Postembryonic de novo organogenesis represents an important competence evolved in plants that allows their physiological and developmental adaptation to changing environmental conditions. The phytohormones auxin and cytokinin (CK) are important regulators of the developmental fate of pluripotent plant cells. However, the molecular nature of their interaction(s) in control of plant organogenesis is largely unknown. Here, we show that CK modulates auxin-induced organogenesis (AIO) via regulation of the effluxdependent intercellular auxin distribution. We used the hypocotyl explants-based in vitro system to study the mechanism underlying de novo organogenesis. We show that auxin, but not CK, is capable of triggering organogenesis in hypocotyl explants. The AIO is accompanied by endogenous CK production and tissue-specific activation of CK signaling. CK affects differential auxin distribution, and the CKmediated modulation of organogenesis is simulated by inhibition of polar auxin transport. CK reduces auxin efflux from cultured tobacco cells and regulates expression of auxin efflux carriers from the PIN family in hypocotyl explants. Moreover, endogenous CK levels influence PIN transcription and are necessary to maintain intercellular auxin distribution in planta. Based on these findings, we propose a model in which auxin acts as a trigger of the organogenic processes, whose output is modulated by the endogenously produced CKs. We propose that an important mechanism of this CK action is its effect on auxin distribution via regulation of expression of auxin efflux carriers.PIN expression ͉ two-component signalling ͉ root meristem ͉ auxin maxima P ostembryonic de novo organogenesis represents an important developmental adaptation evolved in plants. Regeneration of entire bodies in hydras (1) or organs in amphibians (2) has been described. However, in the animal kingdom, these examples are rather exceptional. In contrast, plants evolved postembryonic formation of new organs from differentiated tissues as a strategy that allows physiological and developmental adaptation to changing environmental conditions. However, this strategy requires action by factors that are specifically able to induce developmental programs, leading to the formation of entire organs from virtually differentiated cells.The interaction of auxin and cytokinin (CK) during plant organogenesis is a phenomenon known for a long time. In their pioneering work, Skoog and Miller (3) identified auxin-to-CK concentration ratios as an important factor regulating the developmental fate of plant tissue explants. Since that time, the role of both growth factors in plant development has been extensively studied. For auxin action, a model involving a spatial and temporal pattern of intercellular auxin distribution and concentration maxima is well established, and the molecular and cellular factors mediating auxin distribution have been identified (4, 5). Differential auxin distribution has been shown to mediate multiple aspects of plant development, such as apical...
trans-Zeatin is a major and ubiquitous cytokinin in higher plants. cis-Zeatin has traditionally been viewed as an adjunct with low activity and rare occurrence. Recent reports of cis-zeatin and its derivatives as the predominant cytokinin components in some plant tissues may call for a different perspective on cis-isomers. The existence of a maize (Zea mays) gene (cisZOG1) encoding an O-glucosyltransferase specific to cis-zeatin (R.C. Martin, M.C. Mok, J.E. Habben, D.W.S. Proc Natl Acad Sci USA 98: 5922-5926) lends further support to this view. Results described here include the isolation of a second maize cisZOG gene, differential expression of cisZOG1 and cisZOG2, and identification of substantial amounts of cis-isomers in maize tissues. The open reading frame of cisZOG2 has 98.3% identity to cisZOG1 at the nucleotide level and 97.8% at the amino acid level. The upstream regions contain common and unique segments. The recombinant enzymes have similar properties, K m values of 46 and 96 m, respectively, for cis-zeatin and a pH optimum of 7.5. Other cytokinins, including N 6 -(⌬ 2 -isopentenyl)adenine, trans-zeatin, benzyladenine, kinetin, and thidiazuron inhibited the reaction. Expression of cisZOG1 was high in maize roots and kernels, whereas cisZOG2 expression was high in roots but low in kernels. cis-Zeatin, cis-zeatin riboside, and their O-glucosides were detected in all maize tissues, with immature kernels containing very high levels of the O-glucoside of cis-zeatin riboside. The results are a clear indication that O-glucosylation of cis-zeatin is a natural metabolic process in maize. Whether cis-zeatin serves as a precursor to the active trans-isomer or has any other unique function remains to be demonstrated.Cytokinins are plant hormones regulating cell division and a range of developmental events such as bud formation, leaf expansion, senescence, seed germination, and chloroplast formation (Mok, 1994). trans-Zeatin is a major and ubiquitous cytokinin in higher plants. Earlier cytokinin analyses detected ciszeatin and its derivatives in trace amounts in some plants, but due to their low activity (Schmitz et al., 1972), cis-isomers were viewed as adjunct to transisomers. Recent analyses, however, showed that the cis-isomers can be the dominant cytokinins at particular stages of development in plants such as chickpea (Cicer arietinum) and lupine (Lupinus albus; Emery et al., 1998Emery et al., , 2000. Moreover, the presence of cis-isomers was associated with male sterility in Mercurialis spp. flowers (Louis et al., 1990;Durand and Durand, 1994). These are indications that cis-isomers may have unique physiological functions. The ability to regulate the levels of cis-zeatin is evidenced by the maize (Zea mays) cisZOG1 gene, encoding an O-glucosyltransferase with specificity to cis-zeatin (Martin et al., 2001).O-Glucosylation is a major step in the metabolism of trans-zeatin ). The resulting O-glucosides seem to serve as storage compounds and are resistant to degradation by cytokinin oxidases (Armstrong, 1994...
The impact of water deficit progression on cytokinin (CK), auxin and abscisic acid (ABA) levels was followed in upper, middle and lower leaves and roots of Nicotiana tabacum L. cv. Wisconsin 38 plants [wild type (WT)]. ABA content was strongly increased during drought stress, especially in upper leaves. In plants with a uniformly elevated total CK content, expressing constitutively the trans-zeatin O-glucosyltransferase gene (35S::ZOG1), a delay in the increase of ABA was observed; later on, ABA levels were comparable with those of WT.As drought progressed, the bioactive CK content in leaves gradually decreased, being maintained longer in the upper leaves of all tested genotypes. Under severe stress (11 d dehydration), a large stimulation of cytokinin oxidase/ dehydrogenase (CKX) activity was monitored in lower leaves, which correlated well with the decrease in bioactive CK levels. This suggests that a gradient of bioactive CKs in favour of upper leaves is established during drought stress, which might be beneficial for the preferential protection of these leaves.During drought, significant accumulation of CKs occurred in roots, partially because of decreased CKX activity. Simultaneously, auxin increased in roots and lower leaves. This indicates that both CKs and auxin play a role in root response to severe drought, which involves the stimulation of primary root growth and branching inhibition.
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