Protein Phosphatase 2A Controls Ethylene Biosynthesis by Differentially Regulating the Turnover of ACC Synthase Isoforms

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Abstract
The gaseous hormone ethylene is one of the master regulators of development and physiology throughout the plant life cycle. Ethylene biosynthesis is stringently regulated to permit maintenance of low levels during most phases of vegetative growth but to allow for rapid peaks of high production at developmental transitions and under stress conditions. In most tissues ethylene is a negative regulator of cell expansion, thus low basal levels of ethylene biosynthesis in dark-grown seedlings are critical for optimal cell expansion during early seedling development. The committed steps in ethylene biosynthesis are performed by the enzymes 1-aminocyclopropane 1-carboxylate synthase (ACS) and 1-aminocyclopropane 1-carboxylate oxidase (ACO). The abundance of different ACS enzymes is tightly regulated both by transcriptional control and by post-translational modifications and proteasome-mediated degradation. Here we show that specific ACS isozymes are targets for regulation by protein phosphatase 2A (PP2A) during Arabidopsis thaliana seedling growth and that reduced PP2A function causes increased ACS activity in the roots curl in 1-N-naphthylphthalamic acid 1 (rcn1) mutant. Genetic analysis reveals that ethylene overproduction in PP2A-deficient plants requires ACS2 and ACS6, genes that encode ACS proteins known to be stabilized by phosphorylation, and proteolytic turnover of the ACS6 protein is retarded when PP2A activity is reduced. We find that PP2A and ACS6 proteins associate in seedlings and that RCN1-containing PP2A complexes specifically dephosphorylate a C-terminal ACS6 phosphopeptide. These results suggest that PP2A-dependent destabilization requires RCN1-dependent dephosphorylation of the ACS6 C-terminus. Surprisingly, rcn1 plants exhibit decreased accumulation of the ACS5 protein, suggesting that a regulatory phosphorylation event leads to ACS5 destabilization. Our data provide new insight into the circuitry that ensures dynamic control of ethylene synthesis during plant development, showing that PP2A mediates a finely tuned regulation of overall ethylene production by differentially affecting the stability of specific classes of ACS enzymes. Like animals, plants produce a number of substances that regulate growth and coordinate developmental transitions and responses to environmental signals. Ethylene gas is one such regulator of the plant life cycle, playing important roles in fruit ripening, pathogen defenses, and the regulation of cell expansion. Because overall plant form is determined largely by the degree and directionality of cell expansion, ethylene is a crucial regulator of morphology, and ethylene production must be maintained at low levels during phases of rapid cell expansion, such as early seedling growth. Recent work has identified molecular mechanisms that target ethylene biosynthetic enzymes for proteolytic degradation; this degradation plays a key role in controlling ethylene production. Here we exploit the molecular genetic resources available in the Arabidopsis thaliana system to identify a highly conserved protein complex that dephosphorylates target proteins as a new component of the mechanism that regulates degradation of ethylene-producing enzymes. Our findings show that protein phosphatase 2A plays a nuanced role in this regulatory circuit, with both positive and negative inputs into the stability of specific proteins that drive ethylene biosynthesis. This work enhances our understanding of the mechanisms that enforce adaptive levels of hormone production in plants.

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