PhcX Is a LqsR-family response regulator that contributes to
Ralstonia solanacearum
virulence and regulates multiple virulence factors
Authors: Liu Qingmei , Li Chuhao , Zhang Xiaohan , Ding Mengfan , Liao Xinyue , Yan Jinli , Hu Ming , Yang Leilei , Wang Xiaoqing , Liao Lisheng , Li Peng , Zhou Xiaofan 
Published: October 2023

As one of the most notorious and successful phytopathogenic bacteria,
Ralstonia solanacearum
controls the transition between long-term survival and pathogenic modes through an intricate regulatory network, the understanding of which remains incomplete despite years of effort. In this study, we identified PhcX, a previously uncharacterized response regulator in
R. solanacearum
, and uncovered its essential functions in modulating virulence and metabolism. The
phcX
deletion mutant exhibited substantial phenotypic alterations, including slower initial growth, altered response to host extract, reduced motilities, polygalacturonase activity, and exopolysaccharide production, elevated biofilm formation, delayed hypersensitive response, and impaired virulence. Moreover, ~16% of all genes were differentially expressed in the mutant, among which the genes associated with virulence, nitrogen metabolism, and regulation were overrepresented (e.g., most T3SS/T3Es genes). Many of these traits and genes were regulated by PhcX and the global virulence regulator PhcA, but 81.4% of the genes were regulated in opposite directions. The functions of PhcX were largely conserved in
R. solanacearum
EP1 and GMI1000 strains. Apparent orthologs of PhcX are widely distributed in Proteobacteria, including the LqsR quorum sensing (QS) response regulator in
Legionella pneumophilia
. Notably, our data suggest that
phcX
was originally part of the Lqs QS system but was decoupled from Lqs in
Ralstonia
/
Cupriavidus
, physically linked to the
phc
QS genes, and connected with the virulence regulatory network in
Ralstonia
during its evolution. The findings of this study contribute to a better understanding of the virulence and metabolism regulation mechanisms in
R. solanacearum
and shed light on the evolution of its complex regulatory network.

IMPORTANCE

The bacterial wilt caused by the soil-borne phytopathogen
Ralstonia solanacearum
is one of the most destructive crop diseases. To achieve a successful infection,
R. solanacearum
has evolved an intricate regulatory network to orchestrate the expression of an arsenal of virulence factors and fine-tune the allocation of energy. However, despite the wealth of knowledge gained in the past decades, many players and connections are still missing from the network. The importance of our study lies in the identification of PhcX, a novel conserved global regulator with critical roles in modulating the virulence and metabolism of
R. solanacearum
. PhcX affects many well-characterized regulators and exhibits contrasting modes of regulation from the central regulator PhcA on a variety of virulence-associated traits and genes. Our findings add a valuable piece to the puzzle of how the pathogen regulates its proliferation and infection, which is critical for understanding its pathogenesis and developing disease control strategies.