Beneficial Role of Plant Growth-Promoting Rhizobacteria in Bioremediation of Heavy Metal(loid)-Contaminated Agricultural Fields

The synergy of plants and microbes is one of the most interesting parts of holobiont research that yet have to be unwrapped before we can understand its implications in agriculture. Environmental stresses on plant ecology have further added to our curiosity in this context. Microorganisms are key players in benefitting plant health. This chapter mainly covers heavy metal and metalloid (HM)-induced phytotoxicity in different crops. We will be describing the role of soil-dwelling plant growth-promoting rhizobacteria (PGPR) in the mitigation of HM-induced damages in plants. We will also consider more generally the influential role of these microbes in biotic stress tolerance and the agricultural adoption of PGPR-involved strategies to combat HMs, which will help us provide adequate food for the world’s human population and the animals on which we depend for food, labor and companionship. Our starting point will be PGPR collected directly from the crop rhizosphere and associated with the lessening of HM content in crops, but excluding those intracellular endophytic microbes and those involved in PGPR-assisted phytoremediation. The principal rationale for these research efforts is to reduce the consumer’s health risks that are directly associated with the mobilisation or immobilisation of HMs inside plant cells. These microbes are possibly the best candidates for bioremediation because of their resilience and ability to withstand high HM levels, their mediation of the limiting effects that recalcitrant metals exert upon plant’s health, our successes of collaboration with the plants and microbes for biocontrol activities and microbial phytostimulation. This elaborative study covers the effect of 10 HMs (viz. Arsenic, Cadmium, Chromium, Cobalt, Copper, Lead, Manganese, Mercury, Nickel and Zinc) on crops and the HM-resistant PGPR discovered since 20 years. In addition, a general account of fundamental principles behind bacterial heavy metal resistance has been elaborated. Hence, this chapter will be of great interest especially to environmental microbiologists.