Summary Cytokinins are hormones that regulate many developmental and physiological processes in plants. Recent work has revealed that the cytokinin signal is transduced by two‐component systems to the nucleus where target genes are activated. Most of the rapid transcriptional responses are unknown. We measured immediate‐early and delayed cytokinin responses through genome‐wide expression profiling with the Affymetrix ATH1 full genome array (Affymetrix Inc., Santa Clara, CA, USA). Fifteen minutes after cytokinin treatment of 5‐day‐old Arabidopsis seedlings, 71 genes were upregulated and 11 genes were downregulated. Immediate‐early cytokinin response genes include a high portion of transcriptional regulators, among them six transcription factors that had previously not been linked to cytokinin. Five plastid transcripts were rapidly regulated as well, indicating a rapid transfer of the signal to plastids or direct perception of the cytokinin signal by plastids. After 2 h of cytokinin treatment genes coding for transcriptional regulators, signaling proteins, developmental and hormonal regulators, primary and secondary metabolism, energy generation and stress reactions were over‐represented. A significant number of the responding genes are known to regulate light (PHYA, PSK1, CIP8, PAT1, APRR), auxin (Aux/IAA), ethylene (ETR2, EIN3, ERFs/EREBPs), gibberellin (GAI, RGA1, GA20 oxidase), nitrate (NTR2, NIA) and sugar (STP1, SUS1) dependent processes, indicating intense crosstalk with environmental cues, other hormones and metabolites. Analysis of cytokinin‐deficient 35S:AtCKX1 transgenic seedlings has revealed additional, long‐lasting cytokinin‐sensitive changes of transcript abundance. Comparative overlay‐analysis with the software tool mapman identified previously unknown cytokinin‐sensitive metabolic genes, for example in the metabolism of trehalose‐6‐phosphate. Taken together, we present a genome‐wide view of changes in cytokinin‐responsive transcript abundance of genes that might be functionally relevant for the many biological processes that are governed by cytokinins.
SummaryNucleobases and derivatives like cytokinins and caffeine are translocated in the plant vascular system. Transport studies in cultured Arabidopsis cells indicate that adenine and cytokinin are transported by a common H -coupled high-af®nity purine transport system. Transport properties are similar to that of Arabidopsis purine transporters AtPUP1 and 2. When expressed in yeast, AtPUP1 and 2 mediate energydependent high-af®nity adenine uptake, whereas AtPUP3 activity was not detectable. Similar to the results from cell cultures, purine permeases (PUP) mediated uptake of adenine can be inhibited by cytokinins, indicating that cytokinins are transport substrates. Direct measurements demonstrate that AtPUP1 is capable of mediating uptake of radiolabeled trans-zeatin. Cytokinin uptake is strongly inhibited by adenine and isopentenyladenine but is poorly inhibited by 6-chloropurine. A number of physiological cytokinins including trans-and cis-zeatin are also ef®cient competitors for AtPUP2-mediated adenine uptake, suggesting that AtPUP2 is also able to mediate cytokinin transport. Furthermore, AtPUP1 mediates transport of caffeine and ribosylated purine derivatives in yeast. Promoter±reporter gene studies point towards AtPUP1 expression in the epithem of hydathodes and the stigma surface of siliques, suggesting a role in retrieval of cytokinins from xylem sap to prevent loss during guttation. The AtPUP2 promoter drives GUS reporter gene activity in the phloem of Arabidopsis leaves, indicating a role in long-distance transport of adenine and cytokinins. Promoter activity of AtPUP3 was only found in pollen. In summary, three closely related PUPs are differentially expressed in Arabidopsis and at least two PUPs have properties similar to the adenine and cytokinin transport system identi®ed in Arabidopsis cell cultures.
