Overexpression of an ABA biosynthesis gene using a stress inducible promoter enhances drought resistance in petunia

Authors: ESTRADA-MELO, ALEJANDRO - University Of California; MA, CHAO - University Of California; REID, MICHAEL - University Of California; Jiang, Cai-Zhong

Submitted to: Horticulture Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/26/2015
Publication Date: 4/8/2015
Publication URL: http://www.nature.com/articles/hortres201513
Citation: Estrada-Melo, A.C., Ma, C., Reid, M.S., Jiang, C. 2015. Overexpression of an ABA biosynthesis gene using a stress inducible promoter enhances drought resistance in petunia. Horticulture Research. doi: 10.1038/hortes.2015.13.

Interpretive Summary: Among the different stress conditions that plants frequently encounter, the abiotic stress, drought, constitutes one that is responsible for major worldwide environmental constraints for agricultural crops. Climate change will increase the occurrence of drought periods in some parts of the world, therefore improving plant growth under water-limited conditions will become one of the most challenging tasks for growers of all plant crops. Plants are sessile and therefore need to cope effectively with environmental stresses. Plant growth regulators often play a role as intermediaries between the perception of an environmental stress and the ultimate morphological response. The plant hormone Abscisic acid (ABA) is involved in seed maturation and dormancy processes, but also enables plants to survive stresses such as drought, cold and high salt in soils. Water stress resulted in an increased ABA levels in shoots followed by stomata closure which decrease water loss by reducing transpiration. ABA also acts as a signal for the induction of dehydration tolerance genes, that in addition to protect the cells from stress, some of these osmotic responsive (OR) genes, that are activated under stress conditions, regulate downstream genes for signal transduction. These genes are involved in the synthesis of Late Embryogenesis Abundant (LEA) proteins, that are associated in desiccation tolerance during seed maturation, in the water transporters such as aquaporins, in the detoxification enzymes like superoxide dismutase, catalase, etc, in protein kinases, in phosphatases as well as in transcription factors that act as regulatory molecules. Experiments in transgenic plants have demonstrated that NCED is a key rate limiting step in ABA biosynthesis, and its overexpression resulted in ABA accumulation and increased drought tolerance in tomato (Solanum Lycopersicon), Arabidopsis thaliana and tobacco (Nicotiana tabacum). These studies all used constitutive promoters to drive expression of NCED genes, with negative pleiotropic effects on plant growth and development. It is therefore necessary to use a promoter which will not result to pleiotropic effects or at least reduce the negative effects of the constitutive promoter. In the present study, we used the stress-inducible promoter Rd29A to drive LeNCED1 expression. Under well-watered conditions, transgenic plants growth was similar than wild-type and the effect of seed dormancy was not observed. When the transgenic plants were exposed to water stress, resulted in an increased NCED mRNA, an elevated ABA content in the leaves and in a significant increase in drought tolerance compared to the wild-type. The transgenic petunia plants also displayed the expected decrement in stomatal conductance, transpiration and photosynthetic rate compared to the wild type. Our data suggest that under water stress is possible to reduce water loss by transpiration in Petunia plants by using a stress inducible promoter which turn on the expression of LeNCED, increases ABA content, induces stomata closure and enhances drought tolerance; and when favorable conditions are present, the LeNCED expression is turned off, thus avoiding the negative phenotypic effects of using a constitutive promoter.

Technical Abstract: Plants respond to drought stress by closing their stomata and reducing transpirational water loss. The plant hormone abscisic acid (ABA) regulates growth and stomatal closure particularly when the plant is under environmental stresses. One of the key enzymes in the ABA biosynthesis of higher plants is 9-cis-epoxy-carotenoid dioxygenase (NCED). It has been shown in tomato that the overexpression of NCED using constitutive promoter resulted in increased ABA which induced stomata closure and increased water use efficiency, however high levels of ABA accumulation also resulted in increased seed dormancy, intervenial leaf flooding and reduced plant growth. Here the LeNCED1 gene was overexpressed in petunia plants under the control of a stress inducible Rd29A promoter. Under well-watered conditions, transgenic plants growth was similar than wild-type and the effect of seed dormancy was not observed. When the transgenic plants were exposed to water stress, resulted in an increased NCED mRNA, an elevated ABA content in the leaves and in a significant increase in drought tolerance compared to the wild-type. The transgenic petunia plants also displayed the expected decrement in stomatal conductance, transpiration and photosynthetic rate compared to the wild type. Our data suggest that under water stress is possible to reduce water loss by transpiration in Petunia plants by using a stress inducible promoter which turn on the expression of LeNCED, increases ABA content, induces stomata closure and enhances drought tolerance; and when favorable conditions are present, the LeNCED expression is turned off, thus avoiding the negative phenotypic effects of using a constitutive promoter.