Putative biological action of oligosaccharides on enzymes involved in cell-wall development.

Introduction Primary plant cell wall is synthesized around protoplast and its architecture is determined by a complex balance in the synthesis of several polysaccharides and glycoproteins which form an interwoven network organized around cellulose microfibrils [ 13. Therefore deposition and synthesis of cell-wall material must accompany cell progressive enlargement, and this necessitates a permanent rearrangement of the already deposited material and of the newly synthesized polymers. The chemical and biological nature of the rearrangements that occur within a given constituent polymer or between polymers, the site of their occurrence within the cell wall, and the physical, physiological and biochemical factors that control them, are still not clearly understood. Among the regulatory processes that control cell expansion, auxin-induced growth [2] involves a proton extrusion into the apoplast which may selectively activate wall-associated hydrolases with low pH optima [3]. Cleavage of hemicelluloses, especially xyloglucans (XGs), which are the predominant hemicelluloses in the primary wall of higher plants, has been observed in auxin-treated and acid-treated pea stems [4,5]. This degradation by wall-bound enzymes may have several consequences that serve cell-wall rearrangement. It may result in the breakage of XG interconnections between microfibrils [ 51, thus accounting for wall loosening, and it may at the same time result in the release of biologically active fragments with various lengths and structures. These biologically active oligosaccharides belong to the so-called oligosaccharins, which all possess signalling and regulatory activities [6].