In many species translocation of sucrose from the mesophyll to the phloem is carrier mediated. A sucrose/H ؉ -symporter cDNA, NtSUT1, was isolated from tobacco (Nicotiana tabacum) and shown to be highly expressed in mature leaves and at low levels in other tissues, including floral organs. To study the in vivo function of NtSUT1, tobacco plants were transformed with a SUT1 antisense construct under control of the cauliflower mosaic virus 35S promoter. Upon maturation, leaves of transformants expressing reduced amounts of SUT1 mRNA curled downward, and strongly affected plants developed chloroses and necroses that led to death. The leaves exhibited impaired ability to export recently fixed 14 CO 2 and were unable to export transient starch during extended periods of darkness. As a consequence, soluble carbohydrates accumulated and photosynthesis was reduced. Autoradiographs of leaves show a heterogenous pattern of CO 2 fixation even after a 24-h chase. The 14 C pattern does not change with time, suggesting that movement of photosynthate between mesophyll cells may also be impaired. The affected lines show a reduction in the development of the root system and delayed or impaired flowering. Taken together, the effects observed in a seed plant (tobacco) demonstrate the importance of SUT1 for sucrose loading into the phloem via an apoplastic route and possibly for intermesophyll transport as well.Photosynthesis in mature leaves produces a surplus of assimilates. Carbohydrates derived from mature leaves are distributed in the plant through the vascular system, mainly in the form of sucrose, to support the growth of heterotrophic tissues such as developing leaves, apices, roots, and reproductive organs. Both active transport by specific carriers across the plasma membrane and symplastic transport via plasmodesmata have been discussed as possible mechanisms for phloem loading (Ap Rees, 1994). Nevertheless, a direct demonstration of the actual role of plasmodesmata in assimilate transport is still missing. Sucrose transport activities have been identified in a number of plant species (for reviews, see Bush, 1993; and have been described as sucrose:proton cotransport with a 1:1 stoichiometry (Bush, 1990;Lemoine et al., 1996).To resolve the question of whether carrier-mediated sucrose transport represents an essential step in phloem loading, the respective genes were identified. A yeast strain was modified so that it could be used as a complementation system to isolate the SUT cDNAs SUT1 from spinach and potato (Solanum tuberosum) (Riesmeier et al., 1992(Riesmeier et al., , 1993. Subsequently, homologous genes were isolated from a number of other plant species (Gahrtz et al., 1994;Sauer and Stolz, 1994; Weig and Komor, 1996; Hirose et al., 1997;Kü hn et al., 1997; Weber et al., 1997).The biochemical properties of the transporters when expressed in yeast were similar to those described in protoplasts or in plasma membrane vesicles from a variety of plant species. Detailed electrophysiological analyses in Xenopus oocytes dem...
Vitamin B1 (thiamin) is an essential compound in all organisms acting as a cofactor in key metabolic reactions and has furthermore been implicated in responses to DNA damage and pathogen attack in plants. Despite the fact that it was discovered almost a century ago and deficiency is a widespread health problem, much remains to be deciphered about its biosynthesis. The vitamin is composed of a thiazole and pyrimidine heterocycle, which can be synthesized by prokaryotes, fungi, and plants. Plants are the major source of the vitamin in the human diet, yet little is known about the biosynthesis of the compound therein. In particular, it has never been verified whether the pyrimidine heterocycle is derived from purine biosynthesis through the action of the THIC protein as in bacteria, rather than vitamin B6 and histidine as demonstrated for fungi. Here, we identify a homolog of THIC in Arabidopsis and demonstrate its essentiality not only for vitamin B1 biosynthesis, but also plant viability. This step takes place in the chloroplast and appears to be regulated at several levels, including through the presence of a riboswitch in the 3-untranslated region of THIC. Strong evidence is provided for the involvement of an iron-sulfur cluster in the remarkable chemical rearrangement reaction catalyzed by the THIC protein for which there is no chemical precedent. The results suggest that vitamin B1 biosynthesis in plants is in fact more similar to prokaryotic counterparts and that the THIC protein is likely to be the key regulatory protein in the pathway.metabolites ͉ plant viability ͉ riboswitch ͉ thiamin ͉ Arabidopsis
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