MECHANICAL PRESSURE BY ERWINIA AMYLOVORA IN RELATION TO WATER POTENTIAL AND ITS POSSIBLE ROLE IN PATHOGENESIS

When ooze of Erwinia amylovora is water saturated or nearly water saturated, it is fluid and appears during exudation as droplets. Under less humid conditions, when the water potential inside the host is between -1 and -2 MPa, the bacterial biomass is pasty (unpublished results) and when it is squeezed out of the host it may appear as strands. Oozing, especially strand formation, points to a mechanical pressure exerted on or by the bacterial biomass.

In the course of pathogenesis, E. amylovora appears in the intercellular spaces of plant tissues. There, the bacteria multiply and form extracellular slime, filling the intercellular space. If the bacterial biomass continues to grow and if it cannot escape, a pressure on the surrounding plant cells comes into existence. As long as E. amylovora grows and produces new biomass the 'growth pressure' increases. Water absorbtion by the bacteria and subsequent volume increase become more and more difficult, due to the counter pressure by the host. The bacterial biomass will then suffer lack of water, so that further production of bacterial dry matter will diminish. Consequently, the growth pressure will reach a maximum value. The maximum value of the growth pressure, hypothetically, equals the actual water potential minus the water potential at which bacterial dry matter production attains its limit when there is no pressure (Schouten, 1988). So, a high water potential (humid circumstances) allows a strong growth pressure.

The mechanical pressure may also rise without increase of the bacterial dry weight: When bacterial biomass is accumulated in the intercellular holes of the host plant and the water potential rises suddenly, for instance due to a rain shower, the bacterial biomass will tend to absorb water and to swell strongly. That swelling without increase of bacterial dry weight may cause a 'swelling pressure' on the surrounding plant cells. The maximum value of the swelling pressure equals Δ, the change in water potential (Schouten, 1988). The strong swelling capacity of the bacterial biomass can be ascribed for a minor part to the cells of E. amylovora and for a major part to the extracellular slime (Schouten, 1989).

The pressure of the bacterial mass on the host tissue might lead to compression of soft host cells, derangement of plasmalemmata and formation of large slime-filled holes in the plant's tissue. Moreover, the expanding slime may force its way to the outside of the plant (exudation) or to healthy parts of the host. It even may pierce developing cork barriers formed by the plant after infection to confine the disease. The resistance reaction of the plant, which is