Only xylem-borne factors can account for systemic wound signalling in the tomato plant

Rapid and systemic defence responses occur in various higher plants, including the tomato (Lycopersicon esculentum L.). The long-distance signalling mechanisms which permit these responses are not clear, but three models are currently considered in the literature: phloem transport, hydraulic dispersal in the xylem, and electrical transmission. Experiments presented here are designed to discriminate between these three models on the basis of some key predictions. It is demonstrated that wound signalling can be prevented by enclosure of the shoots in polythene bags, to generate high humidity. This effect can be reversed by addition of mannitol solution to the roots, showing that it depends on saturation of the plant's water status rather than on changes in the gaseous environment of the shoot. In addition, wound signals are shown to pass freely across heat-killed tissue. These results are predicted by the hydraulic-dispersal model of signalling, but they are not compatible with the other two models. We therefore conclude in favour of hydraulic dispersal.

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