The plant hormone cytokinin regulates growth and development of roots and shoots in opposite ways. In shoots it is a positive growth regulator whereas it inhibits growth in roots. It may be assumed ...that organ-specific regulation of gene expression is involved in these differential activities, but little is known about it. To get more insight into the transcriptional events triggered by cytokinin in roots and shoots, we studied genome-wide gene expression in cytokinin-treated and cytokinin-deficient roots and shoots.
It was found by principal component analysis of the transcriptomic data that the immediate-early response to a cytokinin stimulus differs from the later response, and that the transcriptome of cytokinin-deficient plants is different from both the early and the late cytokinin induction response. A higher cytokinin status in the roots activated the expression of numerous genes normally expressed predominantly in the shoot, while a lower cytokinin status in the shoot reduced the expression of genes normally more active in the shoot to a more root-like level. This shift predominantly affected nuclear genes encoding plastid proteins. An organ-specific regulation was assigned to a number of genes previously known to react to a cytokinin signal, including root-specificity for the cytokinin hydroxylase gene CYP735A2 and shoot specificity for the cell cycle regulator gene CDKA;1. Numerous cytokinin-regulated genes were newly discovered or confirmed, including the meristem regulator genes SHEPHERD and CLAVATA1, auxin-related genes (IAA7, IAA13, AXR1, PIN2, PID), several genes involved in brassinosteroid (CYP710A1, CYP710A2, DIM/DWF) and flavonol (MYB12, CHS, FLS1) synthesis, various transporter genes (e.g. HKT1), numerous members of the AP2/ERF transcription factor gene family, genes involved in light signalling (PhyA, COP1, SPA1), and more than 80 ribosomal genes. However, contrasting with the fundamental difference of the growth response of roots and shoots to the hormone, the vast majority of the cytokinin-regulated transcriptome showed similar response patterns in roots and shoots.
The shift of the root and shoot transcriptomes towards the respective other organ depending on the cytokinin status indicated that the hormone determines part of the organ-specific transcriptome pattern independent of morphological organ identity. Numerous novel cytokinin-regulated genes were discovered which had escaped earlier discovery, most probably due to unspecific sampling. These offer novel insights into the diverse activities of cytokinin, including crosstalk with other hormones and different environmental cues, identify the AP2/ERF class of transcriptions factors as particularly cytokinin sensitive, and also suggest translational control of cytokinin-induced changes.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We have induced callus tissues from one R6/R6 homozygous genotype red-fleshed apple individual which was the hybrid offspring of Malus sieversii f.niedzwetzkyana and ‘Fuji’, and investigated the ...effect of auxin alone and auxin combined with cytokinin or nitrogen deficiency on anthocyanin synthesis. In callus culture, auxin alone significantly inhibited anthocyanin biosynthesis with the increase of auxin concentration. The inhibitory effect of 2,4-dichlorophenoxyacetic acid (2,4-D) on anthocyanin accumulation was about tenfold stronger than naphthalene acetic acid. Anthocyanin regulatory genes (MdMYB10 and MdbHLH3) and structural genes were dramatically suppressed by 0.6 mg/L 2,4-D. The inhibitory effect of auxin on anthocyanin biosynthesis was influenced by cytokinins 6-benzylaminopurine (BAP) and thidiazuron (TDZ) as well as nitrogen deficiency. Auxin and cytokinin displayed the interaction in controlling anthocyanin biosynthesis. Co-treatment of auxin and cytokinin (BAP or TDZ) significantly enhanced the cytokinin-induced increase in anthocyanin levels but too high auxin concentration strongly inhibited anthocyanin synthesis even in the presence of cytokinin. Nitrogen deficiency could reverse the inhibition of anthocyanin accumulation by auxin.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
has become endangered due to reproductive difficulties. Specifically, vegetative reproduction is almost its only way to reproduce, and, under natural conditions, it cannot grow branches, resulting in ...an extremely low reproductive coefficient (reproductive percentage). Here, we performed RNA-Seq and a differentially expressed gene (DEG) analysis of the three stages of lateral bud development in
after decapitation-dormancy (D2), transition (TD2), and emergence (TG2)-and the annual axillary bud natural break (G1) to gain insight into the molecular regulatory network of shoot branching in this plant. Additionally, we applied the auxin transport inhibitors
-1-naphthylphthalamic acid (NPA) and 2,3,5-triiodibenzoic acid (TIBA) to a treated pseudobulb string of
to verify the conclusions obtained by the transcriptome. RNA-Seq provided a wealth of valuable information. Successive pairwise comparative transcriptome analyses revealed 5988 genes as DEGs. GO (Gene Ontology) and KEGG (Kyoto encyclopedia of genes and genomes) analyses of DEGs showed significant enrichments in phytohormone biosynthesis and metabolism, regulation of hormone levels, and a hormone-mediated signaling pathway. qRT-PCR validation showed a highly significant correlation (
< 0.01) with the RNA-Seq generated data. High-performance liquid chromatography (HPLC) and qRT-PCR results showed that, after decapitation, the NPA- and TIBA-induced lateral buds germinated due to rapidly decreasing auxin levels, caused by upregulation of the dioxygenase for auxin oxidation gene (
). Decreased auxin levels promoted the expression of isopentenyl transferase (
) and cytochrome P450 monooxygenase, family 735, subfamily A (
) genes and inhibited two carotenoid cleavage dioxygenases (
and
). Zeatin levels significantly increased after the treatments. The increased cytokinin levels promoted the expression of
(
) and inhibited expression of
(
