The role of PGPR in micronutrient supplement, uptake and plant nutrition

Micronutrients are essential for plant nutrition but are required in very small quantities. The copper, iron, manganese, chlorine, zinc, molybdenum and boron are micronutrients of plants whose availability and utilization is as diverse as plant diversity. They are vital for proper functioning of plant metabolism, chlorophyll and lignin synthesis, water and nutrient transport and translocation, viability of seeds, development of vigor and productivity, alleviation of other mineral deficiencies, regulation of general plant growth and also for elasticity of plant parts. These micronutrients are also important constituents of essential amino acids and enzyme complexes of plants and bacteria. The deficiency one or more micronutrients may cause severe deficiency diseases, increased vulnerability to fungal, viral and bacterial pathogens, chlorosis, necrosis, stunted growth which may all affect plant's health and productivity. They are always limiting in soil for number of reasons such as deficiency or low availability, type of soil drainage, flooded or dry soils, past fertilizer applications, soil texture, soil pH and moisture, seasonal weather conditions. Sometimes minerals are hindered inside soil aggregates as complex organic or inorganic forms and therefore possess low mobility. They also affect soil productivity in various ways. So called unproductiveness of acid or alkali soils is mostly due to the lack of available plant nutrients.

Plant alone is not able to acquire sufficient quantity of these essential micronutrients from the soil just by usual root diffusion process. In addition to provision of major nutrients like nitrogen and phosphorus; Plant Growth Promoting Rhizobacteria (PGPR) are known to increase an availability of these essential micronutrients to their plant hosts via different uptake mechanisms. In addition to this they are also active components of all biogeochemical cycles of nutrients in the biosphere. In addition to micronutrient supplement, rhizobacteria are reported to enhance uptake of nutrients by plants. They may alter plant hormone levels which changes root growth and morphology and increase surface area for nutrient absorption. Such roots absorb more water and nutrients. Free living, nitrogen fixing rhizobacteria and legume rhizobia are known to increase absorption capacity of plants by stimulating morphological changes in plant root system. When PGPR are introduced to a contaminated site, they increase the potential for plants that grow there to sequester heavy metals and to recycle nutrients, maintain soil structure, detoxify chemicals, and control diseases and pests; PGPR also decrease the toxicity of metals by changing their bioavailability in plants. In return, plants provide free amino acids, proteins, carbohydrates, alcohols, vitamins, and hormones in the form of root exudates to microorganisms. But most bacteria are functionally active at pH range 6.0 to 7.5. Soils of pH beyond this range therefore evidently lack functional microbial activities for the provision of essential micronutrients and negatively affect plant productivity. For example, in highly acid soils where pH is low, the availability of iron and manganese is increased to the point to become toxic to plant. On the contrary at alkaline or high pH; supply of iron, manganese and boron is so reduced to unavailability. Functional soil bacterial population and micronutrient availability is also directly positively correlated to soil organic matter content which is nothing but proportionate mixture of mostly all major and micronutrients. Let's read here, how PGPR enable to enrich and uptake various micronutrients in the soil.

Iron (Fe): Iron is extremely limiting micronutrient in the soil and rhizosphere environment. It is taken up by plants as Fe+2 (ferrous) which is obtained by reduction of insoluble Fe+3 (ferric) forms in the soil. Rhizobacteria produce different types of iron chelating compounds or siderophores like hydroxamate, catechol, pyoverdins to sequester and uptake available quantity of essential iron from the soil. The stability of siderophores is variable with their type and soil pH. Hydroxamate siderophores are very stable in acidic soils (pH 3-4) while as catechol type is functional in the alkaline soils of pH 7.4 to 10. Fluorescent pseudomonads, Aeromonas, Arthrobacter, Bacillus, Streptomyces and Nocardia are well known siderophore producing rhizobacteria. Schmidt et al showed that plants assimilate iron either by direct uptake of siderophore-Fe complex or by ligand mediated exchange reactions. In addition to growth promotion, siderophores have important antagonistic potential for phytopathogenic fungi and bacteria. They compete with these phytopathogens and deprive them of iron, limiting their nutrition and growth. Siderophores are also known to sequester elements other than iron, such as aluminium, gallium, lead, cadmium, zinc and even Uranium. These elements, if present at high concentration in the soil, may prove toxic to plant and surrounding biota. Rhizobacterial sequestration, thus actually helps to remediate plant environment for healthy growth.

