ACCUMULATION OF UV-ABSORBING FLAVONOIDS INDUCED BY UV-B RADIATION IN ARABIDOPSIS-THALIANA L .1. MECHANISMS OF UV-RESISTANCE IN ARABIDOPSIS

Irradiation of Arabidopsis with ultraviolet (UV) light resulted in intensity- and wavelength-dependent increases in the levels of a small family of UV-absorbing flavonoids, which accumulate in the aerial parts of the plants. A gradient of sensitivity to UV-B radiation is described in the different leaves of developing Arabidopsis plants whereby the earliest formed leaves become damaged by UV-B faster and more extensively than later-formed leaves. This UV-sensitivity gradient tightly parallels differences in constitutive as well as UV-induced levels of flavonoid accumulation among the various leaves, suggesting a direct role of flavonoids in protection against damage by UV radiation. The level of accumulated flavonoids, both constitutive and UV-induced, in each leaf appear to be dependent on the specific developmental state of each leaf as well as the overall developmental state of the plant. The UV-mediated flavonoid response, along with the observed UV-induced damage, appear not to be systemic in Arabidopsis but restricted very closely to the irradiated areas of leaves.