Physiological and antioxidant responses of Medicago sativa-rhizobia symbiosis to cyanobacterial toxins (Microcystins) exposure

Toxic cyanobacteria in freshwaters can induce potent harmful effects on growth and development of plants irrigated with contaminated water. In this study, the effect of cyanobacteria extract containing Microcystins (MC) on Medicago sativa-rhizobia symbiosis was investigated in order to explore plants response through biomass production, photosynthetic pigment and antioxidant enzymes analysis: Peroxidase (POD), Polyphenoloxidase (PPO) and Catalase (CAT). Alfalfa plants were inoculated with two endosymbiotic rhizobial strains: RhOL1 (MC less sensitive strain) and RhOL3 (MC more sensitive strain), to evaluate the rhizobial contribution on the plant response cultured under cyanobacterial toxins stress. The two rhizobia strains were identified as Ensifer meliloti by sequence analysis of their rrs and atpD genes. The chronic exposure to MC extract showed shoot, root and nodules dry weight decrease, in both symbiosis cultures. The rate of decline in plants inoculated with RhOL3 was higher than that in symbiosis with RhOL1 mainly at 20 mu g L-1 of MC. Cyanotoxins also reduced photosynthetic pigment content and generated an oxidative stress observed at cellular level. POD, PPO and CAT activities were significantly increased in leaves, roots and nodules of alfalfa plants exposed to MC. These enzyme activities were higher in plants inoculated with RhOL3 especially when alfalfa plants were exposed to 20 mu g L-1 of MC. The present paper reports new scientific finding related to the behavior of rhizobia-M. sativa associations to MC (Microcystins) for later recommendation concerning the possible use of these symbiosis face to crops exposure to MC contaminated water irrigation. (C) 2013 Elsevier Ltd. All rights reserved.