AUTHOR=Gu Shin-Yuan , Wang Long-Chi , Cheuh Chiao-Mei , Lo Wan-Sheng 

TITLE=CHITINASE LIKE1 Regulates Root Development of Dark-Grown Seedlings by Modulating Ethylene Biosynthesis in Arabidopsis thaliana

JOURNAL=Frontiers in Plant Science

VOLUME=10

YEAR=2019

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2019.00600

DOI=10.3389/fpls.2019.00600

ISSN=1664-462X

ABSTRACT=<p>The plant hormone ethylene plays a regulatory role in development in light- and dark-grown seedlings. We previously isolated a group of small-molecule compounds with a quinazolinone backbone, which were named acsinones (for <underline>AC</underline>C <underline>s</underline>ynthase <underline>in</underline>hibitor quinazoli<underline>nones</underline>), that act as uncompetitive inhibitors of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS). Thus, the triple response phenotype, which consists of shortened hypocotyls and roots, radial swelling of hypocotyls and exaggerated curvature of apical hooks, was suppressed by acsinones in dark-grown (etiolated) <italic>ethylene overproducer</italic> (<italic>eto</italic>) seedlings. Here, we describe our isolation and characterization of an Arabidopsis <italic><underline>r</underline>evert to <underline>et</underline>o1 9</italic> (<italic>ret9</italic>) mutant, which showed reduced sensitivity to acsinones in etiolated <italic>eto1</italic> seedlings. Map-based cloning of <italic>RET9</italic> revealed an amino acid substitution in <italic>CHITINASE LIKE1</italic> (<italic>CTL1</italic>), which is required for cell wall biogenesis and stress resistance in Arabidopsis. Etiolated seedlings of <italic>ctl1<sup>ret<italic>9</italic></sup></italic> showed short hypocotyls and roots, which were augmented in combination with <italic>eto1-4</italic>. Consistently, <italic>ctl1<sup>ret<italic>9</italic></sup></italic> seedlings showed enhanced sensitivity to exogenous ACC to suppress primary root elongation as compared with the wild type. After introducing <italic>ctl1<sup>ret<italic>9</italic></sup></italic> to mutants completely insensitive to ethylene, genetic analysis indicated that an intact ethylene response pathway is essential for the alterations in root and apical hook but not hypocotyl in etiolated <italic>ctl1<sup>ret<italic>9</italic></sup></italic> seedlings. Furthermore, a mild yet significantly increased ethylene level in <italic>ctl1</italic> mutants was related to elevated mRNA level and activity of ACC oxidase (ACO). Moreover, genes associated with ethylene biosynthesis (<italic>ACO1</italic> and <italic>ACO2</italic>) and response (<italic>ERF1</italic> and <italic>EDF1</italic>) were upregulated in etiolated <italic>ctl1<sup>ret<italic>9</italic></sup></italic> seedlings. By characterizing a new recessive allele of <italic>CTL1</italic>, we reveal that CTL1 negatively regulates ACO activity and the ethylene response, which thus contributes to understanding a role for ethylene in root elongation in response to perturbed cell wall integrity.</p>