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      The regulation of ethylene biosynthesis: a complex multilevel control circuitry


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          The gaseous plant hormone ethylene is produced by a fairly simple two‐step biosynthesis route. Despite this pathway’s simplicity, recent molecular and genetic studies have revealed that the regulation of ethylene biosynthesis is far more complex and occurs at different layers. Ethylene production is intimately linked with the homeostasis of its general precursor S‐adenosyl‐ l‐methionine (SAM), which experiences transcriptional and posttranslational control of its synthesising enzymes (SAM synthetase), as well as the metabolic flux through the adjacent Yang cycle. Ethylene biosynthesis continues from SAM by two dedicated enzymes: 1‐aminocyclopropane‐1‐carboxylic (ACC) synthase (ACS) and ACC oxidase (ACO). Although the transcriptional dynamics of ACS and ACO have been well documented, the first transcription factors that control ACS and ACO expression have only recently been discovered. Both ACS and ACO display a type‐specific posttranslational regulation that controls protein stability and activity. The nonproteinogenic amino acid ACC also shows a tight level of control through conjugation and translocation. Different players in ACC conjugation and transport have been identified over the years, however their molecular regulation and biological significance is unclear, yet relevant, as ACC can also signal independently of ethylene. In this review, we bring together historical reports and the latest findings on the complex regulation of the ethylene biosynthesis pathway in plants.

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          Ethylene Biosynthesis and its Regulation in Higher Plants

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            The oxylipin signal jasmonic acid is activated by an enzyme that conjugates it to isoleucine in Arabidopsis.

            Despite its importance in a variety of plant defense responses, our understanding of how jasmonic acid (JA) functions at the biochemical level is limited. Several amino acid conjugates of JA were tested for their ability to complement the JA-insensitive Arabidopsis thaliana mutant jar1-1. Unlike free JA, JA-Ile inhibited root growth in jar1-1 to the same extent as in the wild type, whereas JA-Val, JA-Leu, and JA-Phe were ineffective inhibitors in both genotypes. Thin-layer chromatography and gas chromatography-mass spectrometry (GC-MS) analysis of products produced in vitro by recombinant JAR1 demonstrated that this enzyme forms JA-amido conjugates with several amino acids, including JA-Ile. JA-Val, -Leu, -Ile, and -Phe were each quantified in Arabidopsis seedlings by GC-MS. JA-Ile was found at 29.6 pmole g(-1) fresh weight (FW) in the wild type but was more than sevenfold lower in two jar1 alleles. JA-Leu, -Val, and -Phe were present at only low levels in both genotypes. Expression of wild-type JAR1 in transgenic jar1-1 plants restored sensitivity to JA and elevated JA-Ile to the same level as in the wild type. The ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) conjugated to JA was also found in plant tissue at 18.4 pmole g(-1) FW. JA-ACC was determined not be an effective jasmonate root inhibitor, and surprisingly, was twofold higher in the mutants than in the wild type. This suggests that another JA-conjugating enzyme(s) is present in Arabidopsis. Synthesis of JA-ACC might provide a mechanism to coregulate the availability of JA and ACC for conversion to the active hormones JA-Ile and ethylene, respectively. We conclude that JAR1 is a JA-amino synthetase that is required to activate JA for optimal signaling in Arabidopsis. Plant hormone activation by conjugation to amino acids and the enzymes involved in their formation were previously unknown.
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              Polyamine Function in Plants: Metabolism, Regulation on Development, and Roles in Abiotic Stress Responses

              Polyamines (PAs) are low molecular weight aliphatic nitrogenous bases containing two or more amino groups. They are produced by organisms during metabolism and are present in almost all cells. Because they play important roles in diverse plant growth and developmental processes and in environmental stress responses, they are considered as a new kind of plant biostimulant. With the development of molecular biotechnology techniques, there is increasing evidence that PAs, whether applied exogenously or produced endogenously via genetic engineering, can positively affect plant growth, productivity, and stress tolerance. However, it is still not fully understood how PAs regulate plant growth and stress responses. In this review, we attempt to cover these information gaps and provide a comprehensive and critical assessment of the published literature on the relationships between PAs and plant flowering, embryo development, senescence, and responses to several (mainly abiotic) stresses. The aim of this review is to summarize how PAs improve plants' productivity, and to provide a basis for future research on the mechanism of action of PAs in plant growth and development. Future perspectives for PA research are also suggested.

                Author and article information

                New Phytol
                New Phytol
                The New Phytologist
                John Wiley and Sons Inc. (Hoboken )
                12 September 2020
                January 2021
                : 229
                : 2 ( doiID: 10.1111/nph.v229.2 )
                : 770-782
                [ 1 ] Molecular Plant Hormone Physiology Laboratory Division of Crop Biotechnics Department of Biosystems University of Leuven Willem de Croylaan 42 Leuven 3001 Belgium
                Author notes
                [*] [* ] Author for correspondence:

                Bram Van de Poel

                Email: bram.vandepoel@ 123456kuleuven.be


                These authors contributed equally to this work.

                Author information
                NPH16873 2020-33761
                © 2020 The Authors New Phytologist © 2020 New Phytologist Trust

                This is an open access article under the terms of the http://creativecommons.org/licenses/by/4.0/ License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

                : 26 June 2020
                : 04 August 2020
                Page count
                Figures: 4, Tables: 3, Pages: 13, Words: 12859
                Funded by: Fonds Wetenschappelijk Onderzoek , open-funder-registry 10.13039/501100003130;
                Award ID: G092419N &
                Award ID: G0G0219N
                Funded by: KU Leuven , open-funder-registry 10.13039/501100004040;
                Award ID: C14/18/056
                Research Review
                Research Reviews
                Custom metadata
                January 2021
                Converter:WILEY_ML3GV2_TO_JATSPMC version:5.9.6 mode:remove_FC converted:22.01.2021

                Plant science & Botany
                1‐aminocyclopropane‐1‐carboxylic acid (acc) metabolism,ethylene biosynthesis,posttranslational regulation,s‐adenosyl‐l‐methionine (sam) metabolism,transcriptional regulation,yang cycle


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