Regulation of ACS protein stability by cytokinin and brassinosteroid.

Maureen Hansen,Joseph J Kieber,Hyun Sook Chae

doi:10.1111/j.1365-313x.2008.03711.x

Abstract

A major question in plant biology is how phytohormone pathways interact. Here, we explore the mechanism by which cytokinins and brassinosteroids affect ethylene biosynthesis. Ethylene biosynthesis is regulated in response to a wide variety of endogenous and exogenous signals, including the levels of other phytohormones. Cytokinins act by increasing the stability of a subset of ACC synthases, which catalyze the generally rate-limiting step in ethylene biosynthesis. The induction of ethylene by cytokinin requires the canonical cytokinin two-component response pathway, including histidine kinases, histidine phosphotransfer proteins and response regulators. The cytokinin-induced myc-ACS5 stabilization occurs rapidly (<60 min), consistent with a primary output of this two-component signaling pathway. We examined the mechanism by which another phytohormone, brassinosteroid, elevates ethylene biosynthesis in etiolated seedlings. Similar to cytokinin, brassinosteroid acts post-transcriptionally by increasing the stability of ACS5 protein, and its effects on ACS5 were additive with those of cytokinin. These data suggest that ACS is regulated by phytohormones through regulatory inputs that probably act together to continuously adjust ethylene biosynthesis in various tissues and in response to various environmental conditions.

Highlights

Ethylene is a gaseous plant hormone that is involved in many growth and developmental processes, including germination, abscission, senescence, plant defense and fruit ripening (Abeles et al, 1992)
The protein sequence of the catalytic core is highly conserved, but the C-terminal region of ACC synthase (ACS) proteins is divergent and the proteins may be sub-divided into three groups on that basis: type 1 ACS proteins have the longest C-terminus with a single putative calcium-dependent protein kinase (CDPK) phosphorylation site and three mitogen-activated protein kinase (MAPK) phosphorylation sites, type 2 ACS proteins have an intermediate length C-terminus containing a single putative CDPK phosphorylation site, and type 3 ACS proteins have a very short C-terminus and no predicted kinase phosphorylation sites (Chae and Kieber, 2005; Liu and Zhang, 2004; Sebastià et al, 2004; Tatsuki and Mori, 2001; Yoshida et al, 2005)
We examine the impact of cytokinin-signaling mutants on ethylene biosynthesis and the kinetics of cytokinin-mediated ACS protein stability, and investigate points of intersection between cytokinin, BR and ethylene biosynthesis

Summary

We explore the mechanism by which cytokinins and brassinosteroids affect ethylene biosynthesis. Ethylene biosynthesis is regulated in response to a wide variety of endogenous and exogenous signals, including the levels of other phytohormones. The induction of ethylene by cytokinin requires the canonical cytokinin two-component response pathway, including histidine kinases, histidine phosphotransfer proteins and response regulators. Brassinosteroid acts post-transcriptionally by increasing the stability of ACS5 protein, and its effects on ACS5 were additive with those of cytokinin. These data suggest that ACS is regulated by phytohormones through regulatory inputs that probably act together to continuously adjust ethylene biosynthesis in various tissues and in response to various environmental conditions

Introduction

Results

Discussion

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Experimental procedures