Pseudomonas syringae pv. syringae Uses Proteasome Inhibitor Syringolin A to Colonize from Wound Infection Sites

Abstract

Infection of plants by bacterial leaf pathogens at wound sites is common in nature. Plants defend wound sites to prevent pathogen invasion, but several pathogens can overcome spatial restriction and enter leaf tissues. The molecular mechanisms used by pathogens to suppress containment at wound infection sites are poorly understood. Here, we studied Pseudomonas syringae strains causing brown spot on bean and blossom blight on pear. These strains exist as epiphytes that can cause disease upon wounding caused by hail, sand storms and frost. We demonstrate that these strains overcome spatial restriction at wound sites by producing syringolin A (SylA), a small molecule proteasome inhibitor. Consequently, SylA-producing strains are able to escape from primary infection sites and colonize adjacent tissues along the vasculature. We found that SylA diffuses from the primary infection site and suppresses acquired resistance in adjacent tissues by blocking signaling by the stress hormone salicylic acid (SA). Thus, SylA diffusion creates a zone of SA-insensitive tissue that is prepared for subsequent colonization. In addition, SylA promotes bacterial motility and suppresses immune responses at the primary infection site. These local immune responses do not affect bacterial growth and were weak compared to effector-triggered immunity. Thus, SylA facilitates colonization from wounding sites by increasing bacterial motility and suppressing SA signaling in adjacent tissues.

The Proteasome Acts as a Hub for Plant Immunity and Is Targeted by Pseudomonas Type III Effectors

“…Both effectors belong to the YopJ family, which is widespread among animal and plant pathogens but whose members are absent from Pst (Lewis et al, 2011). Also, Pst does not possess SylA, a secreted toxin produced, for instance, by P. syringae pv syringae, which directly targets the catalytic subunits of the 26S proteasome (Groll et al, 2008;Schellenberg et al, 2010;Baltrus et al, 2011;Misas-Villamil et al, 2013). Thus, the suppression of proteasome activity below the basal level by virulent Pst likely involves a previously unidentified effector.…”

“…Several T3Es from different genera of plant pathogenic bacteria, such as Pseudomonas and Xanthomonas, were shown to suppress plant defense responses by acting as E3 ligases or by promoting ubiquitination and degradation of target proteins (Nomura et al, 2006;Singer et al, 2013;Börnke, 2014, 2015;Banfield, 2015). A more direct way to subvert the UPS is achieved by SylA, a secreted small nonribosomal peptide from Pseudomonas syringae pv syringae, which binds to the catalytic subunits of the 26S proteasome to inhibit its activity and suppress plant immune reactions, including stomatal closure and SA-mediated signaling (Groll et al, 2008;Schellenberg et al, 2010;Misas-Villamil et al, 2013). The first bacterial T3Es identified that directly target the proteasome for defense suppression are XopJ from Xanthomonas campestris pv vesicatoria and HopZ4 from Pseudomonas syringae pv lachrymans.…”

Intervention of Phytohormone Pathways by Pathogen Effectors

“…Syringolin A (SylA), a virulence factor and a proteasome inhibitor produced by P. syringae pv syringae, plays a role in defense suppression as evidenced by the inability of sylA mutants to inhibit SA accumulation and associated resistance responses in bean (Table 1) (Groll et al, 2008;Misas-Villamil et al, 2013). SylA reverts PTIassociated stomatal closure that is also dependent on both SA and NPR1 (NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1), a master regulator of SA signaling (Figure 3) (Schellenberg et al, 2010).…”

Regulation of Stomatal Defense by Air Relative Humidity

“…However, for a bacterial pathogen to cause disease, it must penetrate through the plant epidermis and be able to survive and proliferate inside the plant. The mode and mechanism of penetration into the plant tissue is a critical step for infection, especially for bacterial pathogens that rely on natural openings and accidental wounds on the plant surface to colonize internal tissues (Misas-Villamil et al, 2013). Stomata are an example of such openings, providing one of the main routes through which the foliar pathogen Pseudomonas syringae transitions from avirulent epiphytic to virulent endophytic lifestyles (Melotto et al, 2008).…”