Copper (Cu): Soil Cu occurs as in many forms and mineral states either adsorbed and/or occluded as oxides, in organic residues, as mineral hydroxides and carbonates in soil solution. Cu is very low to deficient in the soils with high organic matter and alkaline pH. This is because at pH above 7.0 Cu is strongly soluble in organic matter resulting in its rapid adsorption and complexation to form CuO, CuCO3 and other mixed OH-CO3 copper minerals. These reactions are typical of calcareous and arid soil regions where growing plants are consistently prone to copper unavailability. Copper is taken up as cupric form (Cu+2) by green plants and rhizobacteria that are active in supply of copper are Pseudomonas, Bacillus, Sphingomonas, Arthrobacter, Stenotrophomonas, etc. These genera are generally prevalently found in Cu deficient soils. In the soils where copper is heavily accumulated, these bacteria also assist in natural phytoremediation and maintain homeostasis in the soil and rhizosphere region.

Manganese (Mn): Manganese is present in the earth's crust as manganese dioxide (MnO2). However, the solubility and availability of all forms of manganese depends heavily on the prevailing pH in the soil and is also critically controlled by soil pH. At pH 5-7, Mn is available in its most assimiable form. As acidity increases, the lower pH more easily allows the ionic bond in MnO to dissociate, releasing Mn+2 into the soil, making the manganese more available to plants and to the many soil microbes. However, at pH below 5.5 the solubility of Mn compounds is considerably increased and too much Mn in soil solution cause a toxic influence on plant growth. Under neutral and alkaline conditions, manganese is usually present in (Mn+4) state; an insoluble form. Hence in soils with pH 7.5 and above, it become unavailable and sometimes produce deficiency diseases like chlorosis in plants. Manganese availability to the plant in need is also dependent on soil rhizobacterial activities. PGPR like Bacillus, Pseudomonas and Geobacter are known to provide Mn in metabolically assimiable Mn+2 (reduced) form to the plant. When large populations of bacteria are present under moist conditions, almost all free manganese available in the soil is reduced to manganese oxide for plant uptake. On the other hand, in dry regions, the manganese is available in the form more suitable to the utilization by rhizobacteria. But otherwise, any excess Mn+2 available is used by plants and rhizobacteria for their respective metabolic purposes in collaboration maintaining Mn homeostasis.

Zinc (Zn): The cation- Zn+2itself is the prevailing form taken up by the plants. Zinc is often supplied as inorganic fertilizer but is rapidly converted into unavailable form by precipitation reactions in the soil. Instead, zinc solubilizing rhizobacteria are potential candidates to supply and fulfill host plants' Zn requirement. However, solubility of Zn is also highly dependent upon soil pH and moisture. Regions devoid of these two conditions such as arid or semiarid areas are therefore, usually zinc-deficient. Zn availability is more in acidic soils than in alkaline soils so as soil pH increases Zn solubility decreases. Different species of rhizobacteria like Rhizobium, Acinetobacter, Pseudomonas and Bacillus are found to be proficient Zn solubilizers. Microbial Zn solubilization is carried out similarly like phosphate solubilization using different mechanisms such as organic acid production or chelation. These bacteria are also known to reduce Zn induced phytotoxicity in Zn contaminated soils.

