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 Arabidopsis and a beneficial Pseudomonas 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.
Sporulation is a specialized developmental program employed by a diverse set of bacteria which culminates in the formation of dormant cells displaying increased resilience to stressors. This represents a major survival strategy for bacteria facing harsh environmental conditions, including nutrient limitation, heat, desiccation, and exposure to antimicrobial compounds. Through dispersal to new environments via biotic or abiotic factors, sporulation provides a means for disseminating genetic material and promotes encounters with preferable environments thus promoting environmental selection. Several types of bacterial sporulation have been characterized, each involving numerous morphological changes regulated and performed by non-homologous pathways. Despite their likely independent evolutionary origins, all known modes of sporulation are typically triggered by limited nutrients and require extensive membrane and peptidoglycan remodeling. While distinct modes of sporulation have been observed in diverse species, two major types are at the forefront of understanding the role of sporulation in human health, and microbial population dynamics and survival. Here, we outline endospore and exospore formation by members of the phyla Firmicutes and Actinobacteria, respectively. Using recent advances in molecular and structural biology, we point to the regulatory, genetic, and morphological differences unique to endo- and exospore formation, discuss shared characteristics that contribute to the enhanced environmental survival of spores and, finally, cover the evolutionary aspects of sporulation that contribute to bacterial species diversification.
Plant root-associated microbes promote plant growth and elicit induced systemic resistance (ISR) to foliar pathogens. In an attempt to find novel growth-promoting and ISR-inducing strains, we previously identified strains of root-associated Pseudomonas spp. that promote plant growth but unexpectedly elicited induced systemic susceptibility (ISS) rather than ISR to foliar pathogens. Here, we demonstrate that the ISS-inducing phenotype is common among root-associated Pseudomonas spp. Using comparative genomics, we identified a single Pseudomonas fluorescens locus that is unique to ISS strains. We generated a clean deletion of the 11-gene ISS locus and found that it is necessary for the ISS phenotype. Although the functions of the predicted genes in the locus are not apparent based on similarity to genes of known function, the ISS locus is present in diverse bacteria, and a subset of the genes were previously implicated in pathogenesis in animals. Collectively, these data show that a single bacterial locus contributes to modulation of systemic plant immunity. IMPORTANCE Microbiome-associated bacteria can have diverse effects on health of their hosts, yet the genetic and molecular bases of these effects have largely remained elusive. This work demonstrates that a novel bacterial locus can modulate systemic plant immunity. Additionally, this work demonstrates that growth-promoting strains may have unanticipated consequences for plant immunity, and this is critical to consider when the plant microbiome is being engineered for agronomic improvement.
17Pseudomonas fluorescens and related plant root-("rhizosphere") associated species contribute to 18 plant health by modulating defenses and facilitating nutrient uptake. To identify bacterial fitness 19 determinants in the rhizosphere of the model plant Arabidopsis thaliana, we performed a Tn-Seq 20 screen using the biocontrol and growth-promoting strain Pseudomonas sp. WCS365. The screen, 21 which was performed in parallel on wild-type and an immunocompromised Arabidopsis, 22 identified 231 genes that positively affect fitness in the rhizosphere of wild-type plants. A subset 23 of these genes negatively affect fitness in the rhizosphere of immunocompromised plants. We 24 postulated that these genes might be involved in avoiding plant defenses and verified 7 25 Pseudomonas sp. WCS365 candidate genes by generating clean deletions. We found that two of 26 these deletion strains, ∆morA (encodes a putative diguanylate cyclase/phosphodiesterase) and 27 ∆spuC (encodes a putrescine aminotransferase) formed enhanced biofilms and inhibited plant 28 growth. Inhibition of plant growth by ∆spuC and ∆morA was the result of pattern triggered 29 immunity (PTI) as measured by induction of an Arabidopsis PTI reporter and FLS2/BAK1-30 dependent inhibition of plant growth. We found that MorA acts as a phosphodiesterase to inhibit 31 biofilm formation suggesting a possible role in biofilm dispersal. We found that both putrescine 32 and its precursor arginine promote biofilm formation that is enhanced in the ∆spuC mutant, 33 which cannot break down putrescine suggesting that putrescine might serve as a signaling 34 molecule in the rhizosphere. Collectively, this work identified novel bacterial factors required to 35 evade plant defenses in the rhizosphere. 36 37 38 39 3 Importance 40While rhizosphere bacteria hold the potential to improve plant health and fitness, little is known 41 about the bacterial genes required to evade host immunity. Using a model system consisting of 42Arabidopsis and a beneficial Pseudomonas sp. isolate, we identified bacterial genes required for 43 both rhizosphere fitness and for evading host immune responses. This work advances our 44 understanding of how evasion of host defenses contributes to survival in the rhizosphere. 45 Introduction 46Plant root-associated commensal microbes confer fitness advantages to plant hosts 47 including growth promotion, nutrient uptake, and resistance to pathogens (1). In order to benefit 48 its plant host, a root-associated microbe must survive in the rhizosphere, compete for plant 49 nutrients, and avoid plant defenses. Despite the importance of rhizosphere competence for 50 microbes to confer important benefits to plants, the mechanisms regulating rhizosphere fitness 51 and evasion of host defenses are poorly understood. 52Symbiotic bacteria must cope with a host immune system, which can robustly recognize 53 microbe-associated molecular patterns (MAMPs) such as flagellin, lipopolysaccharide and 54 chitin, and trigger a defense response. Model plant-associated Pseud...
In Fig. 3B, the labels for the 3rd and 4th data sets previously labeled ΔspeE2ϩpSpeE2 and Δclusterϩvector, respectively, were swapped. The correctly labeled figure appears below. There are no corresponding changes to the text or figure legend.
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