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 ␤-1,3-glucanase (␤Glu) 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 ␤Glu (␤Glu I). In contrast to other proteins of the family of pathogenesis-related proteins, the induction of ␤Glu I by UV correlated with the formation of photoreversible DNA damage, i.e. pyrimidine dimer formation. In conditions that allowed photorepair of this damage, ␤Glu I induction was blocked. Therefore, UV-induced DNA damage seems to constitute a primary signal in the pathway leading to the induction of the ␤Glu I gene(s). The induction was a local response because in partly irradiated leaves ␤Glu I was selectively found in leaf parts exposed to UV. Although short wavelength UV ( Ͻ 295 nm) was most efficient in ␤Glu I induction, longer wavelength UV ( Ͼ 295 nm) as present in natural radiation was still effective. In contrast to UV induction of ␤Glu 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 ␤Glu 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.Plants require sunlight for photosynthesis and thus are constantly also exposed to potentially damaging UV radiation that is present in sunlight. UV-C ( Ͻ 280 nm) is the band with the highest energy and most efficient in damaging DNA and proteins, but only wavelengths in the UV-B (280-320 nm) range greater than 290 nm reach the earth's surface; shorter wavelengths are completely absorbed by the stratospheric ozone layer. UV-C and UV-B cause DNA damage; this radiation induces the formation of pyrimidine dimers of which cyclobutane pyrimidine dimers (CPDs) constitute the major class (Taylor et al., 1997). Plants possess quite effective protection mechanisms against UV-induced DNA-damage. So, photolyase is an enzyme capable to repair CPDs after activation by violet light (photoreactivation; Strid et al., 1994;Taylor et al., 1997). Another important defense mechanism against UV is the production of UV-absorbing flavonoids and phenylpropanoid compounds in leaf epidermal cells in response to UV irradiation (Li et al., 1993; Beggs and Wellmann, 1994). Flavonoids are also induced by a range of other stimuli; including pathogen attack (Harborne and Williams, 2000).Pathogenesis-related (PR) proteins are implicated in plant defense and accumulate in response to pathogen attack or to treatment with other elicitors (Leubner-Metzger and Meins, 1999). Endo-␤-1,3-glucanases (␤Glu; EC 3.2.1.39) are assigned to the PR proteins, where they constitute the PR-2 family. ␤Glu are abundant proteins, widely distributed in seed plant species. Besides plant defense, they have been implicated in several physiological and developmental processes (Doblin et al., 2001;Leubner...

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