Glucuronoyl Esterase of Pathogenic Phanerochaete carnosa Induces Immune Responses in Aspen Independently of Its Enzymatic Activity

Microbial enzymes expressed in plants add new functionalities but occasionally trigger undesirable immune responses. Phanerochaete carnosa glucuronoyl esterase ( Pc GCE) hydrolyses the bond between lignin and 4‐ O ‐methyl‐α‐D‐glucuronic acid substituent of glucuronoxylan. Pc GCE constitutively expressed in Arabidopsis or hybrid aspen ( Populus tremula × tremuloides ) improved saccharification but also induced premature leaf senescence. To understand what triggered this senescence, we characterised Pc GCE‐expressing hybrid aspen by microscopy and omics approaches, supplemented by grafting and recombinant protein application experiments. Pc GCE induced massive immune responses followed by senescence in the leaves. Expressing an inactive ( Pc GCE S217A ) enzyme has led to similar phenotypes, excluding a possibility that damage‐associated molecular patterns (DAMPs) released by glucuronoyl esterase triggered immune responses. Grafting experiments showed that Pc GCE transcripts are not mobile but they induce systemic responses. Recombinant Pc GCE protein applied to leaves did not induce such responses; thus, Pc GCE is probably not perceived as a pathogen‐associated molecular pattern (PAMP). We suggest that the observed high expression of PcGCE from the 35S promoter triggers the unfolded protein response. Indeed, restricting PcGCE expression to short‐lived xylem cells by using the wood‐specific promoter avoided all detrimental effects. Thus, wood‐specific expression is a viable strategy for PcGCE deployment in planta , which might be applicable for other stress‐inducing proteins.