Legume-Rhizobium Symbiosis: Secondary Metabolites, Free Radical Processes, and Effects of Heavy Metals

Abstract

Leguminous plants are able to establish symbiosis with a group of nitrogen-fixing soil bacteria called collectively rhizobia. This symbiosis leads to the formation of root nodules, specialized structures within which bacteria carry out nitrogen fixation. Both rhizobia and host legumes exhibit a strong specificity, which can be a result of their coevolution. Symbiotic specificity is provided by the complex exchange of signals between both symbiotic partners. To initiate symbiosis, legumes produce a cocktail of flavonoids that trigger synthesis and secretion of bacterial lipochgitooligosaccharide molecules called Nod factors. Nod factors together with surface polysaccharides and secreted proteins are proposed to be major rhizobial determinants of host specificity. Much evidence suggests that reactive oxygen species (ROS) play a key role in the formation and functioning of legume-rhizobium symbiosis. Elevated levels of heavy metals in soils can affect rhizobial growth and host legumes as well as impair legume-rhizobium symbiosis, in particular due to enhanced ROS production. On the other hand, if plants form symbiosis with rhizobia, heavy metals are accumulated preferentially in nodules that can be one of the possible ways to reduce toxic effects of heavy metals to legumes.