Virulence-related regulatory network of Pseudomonas syringae

Abstract

Transcription factors (TFs) play important roles in regulating multiple biological processes by binding to promoter regions and regulating the global gene transcription levels. Pseudomonas syringae is a Gram-negative phytopathogenic bacterium harbouring 301 putative TFs in its genome, approximately 50 of which are responsible for virulence-related gene and pathway regulation. Over the past decades, RNA sequencing, chromatin immunoprecipitation sequencing, high-throughput systematic evolution of ligands by exponential enrichment, and other technologies have been applied to identify the functions of master regulators and their interactions in virulence-related pathways. This review summarises the recent advances in the regulatory networks of TFs involved in the type III secretion system (T3SS) and non-T3SS virulence-associated pathways, including motility, biofilm formation, quorum sensing, nucleotide-based secondary messengers, phytotoxins, siderophore production, and oxidative stress. Moreover, this review discusses the future perspectives in terms of TF-mediated pathogenesis mechanisms and provides novel insights that will help combat P. syringae infections based on the regulatory networks of TFs.

Keywords: Pseudomonas syringae; Regulatory networks; Transcription factors; Virulence.

Fig. 1

The T3SS regulatory network in MM (inducingconditions) in P. syringae. T3SS is primarily activated by the HrpRSL pathway, which is under the control of the top TCS RhpRS under inducing conditions (MM or in planta). More than 20 regulators collectively control this regulatory network (CorRS, CvsRS, GacAS, RhpRS, PilRS, HrpRS, HrpL, HrpA, HrpT, AefR, PsrA, RhpP, RpoN, HrpV, HrpG, HrpF, HrpJ, Lon, and PSPPH_3618). In MM, HrpRS binds the hrpL promoter region to induce its expression, and HrpL directly activates the hrp box and the transcription of T3SS-related genes. GacA indirectly inhibits hrpRS transcription, whereas CvsR, HrpA, and AlgU directly induce the transcription of hrpRS. HrpT, PilR, and HrpL repress hrpL expression, whereas AefR and PsrA activate its expression in MM. PilR also suppresses the expression of T3SS effectors, including hrpW1, hopX1, hopG1, and hopF3. Lon inhibits the expression of hopR1, hrpB, and hopAJ1. PSPPH_3618 positively regulates the transcription of avrB2 and rhpP. RhpP directly degrades the HrpL protein. In planta, RhpS senses polyphenols through the functions of the Pro40 residue and inhibits T3SS by reducing the phosphate activity.

Fig. 2

The T3SS regulatory network in KB (inhibitory conditions) in P. syringae. T3SS is inhibited in nutrient-rich media such as KB. RhpS is a histidine kinase. P-RhpR represses the transcription of hrpRS by binding the inverted repeats in the upstream regions. Other regulators, including CbrB2, EnvZ/OmpR, HrpV, and HrpT, also control this pathway under inhibitory conditions. CbrB2 and OmpR suppress the expression of hrpL, hrpK1, hrpZ1, and hrpA2. CbrB2 directly restrains hrpRS transcription. Lon acts as a negative regulator of T3SS by degrading HrpRS.

Fig. 3

The motility regulatory network of P. syringae. Motility in P. syringae majorly occurs via flagella-dependent liquid, surface swimming, and T4P-mediated swarming, allowing the bacterial cells to enter apoplasts. Motility is mainly regulated by the master regulator FleQ. It directly induces the flagellar-related expression of genes (including flgBCDE, flgFGHJKKL, fliH, fliE, fliJ, fliG, fleSR, fleN, fliA, and flhAF). FleQ is induced by CvsR, OmpR, and GacA; but repressed by PilR, CbrB2, RphR, and AlgU. P-RhpR regulates motility by directly inhibiting the transcription of fimA and flhA. AlgU activates the expression of amrZ and pilR and induces the expression of fleQ_as, which directly enhances flagellar motility. PilR regulates motility by inducing pilBCD and inhibiting algU and mucAB. FleR and PSPPH_2720 regulate the transcription of fliH, fliE, fliJ, and fliG.

Fig. 4

Biofilm formation regulatory networks in P. syringae. The biofilm matrix of P. syringae comprises EPS, extracellular DNA, protein, and lipid. EPS primarily consists of alginates produced by alg-related genes, including algD, alg8, alg44, algKEGXL, algIJF, and algA, which are mainly regulated by the sigma factor AlgU. Regulators including MgrA, AmrZ, AlgR, GacA, RpoN, and HrpL also induce alginate synthesis genes (algU, algD, algC, argB, and algR3). Regulators including PilR, OmpR, CvsR, RhpR, and PhoP negatively regulate alginate synthesis, thereby inhibiting the expression of algU and algD in P. syringae. GacA positively regulates EPS production by inducing salA expression.

Fig. 5

Other virulence-related regulatory networks in P. syringae. Other virulence-related pathways, including nucleotide-based secondary messengers (cAMP, (p)ppGpp, and c-di-GMP), QS, phytotoxins, siderophore production, and ROS resistance, play roles in the pathogenesis of P. syringae. Vfr induces cAMP production and suppresses the expression of multidrug resistance genes such as mexAB and oprM. c-di-GMP is regulated by MetR, PSPPH_1800, and PSPPH_2693. c-di-GMP regulates the T3SS, siderophore production, biofilm formation, and ROS resistance by affecting the expression of hrpL, pvdE, pvdP, pvsA, alg8, alg44, sodA, and sodB. AhlR controls the QS system in P. syringae by binding to the promoter region of ahlI with the assistance of GacA and AefR. Six regulators, including CorRS, CorP, SyrP, KdpE, PSPPH_5204, and PSPPH_2800, regulate phytotoxin production via the function of the genes syrB1B2, syrC, syrE, PSPPH_A0071, and PSPPH_A0005. Siderophore production-related genes involved in PSPPH_5184/5185, PSPPH_1923/1924/1925, and fecBCDE are controlled by nine regulators (GltR, CbrB2, Fur, GasAS, SalA, PilR, PSPPH_3800, PSPPH_4828, and PSPPH_3577). The TFs OxyR, PSPPH_1960, LexA1, and PSPPH_3004 regulate ROS resistance by targeting katE, katB, katG, sodA, and sodB.