The intron of the Arabidopsis thaliana COX5c gene is able to improve the drought tolerance conferred by the sunflower Hahb-4 transcription factor.

Hahb-4 is a member of Helianthus annuus (sunflower) subfamily I of HD-Zip proteins. Transgenic Arabidopsis thaliana plants constitutively expressing this gene exhibit a strong tolerance of water stress in concert with morphological defects and a delay in development. In order to obtain a drought-tolerant phenotype without morphological associated phenotype, several stress inducible promoters were isolated and transgenic plants expressing Hahb-4 controlled by them were obtained and analyzed. These plants showed unchanged morphology in normal growth conditions and enhanced drought tolerance compared with non-transformed plants, but no as high as the one exhibited by the constitutively transformed genotype. A chimerical construction between the Hahb-4 promoter and the leader intron of the Arabidopsis Cox5c gene was made either directing gus or Hahb-4 expression. GUS activity increased in transgenic plants after induction, showing the same distribution pattern as in plants transformed with a construction lacking the intron. Transgenic plants, bearing the chimerical construct, are indistinguishable from wild type plants in normal growth conditions whereas the water stress tolerance achieved was as strong as the one shown by the constitutive genotype. This enhanced stress tolerance seemed to be due to a combination of an increase in transcription and translation rates in comparison to those of plants transformed with the Hahb-4 promoter. Similar strategies could be applied in the future for the obtaining of suitable promoters responsive to other external agents.