Plants have evolved complex signaling pathways to coordinate responses to developmental and environmental information. The oxylipin pathway is one pivotal lipid-based signaling network that determines the plant's ability to adapt to various stimuli. Much of the research on oxylipins to date has focused on the branch pathway involving jasmonates. The parallel, stress-inducible HYDROPEROXIDE LYASE (HPL) branch pathway remains poorly characterized. The HPL enzyme catalyzes the cleavage of fatty acid hydroperoxides into aldehydes and oxoacids. These volatile metabolites are postulated to play key signaling roles both within and between plants, as well as in plant/insect interactions. This project will investigate: (a) the signal transduction machinery responsible for stress-inducible transcriptional regulation of the HPL gene, and (b) the signaling role of the derived HPL-pathway metabolites in mediating intra- and inter-plant stress responses. Preliminary experiments have confirmed that, in wild type (WT) Arabidopsis, the expression of HPL and its cognate pathway metabolites are stress-induced. To begin to identify regulatory components in this signaling network, an HPL::luciferase(LUC)-transgenic line was generated. By using these plants in an imaging based genetic screen mutants were identified that express LUC constitutively, suggesting constitutive activation or derepression of the HPL promoter. To characterize the signaling role of the aldehydes in stress responses, hpl knockout mutants were identified, and transgenic lines that overexpress rice HPL in WT Arabidopsis were generated, as well as in a mutant background incapable of producing jasmonates. These lines produce a range of aldehyde levels, thus providing an experimental system to define the link between endogenous aldehyde levels and target-gene expression. The specific objectives of this proposal are to define the molecular components responsible for stress-inducible transcriptional regulation of HPL, the transcriptional regulatory networks that mediate the aldehyde-induced gene expression program and the role of HPL-pathway metabolites in mediating inter-plant stress-responses. This project should provide important insights into the regulatory mechanisms controlling a complex and crucial signaling network that underlies plant responses to stress. Broader impacts of the project include the training of undergraduate and graduate students in state-of-the-art interdisciplinary research; discovery of novel components regulating production of commercially important aldehydes, and generation of tools for biotechnological applications in agronomically important crops; recruitment of underrepresented minority graduate students; collaboration between the private and academic sectors; broad dissemination of our research results in papers, seminars, conference presentations, and short courses on stress-induced signaling pathways.
The three most interesting outcomes of this project are as follows: 1- We demonstrated that jasmonates are indispensable metabolites in mediating the activation of direct plant-defense responses, whereas the HPL-derived metabolites are the predominant wound-inducible volatile signals that mediate indirect defense responses by directing tritrophic (plant-insect-herbivore) interactions 2-Manipulation of the levels of HPL-derived metabolites can be used as a tool to enhance plant tolerance to abiotic stresses such as drought. Based on these findings we have filed a patent through funds provided by Arcadia, a Davis-based biotechnology company. We are currently in the process of transforming a range of plant species with HPLs in order to test the efficacy of these genes in conferring drought- and salt-tolerance in agronomically important crops. 3- Oomycete pathogens contain arachidonic acid (AA), an elicitor of defense responses in plants. We demonstrated that exogenous application or endogenous production of AA leads to altered resistance of plants to biotic challengers through AA’s action on salicylate and jasmonate stress-signaling networks. Our data further expand the repertoire of signaling molecules known to trigger plant defenses and provide evidence that AA also regulates the general-stress transcriptional networks. Thus AA can be used as a vehicle to enhance plant tolerance to a range of biotic stresses. .
