Type III Secretion and Effectors Shape the Survival and Growth Pattern of Pseudomonas syringae on Leaf Surfaces    

Abstract

The bacterium Pseudomonas syringae pv syringae B728a (PsyB728a) uses a type III secretion system (T3SS) to inject effector proteins into plant cells, a process that modulates the susceptibility of different plants to infection. Analysis of GREEN FLUORESCENT PROTEIN-expressing PsyB728a after spray inoculation without additives under moderate relative humidity conditions permitted (1) a detailed analysis of this strain’s survival and growth pattern on host (Nicotiana benthamiana) and nonhost (tomato [Solanum lycopersicum]) leaf surfaces, (2) an assessment of the role of plant defenses in affecting PsyB728a leaf surface (epiphytic) growth, and (3) the contribution of the T3SS and specific effectors to PsyB728a epiphytic survival and growth. On host leaf surfaces, PsyB728a cells initially persist without growing, and show an increased population only after 48 h, unless plants are pretreated with the defense-inducing chemical benzothiazole. During the persistence period, some PsyB728a cells induce a T3SS reporter, whereas a T3SS-deficient mutant shows reduced survival. By 72 h, rare invasion by PsyB728a to the mesophyll region of host leaves occurs, but endophytic and epiphytic bacterial growths are not correlated. The effectors HopZ3 and HopAA1 delay the onset of epiphytic growth of PsyB728a on N. benthamiana, whereas they promote epiphytic survival/growth on tomato. These effectors localize to distinct sites in plant cells and likely have different mechanisms of action. HopZ3 may enzymatically modify host targets, as it requires residues important for the catalytic activity of other proteins in its family of proteases. Thus, the T3SS, HopAA1, HopZ3, and plant defenses strongly influence epiphytic survival and/or growth of PsyB728a.

Direct and Indirect Visualization of Bacterial Effector Delivery into Diverse Plant Cell Types during Infection

“…These data indicate that pavement cells are sites for effector delivery by P. syringae. These findings are supported by previously published work, which demonstrated that transcription of avrPto in P. syringae was highest on the leaf surface when bacteria were in contact with pavement cells (Lee et al, 2012).…”

“…Previous studies using P. syringae pv syringae B728a detected bacterial effector expression at 24 and 48 hpi on the leaf surface, and bacterial strains with mutations in the TTSS exhibited reduced epiphytic growth (Lee et al, 2012). In a natural infection, P. syringae initially colonizes the leaf surface, with aggregates forming at cell-cell junctions between pavement cells (Monier and Lindow, 2003b; Lee et al, 2012). We observed effector delivery at the pavement cell junctions ( Figure 4B), supporting cell junctions as important environmental niches for P. syringae colonization and initiation points for effector dissemination.…”

Phosphorylation of the Plant Immune Regulator RPM1-INTERACTING PROTEIN4 Enhances Plant Plasma Membrane H+-ATPase Activity and Inhibits Flagellin-Triggered Immune Responses in Arabidopsis

“…Syringolin A, a nonribosomal peptide and polyketide synthase present in some P. syringae pv syringae strains, is able to counteract stomatal immunity (Schellenberg et al, 2010). Furthermore, some P. syringe effectors enhance growth on the surface of plant leaves (Lee et al, 2012). The P. syringae effector HopM1 can suppress stomatal immunity (Lozano-Durán et al, 2014).…”

Quantitative Resistance: More Than Just Perception of a Pathogen

“…Extending these studies to other detection events has been facilitated by the advent of modern genomics approaches where genomics surveys have identified suites of bacterial virulence effectors independent of a specific virulence function (Lee et al, 2012). These effector surveys have shown that pathogens send a large suite of signals into the plant that if each was sensed and responded to by an independent suite of polymorphic genes could lead to a highly polygenic response system.…”