DNA Damage And UV-absorbing Compounds In Plant Cells Under Supplementary Ultraviolet-B Radiation

Enhanced solar UV-B radiation reaching the earth affects the secondary metabolism ofplants, especially the phenylpropanoid pathway. Products of this pathway like flavonoidshave favorable UV-B absorbing properties and are thus very effective as UV-B protectivepigments in plants. An accurate and effective method-HPLC was developed to measureUV-absorbing compounds in Spirodela polyrhiza. The concentrations of some of thecompounds are increased by UV-B radiation, but others are unchanged or even decreased.The single cell gel electrophoresis or comet assay is now widely used to detect DNAdamage in animal cells induced by radiation or chemicals. In the present study, comet assaywas applied to plant cells for measuring UV-B-induced DNA damage. The accepted animalcell protocol for the comet assay was modified to adapt it to plant cells. The majormodifications were conversion of the plant cells to protoplasts and the use of T4endonuclease V. This enzyme is a lesion-specific enzyme, and specifically recognizespyrimidine dimers in DNA strands. As a positive control hydrogen peroxide was applied,eliciting detectable DNA damage at 100μM H₂O₂. The level of this type of DNA damagewas not affected by T4 endonuclease V treatment. This result implies that the mechanism ofH₂O₂ induced DNA damage was different from UV-B induced DNA damage. The results alsoindicate that both UV-A and UV-B radiation can induce DNA single-strand breakage in plantcells, while UV-B was more effective than UV-A for inducing pyrimidine dimer formation.The modified comet assay protocol was applied to investigate sensitivity to UV-Binduced DNA damage in young and mature leaves of Murraya panicuata. Results indicatedthat both UV-A and UV-B radiation induced DNA damages in mesophylls. The level of DNAdamage was much higher in young leaves, which imply that young leaf was more sensitive toUV-B radiation than mature leaf.