CYTOKININ METABOLISM AND ACTION

Structural insights on the molecular recognition patterns between N(6)-substituted adenines and N-(aryl-methyl)iminodiacetate copper(II) chelates

For a better understanding of the metal binding pattern of N(6)-substituted adenines, six novel ternary Cu(II) complexes have been structurally characterized by single crystal X-ray diffraction: [Cu(NBzIDA)(HCy5ade)(H2O)]·H2O (1), [Cu(NBzIDA)(HCy6ade)(H2O)]·H2O (2), [Cu(FurIDA)(HCy6ade)(H2O)]·H2O (3), [Cu(MEBIDA)(HBAP)(H2O)]·H2O (4), [Cu(FurIDA)(HBAP)]n (5) and {[Cu(NBzIDA)(HdimAP)]·H2O}n (6). In these compounds NBzIDA, FurIDA and MEBIDA are N-substituted iminodiacetates with a non-coordinating aryl-methyl pendant arm (benzyl in NBzIDA, p-tolyl in MEBIDA and furfuryl in FurIDA) whereas HBAP, HCy5ade, HCy6ade and HdimAP are N(6)-substituted adenine derivatives with a N-benzyl, N-cyclopentyl, N-cyclohexyl or two N-methyl groups, respectively. Regardless of the molecular (1-4) or polymeric (5-6) nature of the studied compounds, the Cu(II) centre exhibits a type 4+1 coordination where the tridentate IDA-like chelators adopt a mer-conformation. In 1-5 the N(6)-R-adenines use their most stable tautomer H(N9)adenine-like, and molecular recognition consists of the cooperation of the CuN3(purine) bond and the intra-molecular interligand N9H···O(coordinated carboxy) interaction. In contrast, N(6),N(6)-dimethyl-adenine shows the rare tautomer H(N3)dimAP in 6, so that the molecular recognition with the Cu(NBzIDA) chelate consist of the CuN9 bond and the N3H···O intra-molecular interligand interaction. Contrastingly to the cytokinin activity found in the free ligands HBAP (natural cytokinin), HCy5ade and HCy6ade, the corresponding Cu(II) ternary complexes did not show any activity.

Cytokinin oxidase is key enzyme of cytokinin degradation

Cytokinin oxidase (EC 1.5.99.12) is an enzyme that catalyzes the irreversible degradation of cytokinin phytohormones that are extremely necessary for growth, development, and differentiation of plants. Cytokinin oxidase plays an important role in the regulation of quantitative level of cytokinins and their distribution in plant tissues. This review generalizes the available information on the structure, properties, and functional role of this enzyme in plant ontogeny under conditions of normal growth and under the influence of unfavorable environmental factors.

Transgenic barley: a prospective tool for biotechnology and agriculture

Barley (Hordeum vulgare L.) is one of the founder crops of agriculture, and today it is the fourth most important cereal grain worldwide. Barley is used as malt in brewing and distilling industry, as an additive for animal feed, and as a component of various food and bread for human consumption. Progress in stable genetic transformation of barley ensures a potential for improvement of its agronomic performance or use of barley in various biotechnological and industrial applications. Recently, barley grain has been successfully used in molecular farming as a promising bioreactor adapted for production of human therapeutic proteins or animal vaccines. In addition to development of reliable transformation technologies, an extensive amount of various barley genetic resources and tools such as sequence data, microarrays, genetic maps, and databases has been generated. Current status on barley transformation technologies including gene transfer techniques, targets, and progeny stabilization, recent trials for improvement of agricultural traits and performance of barley, especially in relation to increased biotic and abiotic stress tolerance, and potential use of barley grain as a protein production platform have been reviewed in this study. Overall, barley represents a promising tool for both agricultural and biotechnological transgenic approaches, and is considered an ancient but rediscovered crop as a model industrial platform for molecular farming.

Integration of epigenetic and genetic controls of seed size by cytokinin in Arabidopsis

The development of seeds in flowering plants is placed under complex interactions between maternal tissues, the embryo, and the endosperm. The endosperm plays a major role in the regulation of seed size. In Arabidopsis thaliana, endosperm size depends on the coordination of the genetic pathway HAIKU (IKU) with epigenetic controls comprising genome dosage, DNA methylation, and trimethylated lysine 27 on histone H3 (H3K27me3) deposition. However, the effectors that integrate these pathways have remained unknown. Here, we identify a target of the IKU pathway, the cytokinin oxidase CKX2, that affects cytokinin signaling. CKX2 expression is activated by the IKU transcription factor WRKY10 directly and promotes endosperm growth. CKX2 expression also depends on H3K27me3 deposition, which fluctuates in response to maternal genome dosage imbalance and DNA demethylation of male gametes. Hence, the control of endosperm growth by CKX2 integrates genetic and epigenetic regulations. In angiosperms, cytokinins are highly active in endosperm, and we propose that IKU effectors coordinate environmental and physiological factors, resulting in modulation of seed size.

