Abstract
The phytohormone ethylene has numerous effects on plant growth and development. Its immediate precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), is a non-proteinogenic amino acid produced by ACC SYNTHASE (ACS). ACC is often used to induce ethylene responses. Here, we demonstrate that ACC exhibits ethylene-independent signaling in Arabidopsis thaliana reproduction. By analyzing an acs octuple mutant with reduced seed set, we find that ACC signaling in ovular sporophytic tissue is involved in pollen tube attraction, and promotes secretion of the pollen tube chemoattractant LURE1.2. ACC activates Ca2+-containing ion currents via GLUTAMATE RECEPTOR-LIKE (GLR) channels in root protoplasts. In COS-7 cells expressing moss PpGLR1, ACC induces the highest cytosolic Ca2+ elevation compared to all twenty proteinogenic amino acids. In ovules, ACC stimulates transient Ca2+ elevation, and Ca2+ influx in octuple mutant ovules rescues LURE1.2 secretion. These findings uncover a novel ACC function and provide insights for unraveling new physiological implications of ACC in plants.
Highlights
The phytohormone ethylene has numerous effects on plant growth and development
Consistent with ACC being needed in the ovule sporophytic tissue, a GUS reporter fused to the ACS8 promoter was localized to the sporophytic tissue of the ovule (Supplementary Fig. 7g). (Expression of ACS11, the other gene knocked down in the octuple mutant, has not been detected in ovules25.) These results suggested that ACC is an inducing factor in LURE1.2 trafficking, and indicate that prevention of LURE1.2 trafficking may contribute to reduced pollen tube guidance toward the micropyle of acs octuple mutant ovules
Given ACC’s signaling function in plants, we investigated whether ACC is capable of stimulating the plant homologs of ionotropic glutamate receptors (iGluRs) known as GLUTAMATE RECEPTOR-LIKE (GLR) channels
Summary
The phytohormone ethylene has numerous effects on plant growth and development. Its immediate precursor, 1-aminocyclopropane-1-carboxylic acid (ACC), is a non-proteinogenic amino acid produced by ACC SYNTHASE (ACS). Responses to ethylene are often assessed by treating plants with ACC, due to the relative ease of application and rapid conversion of ACC to ethylene[2] Exceptions to this rule, such as ethylene-independent ACC responses in roots[3,4,5], guard cells[6], and reproduction[7] have been reported. In the latter case, an Arabidopsis thaliana octuple mutant of all eight functional ACS genes was generated by expressing an artificial microRNA (amiRNA) that reduced expression of both ACS8 and ACS11 in an acs hextuple knockout mutant background[7].
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