CHASE domain-containing receptors play an essential role in the cytokinin response of the moss Physcomitrella patens.

Klaus Von Schwartzenberg,Ondřej Novák,Martine Gonneau,Fabien Nogué,Njuscha Gruhn,Alexander Heyl,Ann-Cathrin Lindner,Miroslav Strnad,Jan Šimura

doi:10.1093/jxb/erv479

Abstract

While the molecular basis for cytokinin action is quite well understood in flowering plants, little is known about the cytokinin signal transduction in early diverging land plants. The genome of the bryophyte Physcomitrella patens (Hedw.) B.S. encodes three classical cytokinin receptors, the CHASE domain-containing histidine kinases, CHK1, CHK2, and CHK3. In a complementation assay with protoplasts of receptor-deficient Arabidopsis thaliana as well as in cytokinin binding assays, we found evidence that CHK1 and CHK2 receptors can function in cytokinin perception. Using gene targeting, we generated a collection of CHK knockout mutants comprising single (Δchk1, Δchk2, Δchk3), double (Δchk1,2, Δchk1,3, Δchk2,3), and triple (Δchk1,2,3) mutants. Mutants were characterized for their cytokinin response and differentiation capacities. While the wild type did not grow on high doses of cytokinin (1 µM benzyladenine), the Δchk1,2,3 mutant exhibited normal protonema growth. Bud induction assays showed that all three cytokinin receptors contribute to the triggering of budding, albeit to different extents. Furthermore, while the triple mutant showed no response in this bioassay, the remaining mutants displayed budding responses in a diverse manner to different types and concentrations of cytokinins. Determination of cytokinin levels in mutants showed no drastic changes for any of the cytokinins; thus, in contrast to Arabidopsis, revealing only small impacts of cytokinin signaling on homeostasis. In summary, our study provides a first insight into the molecular action of cytokinin in an early diverging land plant and demonstrates that CHK receptors play an essential role in bud induction and gametophore development.

Highlights

Phytohormones regulate many processes in plants such as the development of tissues and organs and the response to changes in the environment
In a complementation assay with protoplasts of receptor-deficient Arabidopsis thaliana as well as in cytokinin binding assays, we found evidence that CHK1 and CHK2 receptors can function in cytokinin perception
Our results show that the proteins can function as cytokinin receptors in different assays, and analysis of single, double, and the triple mutants demonstrated that CHK1, CHK2, and CHK3 are necessary for cytokinin perception by the moss

Summary

Introduction

Phytohormones regulate many processes in plants such as the development of tissues and organs and the response to changes in the environment. For Arabidopsis thaliana, the current model of this signaling pathway predicts that the cytokinin ligand is bound by hybrid histidine kinase receptors via the cyclases/histidine kinases associated sensory extracellular (CHASE) domain (Anantharaman and Aravind, 2001; Mougel and Zhulin, 2001; Heyl et al, 2007). These CHASE domain-containing histidine kinases (CHKs) were shown to localize mainly to the endoplasmic reticulum (ER) (Caesar et al, 2011; Lomin et al, 2011; Wulfetange et al, 2011). Most of the research on this signaling pathway has been done using the model plant Arabidopsis, but work in other plants species contributed to the elucidation of the functioning of the pathway (Heyl et al, 2006a; Hellmann et al, 2010)

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