Investigating the function of an Arabidopsis calmodulin-like gene in defence against pathogens

1 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.