Adenine phosphoribosyl transferase 1 is a key enzyme catalyzing cytokinin conversion from nucleobases to nucleotides in Arabidopsis

In plants, the cytokinin metabolic processes, including cytokinin biosynthesis, interconversion, inactivation, and degradation, play critical roles in the regulation of cytokinin homeostasis and plant development. Purine metabolic enzymes have been implied to catalyze the cytokinin interconversion in previous works. In this study, we report that Adenine Phosphoribosyl Transferase 1 (APT1) is the causal gene of the high-dose cytokinin-resistant mutants. APT1 catalyzes the cytokinin conversion from free bases to nucleotides, and is functionally predominant among the five members of the Arabidopsis Adenine Phosphoribosyl Transferase family. Loss of APT1 activity in plants leads to excess accumulation of cytokinin bases, thus evoking myriad cytokinin-regulated responses, such as delayed leaf senescence, anthocyanin accumulation, and downstream gene expression. Thus, our study defines APT1 as a key metabolic enzyme participating in the cytokinin inactivation by phosphoribosylation.

Identification, expression, and comparative genomic analysis of the IPT and CKX gene families in Chinese cabbage (Brassica rapa ssp. pekinensis)

BACKGROUND

Cytokinins (CKs) have significant roles in various aspects of plant growth and development, and they are also involved in plant stress adaptations. The fine-tuning of the controlled CK levels in individual tissues, cells, and organelles is properly maintained by isopentenyl transferases (IPTs) and cytokinin oxidase/dehydrogenases (CKXs). Chinese cabbage is one of the most economically important vegetable crops worldwide. The whole genome sequencing of Brassica rapa enables us to perform the genome-wide identification and functional analysis of the IPT and CKX gene families.

RESULTS

In this study, a total of 13 BrIPT genes and 12 BrCKX genes were identified. The gene structures, conserved domains and phylogenetic relationships were analyzed. The isoelectric point, subcellular localization and glycosylation sites of the proteins were predicted. Segmental duplicates were found in both BrIPT and BrCKX gene families. We also analyzed evolutionary patterns and divergence of the IPT and CKX genes in the Cruciferae family. The transcription levels of BrIPT and BrCKX genes were analyzed to obtain an initial picture of the functions of these genes. Abiotic stress elements related to adverse environmental stimuli were found in the promoter regions of BrIPT and BrCKX genes and they were confirmed to respond to drought and high salinity conditions. The effects of 6-BA and ABA on the expressions of BrIPT and BrCKX genes were also investigated.

CONCLUSIONS

The expansion of BrIPT and BrCKX genes after speciation from Arabidopsis thaliana is mainly attributed to segmental duplication events during the whole genome triplication (WGT) and substantial duplicated genes are lost during the long evolutionary history. Genes produced by segmental duplication events have changed their expression patterns or may adopted new functions and thus are obtained. BrIPT and BrCKX genes respond well to drought and high salinity stresses, and their transcripts are affected by exogenous hormones, such as 6-BA and ABA, suggesting their potential roles in abiotic stress conditions and regulatory mechanisms of plant hormone homeostasis. The appropriate modulation of endogenous CKs levels by IPT and CKX genes is a promising approach for developing economically important high-yielding and high-quality stress-tolerant crops in agriculture.

