Arbuscular mycorrhizal symbiosis can alleviate drought-induced nodule senescence in soybean plants

Drought stress causes premature senescence in legume root nodules, therefore decreasing their ability for nitrogen fixation. Many physiological plant processes affected by drought stress have also been proposed as inducers of nodule senescence. The objective of this research was to determine whether arbuscular mycorrhizal (AM) symbiosis, already proven to protect host plants against the detrimental effects of drought, can also help legume plants to cope with the premature nodule senescence induced by drought stress. Several parameters related to nodule senescence were evaluated in well watered or drought-stressed soybean plants singly or dually inoculated with Bradyrhizobium japonicum and/or the AM fungi Glomus mosseae or Glomus intraradices. Under drought conditions G. mosseae colonization stimulated nodule d. wt, increased acetylene reductase activity (ARA) by 112%, increased leghemoglobin content by 25% and increased protein content by 15%, relative to plants singly colonized by Bradyrhizobium. Drought considerably enhanced oxidative damage to lipids and proteins in nodules of nonmycorrhizal plants, whereas both mycorrhizal treatments were protected against oxidative damage. We propose that alleviation of oxidative damage is strongly involved in AM protection against nodule senescence. Differential influences of G. mosseae and G. intraradices are also discussed.