Molybdenum (Mo): Naturally, molybdenum is present in farmyard manure, seeds, tubers and corms. But its supply from these sources is always not sufficient and significant for plant growth. Mo is always limiting in soil and rhizosphere region and like all other minerals its availability is chiefly affected by soil pH conditions. Molybdenum is primarily taken up as MoO42- (molybdate ion) by plants. Molybdenum in acid soils (pH 5.5 or less) tends to be unavailable to plants and hence Mo deficiencies are common in acid, rather than neutral or alkaline soils. Thus, soils rich in organic matter and with poor drainage (Marsh lands, swamps) accumulate soluble molybdate than sandy soils from coastal areas. Mo is also required by diazotrophic bacteria as it is important part of functional nitrogenase enzyme complex which catalyses biological nitrogen fixation. Bacterial genera Pseudomonas, Bacillus, Leptothrix, Citrobacter, Acidobacteria, Nitrospira, Firmicutes, Chromobacterium, Azotobacter, Azospirillum and Actinomyces are promising in making Mo available to plants as well as trapping excess of it to be used for themselves. In non-legumes, Mo enables the plant to use the nitrates taken up from the soil; its absence would otherwise accumulate excess nitrate and plant would suffer from severe nitrogen deficiency. On the other hand, root nodule bacteria like Rhizobium infecting legumes rely on Mo up taken by host plant to carry out nitrogen fixation efficiently. Thus availability of Mo is critical in smooth functioning of nitrogen cycle in the ecosystem. It has been suggested that Mo in its accessible (molybdate) form is sequestered similarly like iron by Mo binding siderophores and therefore facilitate its uptake by plants under limiting conditions.

Boron (B): Boron is available to plants as water soluble form that is, boric acid (H3BO3) and borate (H2BO3-). Ideally, boron is in its least concentration (fraction of 1ppm) is essentially required by plants and few more ppm can be otherwise toxic. Since boron is non-mobile in plants, a continuous supply from soil or planting media is required. In mineral soils, release of boron is usually quite slow. Much of the available soil boron is held rather tightly by soil organic material. The reduction in soil pH is the way boron can be released as a soil solution. It is helped by bacterial degradation of organic matter and their production of organic acids. As organic matter decomposition occurs boron is released with a portion being absorbed by plants, the remaining is leached below the root zone area (especially in high rainfall/acid soil areas) for microbial utilization. The remaining boron is again tied up in soil particles under alkaline conditions and thus gets unavailable. Bacteria also sequester excess boron to reduce its toxicity to plants. Soil bacteria such as Gypsophila, Bacillus, and Microbacter etc. sequester boron by forming unreactive precipitate in the mineral form with iron, calcium or phosphate at pH 8-9. Recently, Ahmed et al have isolated and characterized boron tolerant bacterial genera: Bacillus, Arthrobacter, Rhodococcus, Gracilibacillus, Lysinibacillus and Algoriphagus from various soil samples. Of these, Bacillus sp. is not only boron tolerant but also uses boron as chief growth substrate. These genera certainly have great potential to be applied as PGPR for plants growing either in boron deficient soils or boron contaminated sites.

Chlorine (Cl): Plants take up chlorine in the form of chloride ion (Cl-). Plants grown in saline soils which naturally have high content of chlorine are highly dependent on their rhizobacterial counterparts. Chlorine in the soil naturally comes from atmosphere, rain, river, ocean water, agricultural irrigation or fertigation practices but chlorine in its gaseous form do not persist in the soil. Chloride being an anion is highly mobile in soil, water and plant body and easier for uptake by plants. Studies have shown that soil bacteria such as Bacillus, Actinomyces spp. are able to maintain homeostasis between Na+-K+-Cl- under salt stress. These bacteria can confer plant salt tolerance by tissue-specific regulation such that accumulation of salts inside root/shoot is kept at minimum level.

Thus rhizobacteria play a crucial role in transformation, mobilization, solubilization and provision of micronutrients from a limited nutrient pool to plants and thus promote healthy and prolific plant growth.

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