Management of Plant Diseases by PGPR-Mediated Induced Resistance with Special Reference to Tea and Rice Crops

Abstract

Among the biotic stresses, plant pathogens can reduce yield crop which affected potential loss to crop productivity. Plant growth-promoting rhizobacteria (PGPR) can help plants to be resistant against biotic stress via direct antagonism to pathogens or by induction of systemic resistance to pathogens. The presence of high levels of nutrients exuded from various roots of most plants can support bacterial growth and metabolism as well as maintain health of the plant in the growth process. PGPR promote plant growth due to their abilities in phytohormone production, nitrogen fixation, and phosphorus solubilization; produce several substances which are related to pathogen control, i.e., exhibiting competition with plant pathogens, synthesis of antibiotics, antifungal metabolites and defense enzymes, and secretion of iron-chelating siderophores; and trigger induced systemic resistance (ISR) via methyl jasmonate and methyl salicylate in plants. The ISR resembles pathogen-induced systemic acquired resistance (SAR) through the salicylic acid-dependent SAR pathway under conditions where the inducing bacteria and the challenging pathogen remain spatially separated. The use of PGPR combinations of different mechanisms of action, i.e., induced resistance and antagonistic PGPR, might be useful in formulating inoculants leading to a more efficient use for biological control strategies to improve crop productivity. Many PGPR have been isolated from the tissues of many plants, and various species of bacteria, i.e., Azotobacter, Azospirillum, Alcaligenes, Arthrobacter, Bacillus, Burkholderia, Enterobacter, Klebsiella, Pseudomonas, and Serratia, have been reported to control several diseases and enhance plant growth. PGPR belonging to the genera Pseudomonas and Bacillus are also well known for their antagonistic effects and their ability to trigger ISR. An increasingly successful study to reduce disease severity is the use of bacteria, namely, Bacillus subtilis, P. fluorescens, Serratia, and the fungus Trichoderma. Tea and rice plants are cultivated in Indonesia predominantly in Java and Sumatra islands. Major constraints of cultivation include low fertility of soils, poor input management, low germination, and high susceptibility to the diseases. The strategies employed by PGPR provide promising approaches to alter agricultural crops and plantation practices toward sustainable environmental development. Research has been conducted to know the effect of PGPR on tea plant growth that can work optimally as a biological fertilizer and plant-induced resistance to suppress blister blight (Exobasidium vexans Massee), a major disease in tea plantation that can decrease yield loss up to 50%. Individual PGPR strains for in vitro broad-spectrum pathogen suppression and production of several physiological/biochemical activities related to plant growth promotion have been screened. Numerous bacterial isolates have been found to function both as biofertilizers and biological control agents, namely, Chryseobacterium sp. AzII-1, Acinetobacter sp., Alcaligenes sp. E5, Bacillus E65, and Burkholderia E76. Study about synergism among bacteria has been carried out in the laboratory test using four combinations, i.e., (a) Chryseobacterium sp. AzII-1 + Acinetobacter sp., (b) Chryseobacterium sp. AzII-1 + Alcaligenes sp. E5, (c) Chryseobacterium sp. AzII-1 + Bacillus E65, and (d) Chryseobacterium sp. AzII-1 + Burkholderia E76. All bacterial combinations had a synergistic effect. It was shown that the bacterial population was not significantly different with the average of the total bacterial population (4.62 × 108 CFU/ml). The effect of bacterial combinations to blister blight and plant growth under a tea nursery trial revealed that combination of Chryseobacterium sp. AzII-1 75% + Alcaligenes sp. E5 25% could increase the growth of tea plant and suppress the intensity of blister blight up to 1.27%. The disease intensity of blister blight decreased in all treatments under field trial, while the Acinetobacter sp. treatment in tea shoots was 17.26% higher than the control. PGPR have also been isolated from cultivated rice. Serratia SKM, Burkholderia E76, and Bacillus E65 have the potential for controlling rice diseases and induce plant growth promotion. Under in vitro antagonistic assay, it was shown that these isolates could suppress effectively the growth of rice pathogens Xanthomonas oryzae pv. oryzae, the causal agent of bacterial blight (BB). Kaolin formulation of these three isolates was evaluated as a foliar application on rice. PGPR application under experimental plots resulted in enhancement of rice growth and yield, with the yield increment on cv. Sintanur being 12.8 percent higher compared with control (cv. Ciherang). Based on PGPR application technology which is demonstrated in farmers’ plots, the severity of BB disease was reduced to 76.8 percent compared with the untreated plot. The farmers were convinced with the beneficial effects of PGPR on both plant growth and yield and reduction of BB disease incidence. PGPR technologies have the potential to reduce agrochemical application. They can also be exploited as low in input and environmentally friendly for sustainable plant management. PGPR is highly diverse, and in this review, we focus on PGPR in plant growth promotion, as well as understanding the role of PGPR in crop protection.