Vascularization is a general requirement for growth of plant and animal tumours

Solid-tumour growth in animals as in humans depends on angiogenesis. Tumour s that fail to induce the formation of new blood vessels do not enlarge bey ond a few millimetres in diameter. Plant tumours induced by Agrobacterium t umefaciens can reach diameters of more than 100 mm, thus raising the questi on of how they are sufficiently supplied with nutrients and water. Until re cently, these rapidly growing tumours were considered unorganized or partly organized masses. However, in analogy to animal and human tumours, growth of leaf and stem tumours depends on neovascularization. Plant tumour cells induce the formation of a sophisticated vascular network consisting of wate r-conducting vessels and assimilate-transporting sieve elements. Similar to animal and human tumours that overexpress angiogenic growth factors, plant tumours overexpress the T-DNA-encoded vascularization-promoting growth fac tors auxin and cytokinin upon Agrobacterium infection. High auxin levels in duce ethylene emission from the tumours, which has a strong impact on tumou r and host stem, as well as on root structure and function. Ethylene appare ntly stimulates abscisic acid synthesis in the leaves above the tumour, whi ch reduces transpiration and thus protects the host plant from rapid wiltin g. Hence, for the elucidation of phytohormone-dependent vascular developmen t in plants, such tumours are regarded as an excellent model system. The co mparison of analogous requirement of neovascularization for tumour growth i n plants, as in animals and humans, is discussed in terms of interdisciplin ary strategies of possible prevention and therapy.