SOLUTION PROPERTIES OF LEVAN POLYSACCHARIDE FROM PSEUDOMONAS-SYRINGAE PV PHASEOLICOLA, AND ITS POSSIBLE PRIMARY ROLE AS A BLOCKER OF RECOGNITION DURING PATHOGENESIS

Bacterial levan, a highly branched, high molecular weight polymer of fructose, was purified from culture supernatants of Pseudomonas syringae pv. phaseolicola grown in a liquid high-sucrose medium, and the predominance of beta-(2 --> 6) linkages was confirmed by C-13 NMR. The solution properties of this material resembled those of disordered linear polysaccharides in the response to low-amplitude oscillatory shear (frequency dependence of G' and G''); the absence of any detectable conformational change with temperature (as monitored by optical rotation); close superposition of steady-shear viscosity (eta) and complex dynamic viscosity (eta*) at equivalent values of shear-rate (gamma/s(-1)) and frequency (omega/rad s(-1)); a similar form of shear-thinning (giving linear plots of eta versus eta gamma(0.76)); and the onset of semi-dilute behaviour at a closely comparable degree of space-occupancy (c[eta] approximate to 3.6). The intrinsic viscosity, however, was unusually low ([eta] approximate to 0.17 dl g(-1)) and the concentration dependence of 'zero-shear' viscosity in the semi-dilute regime unusually high (eta(0) similar to c(9.3)), as anticipated from the densely packed, branched molecular structure. Solutions of levan and pectin, matched to approximately the same initial viscosity, showed a substantial reduction in viscosity when mixed. Similar behaviour was observed for mixed solutions of levan with locust bean gum (LBG), chosen for its structural similarity to cellulose and hemicelluloses of the plant cell wall. Viscosity reduction was eliminated at low concentrations (indicating that it does not arise from heterologous association), but became very pronounced at high concentrations, and was then accompanied by resolution into levan-rich and LBG-rich solution phases. This evidence of strong thermodynamic incompatibility and exclusion behaviour with (1 --> 4)-linked plant polysaccharides suggests that the primary role of levan during pathogenesis may be as a barrier to intimate morphological contact (recognition) between plant cell walls and those of the parasite, thus inhibiting initiation of a hypersensitive response by the host.