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The leguminous crop Arachis hypogaea L. (peanut) is originally from South America and then was disseminated to tropical and subtropical regions. The dissemination of the crop resulted in peanut plants establishing a symbiotic nitrogen-fixing relationship with a wide diversity of indigenous soil bacteria. We present in this review, advances on the molecular basis for the crack-entry infection process involved in the peanut-rhizobia interaction, the diversity of rhizobial and fungal antagonistic bacteria associated with peanut plants, the effect of abiotic and biotic stresses on this interaction and the response of peanut to inoculation.

Arsenic (As)-polluted groundwater constitutes a serious problem for peanut plants, as roots can accumulate the metalloid in their edible parts. Characterization of stress responses to As may help to detect potential risks and identify mechanisms of tolerance, being the induction of oxidative stress a key feature. Fifteen-day old peanut plants were treated with arsenate in order to characterize the oxidative stress indexes and antioxidant response of the legume under realistic groundwater doses of the metalloid. Superoxide anion (O) and hydrogen peroxide (HO) histochemical staining along with the activities of NADPH oxidase, superoxide dismutase (SOD), catalase (CAT) and thiol (glutathione and thioredoxins) metabolism were determined in roots. Results showed that at 20 μM HAsO, peanut growth was reduced and the root architecture was altered. O and HO accumulated at the root epidermis, while lipid peroxidation, NADPH oxidase, SOD, CAT and glutathione S-transferase (GST) activities augmented. These variables increased with increasing As concentration (100 μM) while glutathione reductase (GR) and glutathione peroxidase/peroxiredoxin (GPX/PRX) were significantly decreased. These findings demonstrated that the metalloid induced physiological and biochemical alterations, being the NADPH oxidase enzyme implicated in the oxidative burst. Additionally, the strong induction of GST activity, even at the lowest HAsO doses studied, can be exploited as suitable biomarker of As toxicity in peanut plants, which may help to detect risks of As accumulation and select tolerant cultivars.