UV radiation induces two major DNA damage products: the cyclobutane pyrimidine dimer (CPD) and the pyrimidine[6-4]pyrimidinone dimer (or [6-4] photoproduct; [6-4]PP). The biological effects of both lesions have been studied in microbial and mammalian systems. Pyrimidine dimers have been shown to act as blocks to the progress of microbial and mammalian DNA polymerases and to inhibit DNA replication both incisand intrans. These dimers have also been shown to inhibit the progress of mammalian RNA polymerases and, as a result, to eliminate the expression of a transcriptional unit. The direct biological effects of UV-induced pyrimidine dimers on DNA replication and transcription have not been well studied in plants. However, it has been well documented that increasing doses of UV radiation can result in slower plant growth, a generalized stress response, or death of the irradiated tissues. UV irradiation of pollen can induce mutations; presumably dimers play a role in this process, as the mutagenic effects of UV radiation are photoreactivatable.
