Fig. 5.

Model for the activation of PR genes in HCT-RNAi and ccr1 Arabidopsis plants. In the proposed model, changes in lignin content in xylem cells of HCT-RNAi or ccr1 Arabidopsis are perceived initially by the cell through activation of plasma membrane-localized cell wall integrity receptors. This results in initiation of a signaling cascade that induces the expression of cell wall remodeling genes, including PECTATE LYASES, XYLOGLUCAN ENDO-TRANSGLYCOSYLASES (XTHs), and ADGP1. ADPG1 activity may contribute to solubilization of pectin, but is necessary for release of elicitor fragments, most likely from RG-II. The soluble elicitors activate expression of PR defense response genes through a signaling pathway involving SA (66). Many of the other transcriptomic changes occurring in the lignin-modified plants, such as the activation of seed-specific genes in stems of ccr1, may result from secondary effects. The modification of pectin is also, at least in part, responsible for the reduced recalcitrance of the biomass.