Brassinosteroids and jasmonates mitigate arsenite- and arsenate-induced morpho-anatomical anomalies in Arabidopsis roots

Arsenic, a toxic metalloid, predominantly exists in soil as inorganic arsenate (AsV) and arsenite (AsIII). Upon root uptake, AsV is extensively reduced to AsIII in the plant. The Arabidopsis root system comprises primary, lateral and adventitious roots. It is unclear whether the inorganic arsenic-form and concentration affect specific components of the Arabidopsis root system. Synergistic and antagonistic interactions of brassinosteroids and jasmonates regulate plant development under stress, as shown by treatments with epibrassinolide (eBL) or methyl jasmonate (MeJA). However, the role of these phytohormones in root response to inorganic arsenic has been poorly studied. This research aimed to determine whether Arabidopsis roots of different type respond similarly or differently to AsIII and AsV, administered as NaAsO2 and Na2HAsO4·7 H2O respectively, and whether these responses are modulated by the application of eBL and/or MeJA. The results demonstrated that AsIII inhibited primary root elongation, in contrast to AsV, but promoted lateral and adventitious root formation, especially when combined with eBL. AsIII, more than AsV, induced irregular cell divisions in the quiescent center and the stem cell niche of root apices, mainly of lateral roots. The AsIII negative impact on lateral/adventitious roots was counteracted by eBL which favoured root formation. Xylogenesis was induced by periclinal divisions in the pericycle of the basal hypocotyl and was promoted by MeJA as a mechanical defense barrier against AsV. Collectively, results suggest that Arabidopsis responds to As by strengthening its root system and applications of eBL and MeJA ameliorate root development in specific ways, depending on the As-form.