Functions and signaling mechanisms of a non-proteinogenic amino acid ACC: the case for a novel plant hormone

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Plants produce hormones such as ethylene that collectively control essentially all aspects of plant biology. This project advances a new hypothesis that a small molecule, which plants use to make ethylene, functions itself as a novel plant hormone that impacts plant growth and development, and further investigates how the hormone functions at the molecular level throughout the evolutionary breadth of plants. The project has the potential to transform the plant hormone field by altering existing dogma, by requiring the reevaluation of several decades of ethylene literature and by impacting how future ethylene experiments are carried out. The knowledge gained from this project could contribute to future strategies for modifying crop plants for the benefit of society. The project broadens participation of underrepresented students by providing two to three student internships per year in the lab of the principal investigator. The internships involve partnerships with Howard University (an Historically Black College or University) and Eleanor Roosevelt High School (a local public school in which over 70% of the students are underrepresented minorities). The interns carry out aspects of the project while receiving valuable training in scientific methods, data analysis and communication. The project also provides training and career preparation for two postdoctoral scientists, two graduate students and two University of Maryland undergraduates. To further broaden participation, the principal investigator co-teaches summer research workshops for Howard University undergraduates. The well-known ethylene precursor in the ethylene biosynthesis pathway is 1-aminocyclopropane-1-carboxylic acid (ACC), a non-proteinogenic amino acid. Compelling new data suggest that ACC itself may be an important signaling molecule that evolutionarily predated the ability of higher land plants to efficiently convert ACC to ethylene. In particular, ACC inhibits cellular differentiation and growth in the liverwort Marchantia polymorpha. This project tests the hypothesis that ACC is a novel plant hormone through analyses of ACC function, including an investigation of ACC signaling mechanisms. Insight into ACC function is provided by studies of ACC synthesis mutants and ACC responses in Marchantia, and in evolutionarily relevant species in the plant lineage (Chlamydomonas reinhardtii, Spirogyra pratensis, Physcomitrella patens and Arabidopsis thaliana), thus addressing the conservation of ACC function, while taking advantage of lower gene copy number in basal plants. In Arabidopsis, preliminary findings indicate that ACC can induce pollen tube growth concomitant with Ca2+ influx and can stimulate primary root growth. ACC activates Ca2+ currents in Arabidopsis root protoplasts, and notably, this activation is dependent on glutamate receptor-like (GLR) ionotropic channels. The project tests the hypothesis that ACC is a GLR ligand using a combination of molecular genetics, patch-clamping, ion-specific vibrating probes and live imaging of Ca2+. Genetic evidence is sought based on phenotypic comparisons of glr mutants and ACC synthesis mutants in the range of plant species. The activation of GLRs by ACC, leading to the regulation of Ca2+ signaling in pollen tube growth and/or root growth, would be a breakthrough in understanding mechanisms of ACC signaling, as well as GLR signaling, and would represent a ligand-operated system of Ca2+ regulation for which no consensual system currently exists in plants. The striking ACC response uncovered in Marchantia provides for a genetic screen for ACC signaling mutants, followed by gene cloning based on T-DNA tagging or whole genome sequencing.