Figure 3.

A diagram of the two-component signaling and secretion systems during interaction of Lonsdalea populi and Populus × euramericana. The bacterium L. populi N-5-1 mainly perceives external signals via two-component systems (TCS) comprising histine kinases (HKs) and response regulators (RRs). This leads to activation of TCS signaling that modulates expression of genes associated with virulence, motility, proliferation, stress response, etc. A well-characterized TCS of L. populi N-5-1, KdpD-KdpE, has been demonstrated to control virulence and stress response (Yang et al., 2018). Moreover, L. populi N-5-1 contains a 23 kb gene cluster encoding a type III secretion system (T3SS), which is deployed along with other secretion systems (SS) to deliver effectors and plant cell wall-degrading enzymes (CWDEs), which may activate poplar plant immunity. In response, the host may release a reactive oxygen species (ROS) burst and other defense pathways, or some effectors in susceptible hosts may promote bacterial infection and cause canker disease. Exopolysaccharides (EPS) also facilitate bacterial virulence. On the other hand, plant pattern recognition receptor (PRR) complexes recognize pathogen-associated molecular patterns (PAMPs), which result in activation of plant innate immunity. In this view, an undetermined number of transmembrane domains of HK is shown by dashed lines. Dashed arrows and lines indicate potential regulation. Question marks indicate predicted characteristics that are yet to be substantiated.