[Show abstract][Hide abstract] ABSTRACT: Cytokinins, a group of mobile phytohormones, play an important role in plant growth and development, and their activity is
finely controlled by environmental factors in the control of morphogenic and metabolic adaptations. Inorganic nitrogen sources,
such as nitrate, are a major factor regulating gene expression of adenosine phosphate-isopentenyltransferase (IPT), a key
enzyme of cytokinin biosynthesis. Modulation of IPT and macronutrient transporter gene expression in response to nitrate, sulphate and phosphate, and cytokinin-dependent repression
of the transporter genes suggest that cytokinins play a critical role in balancing acquisition and distribution of macronutrients.
Biased distribution of trans-zeatin (tZ)-type cytokinins in xylem and N6-(Δ2-isopentenyl)adenine (iP)-type cytokinins in phloem saps suggest that, in addition to acting as local signals, cytokinins
communicate acropetal and systemic long-distance signals, and that structural side chain variations mediate different biological
messages. The compartmentalization of tZ- and iP-type cytokinins implies the involvement of a selective transport system.
Recent studies have raised the possibility of subsets of the purine permease family as a transporter of cytokinin nucleobases
and equilibrative nucleoside transporters (ENT) for cytokinin nucleosides. These biochemical and transgenic data suggest that
AtENT6, an Arabidopsis ENT, could also participate in cytokinin nucleoside transport with a preference for iP riboside in vascular tissue.
[Show abstract][Hide abstract] ABSTRACT: Genome-wide analyses of rice (Oryza sativa L.) cytokinin (CK)-responsive genes using the Affymetrix GeneChip(R) rice genome array were conducted to define the spectrum of genes subject to regulation by CK in monocotyledonous plants. Application of trans-zeatin modulated the expression of a wide variety of genes including those involved in hormone signaling and metabolism, transcriptional regulation, macronutrient transport and protein synthesis. To understand further the function of CK in rice plants, we examined the effects of in planta manipulation of a putative CK signaling factor on morphology, CK metabolism and expression of CK-responsive genes. Overexpression of the CK-inducible type-A response regulator OsRR6 abolished shoot regeneration, suggesting that OsRR6 acts as a negative regulator of CK signaling. Transgenic lines overexpressing OsRR6 (OsRR6-ox) had dwarf phenotypes with poorly developed root systems and panicles. Increased content of trans-zeatin-type CKs in OsRR6-ox lines indicates that homeostatic control of CK levels is regulated by OsRR6 signaling. Expression of genes encoding CK oxidase/dehydrogenase decreased in OsRR6-ox plants, possibly accounting for elevated CK levels in transgenic lines. Expression of a number of stress response genes was also altered in OsRR6-ox plants.
[Show abstract][Hide abstract] ABSTRACT: Inorganic nitrogen is a substrate for nitrogen assimilation and also functions as a signal triggering widespread changes in gene expression that modulate metabolism and development. To integrate the actions of the nitrogen signal at the whole plant level, plants use multiple signaling routes that communicate internal and external nitrogen status. One route depends on nitrate itself and one uses cytokinin as a messenger. Recent genome-wide research has shown that the nitrate-specific signal regulates a wide variety of metabolic processes including nitrogen and carbon metabolism, and cytokinin biosynthesis. Cytokinin-mediated signaling is related to the control of development, protein synthesis and acquisition of macronutrients. The coordination and interaction of both regulatory pathways is important for normal plant growth under variable nitrogen supply conditions.
[Show abstract][Hide abstract] ABSTRACT: We identified four genes for potential equilibrative nucleoside transporters (ENTs) from rice (Oryza sativa; designated OsENT1 through OsENT4). Growth analysis of budding yeast (Saccharomyces cerevisiae) cells expressing OsENTs showed that OsENT2 transported adenosine and uridine with high affinity (adenosine, K(m) = 3.0 microm; uridine, K(m) = 0.7 microm). Purine or pyrimidine nucleosides and 2'-deoxynucleosides strongly inhibited adenosine transport via OsENT2, suggesting that OsENT2 possesses broad substrate specificity. OsENT2-mediated adenosine transport was resistant to the typical inhibitors of mammalian ENTs, nitrobenzylmercaptopurine ribonucleoside, dilazep, and dipyridamole. The transport activity was maximal at pH 5.0 and decreased slightly at lower as well as higher pH. In competition experiments with various cytokinins, adenosine transport by OsENT2 was inhibited by isopentenyladenine riboside (iPR). Direct measurements with radiolabeled cytokinins demonstrated that OsENT2 mediated uptake of iPR (K(m) = 32 microm) and trans-zeatin riboside (K(m) = 660 microm), suggesting that OsENT2 participates in iPR transport in planta. In mature plants, OsENT2 was predominantly expressed in roots. The OsENT2 promoter drove the expression of the beta-glucuronidase reporter gene in the scutellum during germination and in vascular tissues in germinated plants, suggesting a participation of OsENT2 in the retrieval of endosperm-derived nucleosides by the germinating embryo and in the long-distance transport of nucleosides in growing plants, respectively.
