N-glycosylation of a chitin binding effector allows a fungal pathogen to evade the plant immune response.

Can you tell where your child has been by the soccer balls, rubber band bracelets, unmatched socks, and other stuff scattered around? Much like children, some pathogens leave telltale traces of their presence, including proteins and carbohydrates, such as fragments from the fungal cell wall. Indeed, the plant immune system, like an exasperated parent, might ask, “Who left all these chitin fragments lying around?” In rice (Oryza sativa), chitin binds the glycoprotein CEBiP (chitin elicitor binding protein), which functions in cooperation with CERK (chitin elicitor receptor kinase 1), to activate defense signaling (reviewed in Liu et al., 2013). However, rather than scolding, the plant immune system dishes out some serious punishment, in the form of a blast of reactive oxygen species and activation of defense gene expression. Don’t try this at home. To avoid detection, pathogens have evolved strategies to clean up the mess, thereby preventing CEBiP and CERK from activating defense responses. For example, in the rice blast fungal pathogenMagnaporthe oryzae, the effector Slp1 (Secreted LysM Protein1) binds to chitin, competing with the plant CEBiP (Mentlak et al., 2012). In a screen for rice blast mutants defective in infection, Chen et al. (1360–1376) identified mutants in ALG3 (Asn-linked glycosylation gene 3). Fungal strains carrying the Dalg3 mutation do not get far in infecting rice plants before they get in big trouble. The Dalg3 mutants can form an appressorium and penetrate the plant cuticle, but, unlike wild-type strains, Dalg3 mutants trigger a massive release of reactive oxygen species, which halts infection. This phenotype resembles that of fungal strains lacking the chitin binding effector Slp1 (see figure). The authors next address the connection between Alg3 and Slp1. They find that ALG3 encodes an a-1,3-mannosyltransferase and show that Dalg3 mutants produce proteins with reduced N-glycosylation, having Man5-oligosaccharides instead of Man9oligosaccharides. Indeed, Slp1 contains three glycosylation sites and the protein stability and chitin binding ability of Slp1 require glycosylation of all three sites by Alg3. Intriguingly, the Dalg3 mutants also show reduced glycosylation of another effector, indicating a potentially broad role for N-glycosylation in effector function, consistent with prior work that identified other glycosylated effectors. However, further work will be required to reveal the functional significance of glycosylation in effector function and whether Alg3 plays a general or specific role in effector glycosylation. Also, any applications of these findings to human children remain for future research, so be careful and don’t trip over that soccer ball.