Rhizobial ACC deaminase contributes to efficient symbiosis with pea (Pisum sativum L.) under single and combined cadmium and water deficit stress

Abstract

This research aimed to study interactions between nodule bacteria (rhizobia) containing ACC deaminase and legume plants subjected to combined environmental stresses caused by drought and heavy metals (cadmium – Cd). A Cd-tolerant strain of Rhizobium leguminosarum bv. viciae 1066S containing ACC deaminase and resistant to streptomycin was selected, and its ACC deaminase knock-out mutant RlM1 obtained. The pea line SGE and its Cd-tolerant mutant SGECdt were grown in pots and the plants inoculated with 1066S or RlM1. They were grown under well watered or water limited conditions in a sod-podzolic soil supplemented or not with 30 mg kg−1 soil. Pea shoot biomass, nodulation, water use efficiency, shoot Cd content and uptake of fertilizer 15N and mineral nutrients were investigated. Rhizobial ACC deaminase enhanced shoot biomass, nodulation, N2 fixation, WUE, uptake of fertilizer N and mineral nutrients of plants subjected to single stresses (water deficit or Cd toxicity). These effects were accompanied by increased Cd content and total Cd in plant shoots, particularly in the Cd-tolerant pea mutant SGECdt. However, a combined water deficit and Cd stress disturbed the plant/rhizobia interaction, which could not be ameliorated via rhizobial ACC deaminase, especially in the SGE genotype. The nodule bacteria R. leguminosarum bv. viciae commonly have ACC deaminase, which plays an important role in tolerance of pea plants to drought and toxic Cd. However, efficient integration of symbiotic partners and successful "team-work" of plants and bacteria under combined stress requires that both symbionts be tolerant to both stress factors.