) in the cytokinin signal transduction pathway and initiated lateral bud outgrowth. Our data suggest that our theories concerning the regulation of shoot branching and apical dominance is really similar to those observed in annual plants. Auxin inhibits bud outgrowth and tends to inhibit cytokinin levels. The pseudobulb in the plant behaves in a similar manner to that of a shoot above the ground.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Sugars are generally used to extend the vase life of cut flowers. Such beneficial effects have been associated with an improvement of water relations and an increase in available energy for ...respiration by floral tissues. In this study we aimed at evaluating to what extent (i) endogenous levels of sugars in outer and inner tepals, androecium and gynoecium are altered during opening and senescence of lily flowers; (ii) sugar levels increase in various floral tissues after sucrose addition to the vase solution; and (iii) sucrose addition alters the hormonal balance of floral tissues. Results showed that endogenous glucose levels increased during flower opening and decreased during senescence in all floral organs, while sucrose levels increased in outer and inner tepals and the androecium during senescence. Sucrose treatment accelerated flower opening, and delayed senescence, but did not affect tepal abscission. Such effects appeared to be exerted through a specific increase in the endogenous levels of sucrose in the gynoecium and of glucose in all floral tissues. The hormonal balance was altered in the gynoecium as well as in other floral tissues. Aside from cytokinin and auxin increases in the gynoecium; cytokinins, gibberellins, abscisic acid and salicylic acid levels increased in the androecium, while abscisic acid decreased in outer tepals. It is concluded that sucrose addition to the vase solution exerts an effect on flower opening and senescence by, among other factors, altering the hormonal balance of several floral tissues.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•Boron deficiency causes root growth arrest by inihibiting root meristem activity.•Rapid changes in cytokinin signaling were identified in response to B deficiency.•The quiescent center identity loss ...occur at later stages of the stress.
Significant advances have been made in the last years trying to identify regulatory pathways that control plant responses to boron (B) deficiency. Still, there is a lack of a deep understanding of how they act regulating growth and development under B limiting conditions. Here, we analyzed the impact of B deficit on cell division leading to root apical meristem (RAM) disorganization. Our results reveal that inhibition of cell proliferation under the regulatory control of cytokinins (CKs) is an early event contributing to root growth arrest under B deficiency. An early recovery of QC46:GUS expression after transferring B-deficient seedlings to control conditions revealed a role of B in the maintenance of QC identity whose loss under deficiency occurred at later stages of the stress. Additionally, the D-type cyclin CYCD3 overexpressor and triple mutant cycd3;1-3 were used to evaluate the effect on mitosis inhibition at the G1-S boundary. Overall, this study supports the hypothesis that meristem activity is inhibited by B deficiency at early stages of the stress as it does cell elongation. Likewise, distinct regulatory mechanisms seem to take place depending on the severity of the stress. The results presented here are key to better understand early signaling responses under B deficiency.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The root is a dynamic system whose structure is regulated by a complex network of interactions between hormones. The primary root meristem is specified in the embryo. After germination, the primary ...root meristem grows and then reaches a final size that will be maintained during the life of the plant. Subsequently, secondary structures such as lateral roots and root nodules form via the re-specification of differentiated cells. Cytokinin plays key roles in the regulation of root development. Down-regulation of the cytokinin response is required for the specification of a new stem cell niche, during both embryo and lateral root development. In the root meristem, cytokinin signalling regulates the longitudinal zonation of the meristem by controlling cell differentiation. Moreover, cytokinin regulates radial patterning of root vasculature by promoting protophloem cell identity and by spatially inhibiting protoxylem formation. In this review, an effort is made to describe the known details of the role of cytokinin during root development, taking into account also the interactions between cytokinin and other hormones. Attention is given on the dynamicity of cytokinin signalling output during different developmental events. Indeed, there is much evidence that the effects of cytokinin change as organs grow, underlining the importance of the spatiotemporal specificity of cytokinin signalling.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NMLJ, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
Plants continuously maintain pluripotent stem cells embedded in specialized tissues called meristems, which drive long-term growth and organogenesis. Stem cell fate in the shoot apical meristem (SAM) ...is controlled by the homeodomain transcription factor WUSCHEL (WUS) expressed in the niche adjacent to the stem cells. Here, we demonstrate that the bHLH transcription factor HECATE1 (HEC1) is a target of WUS and that it contributes to SAM function by promoting stem cell proliferation, while antagonizing niche cell activity. HEC1 represses the stem cell regulators WUS and CLAVATA3 (CLV3) and, like WUS, controls genes with functions in metabolism and hormone signaling. Among the targets shared by HEC1 and WUS are phytohormone response regulators, which we show to act as mobile signals in a universal feedback system. Thus, our work sheds light on the mechanisms guiding meristem function and suggests that the underlying regulatory system is far more complex than previously anticipated.
