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DEVELOPMENT AND HORMONE ACTION

The Cytokinin Type-B Response Regulator PtRR13 Is a Negative Regulator of Adventitious Root Development in Populus^{1,[C],[W],[OA]}

Plant Molecular and Cellular Biology Program (G.A.R.-C., J.M.D.) and School of Forest Resources and Conservation (A.M.M., C.D., J.M.D.), University of Florida, Gainesville, Florida 32611

Adventitious root formation at the base of plant cuttings is an innate de novo organogenesis process that allows massive vegetative propagation of many economically and ecologically important species. The early molecular events following shoot excision are not well understood. Using whole-genome microarrays, we detected significant transcriptome remodeling during 48 h following shoot removal in Populus tremula x Populus alba softwood cuttings in the absence of exogenous auxin, with 27% and 36% of the gene models showing differential abundance between 0 and 6 h and between 6 and 24 h, respectively. During these two time intervals, gene networks involved in protein turnover, protein phosphorylation, molecular transport, and translation were among the most significantly regulated. Transgenic lines expressing a constitutively active form of the Populus type-B cytokinin response regulator PtRR13 (DDKPtRR13) have a delayed rooting phenotype and cause misregulation of CONTINUOUS VASCULAR RING1, a negative regulator of vascularization; PLEIOTROPIC DRUG RESISTANCE TRANSPORTER9, an auxin efflux transporter; and two APETALA2/ETHYLENE RESPONSE FACTOR genes with sequence similarity to TINY. Inappropriate cytokinin action via DDKPtRR13 expression appeared to disrupt adventitious root development 24 h after shoot excision, when root founder cells are hypothesized to be sensitive to the negative effects of cytokinin. Our results are consistent with PtRR13 acting downstream of cytokinin to repress adventitious root formation in intact plants, and that reduced cytokinin signaling after shoot excision enables coordinated expression of ethylene, auxin, and vascularization pathways leading to adventitious root development.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: John M. Davis (jmdavis{at}ufl.edu).

^[C] Some figures in this article are displayed in color online but in black and white in the print edition.

^[W] The online version of this article contains Web-only data.

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www.plantphysiol.org/cgi/doi/10.1104/pp.109.137505

^* Corresponding author; e-mail jmdavis{at}ufl.edu.

Received February 20, 2009; accepted April 20, 2009; published April 24, 2009.