Auxin plays multiple roles during PtoDC3000 pathogenesis.
The plant hormone auxin is involved in multiple plant-pathogen interactions. In the case of P. syringae, we and others have shown that indole 3 acetic acid (IAA) and other auxins promote bacterial virulence, either when applied exogenously (Chen et al, 2007; Navarro et al, 2006; Wang et al, 2007), or when endogenous IAA levels are elevated (Mutka, et al., 2013; Djami-Tchatchou, et. al, 2020). Furthermore, modulation of host auxin biology is an important virulence strategy for several pathogens including P. syringae (Kunkel and Johnson, 2021). We have made significant progress elucidating the mechanisms through which IAA promotes pathogenesis during infection of A. thaliana by P. syringae strain PtoDC3000 and have demonstrated that IAA promotes pathogenesis via two different mechanisms (Fig. 6). The first is through suppression of SA-mediated host defenses, a process dependent upon the canonical plant auxin signaling pathway. The second mechanism is by directly effecting the pathogen, namely by regulating bacterial gene expression. Thus, IAA acts both as a plant hormone and as a microbial signal to promote pathogenesis (Djami-Tchatchou et al., 2020, 2022).
Auxin acts as a microbial signal to regulate gene expression in PtoDC3000
IAA regulates gene expression in many plant-associated bacteria, including members of the soil microbiome, beneficial symbionts, and pathogens. These include P. putida, Variovorax paradoxus, Rhizobium etli, Azosprillum brasilense, Agrobacterium tumefaciens, Erwinia chrysanthemi, P. savastanoi, and P. syringae. For example, IAA regulates expression of genes involved in IAA catabolism, survival under stress conditions, antibiotic synthesis, and virulence (Kunkel and Johnson, 2021; Duca et al, 2014).
To investigate how IAA impacts gene expression in P. syringae strain PtoDC3000, we performed a global transcriptomic analysis of bacteria grown in culture in the presence and absence of IAA and observed large scale transcriptional reprogramming within 30 minutes of treatment (Djami-Tchatchou, et. al, 2022). Of the >700 genes impacted by IAA, many genes involved in motility and chemotaxis as well as all genes involved in T3SS were down-regulated. IAA treatment also up-regulated expression of genes encoding several known and putative transcriptional regulators, regulators of stress responses, efflux pumps and proteins involved in oxidative and osmotic stress.
We also assessed whether bacterial gene expression is affected by IAA in planta. To do this, we took advantage of an A. thaliana mutant lacking four out of the six TIR1/AFB auxin co-receptors (tir1 afb1 afb4 afb5; tir/afb4x). Although this mutant is altered for several IAA-regulated responses, it develops normally and adult plants exhibit only minor differences in size and leaf shape compared to wild-type Col-0. Of special importance to our studies is the fact that this mutant accumulates elevated levels of IAA in leaf tissue and supports higher levels of PtoDC3000 growth (Djami-Tchatchou, et. al., 2020). We observed that genes upregulated or downregulated by IAA in culture exhibited similar expression patterns in tir/afb4x plants. These findings led us to hypothesize that IAA acts as a signal for bacteria to switch from expressing virulence genes required early during infection (e.g. T3SS-related genes) to expressing genes required at later stages of pathogenesis (Fig. 7). These genes are proposed to be involved in promoting virulence and/or survival of the pathogen under stressful conditions encountered in or on plant leaves (Djami-Tchatchou, et. al., 2022).
We are currently working on elucidating the mechanisms by which PtoDC3000 senses and responds to IAA and characterizing the role of some of these IAA-regulated genes during pathogenesis.
