Mechanism of Wilt Induction by Amylovorin in Cotoneaster Shoots and Its Relation to Wilting of Shoots Infected by Erwinia amylovora

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Abstract
The effects of amylovorin, a wilt-inducing polysaccaride isolated from fire blight ooze, on the water relations and cell permeability of C. pannosa shoots were not related to changes observed in shoots infected by E. amylovora. Rates of solution uptake and transpiration by shoots in amylovorin solution were less than in water. Water potentials and water conductance of stem segments were reduced significantly in amylovorin-wilted shoots relative to non-wilted shoots. The effect of amylovorin solutions on water potentials could be reproduced by dextran solutions or sealing shoot bases with wax. Electrolyte loss from wilted tissues of shoots exposed to amylovorin was no different than loss from nonwilted tissues of shoots held in water. Water potentials of infected shoots were significantly higher than those of noninoculated shoots. Water potentials of apparently healthy tissues distal from infected tissues in shoots inoculated 7 cm behind the shoot apex were not significantly different from comparable tissues of non-inoculated shoots. Electrolytes were lost at a much higher rate from wilted tissues of infected shoots than from healthy tissues of noninoculated shoots. Amylovorin induces wilt by restriction of water movement in xylem, rather than by a directly toxic effect as previously suggested. The changes in water relations of shoots infected by E. amylovora suggest that wilt of such shoots results from a disruption of membrane integrity, rather than restriction of water movement.