SALICYLIC ACID-INDEPENDENT INDUCTION OF PATHOGENESIS-RELATED PROTEIN TRANSCRIPTS BY SUGARS IS DEPENDENT ON LEAF DEVELOPMENTAL STAGE

Soluble sugars have been found to regulate a number of genes involved in functions associated with sink metabolism, defense reactions and ph otosynthesis, As viruses and pathogens induce the expression of pathog enesis-related (PR) protein genes and have also been reported to lead to localized sugar accumulation in leaves, it was investigated whether a salicylic acid-independent but sugar-dependent pathway for PR-prote in gene induction may exist in plant cells, Leaf discs of tobacco plan ts were floated on different sugar solutions, transcript accumulation and salicylic acid (SA) levels were subsequently determined, PR-Q and PAR-1 transcripts were found to be inducible by glucose, fructose and sucrose, No significant change in SA content could be detected, follow ing incubation, On the other hand, SAR8.2 transcripts were repressed b y elevated levels of soluble sugars and sorbitol, respectively, sugges ting sensitivity to turgor pressure, Since leaves undergo sink to sour ce transition during growth, sugar responsiveness was investigated in leaves of different developmental stages, Interestingly, induction of PR-Q and PAR-1 by soluble sugars was essentially restricted to fully e xpanded leaves and was independent of plant age, Induction by salicyla te was not confined to the source capacity of a leaf but was dependent on the age of the respective leaf, Repression of transcripts encoding photosynthetic genes (ribulose-1,5-bisphosphate carboxylase/oxygenase (rbcS) and chlorophyll a/b binding protein (cab) by soluble sugars we re largely independent from the leaf developmental state, These findin gs hint to the possibility of salicylic acid-independent defense react ions of plants against pathogens by induction of a set of PR proteins in source leaves, Furthermore, the data suggest different mechanisms f or the induction of PR-protein genes and the repression of photosynthe tic genes by soluble sugars.