Crown gall
tumors induced by Agrobacterium tumefaciens represent a sink that has to be provided with nutrients and water by the host plant. The lack of an intact epidermis or cuticle results in uncontrolled loss of water. However, neither the
tumor nor the host plant displays wilting. This phenomenon points to drought adaptation in both
tumors and the crown gall host plant. To understand the underlying molecular mechanisms of protection against desiccation the gene expression pattern of Arabidopsis (Arabidopsis thaliana)
tumors was integrated with the profile of stress metabolites: Arabidopsis
tumors accumulated high amounts of
abscisic acid (ABA), the
ethylene precursor aminocyclopropyl
carboxylic acid, osmoprotectants, and form a suberized periderm-like protective layer. Suberization of the outer
tumor cell layers most likely is mediated by ABA since external application of ABA induced suberization of Arabidopsis roots. However, the expression level of the classical marker genes, known to respond to drought stress and/or ABA, was lower in
tumors. Instead another set of drought and/or ABA-inducible genes was more highly transcribed. Elevated transcription of several ABA-dependent
aquaporin genes might indicate that ABA controls the water balance of the
tumor. The retarded
tumor growth on abi and aba mutant plants underlined the importance of a
tumor-specific ABA signaling pathway. Taken together, we propose that ABA is an important signal for protection of
tumors against desiccation and thus supports
tumor development.