Xylella fastidiosaEndoglucanases Mediate the Rate of Pierce’s Disease Development inVitis viniferain a Cultivar-Dependent Manner

Abstract

Xylella fastidiosa is a gram-negative bacterium that causes Pierce’s disease (PD) in grapevine. X. fastidiosa is xylem-limited and interfaces primarily with pit membranes (PMs) that separate xylem vessels from one another and from adjacent xylem parenchyma cells. PMs are composed of both pectic and cellulosic substrates, and dissolution of PMs is facilitated by X. fastidiosa cell wall-degrading enzymes. A polygalacturonase, which hydrolyzes the pectin component of PMs, is required for both movement and pathogenicity in grapevines. Here, we demonstrate that two X. fastidiosa β-1,4-endoglucanases (EGases), EngXCA1 and EngXCA2, also play a role in how X. fastidiosa interfaces with grapevine PMs. The loss of EngXCA1 and EngXCA2 in tandem reduces both X. fastidiosa virulence and population size and slows the rate of PD symptom development and progression. Moreover, we demonstrate that single and double EGases mutants alter the rate of PD progression differently in two grapevine cultivars, Cabernet Sauvignon and Chardonnay, and that Chardonnay is significantly more susceptible to PD than Cabernet Sauvignon. Interestingly, we determined that there are quantitative differences in the amount of fucosylated xyloglucans that make up the surface of PMs in these cultivars. Fucosylated xyloglucans are targets of the X. fastidiosa EGases, and xyloglucan abundance could impact PM dissolution and affect PD symptom development. Taken together, these results indicate that X. fastidiosa EGases and the PM carbohydrate composition of different grape cultivars are important factors that influence PD symptom development and progression.

Xylella fastidiosa causes transcriptional shifts that precede tylose formation and starch depletion in xylem

“…Furthermore, asymptomatic regions of infected grapevines experienced significantly more cavitation prior to symptom development relative to healthy control vines, indicating that the cause of cavitation was probably pathogen‐induced (Perez‐Donoso et al, 2007). X. fastidiosa produces cell wall‐degrading enzymes, specifically a polygalacturonase and at least one endoglucanase, that act in concert with one another to degrade intervessel pit membranes (Ingel et al, 2019; Perez‐Donoso et al, 2010; Roper et al, 2007; Sun et al, 2011), facilitating the spread of air embolisms as pit membranes are breached. The bulk of the evidence suggests that embolism formation probably precedes vessel occlusion in PD‐infected grapevines similar to other species (e.g., McElrone et al, 2008) even though Sun et al (2007) showed that tyloses can form in grapevines in the absence of embolism.…”

“…In addition, the functional category "Cell wall organization and biogenesis" was found enriched among the up-regulated genes at both Phase I and Phase II (Figure 7b and Data set S2). Induction of this biological process suggests the production and/or expansion of the plant cell wall in response to X. fastidiosa infection, possibly to replace the cell wall material damaged by the bacterial cell wall-degrading enzymes (Ingel et al, 2019;Roper et al, 2007) or to form tyloses (De Micco et al, 2016). Interestingly, a higher number of genes belonging to the functional category "Cell wall organization and biogenesis" were up-regulated at Phase I (100 genes) compared to Phase II (47 genes; Figure S4), suggesting that the induction of the biological process occurred prior to the formation of tyloses.…”

“…Pit membranes are porous structures made of primary plant cell wall material that allow the passage of water and small solutes, regulate the cavitation threshold, and prevent the free movement of air embolisms and pathogens (Choat et al, 2008; Sperry et al, 1991; Stevenson et al, 2004; Tyree & Zimmermann, 2002). To systemically colonize the xylem, X. fastidiosa enzymatically degrades xylem pit membranes to move between vessels (Ingel et al, 2019; Perez‐Donoso et al, 2010; Roper et al, 2007; Sun et al, 2011). Movement between hosts is facilitated by xylem‐feeding hemipteran leafhopper insects, such as the glassy‐winged sharpshooter ( Homalodisca vitripennis ) (Hill & Purcell, 1995).…”

Infections of the Xylella fastidiosa subsp. pauca Strain “De Donno” in Alfalfa (Medicago sativa) Elicits an Overactive Immune Response

“…Containment of ROS production and scavenging is carried out by the Monothiol glutaredoxin-S6 (GRX S6) and a protein of the Thioredoxin family, both upregulated in infected tissues, while the need to maintain a phosphate homeostasis is shown by the downregulation of several phosphatases, as phosphorylation is a major regulating/signaling mechanism of enzyme status during immune response. Indeed, a xyloglucan-specific endo-β-1,4-glucanase inhibitor protein (XEGIP) was found to be strongly upregulated, most likely related to the attempt of the plant to inhibit Xylella -produced endo-β-1,4-glucanases, responsible for the degradation of the cell wall pit membranes that facilitate the bacterial movement and its pathogenicity [35].…”

Expansin-related proteins: biology, microbe–plant interactions and associated plant-defense responses

“…Some literature reports (especially those for nematode proteins) have erroneously used the term expansin to refer to proteins with structurally similar domains but with different architecture and phylogenetics, yet they cannot strictly be considered as such according to the rules agreed in Kende et al [9]; to avoid confusion we indicate the type of protein we refer to in each section, but maintaining the name given in the original report. A large majority of reports of experiments with expansin-related-protein mutant strains show defects in pathogenicity or reduction of colonization [4][5][6][13][14][15][16][17][18][19][20], whereas in contrast, overexpression results in greater virulence of pathogenic species [6,13,20]. Due to the absence of enzymatic activity, progress towards understanding their activity upon cell-wall polysaccharides at the molecular and biochemical level has been relatively slow.…”