Abstract

To examine the effects of continuous UV-B irradiation on cucumber (Cucumis sativus L.) cotyledons, sections of cucumber cotyledons were observed and the Cs1-MMP expression was analyzed. Cs1-MMP encodes a putative MMP that is expressed during the transition from senescence to PCD in cucumber cotyledons. The observation of sections revealed that the alignment of chloroplasts in palisade cells became irregular by treatment with UV-B for 8 d. The adaxial surface of the cotyledons became smooth due to flattening out of the epidermal cells by treatment with UV-B for 12 d. The longitudinal length of the palisade layers was reduced after 8−16 d of exposure to UV-B. The thickness of the cell walls of the upper palisade cells was reduced after treatment for 16 d. The Cs1-MMP expression was induced by 8−16 d of UV-B exposure. Thus, there is strong temporal correlation between induction of the Cs1-MMP expression and disruption of the epidermal and upper palisade cells in UV-B-irradiated cucumber cotyledons. These results suggest that Cs1-MMP may be responsible for the UV-B-induced disintegration of the cell walls, leading to PCD in the cucumber cotyledons.