Localisation and expression of zeaxanthin epoxidase mRNA in Arabidopsis in response to drought stress and during seed development

Abscisic acid (ABA) is involved in seed development and plant adaptation to environmental stresses. ABA is synthesized from cleaved xanthophylls and zeaxanthin epoxidase (ZEP) is the enzyme responsible for the conversion of zeaxanthin to violaxanthin. In this study, we have characterized the ABA1 gene ( AtZEP ) of Arabidopsis thaliana L. and show that this complements the aba1 mutant, defective in zeaxanthin epoxidation. The molecular basis for two aba1 mutant alleles has been determined and the reduction in their AtZEP transcript levels correlates with the molecular defect identified. As AtZEP mRNA abundance was not affected in two other ABA-deficient mutants ( aba2 and aba3 ) and in two ABA-insensitive mutants ( abi1 and abi2 ), no feedback regulation of ABA biosynthesis seems to occur at the level of ZEP transcription. Steady state transcript levels increased in roots during rapid water stress as well as progressive drought stress, providing evidence that zeaxanthin epoxidation contributed to the regulation of ABA biosynthesis in roots and consequently to the plant adaptive response to hydric stress. In seeds in situ hybridization analysis detected AtZEP mRNA in the embryo cells from the globular stage to desiccation phase. In contrast, expression of AtZEP in maternal tissues was specific to the maturation phase. These results are discussed in relation to the role of ABA both in response to drought stress and in seed development.