Inhabiting fluorescent Pseudomonas on wheat seed promote bacterial leaf streak disease

Wheat seeds harbor different microbial populations, which can be associated with each other in neutral, positive, or negative interactions. The present study investigated the interaction of fluorescent Pseudomonas and Xanthomonas translucens (Xt) as an important wheat seed-borne pathogen. In addition to P. fluorescens (A7) and P. chloroaphis (A4), which were previously isolated from the rhizosphere of potato as potent biocontrol agents, six more non-pathogenic Pseudomonas strains that were isolated from wheat seeds, were studied. According to the general biochemical tests and partial 16S rRNA sequences alignment, the isolated strains were closely related to the species of P. gessardii, P. orientalis, P. poae, P. koreensis, and P. cedrina. The Pseudomonas strains exhibit different antagonistic activities, such as phosphate solubilization, cellulase, protease, and lipase production. Also, they have an apparent inhibition effect under in vivo conditions against X. translucens. Seed treatment by these strains led to suppressing bacterial leaf streak disease incidence in an early growth stage. However, disease progress enhanced with the seedling growth, resulting in the treated plants' complete death. Only in treated seeds by P. fluorescens (A7), P. chloroaphis(A4), and P. orientalis (Ais119) decrease of AUDPC up to 83%, 74%, and 63% was achieved, respectively, compared with the untreated controls. Our results showed that some fluorescent Pseudomonas strains could cause delay at the beginning of the disease appearance due to competition or producing antimicrobial metabolites during that time. In contrast, some may be considered a threat, enhancing disease development through synergistic effects.