Root Exudates Mediated Regulation of Biocontrol Activity in Plant Growth Promoting Rhizobacteria (PGPR)

Abstract

Pseudomonas spp., a ubiquitous biocontrol agent, protects the plants from phytopathogens by suppressing them directly and/ or reinforcing the plant’s intrinsic defense mechanism. Inconsistent performance of antagonistic rhizobacteria to control phytopathogens is the main concern for sustainable crop production. The successful root colonization on certain plant rhizosphere determines the activity of rhizobacteria. Root exudated phenolics play an important role in establishing the rhizobacteria population and crossing the host boundaries in beneficial plant-microbe interaction. In this study, Pseudomonas sp. HU-8 & HU-9 antagonized the sugarcane red rot pathogen (C. falcatum), chickpea charcoal rot pathogen (Macrophomina phaseolina), chili fusarium wilt pathogen (Fusarium graminearum), and root rot pathogen of wheat (Fusarium oxysporum). They effectively colonized all crops (sugarcane, wheat, chili, and chickpea) rhizosphere and mediated defense response in plants challenged with pathogens (C. falcatum, F. oxysporum, F. graminearum, and M. phaseolina) by regulating the exudation of root phenolics under hydroponic (Hogland nutrient media) and pot experiments in shed house conditions. They colonized the rhizosphere of all crops i.e., wheat (109), sugarcane (108) chickpea, and chili (107). Root exudates secreted by plants play an important role in recruiting rhizobacteria. Hence, exploring this area is ideal to overcome the problem of rhizobacterial inconsistent efficacy. Pseudomonas sp. showed a positive chemotactic response against different concentrations (10-30 μM) of synthetic phenolic acids like p-coumaric, vanillic, and 3,4 di-hydroxybenzoic acid. In this study, phenolic resistant antagonistic Pseudomonas sp. (HU-8, HU-9) significantly controlled the root rot disease in wheat (40 – 50.34 %), chili (46.67 – 56 %), and chickpea (51.24 – 64.45 %). Through significantly induced the activity of the antioxidant enzymes CAT (0.11-1.64 fold), PO (0.11-1.61 fold), PAL (0.11-0.97 fold), and PPO (0.11-3.73 fold) over-challenged plants in all crops root and shoot. xi They also induced the total phenolic contents (TPC) in all crops in the presence of respective pathogen (6.0-10.29 mg/g GAE) over alone pathogen (2.89-8.01 mg/g GAE) quantified through the Folin-Ciocalteu (FC) method. They induced total phenolic contents (TPC) in root exudates of the indigenous host i.e., wheat (0.83 – 1.27 fold) and non-indigenous hosts i.e. sugarcane (0.43-0.86 fold), chili (1.14 – 1.76 fold) and chickpea (0.43 – 0.58 fold) over-challenged controls. A significant positive Pearson’s correlation (p ≤ 0.05) was observed between the disease incidence, root exudated total phenolic contents, antioxidant enzymatic activities, and rhizospheric population of inoculated bacteria in each crop. These Pseudomonas sp. (HU-8, HU-9) could be potential biocontrol agents against the root diseases of different crops. The 16S rRNA and rpoD gene analysis showed sequence conservation (C: 0.707), average number of nucleotide differences (k: 199.816), nucleotide diversity, (Pi): 0.09819), average number of informative nucleotide sites per site (Psi: 0.01275), GC content (0.57), and polymorphic sites (n=656). These diverse Pseudomonas sp. (HU-8, HU-9) could be ideal bio-inoculants for a broad range of hosts.