Recognition of the host plant is a prerequisite for infection by pathogenic bacteria. However, how bacterial cells sense plant-derived stimuli, especially chemicals that function in regulating plant development, remains completely unknown. Here, we have identified a membrane-bound histidine kinase of the phytopathogenic bacterium Xanthomonas campestris, PcrK, as a bacterial receptor that specifically detects the plant cytokinin 2-isopentenyladenine (2iP). 2iP binds to the extracytoplasmic region of PcrK to decrease its autokinase activity. Through a four-step phosphorelay, 2iP stimulation decreased the phosphorylation level of PcrR, the cognate response regulator of PcrK, to activate the phosphodiesterase activity of PcrR in degrading the second messenger 3',5'-cyclic diguanylic acid. 2iP perception by the PcrK-PcrR remarkably improves bacterial tolerance to oxidative stress by regulating the transcription of 56 genes, including the virulence-associated TonB-dependent receptor gene ctrA. Our results reveal an evolutionarily conserved, inter-kingdom signaling by which phytopathogenic bacteria intercept a plant hormone signal to promote adaptation to oxidative stress.

bThe Gram-negative bacterium Stenotrophomonas maltophilia lives in diverse ecological niches. As a result of its formidable capabilities of forming biofilm and its resistance to multiple antibiotic agents, the bacterium is also a nosocomial pathogen of serious threat to the health of patients whose immune systems are suppressed or compromised. Besides the histidine kinase RpfC, the two-component signal transduction system (TCS), which is the canonical regulatory machinery used by most bacterial pathogens, has never been experimentally investigated in S. maltophilia. Here, we annotated 62 putative histidine kinase genes in the S. maltophilia genome and successfully obtained 51 mutants by systematical insertional inactivation. Phenotypic characterization identified a series of mutants with deficiencies in bacterial growth, swimming motility, and biofilm development. A TCS, named here BfmA-BfmK (Smlt4209-Smlt4208), was genetically confirmed to regulate biofilm formation in S. maltophilia. Together with interacting partner prediction and chromatin immunoprecipitation screens, six candidate promoter regions bound by BfmA in vivo were identified. We demonstrated that, among them, BfmA acts as a transcription factor that binds directly to the promoter regions of bfmA-bfmK and Smlt0800 (acoT), a gene encoding an acyl coenzyme A thioesterase that is associated with biofilm development, and positively controls their transcription. Genome-scale mutational analyses of histidine kinase genes and functional dissection of BfmK-BfmA regulation in biofilm provide genetic information to support more in-depth studies on cellular signaling in S. maltophilia, in the context of developing novel approaches to fight this important bacterial pathogen. Histidine kinases (HKs) are the cellular sensors of the twocomponent signal transduction systems (TCSs) employed by most bacteria to detect and respond to environmental stimuli (1). With the exception of a few bacterial species, such as Mycoplasma spp., the bacterial cell generally encodes several to hundreds of TCSs, and the total number of TCSs possessed is a metaphor for the bacterial intelligence quotient (IQ) in terms of a bacterium's ability to adapt to various ecological niches (2). The prototypical TCS consists of a membrane-bound HK and a cytoplasmic response regulator (RR). After monitoring a specific stimulus, HK autophosphorylates itself by hydrolyzing ATP and transfers the phosphoryl group to a conserved histidine residue within the dimerization and histidine phosphotransfer (DHp) domain. Next, the phosphoryl group is transferred to a conserved aspartate residue within the N-terminal receiver domain of its cognate RR (3). Thereafter, the activated RR controls downstream gene expression, cellular behavior, or enzymatic activity, depending on the biochemical property of its C-terminal output domain. Therefore, a bacterium's ability to live in complex environments depends largely on the number, structure, and regulatory function of its HK repertoire (4). In the context of infecti...

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