The Ethylene Biosynthetic Enzymes, 1-Aminocyclopropane-1-Carboxylate (ACC) Synthase (ACS) and ACC Oxidase (ACO): The Less Explored Players in Abiotic Stress Tolerance

Abstract

Ethylene is an essential plant hormone, critical in various physiological processes. These processes include seed germination, leaf senescence, fruit ripening, and the plant's response to environmental stressors. Ethylene biosynthesis is tightly regulated by two key enzymes, namely 1-aminocyclopropane-1-carboxylate synthase (ACS) and 1-aminocyclopropane-1-carboxylate oxidase (ACO). Initially, the prevailing hypothesis suggested that ACS is the limiting factor in the ethylene biosynthesis pathway. Nevertheless, accumulating evidence from various studies has demonstrated that ACO, under specific circumstances, acts as the rate-limiting enzyme in ethylene production. Under normal developmental processes, ACS and ACO collaborate to maintain balanced ethylene production, ensuring proper plant growth and physiology. However, under abiotic stress conditions, such as drought, salinity, extreme temperatures, or pathogen attack, the regulation of ethylene biosynthesis becomes critical for plants' survival. This review highlights the structural characteristics and examines the transcriptional, post-transcriptional, and post-translational regulation of ACS and ACO and their role under abiotic stress conditions. Reviews on the role of ethylene signaling in abiotic stress adaptation are available. However, a review delineating the role of ACS and ACO in abiotic stress acclimation is unavailable. Exploring how particular ACS and ACO isoforms contribute to a specific plant's response to various abiotic stresses and understanding how they are regulated can guide the development of focused strategies. These strategies aim to enhance a plant's ability to cope with environmental challenges more effectively.

Keywords: ACS and ACO regulation; abiotic stress; ethylene biosynthesis; growth and development; nutrient starvation.

Figure 1

Ethylene biosynthesis pathway.

Figure 2

Post-translational regulation of ACS. (a) Arabidopsis type I ACS transcriptional and translational control by stimuli-responsive MAPK3/6 (mitogen-activated protein kinase) pathway. The MPK3/MPK6 phosphorylation of ACS2/ACS6 results in the stability of the ACS protein. In addition, MPK3/MPK6 activation through WRKY33, another MPK3/MPK6 substrate, similarly upregulates the expression of the ACS2 and ACS6 genes. Cellular ACS activity and ethylene production are significantly increased by dual-level modulation of Type I ACS by MAPKs and CDPKs (calcium-dependent protein kinase). There have also been discovered phosphatases (PP2A/C) that play a role in the dephosphorylation of ACS2/ACS6. (b) ETO1 (ETHYLENE OVERPRODUCER 1) containing E3 ligases that are capable of recognizing the TOE (Target of ETO1) domain in the C-termini of Type II ACSs, such as Arabidopsis ACS5, stabilize these proteins. The stability of Type II ACS protein is hypothesized to be controlled by CDPK phosphorylation, which is thought to play a role in this ubiquitination process. (c) Type III ACS isoform phosphorylation and stability modulation. A CDPK may phosphorylate the catalytic domain of the Type III ACS Arabidopsis ACS7, which is thought to be involved in ethylene production during root gravitropism. The ubiquitin-26S proteasome pathway, which requires the XBAT32 (XB3 orthologs 1 in A. thaliana) E3 ligase, may degrade ACS7.