By Zeeshan Sattar, Muhammad Aslam Khan and Muhammad Mohsin Raza
AGRICULTURE CONTRIBUTES to a major share of national income and export earnings in Pakistan, while ensuring food security and employment. Sustainable agriculture is vitally important in todays scenario because it offers the potential to meet our future agricultural needs, something that conventional agriculture will not be able to do. Recently there has been a great interest in eco-friendly and sustainable agriculture. PGPR is known to improve plant growth in many ways when compared to synthetic fertilizers, insecticides and pesticides. They enhance crop growth and can help in sustainability of safe environment and crop productivity. The rhizospheric soil contains diverse types of PGPR communities, which exhibit beneficial effects on crop productivity. Several research investigations are conducted on the understanding of the diversity, dynamics and importance of soil PGPR communities and their beneficial and cooperative roles in agricultural productivity.
The Plant Growth Promoting Rhizobacteria (PGPR) is a group of bacteria that enhances plant growth and yield via various plant growth promoting substances as well as biofertilizers. Given the negative environmental impact of artificial fertilizers and their increasing costs, the use of beneficial soil microorganisms such as PGPR for sustainable and safe agriculture has increased globally during the last couple of decades. PGPR as biofertilizers are well recognized as efficient soil microbes for sustainable agric ulture and hold great promise in the improvement of agricultural yields.
PGPR as Biofertilizers:
Free-living PGPR have shown promise as bio-fertilizers. Many studies and reviews have reported plant growth promotion, increased yield, solubilization of phosphorus (P) or potassium (K), uptake of nitrogen (N) and some other elements through inoculation with PGPR. Additionally, studies have shown that inoculation with PGPR enhances root growth, leading to a root system with large surface area and increased number of root hairs.
A huge amount of artificial fertilizers is used to replenish soil N and P, resulting in high costs and increased environmental pollution. Most of P in the form of insoluble compounds is unavailable to plants. N2-fixing and P-solubilizing bacteria may be important for plant nutrition by increasing N and P uptake by the crop plants, and playing a crucial role in bio-fertilization. N2-fixation and P-solubilization, production of antibiotics, and other plant growth promoting substances are the principal contribution of the PGPR in the agro-ecosystems. More recent research findings indicate that the treatment of agricultural soils with PGPR inoculation significantly increases agronomic yields as compared to uninoculated soils.
Mechanisms of Plant Growth Promotion by PGPR
Several mechanisms have been suggested by which PGPR can promote plant growth such as production of growth stimulating phytohormones (like indole-3-acetic acid (IAA), gibberellic acid (GA3), zeatin, ethylene and abscisic acid (ABA), phosphate Solubilization and siderophore production (microbial Iron-chelating low molecular weight compounds).
PGPR as Biocontrol Agents
PGPR produce substances that also protect them against various diseases. By this capability, PGPR may protect plants against pathogens by direct antagonistic interactions between the biocontrol agent and the pathogen, as well as by induction of host resistance. In recent years, the role of siderophore-producing PGPR in biocontrol of soil-borne plant pathogens have created great interest. Microbiologists have developed techniques for the introduction of siderophore producing PGPR in soil system through seed, soil or root system. PGPR that indirectly enhance plant growth via suppression of phytopathogens do so by a variety of mechanisms. These include:
• The ability to produce siderophores (as discussed above) that chelate iron, making it unavailable to pathogens.
• The capacity to synthesize anti-fungal metabolites such as antibiotics, fungal cell wall-lysing enzymes, or hydrogen cyanide, which suppress the growth of fungal pathogens.
• The ability to successfully compete with pathogens for nutrients or specific niches on the root; and the ability to induce systemic resistance.
The PGPR and bacterial endophytes play a vital role in the management of various fungal diseases. But one of the major hurdles experienced with biocontrol agents is lack of an appropriate delivery system.
Worldwide, considerable progress has been achieved in the area of PGPR biofertilizer technology. It has been also demonstrated and proved that PGPR can be very effective and are potential microbes for enriching the soil fertility and enhancing the agricultural yield. PGPR are excellent model systems which can provide the biotechnologist with novel genetic constituents and bioactive chemicals having diverse uses in agriculture and environmental sustainability. Current and future progress in our understanding of PGPR diversity, colonization ability, mechanisms of action, formulation, and application could facilitate their development as reliable components in the management of sustainable agricultural systems.
The writers are associated with the Department of Plant Pathology, University of Agriculture Faisalabad. They can be reached at email@example.com