Abstract
The soil is a repository of microorganisms such as bacteria, fungi, algae, and protozoa. Among these, more bacteria are found, most of which are located in the rhizosphere region of the soil. The rhizosphere, under the direct control of plant root secretions, is the complex, narrow area of the soil. It is densely populated with microorganisms (mostly bacteria) that interact with the plants. These interactions influence the growth of the plant directly or indirectly. Plant growth-promoting rhizobacteria (PGPR) inhabiting the rhizosphere colonizes the plant roots and increases plant growth via different mechanisms. Iron is an essential micronutrient required by almost all life forms including plants. Oxidation of Fe2+ (soluble) to Fe3+ (insoluble) due to the soil's aerobic conditions limits its bioavailability. Siderophores are selective low molecular weight ferric ion chelators secreted by bacteria to acquire iron from the surrounding. They bind to iron (Fe3+) with high specificity as well as high affinity. By helping the insolubilisation of iron, it promotes the growth and yield.
TopIron: An Essential Microelement
Iron is the fourth most abundant element in the lithosphere and the third most limiting nutrient for plant growth. Iron is an essential micronutrient that is indispensable for plant growth (Tripathi et al., 2018). Plants require it for chlorophyll development and stability, for the catalytic activity of proteins involved in essential cellular metabolic processes like respiration, photosynthesis, DNA synthesis, and defence against ROS (Rout et al., 2015). Despite being present in high quantity, it is not readily available for use by plants growing in aerobic and neutral to alkaline pH soil. This is so because the aerobic and alkaline conditions of soil cause oxidation of Fe2+(soluble) to Fe3+ (insoluble) which form insoluble complexes thereby limiting the availability of the usable form of iron to plants (Rout et al., 2015). For example: in aerobic soil, Fe3+ is present majorly as a constituent of highly insoluble oxyhydroxide polymers (Briat et al., 2007). Plants growing in such soils, therefore, have adopted specific strategies to acquire iron in a usable form that is essential for their growth (Rout et al., 2015).
Key Terms in this Chapter
Phytosiderophores: Fe 3+ chelating compounds (mugineic acids (MAs) or its modified derivatives) released by plants in the rhizosphere under iron deficient conditions.
Bioremediation: Bioremediation involves the use of micro-organisms (living) or plants for reduction or degradation of environmental pollutants into non-toxic or less toxic forms.
Biofertilizer: Biofertilizer is a substance comprising of micro-organisms (living) which, when applied on plant parts or soil provides nutrition to the plants and promotes their growth. It is an environmentally sound way of providing nutrients to the plant instead of harmful chemical-based formulations.
Plant Growth Promoting Bacteria (PGPB): Bacteria those that help in enhancing the growth of the plant are known as Plant Growth Promoting Bacteria (PGPB). These can be free living or symbiotic inhabiting the rhizosphere or colonize inner plant tissues or organs.
Rhizosphere: A dynamic, narrow region of the soil where plant roots are easily accessible and are densely populated with microorganisms (especially bacteria). It is the region where maximum interactions between plant roots and the fauna take place.
Siderophores: Selective low molecular weight (less than 2000 Da), water soluble, organic molecules that binds to ferric ions (Fe 3+ ) with high affinity and specificity. They are secreted by nearly all aerobic bacteria and fungi under iron deficient conditions.
Biocontrol: Biocontrol or biological control is an environmentally friendly way in which certain organisms are used to control or prevent the growth of pests like insects, mites, pathogenic microbes, etc.