Sucrose increases pathogenesis-related PR-2 gene expression in Arabidopsis thaliana through an SA-dependent but NPR1-independent signaling pathway

Pathogenesis-related (PR) protein-coding gene expression was studied in Arabidopsis thaliana grown in liquid medium in the presence of sugars (sucrose or glucose). PR protein transcripts accumulated in the presence of sugar in the medium. A potential effect linked to osmolarity changes induced by sugar addition in the medium was ruled out using osmotica (NaCl or polyethylene glycol). Two major proteins were purified from the culture medium and found to be homologous to A. thaliana PR-2 (acidic form of beta-1, 3-glucanase) and PR-5 (thaumatin-like PR-protein). The expression of the corresponding genes was increased in the presence of sucrose and was detected exclusively in the green parts of the plant. The use of mutants and transgenic plants of A. thaliana indicated that salicylic acid (SA) was involved in the sugar-dependent activation of these PR protein-coding genes. Activation of the PR-2-coding gene was demonstrated not to be hexokinase-dependent and to be linked to a sugar metabolite acting as an internal signal as shown with non-metabolizable sugars, which were inefficient for the induction of the PR-2-coding gene. Moreover, the activation of this gene occurred in the npr1 mutant suggesting that the sugar signal acts either downstream or independently of NPR1.