Strategies to ameliorate abiotic stress-induced plant senescence

The plant senescence syndrome resembles, in many molecular and phenotypic aspects, plant responses to abiotic stresses. Both processes have an enormous negative global agro-economic impact and endanger food security worldwide. Premature plant senescence is the main cause of losses in grain filling and biomass yield due to leaf yellowing and deteriorated photosynthesis, and is also responsible for the losses resulting from the short shelf life of many vegetables and fruits. Under abiotic stress conditions the yield losses are often even greater. The primary challenge in agricultural sciences today is to develop technologies that will increase food production and sustainability of agriculture especially under environmentally limiting conditions. In this chapter, some of the mechanisms involved in abiotic stress-induced plant senescence are discussed. Recent studies have shown that crop yield and nutritional values can be altered as well as plant stress tolerance through manipulating the timing of senescence. It is often difficult to separate the effects of age-dependent senescence from stress-induced senescence since both share many biochemical processes and ultimately result in plant death. The focus of this review is on abiotic stress-induced senescence. Here, a number of the major approaches that have been developed to ameliorate some of the effects of abiotic stress-induced plant senescence are considered and discussed. Some approaches mimic the mechanisms already used by some plants and soil bacteria whereas others are based on development of new improved transgenic plants. While there may not be one simple strategy that can effectively decrease all losses of crop yield that accrue as a consequence of abiotic stress-induced plant senescence, some of the strategies that are discussed already show great promise.

Stabilization of cytokinin levels enhances Arabidopsis resistance against Verticillium longisporum

Verticillium longisporum is a vascular pathogen that infects the Brassicaceae host plants Arabidopsis thaliana and Brassica napus. The soilborne fungus enters the plant via the roots and colonizes the xylem of roots, stems, and leaves. During late stages of infections, Verticillium spp. spread into senescing tissue and switch from biotrophic to a necrotrophic life style. Typical symptoms of V. longisporum-induced disease are stunted growth and leaf chlorosis. Expression analyses of the senescence marker genes SENESCENCE-ASSOCIATED GENE12, SENESCENCE-ASSOCIATED GENE13, and WRKY53 revealed that the observed chlorosis is a consequence of premature senescence triggered by Verticillium infection. Our analyses show that, concomitant with the development of chlorosis, levels of trans-zeatin decrease in infected plants. Potentially, induction of cytokinin oxidase/dehydrogenase expression by Verticillium infection contributes to the observed decreases in cytokinin levels. Stabilization of Arabidopsis cytokinin levels by both pharmacological and genetic approaches inhibits Verticillium proliferation and coincides with reduced disease symptom development. In summary, our results indicate that V. longisporum triggers premature plant senescence for efficient host plant colonization.

A non-synonymous SNP within the isopentenyl transferase 2 locus is associated with kernel weight in Chinese maize inbreds (Zea mays L.)

BACKGROUND

Kernel weight, controlled by quantitative trait loci (QTL), is an important component of grain yield in maize. Cytokinins (CKs) participate in determining grain morphology and final grain yield in crops. ZmIPT2, which is expressed mainly in the basal transfer cell layer, endosperm, and embryo during maize kernel development, encodes an isopentenyl transferase (IPT) that is involved in CK biosynthesis.

RESULTS

The coding region of ZmIPT2 was sequenced across a panel of 175 maize inbred lines that are currently used in Chinese maize breeding programs. Only 16 single nucleotide polymorphisms (SNPs) and seven haplotypes were detected among these inbred lines. Nucleotide diversity (π) within the ZmIPT2 window and coding region were 0.347 and 0.0047, respectively, and they were significantly lower than the mean nucleotide diversity value of 0.372 for maize Chromosome 2 (P < 0.01). Association mapping revealed that a single nucleotide change from cytosine (C) to thymine (T) in the ZmIPT2 coding region, which converted a proline residue into a serine residue, was significantly associated with hundred kernel weight (HKW) in three environments (P <0.05), and explained 4.76% of the total phenotypic variation. In vitro characterization suggests that the dimethylallyl diphospate (DMAPP) IPT activity of ZmIPT2-T is higher than that of ZmIPT2-C, as the amounts of adenosine triphosphate (ATP), adenosine diphosphate (ADP), and adenosine monophosphate (AMP) consumed by ZmIPT2-T were 5.48-, 2.70-, and 1.87-fold, respectively, greater than those consumed by ZmIPT2-C. The effects of artificial selection on the ZmIPT2 coding region were evaluated using Tajima's D tests across six subgroups of Chinese maize germplasm, with the most frequent favorable allele identified in subgroup PB (Partner B).

CONCLUSIONS

These results showed that ZmIPT2, which is associated with kernel weight, was subjected to artificial selection during the maize breeding process. ZmIPT2-T had higher IPT activity than ZmIPT2-C, and this favorable allele for kernel weight could be used in molecular marker-assisted selection for improvement of grain yield components in Chinese maize breeding programs.

