Four cell wall-degrading enzymes of Xanthomonas campestris pv. campestris determine bacterial escape from hydathodes to the leaf vasculature

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Abstract
To colonize plants, pathogenic bacteria modulate the biology of the host employing different bacterial secretion systems. For example, the type II secretion system (T2SS) releases toxins, proteases, lipases and carbohydrate-degrading enzymes into the extracellular environment to promote tissue softening and soft rot. In this way, the T2SS promotes virulence of phytopathogenic Gram-negative bacteria. However, the role of the T2SS and its substrates for vascular disease remains enigmatic. Here, we show that the Xps-T2SS allows Xanthomonas campestris pv. campestris (Xcc) to breach the tissue barrier between hydathodes– the initial bacterial entry point – and xylem thereby gaining access to the leaf vasculature. Yet, Xps-T2SS was dispensable for bacterial multiplication in the leaf apoplast or inside the hydathode cavity suggesting a role beyond plant defense suppression or nutrient acquisition. Using comparative genomics, four plant cell wall-degrading enzymes (CWDEs) were found to be associated with vascular pathogenesis. Testing gene knockout combinations of those enzymes revealed that virulence of only the quadruple CWDE mutant was down to the level of the xps -T2SS mutant. Our results thus demonstrate that the Xps-T2SS and a set of CWDEs that is likely secreted by this system allow Xcc to break this tissue barrier enabling long-distance mobility of Xcc inside the host plant. We thus expand our understanding on how certain bacterial pathogens have specialized towards vascular pathogens.

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