Cytokinins are central regulators of cambial activity

Abstract

The roots and stems of dicotyledonous plants thicken by the cell proliferation in the cambium. Cambial proliferation changes in response to environmental factors; however, the molecular mechanisms that regulate cambial activity are largely unknown. The quadruple Arabidopsis thaliana mutant atipt1;3;5;7, in which 4 genes encoding cytokinin biosynthetic isopentenyltransferases are disrupted by T-DNA insertion, was unable to form cambium and showed reduced thickening of the root and stem. The atipt3 single mutant, which has moderately decreased levels of cytokinins, exhibited decreased root thickening without any other recognizable morphological changes. Addition of exogenously supplied cytokinins to atipt1;3;5;7 reactivated the cambium in a dosedependent manner. When an atipt1;3;5;7 shoot scion was grafted onto WT root stock, both the root and shoot grew normally and trans-zeatin-type (tZ-type) cytokinins in the shoot were restored to WT levels, but isopentenyladenine-type cytokinins in the shoot remained unchanged. Conversely, when a WT shoot was grafted onto an atipt1;3;5;7 root, both the root and shoot grew normally and isopentenyladenine-type cytokinins in the root were restored to WT levels, but tZ-type cytokinins were only partially restored. Collectively, it can be concluded that cytokinins are important regulators of cambium development and that production of cytokinins in either the root or shoot is sufficient for normal development of both the root and shoot.cambium ͉ isopentenyladenine ͉ phytohormone ͉ zeatin

Genome sequencing and population genomic analyses provide insights into the adaptive landscape of silver birch

“…Notably, several key components of cytokinin signaling, a major positive regulator of vascular cambium activity and radial tree-trunk growth 65,78 , show evidence of recent natural selection. Other examples are KAK and MED5A, which can elicit pleiotropic growth and cell wall phenotypes in Arabidopsis.…”

Arabidopsis ABCG14 protein controls the acropetal translocation of root-synthesized cytokinins

“…1i-l). This syndrome largely resembles secondary growth retardation observed in the atipt1;3;5;7 quadruple mutant, where the key genes encoding ATP/ADP:isopentenyltransferases (IPTs) responsible for synthesizing cytokinins are disrupted, leading to low levels of cytokinins and reducing cambial activity in the shoot 4 . The altered secondary thickening of the atabcg14 mutant points to a decline in fascicular-and interfascicular-cambial activity, implying a potential shortage of cytokinins in atabcg14 shoots.…”

“…This affords us anatomical evidence of cytokinin perturbation in the mutant lines, because, as well as regulating root meristems, cytokinins are key regulators of shoot meristem activity, but in an opposite manner 22,26 . Cytokinin deficiencies reportedly diminish the activities of vegetative and floral shoot meristems, including cambium-mediated secondary growth 4,22,26 . Furthermore, the numbers of cells in phloem and xylem in the vascular bundle were reduced; lignin synthesis and deposition (key processes in cell differentiation and secondary cell wall thickening) were considerably delayed, and the numbers of lignified cells were substantially reduced in the interfascicular fibres and xylem bundles of the mutant plants ( Fig.…”

“…Chemical profiling of xylem and phloem sap, and reciprocal grafting of roots and shoots of cytokinin-deficient mutants revealed that tZ-type cytokinins are mainly synthesized in roots, and iP-type cytokinins mainly in shoots. They then, respectively, move acropetally through the xylem and basipetally or systemically through the phloem, and are distributed to the action sites 4,10 . Therefore, cytokinins act as both local and long-distance signals in regulating cell division and differentiation.…”

“…Since their discovery in the 1950s as key promoters of cell division in plants, a plethora of biological functions have been attributed to these signalling molecules, encompassing the regulation of cell division and differentiation 4 , chloroplast development 5 , leaf senescence 5,6 , root-and shootapical dominance 7 , nutrient balances and stress responses 8 . Plants respond to cytokinins through a two-component signalling pathway: cytokinin binding to histidine-kinase receptors induces a phosphorelay that activates two types of response regulators (RRs), and consequently triggers the signal responses 9 .…”