Heavy Metal Resistance in Plants: A Putative Role of Endophytic Bacteria

Abstract

Heavy metals and metalloids have become one of the major environmental concerns which pose a serious threat to plants and animal health. In this context, endophytic bacteria could play an important role in understanding the uptake mechanism of heavy metal ions and providing immunity to plant against metal toxicity. The defensive effects of certain elements in plants are known, but the role of endophytes in providing protection to plants has poorly been investigated. Endophytic bacteria, originating from hyperaccumulator plants, exhibit a comparatively higher level of resistance to heavy metals than the soil and the rhizosphere bacteria. Among bacteria, Methylobacterium spp., as well as the representatives of Gram-positive bacteria, are the most widespread bacterial species in both the hyperaccumulator endosphere and endorhizae. The endophytic microbial populations enhance the resistance capacity of the host plants, which, however, depends on the structure and activity of the community. Moreover, endophytic bacteria including those of legume endophytes are considered a promising biological material for improving the efficiency of phytoremediation and, consequently, growing of clean and safe crops including legumes in metal polluted soils. The recent developments in the putative mechanisms by which endophytic microorganisms affect the plant resistance to heavy metals and how they could affect phytoextraction of metals from contaminated soils are highlighted.