Chickpea (Cicer arietinum) is one of the most important dry land field crops. It is not only important in human diet, but also plays a vital role in improving the soil fertility by fixing atmospheric nitrogen into available form with the help of rhizobium species present in the nodules of its roots. However, under agro-ecological conditions of Pakistan, the nodulation of chickpea is poor, which is the major cause of its low yield. The N2-fixing bacteria of the genus rhizobium that form nodules on the roots of leguminous plants, such as chickpea, lentils etc. are called rhizobia. Mesorhizobium ciceri are well documented to be responsible for nodulation in chickpe.
It has been studied that soil-borne microbes interact with plant roots and soil constituents at the root–soil interface. The great array of root–microbe interactions results in the development of a dynamic environment known as the rhizosphere where microbial communities also interact. The differing physical, chemical, and biological properties of the root-associated soil, compared with those of the root-free bulk soil, are responsible for changes in microbial diversity and for increased numbers and activity of micro-organisms in the rhizosphere micro-environment. Microbial activity in the rhizosphere affects rooting patterns and the supply of available nutrients to plants, thereby modifying the quality and quantity of root exudates.
Microbes which interact with roots include both harmful and beneficial bacteria. Beneficial bacteria are generally referred to as plant growth-promoting rhizobacteria or PGPR, and are capable of promoting plant growth by associating with the plant roots. Plant Growth Promoting Rhizobacteria can be divided into two groups according to their relationship with the plants, symbiotic bacteria and free-living rhizobacteria. Generally, PGPR function in different ways, including, synthesizing particular compounds for the plants, facilitating the uptake of certain nutrients from the environment, and lessening or preventing the plants from diseases. Many plant-associated bacteria promote plant growth by synthesizing low molecular weight compounds or enzymes that can modulate plant growth and development. Microbial ACC-deaminase is the enzyme that is present in some of the PGPR along with some other enzymes and it facilitates plant growth and development by decreasing plant ethylene levels, especially following a variety of environmental stresses. When plants are exposed to stressful conditions, they often respond by producing high concentrations of ethylene which are, in general, inhibitory to root growth. Some of the ACC is exuded from seeds, roots or leaves along with other small molecules normally present in these exudates and may be taken up by the bacteria and subsequently cleaved by the enzyme, ACC-deaminase, to ammonia and α-ketobutyrate. The bacterium acts as a sink for plant ACC and as a result of lowering ACC in plant the amount of ethylene is also reduced. As a direct consequence of lowering plant ethylene levels, PGPR can reduce the extent of ethylene inhibition of plant growth following a wide range of stresses. Thus, plants grown in association with these bacteria should have better roots and shoots and be more resistant to growth inhibition by a variety of ethylene-inducing stresses. Ethylene in higher concentration has also been known as an inhibitor of nodulation in various legumes, including those which produce indeterminate nodules and those that produce determinate nodules. The PGPR are also known to participate in many important ecosystem processes, such as the biological control of plant pathogens, nutrient cycling, and/or seedling growth. Selected strains of PGPR are being used as seed inoculants. Many plant-associated bacteria promote plant growth by synthesizing low molecular weight compounds or enzymes that can modulate plant growth and development.
Co-inoculation improves plant growth by direct stimulation of rhizobial growth/survival in the soil, enlargement of the root system by hormone production for enhanced nutrient uptake and increase in the number of potential colonization sites by rhizobium, phosphate solubilization and pathogen suppression due to production of antibiotics. It has also been reported to increase the number of nodules, shoot dry weight and root dry weight in chickpea, soybean and pigeon pea.
The potential of microbial ACC-deaminase to enhance the Mesorhizobium ciceri efficiency in chickpea in a pot trial, thereby exploring the potential of Mesorhizobium-PGPR co-inoculation to enhance nodulation, growth and yield of chickpea.
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