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

Decades ago, the importance of cytokinins (CKs) during Rhodococcus fascians pathology had been acknowledged, and an isopentenyltransferase gene had been characterized in the fas operon of the linear virulence plasmid, but hitherto, no specific CK(s) could be associated with virulence. We show that the CK receptors AHK3 and AHK4 of Arabidopsis thaliana are essential for symptom development, and that the CK perception machinery is induced upon infection, underlining its central role in the symptomatology. Three classical CKs [isopentenyladenine, trans-zeatin, and cis-zeatin (cZ)] and their 2-methylthio (2MeS)-derivatives were identified by CK profiling of both the pathogenic R. fascians strain D188 and its nonpathogenic derivative D188 -5. However, the much higher CK levels in strain D188 suggest that the linear plasmid is responsible for the virulenceassociated production. All R. fascians CKs were recognized by AHK3 and AHK4, and, although they individually provoked typical CK responses in several bioassays, the mixture of bacterial CKs exhibited clear synergistic effects. The cis-and 2MeS-derivatives were poor substrates of the apoplastic CK oxidase/dehydrogenase enzymes and the latter were not cytotoxic at high concentrations. Consequently, the accumulating 2MeScZ (and cZ) in infected Arabidopsis tissue contribute to the continuous stimulation of tissue proliferation. Based on these results, we postulate that the R. fascians pathology is based on the local and persistent secretion of an array of CKs.phytopathogen ͉ actinomycete ͉ phytohormone T he fine-tuned balance of plant regulators has a key role in the growth and development of plants. Many plant-associated bacteria can influence their hosts by either modulating the phytohormone production or producing the phytohormones themselves. The main advantages for the bacteria are increased nutrient release, suppression of defense, and/or disease establishment (1, 2). Hyperplasia-inducing bacteria, such as Pantoea agglomerans and Pseudomonas savastanoi, secrete high amounts of cytokinins (CKs) and auxins to facilitate or initiate gall development (3, 4), and Agrobacterium tumefaciens genetically transforms plant cells to convert them into CK and auxin (and opine) factories (5).In contrast to the undifferentiated galls induced by the bacteria mentioned above, the Actinomycete Rhodococcus fascians that shares persistence strategies with the closely related human pathogen Mycobacterium tuberculosis (6) provokes the formation of differentiated leafy galls, consisting of numerous shoot primordia whose further outgrowth is inhibited (7). The shooty symptoms can be partially mimicked by exogenous addition of CKs (8, 9), and analyses of culture supernatants of different nonisogenic virulent and avirulent R. fascians strains grown under rich culture conditions identified 11 different CKs: methylaminopurine, 2-methylthioisopentenyladenine (2MeSiP), iP, cis-zeatin (cZ), trans-zeatin (tZ), dihydrozeatin (DZ), 2MeScZ, and their respective ribosides (10-14). Except for iP, the p...

Plant hormones, cytokinins (CKs), have been for a long time considered to be involved in plant responses to stress. However, their exact roles in processes linked to stress signalization and acclimatization to adverse environmental conditions are unknown. In this study, expression profiles of the entire gene families of CK biosynthetic and degradation genes in maize (Zea mays) during development and stress responses are described. Transcript abundance of particular genes is discussed in relation to the levels of different CK metabolites. Salt and osmotic stresses induce expression of some CK biosynthetic genes in seedlings of maize, leading to a moderate increase of active forms of CKs lasting several days during acclimatization to stress. A direct effect of CKs to mediate activation of stress responses does not seem to be possible due to the slow changes in metabolite levels. However, expression of genes involved in cytokinin signal transduction is uniformly down-regulated within 0.5 h of stress induction by an unknown mechanism. cis-Zeatin and its derivatives were found to be the most abundant CKs in young maize seedlings. We demonstrate that levels of this zeatin isomer are significantly enhanced during early stress response and that it originates independently from de novo biosynthesis in stressed tissues, possibly by elevated specific RNA degradation. By enhancing their CK levels, plants could perhaps undergo a reduction of growth rates maintained by abscisic acid accumulation in stressed tissues. A second role for cytokinin receptors in sensing turgor response is hypothesized besides their documented function in CK signaling.