Figure 1.

Secreted glycoside hydrolase (GH) proteins from plant-associated fungi and oomycetes play diverse roles in promoting plant colonization and/or activating plant immune responses. These roles include: (1) the modification of surface-associated carbohydrates present in their own cell walls to enable the remodeling of hyphal surfaces or infection structures produced during plant colonization; (2) the induction of plant immune responses, such as plant cell death, following their recognition as microbe-associated molecular patterns (MAMPs; 2A), or the recognition of cell wall carbohydrate fragments (e.g., chitin or β-glucan oligomers) released from their own cell walls as a consequence of their activity [e.g. in (1); 2B], by pattern recognition receptors (PRRs) at the plant cell surface to provide plant resistance or susceptibility. Regarding the latter, plant cell death could, for example, result in a release of nutrients to support the growth of fungal or oomycete pathogens with a necrotrophic lifestyle, or drive a switch from biotrophy to necrotrophy for fungal or oomycete pathogens with a hemibiotrophic lifestyle; (3) nutrient acquisition through the release of carbohydrate fragments from plant cell walls or the breakdown of entire plant cells; (4) the induction of plant immune responses, such as cell death, following the recognition of plant cell wall carbohydrate fragments, generated as a consequence of their activity [e.g., in (3)] by PRRs at the plant cell surface to provide plant resistance or susceptibility. Again, plant cell death could support necrotrophy (as in 2B); (5) the sequestration (5A), modification (5B) or degradation (5C) of MAMPs or DAMPs to prevent their recognition by PRRs at the plant cell surface to prevent activation of plant immune responses; (6) acting as a decoy to bind host-produced proteins that would otherwise inhibit GH proteins produced by plant-associated fungi or oomycetes; (7) the detoxification of antimicrobial compounds produced by plants or microbial competitors (e.g., through the removal of a sugar group from enzymatic or non-enzymatic proteins or secondary metabolites); (8) functions that promote host colonization upon uptake into plant cells (currently uncharacterized); (9) manipulation of the host microbiome through, for example, breaking down the cell walls of microbial competitors. Figure created with BioRender.com.