Altered gene expression brought about by inter- and intracellularly formed hexoses and its possible implications for plant-pathogen interactions

It is commonly accepted that sugars directly or indirectly modulate the expression of genes involved in photosynthesis, storage functions and defense. Generally, studies on gene expression made use of experimental systems where sugars were supplied from the outside of cells such as in protoplasts, cell suspension cultures etc. Using sugar-accumulating transgenic tobacco plants which express yeast invertase either in the cytosol, vacuole or cell wall (cytInv, vacInv, cwInv) we aim at distinguishing between intracellular and extracellular events of hexose sensing leading to altered gene expression. Invertase activity in the apoplast or vacuole but not in the cytosol leads to the repression of photosynthetic genes and to induction of some PR-protein genes. The induction of these PR-protein genes can equally be achieved by treating leaves of wildtype tobacco plants with sugar solutions. The sugar-mediated accumulation of PR-protein transcripts has been found to be dependent on leaf maturity and to occur independent from salicylic acid. On the other hand, a separate set of genes has been observed to be up-regulated in cytlnv plants. By differential hybridization of a cytlnv cDNA library five different cDNAs were isolated. By sequence analysis two cDNAs (cyt7 and cyt28) could be identified as plastidic aldolase and water stress related intrinsic membrane protein (PIP), respectively. One clone (cyt26) was highly homologous to S-adenosylmethionine decarboxylase from potato, the key enzyme in polyamine biosynthesis. Cyt22 showed 71% homology to Arabidopsis lycopene epsilon cyclase, the crucial enzyme in ct-carotene biosynthesis, and cyt19 had 60% homology to tomato 1-aminocyclopropane-1-carboxylate oxidase homolog. Obviously, there are principal differences between sensing of and adaptation to sugars between intracellularly and extracellularly formed hexoses leading to diverse patterns of gene expression. An inverse correlation between photosynthetic gene and PR-protein gene expression has not only be observed in cwlnv and vaclnv plants, but also in an interaction between wildtype tobacco plants and potato virus Y. The expression pattern of this interaction might be a consequence of extracellular hexose sensing.