Recycling of pyridoxine (vitamin B6) by PUP1 in Arabidopsis

Vitamin B6 is a cofactor for more than 140 essential enzymatic reactions and was recently proposed as a potent antioxidant, playing a role in the photoprotection of plants. De novo biosynthesis of the vitamin has been described relatively recently and is derived from simple sugar precursors as well as glutamine. In addition, the vitamin can be taken up from exogenous sources in a broad range of organisms, including plants. However, specific transporters have been identified only in yeast. Here we assess the ability of the family of Arabidopsis purine permeases (PUPs) to transport vitamin B6. Several members of the family complement the growth phenotype of a Saccharomyces cerevisiae mutant strain impaired in both de novo biosynthesis of vitamin B6 as well as its uptake. The strongest activity was observed with PUP1 and was confirmed by direct measurement of uptake in yeast as well as in planta, defining PUP1 as a high affinity transporter for pyridoxine. At the tissue level the protein is localised to hydathodes and here we use confocal microscopy to illustrate that at the cellular level it is targeted to the plasma membrane. Interestingly, we observe alterations in pyridoxine recycling from the guttation sap upon overexpression of PUP1 and in a pup1 mutant, consistent with the role of the protein in retrieval of pyridoxine. Furthermore, combining the pup1 mutant with a vitamin B6 de novo biosynthesis mutant (pdx1.3) corroborates that PUP1 is involved in the uptake of the vitamin.

In planta analysis of a cis-regulatory cytokinin response motif in Arabidopsis and identification of a novel enhancer sequence

The phytohormone cytokinin plays a key role in regulating plant growth and development, and is involved in numerous physiological responses to environmental changes. The type-B response regulators, which regulate the transcription of cytokinin response genes, are a part of the cytokinin signaling system. Arabidopsis thaliana encodes 11 type-B response regulators (type-B ARRs), and some of them were shown to bind in vitro to the core cytokinin response motif (CRM) 5'-(A/G)GAT(T/C)-3' or, in the case of ARR1, to an extended motif (ECRM), 5'-AAGAT(T/C)TT-3'. Here we obtained in planta proof for the functionality of the latter motif. Promoter deletion analysis of the primary cytokinin response gene ARR6 showed that a combination of two extended motifs within the promoter is required to mediate the full transcriptional activation by ARR1 and other type-B ARRs. CRMs were found to be over-represented in the vicinity of ECRMs in the promoters of cytokinin-regulated genes, suggesting their functional relevance. Moreover, an evolutionarily conserved 27 bp long T-rich region between -220 and -193 bp was identified and shown to be required for the full activation by type-B ARRs and the response to cytokinin. This novel enhancer is not bound by the DNA-binding domain of ARR1, indicating that additional proteins might be involved in mediating the transcriptional cytokinin response. Furthermore, genome-wide expression profiling identified genes, among them ARR16, whose induction by cytokinin depends on both ARR1 and other specific type-B ARRs. This together with the ECRM/CRM sequence clustering indicates cooperative action of different type-B ARRs for the activation of particular target genes.

Spatiotemporal changes in the role of cytokinin during root development

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.

Getting to the roots of it: Genetic and hormonal control of root architecture

Root system architecture (RSA) - the spatial configuration of a root system - is an important developmental and agronomic trait, with implications for overall plant architecture, growth rate and yield, abiotic stress resistance, nutrient uptake, and developmental plasticity in response to environmental changes. Root architecture is modulated by intrinsic, hormone-mediated pathways, intersecting with pathways that perceive and respond to external, environmental signals. The recent development of several non-invasive 2D and 3D root imaging systems has enhanced our ability to accurately observe and quantify architectural traits on complex whole-root systems. Coupled with the powerful marker-based genotyping and sequencing platforms currently available, these root phenotyping technologies lend themselves to large-scale genome-wide association studies, and can speed the identification and characterization of the genes and pathways involved in root system development. This capability provides the foundation for examining the contribution of root architectural traits to the performance of crop varieties in diverse environments. This review focuses on our current understanding of the genes and pathways involved in determining RSA in response to both intrinsic and extrinsic (environmental) response pathways, and provides a brief overview of the latest root system phenotyping technologies and their potential impact on elucidating the genetic control of root development in plants.

