Cool temperature triggers HktS-HktR signaling to promote RpoD-mediated activation of T3SS virulence in Pseudomonas syringae pv. actinidiae

Abstract

Kiwifruit bacterial canker (KBC), caused by Pseudomonas syringae pv. actinidiae (Psa), severely impacts kiwifruit production. Previous studies show that Psa exhibits enhanced virulence under cool temperatures; however, the underlying regulatory mechanisms remain unclear. Given the crucial role of histidine kinases (HKs) in bacterial responses to environmental signals, bioinformatics analysis predicted a total of 69 HKs in Psa, among which hktS was identified as the key cool-temperature-responsive gene. Structural analysis and co-transcription experiments revealed that HktS and its corresponding response regulator HktR constitute a functional two-component system (TCS). We found that mutations in the key phospho‑related residues of HktS and HktR impair cool temperature sensing and reduce the virulence of Psa. Further research demonstrated that HktR directly interacts with the transcription factor RpoD and binds to a conserved motif in the promoter region of hrpRS, thereby activating expression of the type III secretion system (T3SS). Genetic and expression analyses showed that the HktS‑HktR system displays a conserved cool-temperature-responsive expression pattern across the tested strains and is highly conserved within the genus Pseudomonas. Thus, we identified a histidine kinase essential for Psa virulence under cool temperatures and elucidated the underlying molecular mechanism. Our findings provide a critical theoretical basis and potential molecular targets for developing novel control strategies for KBC.