Virulence Regulation and Lifestyle Transitions: The Role of c-di-GMP and Two-Component Systems in Erwinia amylovora and Their Evolutionary Context Within Enterobacterales.

Erwinia amylovora, the causative agent of fire blight, poses a threat to pome fruit production worldwide. Disease initiation involves a transition from an epiphytic to an endophytic growth stage, with early activation of the type III secretion system (T3SS) on floral stigmas enabling host tissue invasion. Among the diverse arsenals of E. amylovora, the T3SS and its effectors (T3Es), along with the exopolysaccharide amylovoran, represent core pathogenicity determinants. Systemic colonisation and persistence within the host are orchestrated by a complex regulatory network that coordinates transitions between motile planktonic and sessile biofilm-associated states. Central to this regulation are four conserved two-component systems (HrpX/HrpY, RcsC/RcsB, GrrS/GrrA and EnvZ/OmpR) and the second messenger cyclic-di-GMP (c-di-GMP). This review synthesises current knowledge across four interrelated themes: (i) the disease cycle and host colonisation strategies of E. amylovora and host defence response, (ii) pathogenicity and virulence factors of E. amylovora and their regulatory mechanisms, (iii) regulatory circuits modulating lifestyle transitions during systemic colonisation by the pathogen focusing on the role of c-di-GMP and two-component systems and (iv) evolutionary perspectives on the two-component systems and c-di-GMP along with comparisons to related enterobacterial pathogens. These insights advance our understanding of the molecular basis of fire blight pathogenesis and adaptative strategies employed by E. amylovora to thrive in diverse host environments.