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

Solid-tumour growth in animals as in humans depends on angiogenesis. Tumours that fail to induce the formation of new blood vessels do not enlarge beyond a few millimetres in diameter. Plant tumours induced by Agrobacterium tumefaciens can reach diameters of more than 100 mm, thus raising the question of how they are sufficiently supplied with nutrients and water. Until recently, 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 water-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 factors auxin and cytokinin upon Agrobacterium infection. High auxin levels induce ethylene emission from the tumours, which has a strong impact on tumour and host stem, as well as on root structure and function. Ethylene apparently stimulates abscisic acid synthesis in the leaves above the tumour, which reduces transpiration and thus protects the host plant from rapid wilting. Hence, for the elucidation of phytohormone-dependent vascular development in plants, such tumours are regarded as an excellent model system. The comparison of analogous requirement of neovascularization for tumour growth in plants, as in animals and humans, is discussed in terms of interdisciplinary strategies of possible prevention and therapy.