MgCBP1 Synergistically Enhances MgENG2-Mediated Cellulose Degradation to Promote Parasitism in Meloidogyne graminicola.

Plant-parasitic nematodes secrete effector proteins, including cell wall-degrading enzymes (CWDEs) and cell wall-modifying proteins (CWMPs), to facilitate infection. However, whether cellulose-binding proteins (CBPs), which are a class of CWMPs, can synergize with cellulases (a subgroup of CWDEs) to promote parasitism remains unclear. Here, we identified and characterized a CBP (MgCBP1) and a β-1,4-endoglucanase (MgENG2) from the rice root-knot nematode Meloidogyne graminicola. MgCBP1 contains a conserved cellulose-binding module (CBM) and is specifically expressed in the subventral oesophageal glands, with transcript levels markedly up-regulated in pre-parasitic second-stage juveniles (pre-J2s) and during early infection. Recombinant MgCBP1 bound cellulose, disrupted fibre structure and enhanced cellulase-mediated hydrolysis via its intact CBM, whereas a CBM-disrupted mutant lack both activities. MgENG2, which lacks a CBM, exhibited strong endoglucanase activity and displayed an expression profile similar to that of MgCBP1. MgCBP1 significantly enhanced the hydrolytic efficiency of MgENG2, whereas its CBM-disrupted mutant did not. Transgenic rice overexpressing MgCBP1 showed enhanced susceptibility to M. graminicola. RNAi-mediated individual silencing of MgCBP1 or MgENG2 both reduced infection, with MgENG2 silencing showing a significantly greater effect. Co-silencing of both genes yielded the greatest reduction in gall formation and parasitic nematode numbers, which was significantly greater than that of MgCBP1-only silencing, but did not differ significantly from MgENG2-only silencing. Collectively, these findings demonstrate that MgCBP1 promotes nematode parasitism by binding cellulose, disrupting fibril surface integrity and enhancing the cellulolytic efficiency of MgENG2, providing the first experimental evidence that a CBP from a plant pathogen can synergistically facilitate infection by boosting endoglucanase-mediated cellulose degradation.