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

Abstract

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 ROCK4) or of a gene encoding a constitutively active cytokinin receptor variant (ROCK3). Both genes are derived from Arabidopsis thaliana and were expressed in poplar trees under the control of their own promoter or the cambium-specific pHB8 promoter. pIPT3:ROCK4- and pHB8:ROCK4-expressing plants were smaller than wild-type plants and formed more lateral branches; pHB8:ROCK4 transgenic plants additionally showed an increased stem diameter. In contrast, pAHK3:ROCK3- and pHB8:ROCK3-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 ROCK3 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 ROCK3 as a novel gene to enhance biomass formation in woody plants.

Wood of trees: Cellular structure, molecular formation, and genetic engineering

“…Transgenic Populus expressing cytokinin oxidase/dehydrogenase 2 ( CKX2) , encoding an enzyme to degrade cytokinin, driven by the phloem‐specific CLE41b promoter shows disruption of the cytokinin gradient pattern and restriction of the cambial activity (Fu et al, 2021), and expressing CKX2 under the promoter of a birch CRE1 gene in Populus impairs secondary growth (Nieminen et al, 2008). In another study, expression of the cytokinin biosynthesis genes ROCK4 and ROCK3 under the control of the cambium‐specific HB8 promoter in Populus led to increased radial stem growth (Riefler et al, 2022). These studies indicate that the cytokinin level in the cambium area is critical to cambium activity.…”