Review on increasing iron availability in soil and its content in cowpea (<i>Vigna unguiculata</i>) by plant growth promoting rhizobacteria

Abstract

Iron (Fe) deficiency constitutes a serious health challenge in humans especially in children and pregnant women, causing anemia and lower  cognitive ability. In sub-Saharan Africa, consumption of cowpea (Vigna unguiculata) is suggested as a means to tackle Fe deficiency in humans better than other plants including vegetables. This is because cowpeas are the staple food, they contain other nutrients including protein,
carbohydrate and vitamins and their cultivation sustains the environment through nitrogen fixation. Iron though rich in soils exhibit low solubility thereby limiting phytoaccumulation in cowpea, leading to the production of poor Fe cowpea. Plants that are Fe efficient developed two strategies to increase Fe availability and absorption. As a strategy 1 crop, cowpea acquires iron significantly through rhizosphere acidification; which reduces Fe⁺³ to Fe⁺². Such mechanisms do not deliver enough Fe to the crop to meet their demands. Application of inorganic Fe fertilizer to soil though not sustainable also results in fixation and unavailability of Fe to plants. This review surveys available literature on the use of plant growth-promoting rhizobacteria (PGPR), which are costeffective, eco-friendly and sustainable for enhancing Fe mobility for increased absorption by the plant. Recent biotechnological advancements in rhizosphere organisms have revealed various techniques involved in growth enhancement by PGPR. Such growth enhancement involves mobilization of Fe in soils for ease of absorption by plants. Iron solubilization by PGPR as a direct mechanism is facilitated by  several microbes including Fluorescent pseudomonad, Bacillus, Agrobacterium, Sinorhizobium and Bradyrhizobium japonica. They do this through different mechanisms, which include bioleaching, chelation, biomethylation and siderophores mediated iron uptake. The functional groups responsible for the binding of iron to siderophores are mostly the hydroxymates and catechols. Siderophores are iron binding compounds which are released by both plants and microbes. Cowpea like other non-gramminacae crops, interact with microorganisms to acidify the rhizosphere.  Thus, chelates of ferric reductase and iron (II) transporters are activated in the roots. Microorganisms especially bacteria and fungi now chelate insoluble Fe+3 and make them available for plant uptake. Plants are capable of utilizing siderophores produced by microorganisms for Fe uptake.
Therefore, these bacteria should be inculcated into the cultivation of cowpea so that its Fe content will be enriched. With the consumption of iron-rich cowpea, Fe deficiency will be minimized if not completely eradicated.

Key words: Biofortification, microbes, iron-deficiency, siderophore, bioleaching, biomethylation, bioavailability, catechol