It is commonly accepted that sugars directly or indirectly modulate the expression of genes involved in photosynthesis, storage functions and defense. Using sugar-accumulating transgenic plants which express yeast invertase either in the cytosol, vacuole or cell wall (cytInv, vacInv,cwInv) we aim at distinguishing between intracellular and extracellular hexose sensing events leading to altered gene expression. Invertase action in the apoplast or vacuole leads to repression of photosynthetic and induction of some PR-protein genes (Herbers et al. 1996a). In addition systemic acquired resistance is induced in these transgenic plants [1]. In wildtype tobacco plants induction of PR-protein gene expression can be achieved by treating leaves with sugar solutions [2]. The sugar response has been found to be dependent on leaf maturity and to occur independent from salicylic acid. An inverse correlation between photosynthetic gene and PR-protein gene expression has also been observed in an interaction between wildtype tobacco and potato virus Y [3]. The expression pattern of this interaction might be a consequence o extracellular hexose sensing. Therefore, it has been suggested that elevated hexose levels could lead to increased expression of defense-related functions and might possibly explain the phenomenon of high sugar resistance in plants [4]. On the other hand, a separate set of genes have been observed to be up-regulated in cytInv plants. By differential hybridization of a cytInv cDNA library five different cDNAs were isolated. By sequence analysis two cDNAs could be identified as plastidic aldolase and a water stress related intrinsic membrane protein (PIP), respectively. One clone was highly homologous to S-adenosylmethionine decarboxylase from potato, key enzymein polyamine biosynthesis. Cyt22 showed 71 % homology to Arabidopsis lycopene epsilon cyclase, important enzyme in a-carotene biosynthesis, and cyt19 60% homology to tomato 1-aminocyclopropane-1-carboxylate oxidase homologue. Obviously, there are fundamental differences between sensing of and adaptation to sugars between intracellularly and extracellularly formed hexoses leading to diverse patterns of gene expression. The relevance of sugar-sensing for plant pathogen interactions will be discussed.

1. Herbers K, Moenke G, Badur R, Sonnewald U, 1995. Plant Mol. Biol. 29, 1027-1038.
2. Herbers K, Meuwly P, Metraux JP, Sonnewald U, 1996. FEBS Lett. 397, 239-244.
3. Herbers K, Meuwly P, Frommer WP, Metraux JP, Sonnewald U, 1996. Plant Cell 8, 793-803.
4. Herbers K, Sonnewald U, 1998. J Plant Res (in press).