[Show abstract][Hide abstract] ABSTRACT: The plant phytohormone cytokinin plays an important role in many facets of plant growth and development by regulating cell division and differentiation. Recent studies have shed significant light into the mechanisms of cytokinin metabolism and signaling. However, little is known about how the hormone is transported in planta, although it has been proposed that the hormone is presumably transported in nucleoside-conjugated forms. Here, we report the identification and characterization of cytokinin transporters in Arabidopsis. We previously reported that a gain-of-function mutation in the PGA22/AtIPT8 gene caused overproduction of cytokinins in planta. In an effort to screen for suppressor of pga 22/atipt 8 (soi) mutants, we identified a mutant soi33-1. Molecular and genetic analyses indicated that SOI33 encodes a putative equilibrative nucleoside transporter (ENT), previously designated as AtENT8. Members of this small gene family are presumed to be involved in the transport of nucleosides in eukaryotic cells. Under conditions of nitrogen starvation, loss-of-function mutations in SOI33/AtENT8 or in a related gene AtENT3 cause a reduced sensitivity to the nucleoside-type cytokinins isopentenyladenine riboside (iPR) and transzeatin riboside (tZR), but display a normal response to the free base-type cytokinins isopentenyladenine (iP) and trans-zeatin (tZ). Conversely, overexpression of SOI33/AtENT8 renders transgenic plants hypersensitive to iPR but not to iP. An in planta measurement experiment indicated that uptake efficiency of 3H-labeled iPR was reduced more than 40% in soi33 and atent3 mutants. However, a mutation in AtENT1 had no substantial effect on the cytokinin response and iPR uptake efficiency. Our results suggest that SOI33/ AtENT8 and AtENT3 are involved in the transport of nucleoside-type cytokinins in Arabidopsis.
(Managing editor: Ya-Qin HAN)
[Show abstract][Hide abstract] ABSTRACT: After growth for 26 d in water, rice seedlings were transferred to a medium containing 1 mM NH4Cl. The mRNA for NADH-glutamate synthase was markedly increased in the roots within 3 h. Methionine sulfoximine completely inhibited this accumulation, suggesting that NH4+ is not a direct inducer of this process.
[Show abstract][Hide abstract] ABSTRACT: When rice seedlings, after the growth for 26 days in water alone, were transferred to nutrient medium contained 1 mM NH 4 Cl, the level of NADH-dependent glutamate synthase (GOGAT) protein and the activity of the enzyme increased more than 10-fold in root, but not in shoots. Both the level of the protein and the activity reached a maximum within 24 h. NH 4 Cl was effective at concentrations as low as 50 μM. A supply of either 1 mM NaNO 3 or 0.5 mM NH 4 NO 3 also caused such increases, but NHa 4 Cl was most effective. A supply of glutamine or glutamate was less effective. The increase was specific to NADH-GOGAT and little change was observed in the levels of ferredoxin-GOGAT and glutamine synthetase isoproteins in roots. These inducible increases in the levels of NADH-GOGAT protein and in its activity were greater in the root-tip region than at the base of the root. Both 6-methylpurine and cycloheximide completely inhibited the effects of NH 4 Cl. Moreover, the mRNA for NADH-GOGAT in rice roots accumulated markedly within 12 h of the start of a supply of NH 4 Cl. A possible role for the rapid response of NADH-GOGAT to a supply of NH 4 C1 is discussed.
Plant and Cell Physiology 01/1995; 36. · 4.93 Impact Factor