The Transcription Factor Si ROCX Controls a Monocot-Specific CAZyme Program for Host Cell Wall Remodeling During Symbiotic Root Colonization

Abstract Intracellular accommodation of beneficial fungi requires controlled remodeling of host cell walls while avoiding activation of plant immune responses. During colonization of monocot roots, the endophytic fungus Serendipita indica induces a suite of carbohydrate-active enzymes (CAZymes) targeting xylan and cellulose. Weighted gene co-expression network analysis identified the basidiomycete-specific transcription factor Si ROCX as a key regulator of this monocot-adapted program. DNA affinity purification sequencing (DAP-seq) defined the Si ROCX binding motif and revealed a CAZyme-enriched target regulon. Inducible Si ROCX overexpression selectively activated motif-containing CAZyme genes and markedly enhanced xylanase and cellulase activities on xylan and native barley root cell walls. Premature activation of this program triggered expression of a plant immune marker despite comparable fungal biomass, indicating that precise temporal control of CAZyme deployment is required to maintain symbiotic compatibility. Together, these findings identify Si ROCX as a central regulator of a monocot-adapted cell wall–degrading secretome and reveal a basidiomycete-specific regulatory module that coordinates host cell wall remodeling with immune-compatible symbiotic colonization. Significance Beneficial fungi colonize living plant cells, requiring host cell wall remodeling while avoiding immune activation. However, the key transcriptional regulators coordinating this process in basidiomycetes during symbiotic root colonization remain unknown. We identify Si ROCX, a conserved basidiomycete-specific transcription factor, as a master activator of a monocot-adapted xylan/cellulose degradation program in the root endophyte Serendipita indica . By integrating in planta co-expression networks, DAP-seq, secretome proteomics, and enzymatic assays, we show that Si ROCX overexpression markedly enhances secretion and activity of xylan- and cellulose-degrading enzymes, boosting sugar release from monocot cell walls. These findings reveal a basidiomycete-specific regulatory module for immune-compatible host cell wall remodeling, providing a framework to engineer fungal CAZyme programs for crop symbiosis and biomass conversion.