Xylella fastidiosa requires the Type II secretion system for pathogenicity and survival in grapevine

Authors: INGEL, BRIAN - University Of California, Riverside; CASTRO, CLAUDIA - University Of California, Riverside; Burbank, Lindsey; DE ANDA, NORMA - University Of California, Riverside; WAY, HANNAH - University Of California, Riverside; WANG, PENG - University Of California, Riverside; ROPER, CAROLINE - University Of California, Riverside

Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/10/2023
Publication Date: 10/25/2023
Citation: Ingel, B., Castro, C., Burbank, L.P., De Anda, N., Way, H., Wang, P., Roper, C. 2023. Xylella fastidiosa requires the Type II secretion system for pathogenicity and survival in grapevine. Molecular Plant-Microbe Interactions. 36(10):636-646. https://doi.org/10.1094/MPMI-03-23-0027-R.
DOI: https://doi.org/10.1094/MPMI-03-23-0027-R

Interpretive Summary: Xylella fastidiosa is a bacterial pathogen that causes Pierce’s Disease (PD) of grapevine. In host plants, this bacterium exclusively colonizes the plant water-conducting tissue (xylem) which is primarily constructed of non-living cells in mature plants. Understanding how X. fastidiosa interacts with the plant tissue in this context is important for further understanding the disease process. Unlike many bacterial plant pathogens, X. fastidiosa lacks secreted proteins known as Type III effectors that aid in invasion and damage to plant tissues. Instead, X. fastidiosa uses plant cell wall degrading enzymes and lipase enzymes as part of its infection strategy. Several of these enzymes are predicted to be secreted out of the bacterial cell via a structure known as a Type II secretion system (T2SS). In this study, bacterial mutants were constructed in two genes involved in the T2SS, xpsE and xpsG. Both mutants were non-pathogenic and unable to effectively colonize Vitis vinifera grapevines demonstrating that the T2SS is required for X. fastidiosa infection processes. Furthermore, mass spectrometry was used to identify novel Type II-dependent proteins secreted by X. fastidiosa. In vitro, six Type II-dependent proteins were identified that included three lipases, a ß-1,4-cellobiohydrolase, a protease, and a protein of unknown function.

Technical Abstract: Xylella fastidiosa is a xylem-limited bacterial pathogen that causes Pierce’s Disease (PD) of grapevine. In host plants, this bacterium exclusively colonizes the xylem, a tissue that is primarily non-living at maturity. Understanding how X. fastidiosa interfaces with this specialized conductive tissue is at the forefront of investigation for this pathosystem. Unlike many bacterial plant pathogens, X. fastidiosa lacks a Type III secretion system and cognate effectors that aid in host colonization. Instead, X. fastidiosa utilizes plant cell wall hydrolytic enzymes and lipases as part of its xylem colonization strategy. Several of these virulence factors are predicted to be secreted via the Type II secretion system (T2SS), the main terminal branch of the Sec-dependent general secretory pathway. In this study, we constructed null mutants in xpsE and xpsG, that encode for the ATPase that drives the T2SS and the major structural pseudopilin of the T2SS, respectively. Both mutants were non-pathogenic and unable to effectively colonize Vitis vinifera grapevines demonstrating that the T2SS is required for X. fastidiosa infection processes. Furthermore, mass spectrometry was utilized to identify Type II-dependent proteins in the X. fastidiosa secretome. In vitro, we identified six Type II-dependent proteins in the secretome that included three lipases, a ß-1,4-cellobiohydrolase, a protease, and a conserved hypothetical protein.