The Polar Flagellin of Azospirillum brasilense REC3 Induces a Defense Response in Strawberry Plants Against the Fungus Macrophomina phaseolina

Microbe-associated molecular patterns (MAMPs) are conserved molecules able to trigger plant resistance. The aim of this work was to evaluate the capacity of Azospirillum brasilense REC3 polar flagellin AzFlap as a MAMP, eliciting biochemical, histological, and molecular defense responses that can provide strawberry plants protection against the pathogenic fungus Macrophomina phaseolina. Strawberry plants were treated with AzFlap on leaves or with the isolate REC3 on leaves or roots. Salicylic acid content, biofilm formation, callose and lignin depositions, stomatal closure, ROS, and the expression of defense-related genes such as FaPR1, FaCAT, FaRBOH-D, FaRBOH-F, FaCHI23, FaCHI2-2, and FaGSL5 were evaluated. Phytopathogenic assays in plants treated with AzFlap or REC3 and infected with M. phaseolina were also performed. Results showed that plants leaf treated with AzFlap or root treated with REC3 caused the accumulation of ROS, salicylic acid, callose, lignin, the increase of biofilm formation on leaves, and stomatal closure. The evaluation of the expression of genes associated to defense response indicated the activation of the innate immunity of strawberry plants. The level of gene expression was strongly time and treatment dependent, suggesting a complex regulation of defense signaling. Root inoculations with REC3 or foliar treatment with AzFlap were able to reduce plant mortality, showing the effectiveness of both treatments to control M. phaseolina. These results indicate that flagellin AzFlap from A. brasilense REC3 behaves as a MAMP that activates a defense response against M. phaseolina in strawberry plants.