Cytokinin signaling regulates cambial development in poplar

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 OXI-DASE 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.cambial activity ͉ cambium ͉ secondary development ͉ Populus ͉ CYTOKININ OXIDASE

Mobile Gibberellin Directly StimulatesArabidopsisHypocotyl Xylem Expansion

“…Other hormone pathways that impinge on vascular development in Arabidopsis have been shown to play an equally important role in Populus (Schrader et al, 2003;Dettmer et al, 2009;Nieminen et al, 2008;Nilsson et al, 2008). Moreover, despite being an annual, herbaceous plant, Arabidopsis displays significant secondary growth, including lignification of cells walls, suggesting that Arabidopsis can serve as a model to investigate this trait (Chaffey et al, 2002;Nieminen et al, 2004).…”

TDIF Peptide Signaling Regulates Vascular Stem Cell Proliferation via theWOX4Homeobox Gene inArabidopsis

“…Various signaling molecules, such as plant hormones and secretory peptides, have been implicated in the regulation of this process (De Smet et al, 2009;Wolters and Jü rgens, 2009). Similarly, cellular proliferation and differentiation in the vascular meristem are regulated by hormonal signals (Uggla et al, 1996;Fukuda, 2004;Bjö rklund et al, 2007;Matsumoto-Kitano et al, 2008;Nieminen et al, 2008;Nilsson et al, 2008;Elo et al, 2009;Love et al, 2009). Recent studies have uncovered the importance of an intercellular signaling system consisting of a small peptide TDIF and its receptor PXY/TDR in determining the fates of procambial cells, although it remains elusive whether the TDIF signal regulates directly the identity of stem cells (Ito et al, 2006;Fisher and Turner, 2007;Hirakawa et al, 2008;Whitford et al, 2008).…”

Signaling and gene regulatory programs in plant vascular stem cells

“…Ectopic expression experiments of cytokinin oxidase 2 in Populus suggest a positive correlation between cytokinin levels and cell division activities in the vascular stem cells (Nieminen et al, 2008). Transgenic Populus with a reduced endogenous cytokinin level severely compromised vascular stem cell divisions and the secondary growth, leading to the development of thinner stems.…”