A Genome-Wide Screen Identifies Genes in Rhizosphere-Associated Required to Evade Plant Defenses.

and related plant root ("rhizosphere")-associated species contribute to plant health by modulating defenses and facilitating nutrient uptake. To identify bacterial fitness determinants in the rhizosphere of the model plant , we performed a high-throughput transposon sequencing (Tn-Seq) screen using the biocontrol and growth-promoting strain sp. WCS365. The screen, which was performed in parallel on wild-type and immunocompromised plants, identified 231 genes that increased fitness in the rhizosphere of wild-type plants. A subset of these genes decreased fitness in the rhizosphere of immunocompromised plants. We hypothesized that these genes might be involved in avoiding plant defenses and verified 7 sp. WCS365 candidate genes by generating clean deletions. We found that two of these deletion mutants, Δ (encoding a putative diguanylate cyclase/phosphodiesterase) and Δ (encoding a putrescine aminotransferase), formed enhanced biofilms and inhibited plant growth. We found that mutants and induced pattern-triggered immunity (PTI) as measured by induction of an PTI reporter and /dependent inhibition of plant growth. We show that MorA acts as a phosphodiesterase to inhibit biofilm formation, suggesting a possible role in biofilm dispersal. We found that both putrescine and its precursor arginine promote biofilm formation that is enhanced in the mutant, which cannot break down putrescine, suggesting that putrescine might serve as a signaling molecule in the rhizosphere. Collectively, this work identified novel bacterial factors required to evade plant defenses in the rhizosphere. While rhizosphere bacteria hold the potential to improve plant health and fitness, little is known about the bacterial genes required to evade host immunity. Using a model system consisting of and a beneficial sp. isolate, we identified bacterial genes required for both rhizosphere fitness and for evading host immune responses. This work advances our understanding of how evasion of host defenses contributes to survival in the rhizosphere.