Cytokinin signaling regulates cambial development in poplar

doi:10.1073/pnas.0805617106

Cytokinin signaling regulates cambial development in poplar

^aDepartment of Biological and Environmental Sciences/Institute of Biotechnology, University of Helsinki, 00790 Helsinki, Finland;
^bDepartment of Biochemistry, Faculty of Science, Palacky University, Slechtitelu 11, 783 71 Olomouc, The Czech Republic;
^cLaboratory of Growth Regulators, Palacky University and Institute of Experimental Botany ASCR, Slechtitelu 11, 783 71 Olomouc, The Czech Republic;
^dUmeå Plant Science Center, Department of Forest Genetics and Plant Physiology, 901 83 Umeå, Sweden;
^eFaculty of Biosciences, University of Joensuu, 80101 Joensuu, Finland;
^fDepartment of Biological Sciences, State University of New York, Buffalo, NY 14260; and
^gUmeå Plant Science Center, 901 83 Umeå, Sweden

Edited by Ronald R. Sederoff, North Carolina State University, Raleigh, NC, and approved October 31, 2008
↵¹M.L., L.K., P.T., and K.D. contributed equally to this work. (received for review June 10, 2008)

Abstract

Although a substantial proportion of plant biomass originates from the activity of vascular cambium, the molecular basis of radial plant growth is still largely unknown. To address whether cytokinins are required for cambial activity, we studied cytokinin signaling across the cambial zones of 2 tree species, poplar (Populus trichocarpa) and birch (Betula pendula). We observed an expression peak for genes encoding cytokinin receptors in the dividing cambial cells. We reduced cytokinin levels endogenously by engineering transgenic poplar trees (P. tremula × tremuloides) to express a cytokinin catabolic gene, Arabidopsis CYTOKININ OXIDASE 2, under the promoter of a birch CYTOKININ RECEPTOR 1 gene. Transgenic trees showed reduced concentration of a biologically active cytokinin, correlating with impaired cytokinin responsiveness. In these trees, both apical and radial growth was compromised. However, radial growth was more affected, as illustrated by a thinner stem diameter than in WT at same height. To dissect radial from apical growth inhibition, we performed a reciprocal grafting experiment. WT scion outgrew the diameter of transgenic stock, implicating cytokinin activity as a direct determinant of radial growth. The reduced radial growth correlated with a reduced number of cambial cell layers. Moreover, expression of a cytokinin primary response gene was dramatically reduced in the thin-stemmed transgenic trees. Thus, a reduced level of cytokinin signaling is the primary basis for the impaired cambial growth observed. Together, our results show that cytokinins are major hormonal regulators required for cambial development.

Footnotes

²To whom correspondence should be addressed at:
Department of Biological and Environmental Sciences/Institute of Biotechnology, University of Helsinki, 00790 Helsinki, Finland.
E-mail: yrjo.helariutta{at}helsinki.fi
Author contributions: K.N., J.I., A.E., M.D., K.L., R.P.B., K.K., and Y.H. designed research; K.N., J.I., M.L., L.K., P.T., K.D., and S.T. performed research; K.N., J.I., P.T., K.D., and V.A.A. analyzed data; and K.N., J.I., V.A.A., and Y.H. wrote the paper.
Data deposition: The sequences reported in this paper have been deposited in the GenBank database [Betula pendula CYTOKININ RECEPTOR 1 (BpCRE1) genomic EU583454, cDNA EU583455; HISTIDINE KINASE 2 (BpHK2) cDNA EU583456; HISTIDINE KINASE 3 (BpHK3) cDNA EU583457; and TUBULIN ALPHA (BpTUA) cDNA FJ228477].
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0805617106/DCSupplemental.