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•The bHLH factor HECATE1 is a direct target of the plant stem cell regulator WUSCHEL•HECATE1 interacts with other bHLH factors to control stem cell proliferation•WUSCHEL and HECATE1 activities converge on shared transcriptional targets•Mobile cytokinin response factors define a feedback system downstream of HECATE1
Plants maintain continuously active stem cell niches. Schuster et al. show that the bHLH factor HECATE1 promotes stem cell proliferation but inhibits niche activity. HECATE1 antagonizes stem cell regulators and controls genes with functions in metabolism and hormone signaling, including mobile cytokinin response regulators, which define a universal feedback system.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Cytokinins are hormones that regulate plant development and their environmental responses. Their levels are mainly controlled by the cytokinin oxidase/dehydrogenase (CKO), which oxidatively cleaves ...cytokinins using redox‐active electron acceptors. CKO belongs to the group of flavoproteins with an 8α‐N1‐histidyl FAD covalent linkage. Here, we investigated the role of seven active site residues, H105, D169, E288, V378, E381, P427 and L492, in substrate binding and catalysis of the CKO1 from maize (Zea mays, ZmCKO1) combining site‐directed mutagenesis with kinetics and X‐ray crystallography. We identify E381 as a key residue for enzyme specificity that restricts substrate binding as well as quinone electron acceptor binding. We show that D169 is important for catalysis and that H105 covalently linked to FAD maintains the enzyme's structural integrity, stability and high rates with electron acceptors. The L492A mutation significantly modulates the cleavage of aromatic cytokinins and zeatin isomers. The high resolution X‐ray structures of ZmCKO1 and the E381S variant in complex with N6‐(2‐isopentenyl)adenosine reveal the binding mode of cytokinin ribosides. Those of ZmCKO2 and ZmCKO4a contain a mobile domain, which might contribute to binding of the N9 substituted cytokinins.
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BFBNIB, DOBA, FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, SIK, UILJ, UKNU, UL, UM, UPUK
While the molecular basis for cytokinin action is quite well understood in flowering plants, little is known about the cytokinin signal transduction in early diverging land plants. The genome of the ...bryophyte Physcomitrella patens (Hedw.) B.S. encodes three classical cytokinin receptors, the CHASE domain-containing histidine kinases, CHK1, CHK2, and CHK3. In a complementation assay with protoplasts of receptor-deficient Arabidopsis thaliana as well as in cytokinin binding assays, we found evidence that CHK1 and CHK2 receptors can function in cytokinin perception. Using gene targeting, we generated a collection of CHK knockout mutants comprising single (Δchk1, Δchk2, Δchk3), double (Δchk1,2, Δchk1,3, Δchk2,3), and triple (Δchk1,2,3) mutants. Mutants were characterized for their cytokinin response and differentiation capacities. While the wild type did not grow on high doses of cytokinin (1 μM benzyladenine), the Δchk1,2,3 mutant exhibited normal protonema growth. Bud induction assays showed that all three cytokinin receptors contribute to the triggering of budding, albeit to different extents. Furthermore, while the triple mutant showed no response in this bioassay, the remaining mutants displayed budding responses in a diverse manner to different types and concentrations of cytokinins. Determination of cytokinin levels in mutants showed no drastic changes for any of the cytokinins; thus, in contrast to Arabidopsis, revealing only small impacts of cytokinin signaling on homeostasis. In summary, our study provides a first insight into the molecular action of cytokinin in an early diverging land plant and demonstrates that CHK receptors play an essential role in bud induction and gametophore development.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Summary
The hemiparasitic plant Phtheirospermum japonicum (Phtheirospermum) is a nutritional specialist that supplements its nutrient requirements by parasitizing other plants through haustoria. ...During parasitism, the Phtheirospermum haustorium transfers hypertrophy‐inducing cytokinins (CKs) to the infected host root. The CK biosynthesis genes required for haustorium‐derived CKs and the induction of hypertrophy are still unknown.
We searched for haustorium‐expressed isopentenyltransferases (IPTs) that catalyze the first step of CK biosynthesis, confirmed the specific expression by in vivo imaging of a promoter‐reporter, and further analyzed the subcellular localization, the enzymatic function and contribution to inducing hypertrophy by studying CRISPR‐Cas9‐induced Phtheirospermum mutants.
PjIPT1a was expressed in intrusive cells of the haustorium close to the host vasculature. PjIPT1a and its closest homolog PjIPT1b located to the cytosol and showed IPT activity in vitro with differences in substrate specificity. Mutating PjIPT1a abolished parasite‐induced CK responses in the host. A homolog of PjIPT1a also was identified in the related weed Striga hermonthica.
With PjIPT1a, we identified the IPT enzyme that induces CK responses in Phtheirospermum japonicum‐infected Arabidopsis roots. We propose that PjIPT1a exemplifies how parasitism‐related functions evolve through gene duplications and neofunctionalization.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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