective bioinoculant formulation for maize crops exposed to adverse environmental conditions.

Automated summary

The presence of arsenic in groundwater poses a potential agronomic problem by accumulating in the food chain. Using plant growth-promoting bacteria (PGPB) is an important tool to reduce metal toxicity. When maize plants were exposed to arsenate, different strains of bacteria led to morphological changes in root exodermis, decreased photosynthetic pigments, and induced oxidative stress, with differential antioxidant responses observed.