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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.
Highlights
Wounding of plants by hard wind, hail, heavy rain, sand storms, and frost is common in nature
We recently discovered that green fluorescent protein (GFP)expressing P. syringae can escape from wound infection sites and colonize adjacent tissues in the wild tobacco plant Nicotiana benthamiana [9], which has become an important model plant for P. syringae infections [3,10,11,12,13,14,15,16]
At 5 d after wound inoculation (5 dpi) with this strain, fluorescent colonies appeared along the vasculature at regular intervals (Figure 1A), which were absent in the untransformed control not expressing GFP (Figure 1B)
Summary
Wounding of plants by hard wind, hail, heavy rain, sand storms, and frost is common in nature. Syringae (Psy) causes brown spot on bean plants and blossom blight in pear trees [1], which are serious diseases responsible for significant yield losses in agricultural industries in the US, Africa, and Australia. Psy strains are a common and dominant component of the microflora on pear trees, blossom blight disease only occurs after frost injury, which explains the strong seasonal variation of disease outbreaks [8]. These data illustrate that P. syringae takes advantage of natural wound sites to enter host tissue and cause disease.
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