The plant cell wall is a complex structure and its functional integrity is maintained during interactions with the environment. Plants have cell wall integrity sensing mechanism by which they may detect pathogen attack triggering signaling pathways that induces rapid defense responses (Hématy et al., 2009). The first barrier that the pathogen encounter is the cuticle and the cell wall. In order to gain entry into the plant cell the pathogen disrupt the host cuticle by secreting oxidases, esterases, cutinases, and lipases (Feng et al., 2011). Infection by fungus includes attachment of conidia to the host surface followed by germination on favorable condition producing germ tube that penetrates the host surface killing cells leading to lesion formation, this may activate defense responses. The pathogen have the ability to overcome the defense mechanisms and are able to infect and colonize the plant i.e. fungus grows resulting into tissue maceration followed by sporulation (Prins et al., 2000). Plant cell walls are mainly composed of carbohydrate (polysaccharides such as cellulose, hemicelluloses and pectins), lignin and contain proteins involved in the cell dynamics through diverse functions such as growth, environmental sensing, signaling, and defense (Showalter 1993).
Plants on sensing pathogens and loss of cell wall integrity, activate plant defense responses that lead to cell wall remodeling required to prevent the disease (Bellincampi et al., 2014). Upon invasion by the pathogen, plant improvise defense by strengthening cell wall with lignin, callose and hydroxyl proline-rich-glycoprotein or produce phytoalexin and hydrolytic enzymes. Lignin deposition strengthens the cell wall and maintains functional integrity (Denness et al., 2011). Fortification of cell wall with papilla, callose, lignin and cell wall protein are defense response of plant on sensing pathogen attack.
The Papillae are heterogeneous material deposited between the plasma membrane and inside of the plant cell wall. These papillae are deposited by plants in response to pathogen penetration and mechanical injury. Papillae composition varies between different plant species. Papillae contain (1,3)-β-glucan callose (most abundant constituent), cellulose, hemicelluloses, pectins, lignin, and structural proteins such as arabinogalactan proteins and hydroxyproline-rich-glycoproteins (HRGPs) (Aist 1976; Celio et al., 2004; Voigt 2014). Different papilla-forming transport processes ensures plant defense (Voigt 2014). Callose-rich papillae is found at sites of pathogen penetration and provides resistance (Aist 1976).
Callose is a polysaccharide present in the cell walls of a many higher plants. It is in the form of (1,3)-β-glucan with some β-1,6-branches and is produced by callose synthases (Chen and Kim 2009). Callose is the major component of papillae deposited between the plasma membrane and cell wall to act as a physical barrier to the invading pathogen and is also induced in response to abiotic and biotic stresses (Ellinger and Voigt 2014). Furthermore it is deposited at plasmodesmata to regulate the cell-to-cell movement of molecules by decreasing the size exclusion limit of plasmodesmata (Iglesias and Meins 2000; Bucher et al., 2001) and plays an important role in plant development.
Lignification is the plant defense response and acts as a physical barrier to pathogen invasion providing resistance against the pathogen. Lignin is deposited in secondary wall of most plant cells providing rigidity and mechanical support. Chemically lignin is an aromatic polymer composed of phenylpropanoid monomer mainly of p-coumaryl, coniferyl and sinapyl alcohol. The phenylpropanoid pathway responsible for lignin biosynthesis serves to support the synthesis of other phenolic compounds such as phytoalexins, coumarins, and flavonoids which are involved in plant defense (Dicko et al., 2005; Frasera and Chapplea 2011). Role of lignification in disease resistance has been reviewed by Nicholson and Hammerschmidt (1992). Several component of fungal cell wall are elicitors of lignification.
Cell wall Structural protein:
Structural proteins in plant cell walls are the hydroxyproline-rich-glycoproteins (HRGPs). There are four major group of hydroxyproline-rich-glycoproteins (HRGPs) in plants (Sommer-Knudsen et al., 1998):
- Hydroxy/ Proline-rich-proteins (H/PRPs)
- The solanaceous lectins
- Arabinogalactans protein (AGPs)
HRGPs strengthen the cell wall in response to plant defense. The condition regulating the expression of the five major classes of cell wall proteins [extensins, glycine-rich-proteins (GRPs), PRPs, AGPs and solanaceous lectins] are wounding, fungal/ viral infection, fungal/endogenous elicitors, abiotic stress and development (Showalter 1993).
The multicomposition of plant cell wall can provide some insight to pant-pathogen interaction.
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