The gaseous hormone ethylene triggers numerous responses in plants. These responses are often assessed in flowering plants by treating with the precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), due to the rapid conversion of ACC to ethylene. However, there have been several reports of ethylene-independent ACC responses. By analyzing ACC SYNTHASE (acs) mutants that produce fewer seeds, we uncovered ethylene-independent ACC signaling during reproduction in Arabidopsis thaliana. We found that acs octuple mutants have a high proportion of unfertilized ovules due to a defect in the female sporophyte. In pollen tube guidance assays, we showed that ovular ACC has a signaling role in pollen tube targeting of ovules. The acs octuple mutant displays reduced trafficking of the LURE1.2 peptide (a known pollen tube chemoattractant) from synergid cells to the filiform apparatus and micropyle, suggesting a possible basis for reduced attraction. Treatment with ACC, but not ethylene, rescued both LURE1.2 trafficking and pollen tube attraction. Although not produced by animals, ACC is a partial agonist of glutamate receptors, which are ligand-gated ion channels in neuronal Ca2+ signaling. We therefore tested whether ACC, which is a non-proteinogenic amino acid, could be a ligand of plant GLUTAMATE RECEPTOR-LIKE (GLR) channels. Patch-clamp experiments showed that ACC activates GLR-dependent ion currents in Arabidopsis protoplasts, and Ca2+ imaging in mammalian COS cells showed that ACC induces GLR-dependent Ca2+ elevation. By screening ACC and all 20 proteinogenic amino acids for the ability to stimulate Physcomitrella patens GLR1 in COS cells, ACC induced the highest PpGLR-dependent cytosolic Ca2+ elevation. In ovules, ACC induced transient Ca2+ elevation, and treating with a Ca2+ ionophore alone rescued LURE1.2 trafficking in acs octuple mutant ovules, suggesting ACC-induced Ca2+ elevation is involved in pollen tube attraction. These findings uncover a novel ACC signaling function and provide insights for studying the physiological roles of ACC in plants.
Coauthors: Wangshu Mou, PhD – University of Maryland;Yun-Ting Kao, PhD – University of Maryland;Erwan Michard, PhD – University of Maryland;Alexander Simon, Bachelors Degree – University of Maryland;Dongdong Li, PhD – University of Maryland;Michael Wudick, PhD – University of Maryland;Michael Lizzio – University of Maryland;José Feijó, PhD – University of Maryland