Plant Growth Promoting Rhizobacteria (PGPR) are beneficial soil bacteria that can live either symbiotically with plants at rhizosphere or as endophytes living on or inside of the host plants. There are two main mechanisms via PGPR contribute to the plant growth. Direct mechanism consists of phytohormone production (i.e. auxins (IAA), cytokinins and gibberellins), biological nitrogen fixation, solubilizing inorganic phosphates, mineralizing organic phosphate and producing organic matter such as amino acids. As indirect mechanisms, PGPR aid plants in combat against the pathogen microorganisms by means of stimulating the disease-resistance mechanism of plants, promote favorable symbiosis, decontaminate the soil of xenobiotics. PGPR can also help plants to cope against abiotic stress by lowering ethylene levels, or against pathogenic microorganism by means of secreting antibacterial/antifungal substances. Exact mechanisms of PGPR characteristics which stimulate the plant growth or product formation are still under investigation, yet in agriculture, PGPR are used as environmental friendly biofertilizers, biocontrol agents or biostimulants. These beneficial bacteria are usually introduced to the plants either in powder or liquid form or the seeds are covered with the inoculants before sowing. Plants are subject to many different environmental elements. Abiotic factors such as drought or water stress have been one of the main plant growth limiting factors. Agricultural PGPR application is an alternative solution against loss due to the environmental stresses, since breeding a plant with stress resistance trait is a very long and tricky process due to the fact that such traits are controlled by multiple genes. PGPR phytohormone and enzyme (i.e. ACC deaminase) production can decrease the stress levels of plants while enhancing the root structures.
Part of the book: Plant Growth
Soil is the basis of agriculture and consists of organic matters, minerals, water, and several gasses. All plants require soil both as an anchor to attach and as water and nutrient source. Unfortunately, lifestyles of humans, industrial progress, chemicals used in agriculture contaminate soil and cause soil pollution. A pollutant may be natural or human‐made in origin such as petroleum hydrocarbons, pesticides, heavy metals, and solvents. Since the quality of the soil affects the growth and product yield of plants, soil pollution is a crucial problem needs to be addressed urgently. Plant growth promoting rhizobacteria (PGPR) are microorganisms living in soil, on the plants roots, or inside the plant. PGPRs synthesize chemicals to stimulate plant growth and promote nutrient uptake, help degrading soil pollutants and fending off pathogens. While some pollutants can be degraded by enzymes produced by bacteria and fungi, degradation of heavy metals requires alternative methods. In this chapter, three enzymes produced by PGPRs are reviewed briefly. Aminocyclopropane‐1‐carboxylate (ACC) deaminase is responsible of lowering the ethylene levels of plants during stress conditions, whereas nitrogenase is responsible for N2 reduction to NH3. Moreover, phytase enables the degradation of phytate which is a main storage form of phosphate in plants.
Part of the book: Soil Contamination
Soil contains enzymes, constantly interacting with soil constituents, e.g. minerals, rhizosphere and numerous nutrients. Enzymes, in turn, catalyse important biochemical reactions for rhizobacteria and plants, stabilize the soil by degrading wastes and mediate nutrient recycling.The available enzymes inside soil could originate from plants, animals or microbes. The enzymes that are produced from these organism could exhibit intracellular activities, at the cell membrane, interacting therefore with soil and its constituents, or extracellularly (so freely available). Therefore, vis-à-vis to plant nutrition, the (extra or sub) cellular localization has a key role. Typical major enzymes available in soil can be listed as dehydrogenases, hydrogenases, oxidases, catalases, peroxidases, phenol o-hydroxylase, dextransucrase, aminotransferase, rhodanese, carboxylesterase, lipase, phosphatase, nuclease, phytase, arylsulphatase, amylase, cellulase, inulase, xylanase, dextranase, levanase, poly-galacturonase, glucosidase, galactosidase, invertase, peptidase, asparaginase, glutaminase, amidase, urease, aspartate decarboxylase, glutamate decarboxylase and aromatic amino acid decarboxylase. An interesting strategy for improving the nutritional quality of the soil would be to inoculate microorganism to soil while giving attention to mineral or other compounds that affect enzyme activity in soil. Since, some elements or compounds could show both activation and inhibitory effect, such as Fe, Na, etc. metals, the regulation of their bioavailability is crucial.
Part of the book: Enzyme Inhibitors and Activators