Pseudomonas spp. Mediate defense response in sugarcane through differential exudation of root phenolics

Pseudomonas spp., a ubiquitous biocontrol agent, protects the plants from phytopathogens by suppressing them directly by reinforcing the plant's intrinsic defense mechanism. Root exudated phenolics play an important role in establishing the rhizobacteria population and cross the host boundaries in beneficial plant-microbe interaction. In this study, Pseudomonas spp. HU-8 & HU-9 antagonized the sugarcane red rot pathogen (C. falcatum) and 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 a pot experiment, they effectively colonized the sugarcane rhizosphere and mediated defense response in sugarcane plants challenged with red rot pathogen C. falcatum by regulating the exudation of root phenolics under hydroponic conditions. They significantly induced the activity of the antioxidant enzymes CAT (1.24-1.64 fold), PO (0.78-1.61 fold), PAL (0.77-0.97 fold), and PPO (3.67-3.73 fold) over untreated plants in sugarcane. They also induced the total phenolic contents (TPC) in sugarcane in the presence (6.56-10.29 mg/g GAE) and absence (2.89-4.16 mg/g GAE) of the pathogen quantified through the Folin-Ciocalteu (FC) method. However, their effect was lower than that of the pathogen (4.34-8 mg/g GAE). The Pseudomonas spp. significantly colonized the sugarcane rhizosphere by maintaining a cell population of (1.0E + 07-1.3E + 08 CFU/mL). A significant positive Pearson's correlation was observed between the root exudated total phenolic contents, antioxidant enzymatic activities, and rhizospheric population of inoculated bacteria. 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 spp. could be an ideal bio-inoculants for a broad range of hosts especially graminaceous crops.