Pseudomonas syringae is a Gram-negative plant pathogen that causes a wide variety of diseases, including blights, leaf spots, and galls (Xin et al, 2018). Infection by P. syringae involves several steps, including epiphytic colonization of plant surfaces, entry into the leaf via natural openings such as stomata, and establishment of infection sites in the apoplast (Fig. 2). Successful colonization of the spaces between the leaf cells, also known as the apoplast, of susceptible plants requires that P. syringae evade or suppress basal defense responses and obtain nutrients and water before it can multiply to high levels (Xin et al, 2018). High levels of growth within the plant often results in production of disease symptoms, which can consist of localized necrotic or water-soaked lesions surrounded by chlorosis (Fig. 1, on previous page; Fig. 2E & Fig. 3 B).
Several decades of genetic analyses have identified many important virulence determinants in P. syringae, including the Type III Secretion System (T3SS) and phytotoxins (Xin et al., 2018; Melotto et al, 2013). The T3SS is central to pathogenicity, as it mediates transfer of bacterial virulence proteins (aka “effectors”) into host cells, where they modulate host defense signaling and other cellular processes to promote pathogen colonization and growth in susceptible plants (Fig. 2F). In host plants that carry resistance genes that confer recognition of one or more effector proteins or their activity in the host cell, the effector proteins trigger rapid activation of defense (Zhou et al., 2020).
Figure 2. Pseudomonas syringae pathogenesis. A) P. syringae is present on leaf surfaces, as a member of the phyllosphere microbial community. B-E) Bacteria enter leaves through stomata and attempt to colonize the apoplast, the space between plant cells. F) Pathogen entry often leads to induction of basal host defenses, mediated by recognition of Pathogen Associated Molecular Patterns (PAMPs). To successfully colonize the apoplast, P. syringae suppress basal host defenses by delivering virulence factors into the host cells via a Type III secretion system (T3SS). D) Levels of the plant hormone auxin Indole Acetic Acid (IAA) increase in infected leaf tissue 24-48 hours after infection. E) Bacteria grow to very high levels by 3-4 days post infection and being to cause plant cell death and chlorosis.
The T3SS is one of the best-described P. syringae pathogenicity factors and is clearly important early during the infection process to suppress basal defense responses. However, once the pathogen has started to multiply within plant tissue many of the resulting cells populating the apoplast are not in direct contact with host cells (Fig. 2E & Fig. 3 A), and presumably the T3SS is no longer needed in these cells. Rather, other virulence factors important during intermediate or later stages of pathogenesis are likely to be expressed. Very little is currently known about the virulence factors or processes involved during these later stages of pathogenesis. However, they are likely to be involved in important processes such as altering the physiology of host cells to render the apoplastic space suitable for supporting pathogen growth in the apoplast (e.g. genes involved in stress response, tolerance of antimicrobial compounds, etc.). Consistent with this hypothesis, recent studies report that expression of T3SS-related genes are down-regulated in plant tissue at later stages of infection (Djami-Tchatchou, et al, 2022; McAtee et al., 2018). An overall objective of research in the Kunkel Lab is to elucidate the mechanisms by which PtoDC3000 virulence is regulated during pathogenesis.
Figure 3. Pseudomonas syringae pathogenesis. A) Scanning electron micrograph of a bean leaf colonized by P. syringae; photo by J. W. Mansfield & I. R. Brown B) Bacterial speck symptoms on tomato.