N9-substituted aromatic cytokinins with negligible side effects on root development are an emerging tool for in vitro culturing

Natural cytokinins as well as the majority of their synthetic derivatives show negative effects on root growth and development. Changes in morphology, primarily linked to the inhibition of the cell division in the meristematic zone, are manifested as thickening and shortening of the primary root and impaired lateral root branching. Rational design of cytokinin derivatives can partially overcome these drawbacks and reduce the negative effects. Using our database of cytokinin derivatives, we selected several aromatic cytokinin analogs with modifications at the N9 atom of the adenine moiety. We found that tetrahydropyranyl and tetrahydrofuranyl substitutions at the N9 atom led to enhanced acropetal transport of the modified cytokinin, and both derivatives also showed weak anticytokinin activity. Consequently, changes in the distribution of the active cytokinin pool together with gradual metabolic conversion of the modified cytokinin to its free form prevent root growth inhibition that limits cytokinin utilization in micropropagation techniques.

Endogenous cytokinin and auxin profiles during in vitro organogenesis from vegetative buds of Pinus radiata adult trees

In Pinus radiata D. Don, the transition from the juvenile to the mature phase is characterized by a reduction in the tree's organogenic potential, which is usually reverted in breeding programs by reinvigoration procedures to enable vegetative propagation. In this work, we have determined the best culture conditions for in vitro reinvigoration of radiata pine buds, tested different cytokinin (CK) types [N⁶-benzyladenine (BA), meta-topolin (mT) and trans-zeatin] and concentrations (25 and 50 µM), and studied the effect of culture conditions on endogenous CK and indole-3-acetic acid (IAA) levels at different stages of the organogenic process. To this end, the levels of 43 CKs and IAA were determined in P. radiata buds before and during the reinvigoration process. When BA or mT was applied to the induction medium, we did not observe any significant increase or decrease in endogenous isoprenoid CK content. We also report for the first time the presence of O-glucosides in non-treated P. radiata explants from the field and remark the importance of O-glucosides as storage forms.

Proteomics and transcriptomics of broccoli subjected to exogenously supplied and transgenic senescence-induced cytokinin for amelioration of postharvest yellowing

Previously, we investigated transgenic broccoli harboring senescence-associated-gene (SAG) promoter-triggered isopentenyltransferase (ipt), which encodes the key enzyme for cytokinin (CK) synthesis and mimics the action of exogenous supplied CK in delaying postharvest senescence of broccoli. Here, we used proteomics and transcriptomics to compare the mechanisms of ipt-transgenic and N(6)-benzylaminopurine (BA) CK treatment of broccoli during postharvest storage. The 2 treatments conferred common and distinct mechanisms. BA treatment decreased the quantity of proteins involved in energy and carbohydrate metabolism and amino acid metabolism, and ipt-transgenic treatment increased that of stress-related proteins and molecular chaperones and slightly affected levels of carbohydrate metabolism proteins. Both treatments regulated genes involved in CK signaling, sugar transport, energy and carbohydrate metabolism, amino acid metabolism and lipid metabolism, although ipt-transgenic treatment to a lesser extent. BA treatment induced genes encoding molecular chaperones, whereas ipt-transgenic treatment induced stress-related genes for cellular protection during storage. Both BA and ipt-transgenic treatments acted antagonistically on ethylene functions. We propose a long-term acclimation of metabolism and protection systems with ipt-transgenic treatment of broccoli and short-term modulation of metabolism and establishment of a protection system with both BA and ipt-transgenic treatments in delaying senescence of broccoli florets.

BIOLOGICAL SIGNIFICANCE

Transgenic broccoli harboring senescence-associated-gene (SAG) promoter-triggered isopentenyltransferase (ipt), which encodes the key enzyme for cytokinin (CK) synthesis and N(6)-benzylaminopurine (BA) CK treated broccoli both showed retardation of postharvest senescence during storage. The mechanisms underlying the two treatments were compared. The combination of proteomic and transcriptomic evidences revealed that the 2 treatments conferred common and distinct mechanisms in delaying senescence of broccoli florets. We propose a long-term acclimation of metabolism and protection systems with ipt-transgenic treatment of broccoli and short-term modulation of metabolism and establishment of a protection system with both BA and ipt-transgenic treatments in delaying senescence of broccoli florets. This article is part of a Special Issue entitled: Translational Plant Proteomics.

Crossing paths: cytokinin signalling and crosstalk

Cytokinins are a major class of plant hormones that are involved in various aspects of plant development, ranging from organ formation and apical dominance to leaf senescence. Cytokinin and auxin have long been known to interact antagonistically, and more recent studies have shown that cytokinins also interact with other plant hormones to regulate plant development. A growing body of research has begun to elucidate the molecular and genetic underpinnings of this extensive crosstalk. The rich interconnections between the synthesis, perception and transport networks of these plant hormones provide a wide range of opportunities for them to modulate, amplify or buffer one another. Here, we review this exciting and rapidly growing area of cytokinin research.

