Decoding the chitin signatures generated by Xylella fastidiosa

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The ability to degrade chitin is essential for the survival and colonization success of fungal and bacterial pathogens, many of which are causal agents of agronomically-important diseases. For bacterial pathogens, which lack chitin, the degradation of chitin plays a major role in opposing host-colonizing fungi and surviving in its vector, if transmitted by insects. One of the pathogens ranked in the “list of priority pests” for Europe is Xylella fastidiosa. This insect-transmitted bacterium is the cause of the OLIVE QUICK DECLINE SYNDROME (OQDS) and important for outbreaks over the past ten years. The urgency to develop an understanding how X. fastidiosa colonizes its hosts is apparent from the lack of disease control. Previously it has been reported that a proposed chitinase, chiA, is secreted by X. fastidiosa and i) confers utilization of chitin as a carbon source, potentially from its insect vectors and xylem-colonizing fungi, ii) promotes transmission by insect vectors, and iii) is required for systemic infection in plants, maybe opposing plant-colonizing fungi. Our focus is chiA, because this bacterial chitinase iv) carries an unusual loop in its modelled 3D structure that could determine substrate specificity, and v) likely releases chitin oligomers that could modulate immune responses in hosts of X. fastidiosa. Although chitinases have been studied since decades, we have little knowledge on substrate-structure-product-bioactivity relationships, which are key to understanding its role as virulence factors. Here, our goal is to answer two key questions: 1. How does X. fastidiosa degrade chitin? Profiling chiA activity and specificity using a series of defined chitin substrates and different organismal sources will reveal degradation products then subjected to bioactivity analysis. Structure-function relationships will be explored in silico, in vitro and in vivo. 2. What is the immunomodulatory potential of chitin oligomers produced by X. fastidiosa? Comparing different immune responses to chiA products in model and crop hosts, together with assessing pattern recognition receptor-mediated immunity are likely to provide insights into this question. Being an important virulence factor, our overall aim is to better understand the molecular details of chiA function and clarify its role in systemic plant infection. We envisage that this knowledge could be useful for biotechnological purposes.