Plants have developed survival strategies, including the strengthening of plant cell walls, to protect themselves from continuous abiotic and biotic stresses. As the first battlefield of the plant-pathogen interaction, plant cell walls, which are mainly composed of various polysaccharides such as cellulose and hemicelluloses, provide tensile strength to plant cells and protect them from biotic invasion [1, 2].
During infection, plant pathogens secrete numerous cell-wall degradation enzymes (CWDE) such as cellulases (e.g., endo-β-1,4-glucanase), pectinases (e.g., pectin lyase, polygalacturonase), and endo-β-1,4-xylanase to degrade the cell wall and allow the pathogen to enter in the cell. The majority of these CWDEs belong to the glycosyl hydrolase (GH) family, based on similarities in amino acid sequence, catalytic domains, protein folds, and overall architecture [3, 4]. Recently, CWDEs have attracted interest because of their utility in biotechnological processes, enhancing processes such as bread making  and animal feed production , as well as their role as pathogenicity factors in plant pathogenic microbes . The microbial GH endoxylanase is an important enzyme in the hydrolysis of xylans, catalyzing the hydrolysis of β-1,4-glycosidic linkages between the xylofuranosyl units in the xylan main chain in both cereals and hardwoods . Comprehensive functional studies based on structural, biochemical, and molecular properties of these enzymes have been reported [9, 10].
At the same time, plants secrete a group of proteinaceous xylanase inhibitors to suppress pathogenic xylanases. These proteins are thought as “defense molecules” that protect plant cells from attack by pathogenic hydrolytic enzymes . Two types of plant xylanase inhibitors have been well studied, i.e., xylanase-inhibiting protein (XIP)-type inhibitors  and Triticum aestivum xylanase inhibitor (TAXI)-type inhibitors [13, 14]. Biochemical analysis revealed that a wheat XIP specifically inhibits the expression of family-10 and −11 xylanases from Aspergillus nidulans and Aspergillus niger, respectively , while a TAXI inhibits the expression of the family-11 xylanase (but not the family-10 xylanase) of A. niger and Bacillus subtilis[13, 16].
Recently, three rice XIPs, including OsXIP, rice XIP, and a putative rice xylanase inhibitor (RIXI), were found to be differentially expressed during various developmental stages and under stress conditions [17–19]. Among these, OsXIP was predicted to be a class III chitinase, based on bioinformatics analysis; however, no chitinase activity was detected for this protein. Instead, the protein showed dosage-dependent xylanase inhibitor activity [17, 20].
In this study, we functionally characterized a chitinase-like protein from rice (OsCLP) that is expressed at both the transcriptional and translational levels during fungal pathogen infection. The recombinant OsCLP protein has strong chitinase activity and can dissolve cell walls, leading to the release of cytosolic contents. This is the first report on TAXI-like molecule with chitinase activity. This work provides new insights into the function of OsCLP in plant defense mechanisms.