The plant vascular system: evolution, development and functions

The emergence of the tracheophyte-based vascular system of land plants had major impacts on the evolution of terrestrial biology, in general, through its role in facilitating the development of plants with increased stature, photosynthetic output, and ability to colonize a greatly expanded range of environmental habitats. Recently, considerable progress has been made in terms of our understanding of the developmental and physiological programs involved in the formation and function of the plant vascular system. In this review, we first examine the evolutionary events that gave rise to the tracheophytes, followed by analysis of the genetic and hormonal networks that cooperate to orchestrate vascular development in the gymnosperms and angiosperms. The two essential functions performed by the vascular system, namely the delivery of resources (water, essential mineral nutrients, sugars and amino acids) to the various plant organs and provision of mechanical support are next discussed. Here, we focus on critical questions relating to structural and physiological properties controlling the delivery of material through the xylem and phloem. Recent discoveries into the role of the vascular system as an effective long-distance communication system are next assessed in terms of the coordination of developmental, physiological and defense-related processes, at the whole-plant level. A concerted effort has been made to integrate all these new findings into a comprehensive picture of the state-of-the-art in the area of plant vascular biology. Finally, areas important for future research are highlighted in terms of their likely contribution both to basic knowledge and applications to primary industry.

TDZ pulsing evaluation on the in vitro morphogenesis of peach palm

Peach palm (Bactris gasipaes Kunth.) cropping is an excellent alternative to native species exploitation; nevertheless, the problems with seed germination and conventional propagation justify the use of in vitro culturing. Aiming to asses TDZ pulsing effect on B. gasipaes morphogenesis, explants obtained from unarmed microplants were maintained in two treatments, half of them in MS free medium (without growth regulator) and the other half in MS with TDZ (0.36 μM). Both groups were transferred to growth regulator-free MS medium following 14 days of culture. After 84 days of culture, TDZ pulsing increased the growth and development of the shoots, restricted the growth and development of the roots, with no influence on adventitious bud induction or somatic embryogenesis. Furthermore, development of prickles, thickening of roots and chlorotic leaves were noted under TDZ pulsing. Leaf sheath histological analysis showed an epidermal origin and no vascularization of these prickles.

Overexpression of glucosyltransferase UGT85A1 influences trans-zeatin homeostasis and trans-zeatin responses likely through O-glucosylation

Trans-zeatin is a kind of cytokinins that plays a crucial role in plant growth and development. The master trans-zeatin O-glucosyltransferase of Arabidopsis thaliana, UGT85A1, has been previously identified through biochemical approach. To determine the in planta role of UGT85A1 gene, the characterization of transgenic Arabidopsis plants overexpressing UGT85A1 was carried out. Under normal conditions, transgenic Arabidopsis did not display clearly altered phenotypes. A remarkable alteration is that the accumulation level of the trans-zeatin O-glucosides was significantly increased in UGT85A1 overexpressing transgenic Arabidopsis, while other forms of cytokinins kept the similar concentrations compared to the wild type. When treated with exogenously applied trans-zeatin, UGT85A1 overexpressing Arabidopsis showed much less sensitivity to trans-zeatin in primary root elongation and lateral root formation. Meanwhile, the chlorophyll content of detached leaves of transgenic Arabidopsis was much lower than wild type. Studies of spatial-temporal expression patterns showed that UGT85A1 was mainly expressed in the early seedlings and developing seeds. Analysis of subcellular localization suggested that UGT85A1 was localized to cytoplasm and nucleus. Taken together, our data suggest that overexpression of Arabidopsis glucosyltransferase UGT85A1 influences trans-zeatin homeostasis and trans-zeatin responses likely through O-glucosylation in planta.

