Grain and Forage Legumes in an Arsenic-Polluted Agricultural Scenario

Grain and forage legumes have the potential to be a key component of climate-smart agriculture, even under arsenic (As) stress. Recent publications seem to indicate that agricultural practices with legumes can help in climate change mitigation and simultaneously in remediation of As contaminated areas. The legume-rhizobia symbiosis is one of the most beneficial interactions in the agricultural environment. First, since this symbiosis can improve soil structure, fertility and moisture retention through the incorporation of nitrogen, utilizing the biological nitrogen fixation (BNF) process, legumes have unique advantages to be used in crop rotation and intercropping schemes in pursuit of sustainability. On the other hand, legumes are able to grow in unfavorable environments, like those contaminated with toxic metal(loids). Among them, As is one of the most worldwide distributed non-essential metalloids, that induces different phytotoxic effects on plants and bacteria depending on its speciation, redox state, concentration, plant species, exposure time and repeated exposition. Recently, an interesting link between As and plant water status has been established and this aspect is particularly deepened here. Furthermore, inoculation of legumes with As-resistant rhizobia and/or other As-resistant plant growth promoting bacteria (PGPB) is increasingly being exploited as an As mitigation strategy. In this review, we provide a comprehensive description of the main effects of As on grain and forage legumes with a focus on water status changes and recent advances related with the potential of legume-rhizobia symbiosis under As stress in the climate-smart agriculture.