A Constitutively Active Cytokinin Receptor Variant Increases Cambial Activity and Stem Growth in Poplar.

The cambial meristem is responsible for bark and wood formation in woody plants. The activity of the cambial meristem is controlled by various factors; one of them is the plant hormone cytokinin. Here, we have explored different approaches to genetically engineering cambial activity in poplar plants by the ectopic expression of a cytokinin biosynthesis gene with enhanced activity (named ) or of a gene encoding a constitutively active cytokinin receptor variant (). Both genes are derived from and were expressed in poplar trees under the control of their own promoter or the cambium-specific promoter. - and -expressing plants were smaller than wild-type plants and formed more lateral branches; transgenic plants additionally showed an increased stem diameter. In contrast, - and -expressing plants grew taller than wild type without an altered branching pattern and formed more cambial cells, leading to increased radial stem growth. The effectivity of when expressed in either secondary phloem cells or in cambial cells is consistent with a dual, tissue-autonomous and non-autonomous activity of cytokinin in regulating cambial activity. We propose as a novel gene to enhance biomass formation in woody plants.