Hydroxyproline-rich Glycoproteins and Plant Defence

The distinguished plant cell wall component referred to as hydroxyproline-rich glycoproteins (HRGPs) exists in two forms: soluble in the symplast and insoluble in the apoplast. Insolubilization of HRGPs in cell walls through oxidative cross-linking which is elicited by stress represents a characteristic feature exhibited by two classes of HRGPs, namely, extensins and proline/HRGPs. Cross-linking of these HRGPs is an important process to strengthen the cell walls that contributes to plant defence reactions. In this review, the available information on these proteins is analysed with respect to their roles in host-pathosystems and the various techniques applied for their characterization. Future prospects on strengthening of cell walls through gene regulation and transgenic approaches are also addressed.