Pseudomonas syringae Differentiates into Phenotypically Distinct Subpopulations During Colonization of a Plant Host

Abstract

Summary Bacterial microcolonies with heterogeneous sizes are formed during colonization of Phaseolus vulgaris by Pseudomonas syringae. Heterogeneous expression of structural and regulatory components of the P. syringae type III secretion system (T3SS), essential for colonization of the host apoplast and disease development, is likewise detected within the plant apoplast. T3SS expression is bistable in the homogeneous environment of nutrient‐limited T3SS‐inducing medium, suggesting that subpopulation formation is not a response to different environmental cues. T3SS bistability is reversible, indicating a non‐genetic origin, and the T3SSHIGH and T3SSLOW subpopulations show differences in virulence. T3SS bistability requires the transcriptional activator HrpL, the double negative regulatory loop established by HrpV and HrpG, and may be enhanced through a positive feedback loop involving HrpA, the main component of the T3SS pilus. To our knowledge, this is the first example of phenotypic heterogeneity in the expression of virulence determinants during colonization of a non‐mammalian host.

Confocal microscopy reveals in planta dynamic interactions between pathogenic, avirulent and non‐pathogenic Pseudomonas syringae strains

“…The uneven distribution of fluorescence sometimes apparent within expanding microcolonies did not appear to be associated with differential expression by individual bacteria (Fig. ), but rather to be an artefact of the three‐dimensional structure of the microcolonies in association with quenching of the fluorescent signal (Rufián et al ., ).…”

“…In addition, this type of collaboration would be expected to take place between bacterial variants arising during cell division within a clonal microcolony, regardless of whether they originate through genetic or non‐genetic means, and may therefore have a strong impact on their adaptation to the host. Indeed, bacterial variants not expressing the T3SS, generated within a genetically homogeneous population through stochastic phenotypic heterogeneity during bacterial replication within an apoplastic microcolony (Rufián et al ., ), could thus proliferate and even spread as a result of the defence suppression capabilities of nearby T3SS‐expressing bacteria.…”

“…Recently, phenotypic diversity has also been shown to arise through non‐genetic mechanisms within apoplastic populations of P. syringae (Rufián et al ., ). Work from our team has shown that P. syringae populations display heterogeneous expression of the T3SS genes within the apoplast.…”

Negative Autogenous Control of the Master Type III Secretion System Regulator HrpL inPseudomonas syringae

“…Regulatory motifs such as feedback loops can influence the population-level behaviors of infecting pathogens. In particular, bistable expression of the T3SS drives heterogeneity and division of labor in both plant and animal pathogens (20, 21), including P. syringae (22). New insights into the regulatory networks underlying the T3SS and other bacterial virulence factors promise to inform strategies to manage plant disease.…”

A Career on Both Sides of the Atlantic: Memoirs of a Molecular Plant Pathologist

“…The regulatory complexity of these systems appears to be even more enigmatic: a recent study proposes a "bistability" model for hrp/hrc-I systems involving the action of the doublenegative regulatory feedback loop involving HrpV and HrpG and a positive loop involving the main component of the pilus, HrpA (95). It appears that protein secretion through the T3SS is a highly regulated process, involving multiprotein interaction networks coupling expression regulation to secretion, making the T3SS one of the most complex bacterial protein secretion systems known (88).…”