Glucosyltransferase UGT76C1 finely modulates cytokinin responses via cytokinin N-glucosylation in Arabidopsis thaliana

Cytokinins are master regulators of plant growth and development. The glucosyltransferase UGT76C1 capable of N-glucosylation of different cytokinins at the N(7)- and N(9)-position was previously identified in Arabidopsis thaliana, but its physiological relevance in plants remains unclear. In the present work, we investigated the physiological characteristics of UGT76C1 mutant (ugt76c1) and its overexpressors. Under normal growth conditions, although ugt76c1 plants and UGT76C1 overexpressors did not display obvious phenotypic alteration, ugt76c1 plants significantly reduced the accumulation of cytokinin N-glucosides, whereas UGT76C1 overexpressors increased cytokinin N-glucosides. Unexpectedly, the concentrations of free forms of cytokinins (mainly trans-zeatin and N(6)-isopentenyladenine) were comparable to those of the wild type. Upon application of exogenous cytokinin, the mutant showed the same tendency of more sensitive cytokinin response in primary root elongation, chlorophyll retention and anthocyanin accumulation. In contrast, overexpressors showed a tendency of less sensitive cytokinin response in most tests. Furthermore, cytokinin-related genes were investigated for their expression; and the expression levels of AHK3, ARR1, CYP735A2 and LOG2 noticeably changed in ugt76c1 plants, suggesting that plants employ a set of cytokinin regulation mechanisms to coordinate the loss-of-function of UGT76C1. Tissue-specific expression of UGT76C1 showed a high level of expression in germinating seeds and young seedlings. Taken together, our data suggest that the glucosyltransferase UGT76C1 could finely modulate cytokinin responses in planta via N-glucosylation of cytokinins.

AXR1 promotes the Arabidopsis cytokinin response by facilitating ARR5 proteolysis

The plant hormone cytokinin plays essential roles in many aspects of growth and development. The cytokinin signal is transmitted by a multi-step phosphorelay to the members of two functionally antagonistic classes of Arabidopsis response regulators (ARRs): type B ARRs (response activators) and type A ARRs (negative-feedback regulators). Previous studies have shown that mutations in AXR1, encoding a subunit of the E1 enzyme in the RUB (related to ubiquitin) modification pathway, lead to decreased cytokinin sensitivity. Here we show that the cytokinin resistance of axr1 seedlings is suppressed by loss of function of the type A ARR family member ARR5. Based on the established role of the RUB pathway in ubiquitin-dependent proteolysis, these data suggest that AXR1 promotes the cytokinin response by facilitating type A ARR degradation. Indeed, both genetic (axr1 mutants) and chemical (MLN4924) suppression of RUB E1 increased ARR5 stability, suggesting that the ubiquitin ligase that promotes ARR5 proteolysis requires RUB modification for optimal activity.

Uniconazole, a cytochrome P450 inhibitor, inhibits trans-zeatin biosynthesis in Arabidopsis

Cytokinin (CK) is a plant hormone that plays important regulatory roles in many aspects of plant growth and development. Although functions of CK and its biosynthesis pathway have been studied extensively, there is still no efficient biosynthesis inhibitor, which would be useful for studying CK from a chemical genetic approach. Here, CK biosynthesis inhibitor candidates were searched for using a systematic approach. In silico screening of candidates were carried out using genome-wide gene expression profiles and prediction of target sites using global CK accumulation profile analysis. As a result of these screenings, it was found that uniconazole, a well known inhibitor of cytochrome P450 monooxygenase, prevents the biosynthesis of trans-zeatin, and that its target is CYP735As in Arabidopsis.

Proteasome-dependent proteolysis has a critical role in fine-tuning the feedback inhibition of cytokinin signaling

The ubiquitin/26S proteasome-dependent proteolysis of response regulators is a critical element of many plant hormone signaling pathways. We have recently shown that cytokinin signaling requires the AXR1 component of the related to ubiquitin (RUB) protein modification pathway to promote the proteasome-dependent degradation of the cytokinin response inhibitor ARR5. Here, we show that ARR5 also accumulates in the 26S proteasome mutant rpn12a-1, and leads to a marked resistance to cytokinins. Collectively, these results suggest that proteasome-dependent proteolysis of feedback inhibitors such as ARR5 is essential for the maintenance of optimal responsivity and plasticity in cytokinin signaling.

Indeterminate domain 10 regulates ammonium-mediated gene expression in rice roots

Indeterminate domain (IDD) genes are a family of plant transcriptional regulators that function in the control of development and metabolism during growth. Here, the function of Oryza sativa indeterminate domain 10 (OsIDD10) has been explored in rice plants. Compared with wild-type roots, idd10 mutant roots are hypersensitive to exogenous ammonium. This work aims to define the action of IDD10 on gene expression involved in ammonium uptake and nitrogen (N) metabolism. The ammonium induction of key ammonium uptake and assimilation genes was examined in the roots of idd10 mutants and IDD10 overexpressors. Molecular studies and transcriptome analysis were performed to identify target genes and IDD10 binding cis-elements. IDD10 activates the transcription of AMT1;2 and GDH2 by binding to a cis-element motif present in the promoter region of AMT1;2 and in the fifth intron of GDH2. IDD10 contributes significantly to the induction of several genes involved in N-linked metabolic and cellular responses, including genes encoding glutamine synthetase 2, nitrite reductases and trehalose-6-phosphate synthase. Furthermore, the possibility that IDD10 might influence the N-mediated feedback regulation of target genes was examined. This study demonstrates that IDD10 is involved in regulatory circuits that determine N-mediated gene expression in plant roots.

