Tissue- and Cell-Specific Cytokinin Activity in Populus × canescens Monitored by ARR5::GUS Reporter Lines in Summer and Winter

Abstract

Cytokinins play an important role in vascular development. But knowledge on the cellular localization of this growth hormone in the stem and other organs of woody plants is lacking. The main focus of this study was to investigate the occurrence and cellular localization of active cytokinins in leaves, roots, and along the stem of Populus × canescens and to find out how the pattern is changed between summer and winter. An ARR5::GUS reporter construct was used to monitor distribution of active cytokinins in different tissues of transgenic poplar lines. Three transgenic lines tested under outdoor conditions showed no influence of ARR5::GUS reporter construct on the growth performance compared with the wild-type, but one line lost the reporter activity. ARR5::GUS activity indicated changes in the tissue- and cell type-specific pattern of cytokinin activity during dormancy compared with the growth phase. ARR5::GUS activity, which was present in the root tips in the growing season, disappeared in winter. In the stem apex ground tissue, ARR5::GUS activity was higher in winter than in summer. Immature leaves from tissue-culture grown plants showed inducible ARR5::GUS activity. Leaf primordia in summer showed ARR5::GUS activity, but not the expanded leaves of outdoor plants or leaf primordia in winter. In stem cross sections, the most prominent ARR5::GUS activity was detected in the cortex region and in the rays of bark in summer and in winter. In the cambial zone the ARR5::GUS activity was more pronounced in the dormant than in growth phase. The pith and the ray cells adjacent to the vessels also displayed ARR5::GUS activity. In silico analyses of the tissue-specific expression patterns of the whole PtRR type-A family of poplar showed that PtRR10, the closest ortholog to the Arabidopsis ARR5 gene, was usually the most highly expressed gene in all tissues. In conclusion, gene expression and tissue-localization indicate high activity of cytokinins not only in summer, but also in winter. The presence of the signal in meristematic tissues supports their role in meristem maintenance. The reporter lines will be useful to study the involvement of cytokinins in acclimation of poplar growth to stress.

Phytohormone metabolism in human cells: Cytokinins are taken up and interconverted in HeLa cell culture

“…However, the detection of the nucleotide fraction of iP, iPRP, within the cell pellet, as well as the other FB, R, and 2MeS CK forms suggests that the complexity of mammalian CK production and metabolism is greater than previously understood. Furthermore, in the recent report on CK profiles in canine tissues, a tissue‐ and organ‐specific trend was observed, which to some extent resembles the organ‐specific CK dynamics in plants . Coupling these new findings about mammalian CKs, we suggest there is a greater role for the breakdown product of iPR‐tRNA turnover as is evidenced by the numerous CK forms detected both intracellularly and extracellularly.…”

“…Furthermore, in the recent report on CK profiles in canine tissues, a tissue-and organ-specific trend was observed, which to some extent resembles the organ-specific CK dynamics in plants. 32,46,47 Coupling these new findings about mammalian CKs, we suggest there is a greater role for the breakdown product of iPR-tRNA turnover as is evidenced by the numerous CK forms detected both intracellularly and extracellularly.…”

What Makes the Wood? Exploring the Molecular Mechanisms of Xylem Acclimation in Hardwoods to an Ever-Changing Environment

“…In poplar, overexpression of the Arabidopsis CYTOKININ OXIDASE 2, an enzyme involved in cytokinin catabolism, resulted in non-detectable levels of the cytokinin trans-zeatin and its storage form zeatin-O-glucoside and lead to a reduced number of cambial cells, resulting in impaired wood formation [87]. Moreover, cytokinin activity was high in primary meristems supporting its function in early wood formation processes [88] and reduced in the cambial zone of drought stressed poplar, indicating a regulatory function of cytokinins under drought stress [89]. There is now increasing evidence that cytokinins interact with jasmonic acid.…”

Drought effects on the tissue- and cell-specific cytokinin activity in poplar

“…In this pathway, type-A ARRs ( Arabidopsis response regulators) are the primary response genes for cytokinins in Arabidopsis ( D’Agostino et al 2000 ). Expression of an Arabidopsis ARR5::GUS reporter construct in poplar indicated conserved functions of the response regulator in both species ( Paul et al 2016 ). A total of 10 type-A RR genes are present in Arabidopsis ( D’Agostino et al 2000 ; Schaller et al 2008 ; Pils and Heyl 2009 ) and 11 were reported in Populus trichocarpa ( Ramírez-Carvajal et al 2008 ; Immanen et al 2013 ).…”