Unraveling Aspects of Bacillus amyloliquefaciens Mediated Enhanced Production of Rice under Biotic Stress of Rhizoctonia solani

Abstract

Rhizoctonia solani is a necrotrophic fungi causing sheath blight in rice leading to substantial loss in yield. Excessive and persistent use of preventive chemicals raises human health and environment safety concerns. As an alternative, use of biocontrol agents is highly recommended. In the present study, an abiotic stress tolerant, plant growth promoting rhizobacteria Bacillus amyloliquefaciens (SN13) is demonstrated to act as a biocontrol agent and enhance immune response against R. solani in rice by modulating various physiological, metabolic, and molecular functions. A sustained tolerance by SN13 primed plant over a longer period of time, post R. solani infection may be attributed to several unconventional aspects of the plants' physiological status. The prolonged stress tolerance observed in presence of SN13 is characterized by (a) involvement of bacterial mycolytic enzymes, (b) sustained maintenance of elicitors to keep the immune system induced involving non-metabolizable sugars such as turanose besides the known elicitors, (c) a delicate balance of ROS and ROS scavengers through production of proline, mannitol, and arabitol and rare sugars like fructopyranose, β-D-glucopyranose and myoinositol and expression of ferric reductases and hypoxia induced proteins, (d) production of metabolites like quinazoline and expression of terpene synthase, and (e) hormonal cross talk. As the novel aspect of biological control this study highlights the role of rare sugars, maintenance of hypoxic conditions, and sucrose and starch metabolism in B. amyloliquefaciens (SN13) mediated sustained biotic stress tolerance in rice.

Keywords: B. amyloliquefaciens; Rhizoctonia solani; biological control; plant growth promotion; sheath blight.

FIGURE 1

(A) Biocontrol efficacy of Bacillus amyloliquefaciens (SN13) against Rhizoctonia solani (R. solani) in dual culture method (I) and broth conditions (II) (primary axis) SN13 growth, (secondary axis) R. solani biomass. (B) Effect of SN13 on plant growth and biocontrol of rice sheath blight (a) 45 dpi (b) close view of lesion development in RS and SN13 + RS at 15 dpi. CON, control; SN13, biocontrol agent; R. solani, pathogen; dpi, days post infection.

FIGURE 2

Enzymatic profile of medium supernatant during in vitro interactions of SN13 and R. solani.

FIGURE 3

Stem cross section showing effect of B. amyloliquefaciens (SN13) on aerenchyma formation in rice, 15 dpi. “First’ and ‘second’ indicate the sheath layer from core and the encircled area shows the extent of aerenchyma formation. CON, control; SN13, biocontrol agent; R. solani, pathogen; dpi, days post infection. Magnification 40×.

FIGURE 4

Effect of B. amyloliquefaciens (SN13) inoculation on cell wall degrading enzyme (A) and defense enzyme (B) activities 15 and 45 dpi in rice leaves. CON, control; SN13, biocontrol agent; R. solani, pathogen.

FIGURE 5

Effect of B. amyloliquefaciens (SN13) inoculation on hormone synthesis at 15 dpi in rice leaves. CON, control; SN13, biocontrol agent; R. solani, pathogen.

FIGURE 6

Heat map showing expression profiling of differentially expressed genes of primary metabolism (A), stress response (B), and hormone biosynthetic pathway (C) in rice leaves. CON, control; SN13, biocontrol agent; R. solani, pathogen. Black arrows indicate the marked genes restored as control as compared to R. solani infection. Blue arrows indicate some of the genes randomly selected for RT-PCR analysis.

FIGURE 7

RT-PCR analysis of randomly selected genes in rice leaves 45 dpi. CON, control; SN13, biocontrol agent; R. solani, pathogen.

FIGURE 8

Principal component analysis (PCA) differentiating the components of GC–MS metabolic profiles of O. sativa leaves 45 dpi in the four treatments, control, pathogen infected (R. solani), Bacillus amyloliquefaciens treated (SN13) and pathogen + B. amyloliquefaciens (SN13 + R. solani).