Because the level of epoxy-carotenoids in green leaves is high relative to the amount of ABA synthesized, it is considered unlikely that the zeaxanthin epoxidase (ZEP) has a regulatory role in these tissues. The expression of ZEP transcripts in green tissue does not increase in tobacco,tomato, or cowpea plants that are subjected to osmotic stress. In one study, it was found that the level of AtZEP mRNA was induced by drought stress in root tissues, but not in shoots (4). In another study, it was reported that the expression of AtZEP mRNA increased in response to osmotic stress or ABA treatment in both roots and shoots