Distinct Ultraviolet-Signaling Pathways in Bean Leaves. DNA Damage Is Associated with β-1,3-Glucanase Gene Induction, But Not with Flavonoid Formation

Abstract

The enzyme β-1,3-glucanase (βGlu) was found to be strongly induced by ultraviolet (UV-B; 280-320 nm) radiation in primary leaves of French bean (Phaseolus vulgaris). This was demonstrated on the level of gene transcription, protein synthesis, and enzyme activity and was due to the expression of bean class I βGlu (βGlu I). In contrast to other proteins of the family of pathogenesis-related proteins, the induction of βGlu I by UV correlated with the formation of photoreversible DNA damage, i.e. pyrimidine dimer formation. In conditions that allowed photorepair of this damage, βGlu I induction was blocked. Therefore, UV-induced DNA damage seems to constitute a primary signal in the pathway leading to the induction of the βGlu I gene(s). The induction was a local response because in partly irradiated leaves βGlu I was selectively found in leaf parts exposed to UV. Although short wavelength UV (λ < 295 nm) was most efficient in βGlu I induction, longer wavelength UV (λ > 295 nm) as present in natural radiation was still effective. In contrast to UV induction of βGlu I, the induction of flavonoids in bean leaves was optimally triggered by much more moderate fluences from the UV wavelength range no longer effective in βGlu I induction. UV induction of the flavonoid pathway shows no correlation with DNA damage and thus should be mediated via a different signal transduction pathway.