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.

In seed plants, the canonical role of 1-aminocyclopropane-1-carboxylic acid (ACC) is to serve as the precursor in the biosynthesis of the phytohormone ethylene, and indeed, ACC treatment is often used as a proxy for ethylene treatment. Increasing evidence suggests that ACC can also act independently of ethylene to regulate various aspects of plant growth and development. Here, we explore the effects of ACC on Arabidopsis thaliana root growth and the mechanisms by which it acts. ACC inhibits growth of the primary root in Arabidopsis seedlings when ethylene signaling is blocked, which becomes evident after 36 h of treatment with ACC. This reduced root growth is in part the result of suppressed cell proliferation in the root meristem resulting from altered expression of a key regulator of stem cell niche activity, WOX5. ACC also promotes lateral root (LR) development, in contrast to ethylene, which inhibits LR formation. Transcriptomic analysis of roots revealed no significant changes in gene expression after 45 min or 4 h of ACC treatment, but longer treatment times revealed a large number of differentially expressed genes, including the downregulation of the expression of a small group of phylogenetically related CLE peptides. Reduced expression of these group 1 CLEs in response to ACC leads to the activation of a transcription factor, LBD18, which promotes LR development. These results suggest that ACC acts to modulate multiple aspects of Arabidopsis root growth independently of ethylene via distinct transcriptional effects in the root meristem and LR precursor cells.

In seed plants, ethylene is produced from 1-aminocyclopropane-1-carboxylic acid (ACC) by the enzyme ACC oxidase (ACO). Despite the critical role of ACO in ethylene biosynthesis, the ACO gene family has not been fully characterized in Arabidopsis (Arabidopsis thaliana). This study investigated the 5 ACO genes in Arabidopsis, revealing distinct tissue-specific and developmental expression patterns. Each ACO enzyme exhibited a unique enzymatic capacity for ethylene production, facilitating isoform-specific regulation of ethylene biosynthesis. At the subcellular level, ACO localized predominantly in the cytosol, where ethylene biosynthesis likely occurs, but, unexpectedly, also in the nucleus. Through reverse genetics, including single and higher-order aco mutants, we observed a high degree of gene redundancy, sustaining ethylene biosynthesis. Disruption of all 5 ACO genes resulted in plants unable to produce ethylene but did not adversely affect seedling, vegetative, or reproductive development. However, some development processes associated with high rates of ethylene production, such as germination and petal abscission, were impaired in the aco quintuple mutant, while others, such as leaf senescence, were not. This suggests that modulation of ethylene emission rates by ACOs is key in determining specific developmental processes. Furthermore, the aco quintuple mutant showed impaired responses to abiotic (eg nutrient deficiency and metal toxicity) and biotic stress (eg Botrytis cinerea), akin to ethylene-insensitive plants. This highlights the pivotal role of ethylene in modulating stress responses. In conclusion, the ACO gene family plays a vital role in fine-tuning ethylene biosynthesis in a spatial–temporal way, thereby modulating plant development and stress resilience.

The plant hormone ethylene elicits crucial regulatory effects on plant growth, development, and stress resistance. As the enzyme that catalyzes the final step of ethylene biosynthesis, 1-Aminocyclopropane-1-carboxylic acid oxidase (ACO) plays a key role in precisely controlling ethylene production. However, the functional characterization of the ACO gene family in rice remains largely unexplored. In this study, we performed a phylogenetic analysis of seven OsACO genes (OsACO1–OsACO7), which were classified into three subfamilies (Types I, II, and III). The members within the same clades exhibited similar tertiary structures and conserved protein motifs. We conducted inter/intraspecies covariance assays of OsACOs to elucidate their evolutionary and duplication events. Numerous cis-acting elements identified in OsACO promoter regions are associated with development, hormonal stimuli, and environmental responses. The expression assay by RT-qPCR revealed that OsACO genes exhibited tissue-specific expression and were significantly altered under various abiotic stresses, indicating their potential involvement in these processes regulated at the transcriptional level. Additionally, we predicted candidate-targeting miRNAs and identified putative cysteine sites of S-nitrosylation (SNO) and S-sulfhydration (SSH) in OsACOs, providing insights into their post-transcriptional and post-translational regulatory mechanisms. These findings pave the way for the further investigation of OsACO functions and their potential applications in improving rice growth and stress resilience by modulating ethylene biosynthesis.