Defense responses of plant cell wall non-catalytic proteins against pathogens

• Cell wall structural proteins (CWPs) provide plants basal defense against pathogens.
• Cross-linking and agglutination are the physical defense responses.
• Degradation of genetic materials of pathogens is the biochemical defense response.
• Triggering of pathogenesis-related gene expression is the molecular defense response.
• Manipulation of CWP genes by reverse genetics is suggested for disease resistance.

Plant cell wall (CW) associated non-enzymatic proteins (CWPs) are composed mainly of glycoproteins containing a polypeptide backbone attached with carbohydrate side chains. These CWPs typically include hydroxyproline-rich glycoproteins (HRGPs), proline-rich proteins (PRPs), glycine-rich proteins (GRPs), and arabinogalactan proteins (AGPs). The CWPs have been implicated in plant defense against pathogens. The defense responses appear to be accomplished by several mechanisms. The major mechanisms include: (i) CW hardening through insolubilization and oxidative cross-linking of extensins and PRPs mediated via H2O2 and peroxidases, (ii) secretion and agglutination of AGPs at the sites of pathogenic infection, (iii) degradation of the genetic materials of the pathogens by binding of GRPs to the RNA of the pathogens, and (iv) activation of the pathogenesis-related (PR) gene expression using AGPs as a soluble molecular signal. This review summarizes the aforementioned defense responses, provides an update on classification, and explores future research opportunities of CWPs.