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

Kyle R Skottke,Gyeong Mee Yoon,Joseph J Kieber,Alison Delong,Joseph R Ecker

doi:10.1371/journal.pgen.1001370

Kyle R Skottke, Gyeong Mee Yoon + Show 3 more

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https://doi.org/10.1371/journal.pgen.1001370

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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.

Highlights

Ethylene gas is a crucial regulator of numerous aspects of plant development and physiology, including germination, seedling growth and morphology, organ senescence and fruit ripening, as well as stress and defense responses [1]
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
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

Summary

Introduction

Ethylene gas is a crucial regulator of numerous aspects of plant development and physiology, including germination, seedling growth and morphology, organ senescence and fruit ripening, as well as stress and defense responses [1]. The biosynthetic capacity for ethylene production is nearly ubiquitous throughout the plant body, but biosynthesis is generally maintained at low levels through regulatory circuitry that confers tight control while allowing rapid and dramatic increases under conditions such as wounding or fruit ripening. Ethylene is derived from methionine via a well-characterized biosynthetic pathway (reviewed in references [4,5,6,7]) in which the first committed step, conversion of S-adenosyl methionine to 1aminocyclopropane 1-carboxylate (ACC), is performed by the enzyme ACC synthase (ACS; see Figure 1). Under some conditions ( in fruit and flowers), ACO activity may be rate limiting, but ACC synthesis is generally the rate-limiting step for ethylene production during vegetative growth. In seedlings, increasing ACS protein levels drive ethylene synthesis to high levels [8,9,10]

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