The invisibility cloak: Chitin binding protein ofVerticillium nonalfalfaedisguises fungus from plant chitinases

Abstract

During fungal infections, plant cells secrete chitinases that digest chitin in the fungal cell walls. The recognition of released chitin oligomers via lysin motif (LysM)-containing immune receptors results in the activation of defence signalling pathways. We report here that Verticillium nonalfalfae, a hemibiotrophic xylem-invading fungus, prevents this recognition process by secreting a CBM18 (carbohydrate binding motif 18)-chitin binding protein, VnaChtBP, which is transcriptionally activated specifically during the parasitic life stages. VnaChtBP is encoded by the Vna8.213 gene which is highly conserved within the species, suggesting high evolutionary stability and importance for the fungal lifestyle. In a pathogenicity assay, however, Vna8.213 knockout mutants exhibit wilting symptoms similar to the wild type fungus, suggesting that Vna8.213 activity is functionally redundant during fungal infection of hop. In binding assay, recombinant VnaChtBP binds chitin and chitin oligomers in vitro with submicromolar affinity and protects fungal hyphae from degradation by plant chitinases. Using a yeast-two-hybrid assay, homology modelling and molecular docking, we demonstrated that VnaChtBP forms dimers in the absence of ligands and that this interaction is stabilized by the binding of chitin hexamers with a similar preference in the two binding sites. Our data suggest that, in addition to chitin binding LysM (CBM50) and Avr4 (CBM14) fungal effectors, structurally unrelated CBM18 effectors have convergently evolved to prevent hydrolysis of the fungal cell wall against plant chitinases.

Looking for a Cultured Surrogate for Effectome Studies of the Clubroot Pathogen

“…In a recent study, our group identified two effectors, PbChiB2 and PbChiB4, able to bind to chitin oligomers in vitro, and suppress chitin-triggered immunity (Muirhead and Pérez-López, 2020). These effectors were able to bind chitin through a CBM18 domain as previously described for the soil-borne pathogen Verticillium dahliae (Volk et al, 2020). Like P. brassicae, V. dahliae can affect Arabidopsis, with an hemibiotrophic lifestyle.…”

Comparative Genomic Data Provide New Insight on the Evolution of Pathogenicity in Sporothrix Species

“…CBM48, similar to CBM20, is mostly involved in starch biosynthesis (Wilkens et al, 2018). CBM50, also known as LysM domains, plays an important role in inhibiting host defenses (Volk et al, 2019). Peptidases provide alternative carbon sources for glucose starving cells through degrading proteins (Handtke et al, 2018), and promote various progresses, such as cell growth and differentiation, apoptosis, cell cycle, and signaling (Ng et al, 2009).…”

The right microbe-associated molecular patterns for effective recognition by plants

“…Tomato leaf mold fungus produced Avr4 binds specifically to chitin in the fungal cell walls to protect it from plant chitinases (van den Burg et al, 2006). Similarly, xylem-invading fungus Verticillium nonalfalfae prevents chitin from hydrolysis by secreting VnaChtBP that binds chitin and suppresses chitin-induced immunity (Volk et al, 2019). Interestingly, the fungal vascular wilt pathogen V. dahliae strain VdLs17 could secret a lineagespecific LysM effector, Vd2LysM, which mimics the host PRR to bind chitin and suppresses chitin-induced immune responses (Akcapinar et al, 2015).…”