**4. DNA repair mechanism in plants**

In plants, UV radiation-induced DNA damage can lead to DNA replication inhibition. Although plants exhibit mechanisms by which they are able tolerate some DNA damage, DNA mutation can still lead to transcription and replication blocks. Since plants, like humans, are subjected to oxidative stress (including that induced by UV radiation), an investigation into the evolutionary response by plants to this stress may allow us to apply these findings in a clinical setting.

By their nature, plants are subjected to solar UV radiation indiscriminately. They must necessarily, then, possess inherent methods to prevent and repair DNA damage. While plants are better able to absorb the high energy UV radiation through various photonabsorbing structures, they are still at high risk for oxidative modification of DNA. DNA repair mechanisms in plants such as Arabidopsis thaliana have been investigated and serve as a foundation for the search of a botanical extract that can effectively combat UV-induced DNA damage.

Repair mechanisms increase the likelihood of the accurate transmission of genetic information from parent to daughter cell and thus the survival of the species. Although plants have developed methods to minimize the toxic effects of DNA damage including DNA translesion synthesis and recombination (Schmitz-Hoerner and Weissenbock, 2003), they also have active repair pathways. These pathways include photoreactivation (via photolyase), nucleotide excision repair, base excision repair, and mismatch repair (Kimura et al., 2004).

Plants and some other organisms are able to use light energy (UV and visible) to reverse DNA damage. The enzyme photolyase binds to CPDs and via photoreactivation removes UV-induced lesions. Additionally, excision repair pathways work by replacing damaged DNA with new nucleotides. Base excision repair (BER) employs various DNA glycosylases to remove modified DNA. On the other hand, nucleotide excision repair (NER) is essential in solar radiation protection and in repairing a wide range of DNA lesions. A complex array of proteins recognize, bind to, excise, and repair DNA irregularities in both excision repair pathways.

Humans share repair pathways with plants, particularly nucleotide excision repair (NER). NER is essential in removing major damage to DNA which interferes with the genetic code. Due to similarities in DNA damage and repair mechanisms in plants and humans, metabolites produced by plants may provide beneficial effects in humans.
