"1. INTRODUCTION THE PLANT CELL WALL The cell wall"
1. INTRODUCTION THE PLANT CELL WALL The cell wall is the external component of plant cells and consists of cellulose microfibrils coated by xyloglucans and embedded in a complex matrix of pectic polysaccharides. Both the middle lamella and primary cell wall are synthesized in actively growing cells, whereas secondary cell wall is deposited after that cell expansion stops. the middle lamella and the primary cell wall are prevalently composed by pectin (a group of polysaccharides containing galatturonic acid, galactose, ramnose, arabinose), the most water- soluble component of cell wall. In primary cell wall, cellulose forms an oriented network embedded in a gel matrix formed by pectic components and hemicellulose. Cellulose microfibrils, the major component of the secondary cell wall, are embedded in lignins, complex polymeric compounds chemically resistant, mainly composed of three aromatic alcohols (Carpita and Gibeaut, 1993). For a long time, the plant cell wall was considered only as a static support for the whole plant structure. More recently it was found that cell wall is liable of a great amount of functions and contains many proteins with both structural and enzymatic functions. It is a dynamic compartment whose composition and structure are constantly modified during growth and development, to support cell growth, elongation and defence against pathogens attack. 1 Defence depends on the presence of both constitutive defence mechanisms and specific responses induced upon interaction with pathogens. The first line of defence is represented by cell wall, the physical barrier that plant cells use to oppose bacterial and fungal entry. PATHOGEN PERCEPTION Plants have evolved sophisticated perception mechanisms to recognize microbial invaders and activate defence responses (Gomez- Gomez, 2004). They are able to detect various molecules deriving from the pathogens or from the plant itself: the molecules which induce a plant defence response are defined elicitor. There are general elicitors and race- specific elicitors. Chitin fragments, bacterial flagellin, lipolysaccharide and glucans are general elicitors and are detected by receptors situated at plasma membrane (Jones and Takemoto, 2004). Race specific elicitors are in many cases proteins encoded by pathogen avirulence genes (Avr) and are recognized by the corresponding plant resistence genes (R). IMPORTANCE OF PECTIN DEGRADATION IN PATHOGENESIS Pathogens enter the plant tissues in three major ways: digesting the cell wall, entering from wounds and invading through natural openings such as stomata. Pectins are one of the first target of digestion by invading pathogens (Ridley et al, 2000). Most pathogenic fungi secrete a number of 2 hydrolytic enzymes capable of degrading cell wall polymers. Polygalacturonases (PGs), the first enzymes to be secreted during infection processes, cleave the α-1-4 linkages between D-galacturonic acid residues in non- methylated homogalacturonan polymers. The requirement of PG activity for full virulence has been demonstrated for the fungus Botrytis cinerea (ten Have et al., 2000) and for other fungal and bacterial pathogens. Most fungi produce multiple isoenzymes that differ in their enzymatic properties (substrate specificity, pH optimum, mode of action), molecular way and regulation (De Lorenzo et al., 1997). PGs may have a single- attack mode which generates long polymers, gradually converted into smaller fragments, or a processive mode which generates only small oligomers (fig. 1). The activity of PGs is counteracted by PGIPs, leucine-rich repeat proteins (LRR) located in the cell wall (De Lorenzo et al., 2001). A role for PGIPs in plant defence has been demonstrated by showing that transgenic Arabidopsis plants overexpressing PGIPs exhibit enhanced resistence to Botrytis cinerea (Ferrari et al., 2003). The inhibition of PGs activity by PGIPs is thought to cause the accumulation in the plant aposplast of oligogalacturonides (OGs) with a degree of polymerization between 10-15 that are active as elicitors (Cervone et al., 1986). 3