Enhancing Water Stress Tolerance in Floriculture Crops

Water deficit is one of the major constraints on plant growth and development, causing reduction of crop productivity. To minimize water loss, among many adaptation strategies, plants close their stomata to reduce transpiration. The stomatal closure is regulated by light, internal CO2 concentration, and plant hormones, mainly abscisic acid. Plants’ response mechanisms to water deficit are complex processes involving numerous genes and various signaling pathways. Floriculture crops are often exposed to water deficit during shipping and retailing, and these periods often result in damaged crops and profit loss. Understanding of plant responses to water deficit stress will provide us an opportunity to develop floriculture crops with enhanced water deficit tolerance. The long-term goal of this research is to enhance water deficit stress tolerance in floriculture crops employing physiological and genetic technologies. As a practical approach, five antitranspirants, chemicals to reduce transpirational water loss, were evaluated for enhancing temporary water deficit tolerance in bedding plants. One physical [β-pinene polymer (βP)] and one physiological [a biological active form of abscisic acid (s-ABA)] antitranspirants enhanced tolerance to water deficit stress by blocking stomatal and inducing stomatal closure, respectively. However, βP caused floral damage and s-ABA caused chlorosis on the margin of leaves in some cultivars. To develop a more effective tool with minimal damage to plants, osmotic treatments was examined in viola. The osmotic treatment with high concentration of CaCl2 induced stomatal closure, resulting in delayed plant wilting through reduction of transpirational water loss and maintenance of high water content in plant leaves. In addition, CaCl2 treatment did not show any visual damage on plants. The osmotic treatment will allow floriculture crops to temporary tolerate water deficit stress without significant loss of postproduction quality. To investigate mechanisms of water stress responses at the molecular level, time-course transcriptome analysis was performed in petunia leaves at the early stage of water deficit. A total 6670 genes were differentially expressed under water deficit stress conditions. Gene Ontology analysis revealed that redox homeostasis processes with sulfur metabolism were enriched under water deficit stress. Genes encoding components of plant hormones, abscisic acid and ethylene, biosynthesis and signal transduction pathways were differentially expressed at the early stage of water deficit. Thirty-four transcription factor families were identified in petunia, and one member of AP2/ERF family, PhERF039, were selected for functional analysis. Under-expression of PhERF039 reduced stomatal conductance in addition to phenotypes related to development: delayed flowering and development of a rosette morphology. Although the result of stomatal conductance seemed to indicate that PhERF039 might play as a negative regulator in stomatal closure under water deficit, further analysis is required using additional independent transgenic lines. Transcriptomic data may provide insights into understanding water stress-responsive networks as well as opportunities to engineer floricultural crops with the enhanced water deficit tolerance traits