Transcriptome analysis of cytokinin response in tomato leaves

Tomato is one of the most economically and agriculturally important Solanaceous species and vegetable crops, serving as a model for examination of fruit biology and compound leaf development. Cytokinin is a plant hormone linked to the control of leaf development and is known to regulate a wide range of genes including many transcription factors. Currently there is little known of the leaf transcriptome in tomato and how it might be regulated by cytokinin. We employ high throughput mRNA sequencing technology and bioinformatic methodologies to robustly analyze cytokinin regulated tomato leaf transcriptomes. Leaf samples of two ages, 13d and 35d were treated with cytokinin or the solvent vehicle control dimethyl sulfoxide (DMSO) for 2 h or 24 h, after which RNA was extracted for sequencing. To confirm the accuracy of RNA sequencing results, we performed qPCR analysis of select transcripts identified as cytokinin regulated by the RNA sequencing approach. The resulting data provide the first hormone transcriptome analysis of leaves in tomato. Specifically we identified several previously untested tomato orthologs of cytokinin-related genes as well as numerous novel cytokinin-regulated transcripts in tomato leaves. Principal component analysis of the data indicates that length of cytokinin treatment and plant age are the major factors responsible for changes in transcripts observed in this study. Two hour cytokinin treatment showed a more robust transcript response indicated by both greater fold change of induced transcripts and the induction of twice as many cytokinin-related genes involved in signaling, metabolism, and transport in young vs. older leaves. This difference in transcriptome response in younger vs. older leaves was also found to a lesser extent with an extended (24 h) cytokinin treatment. Overall data presented here provides a solid foundation for future study of cytokinin and cytokinin regulated genes involved in compound leaf development or other developmental processes in tomato.

Nitrogen and phosphorus interaction and cytokinin: responses of the primary root of Arabidopsis thaliana and the pdr1 mutant

Nitrogen (N) and phosphorus (P) are the two most limiting nutrients for plant yield. Plants modify their metabolism and growth to cope with resources availability, consequently the integration of diverse signals is required. There is mounting evidence of N and P interaction, however, the sharing components of their signaling pathways have not been revealed yet. The pdr1 mutant has proved potentially useful in understanding the responses to nitrate (Ni), P and cytokinin. The mutation conferred pdr1 reduced root length in response to Ni under P deficiency and no effect of low cytokinin concentration. High N availability and high cytokinin caused strong root growth inhibition by different paths in wild type. Cytokinin repressed cell division, exhausted the quiescent center, caused changes in the pattern of AtPT1 expression and reduced AtACP5 expression. On the contrary, high N induced cell division as well as increased the expression of AtPT1 and AtACP5 even at high P availability. The data indicated interaction in the root modulation by N and P; and PDR1 is probably a signaling component of the nutritional status in Arabidopsis thaliana that modulates the response to N and P only partially mediated by cytokinin.

Getting to the roots of it: Genetic and hormonal control of root architecture

Root system architecture (RSA) - the spatial configuration of a root system - is an important developmental and agronomic trait, with implications for overall plant architecture, growth rate and yield, abiotic stress resistance, nutrient uptake, and developmental plasticity in response to environmental changes. Root architecture is modulated by intrinsic, hormone-mediated pathways, intersecting with pathways that perceive and respond to external, environmental signals. The recent development of several non-invasive 2D and 3D root imaging systems has enhanced our ability to accurately observe and quantify architectural traits on complex whole-root systems. Coupled with the powerful marker-based genotyping and sequencing platforms currently available, these root phenotyping technologies lend themselves to large-scale genome-wide association studies, and can speed the identification and characterization of the genes and pathways involved in root system development. This capability provides the foundation for examining the contribution of root architectural traits to the performance of crop varieties in diverse environments. This review focuses on our current understanding of the genes and pathways involved in determining RSA in response to both intrinsic and extrinsic (environmental) response pathways, and provides a brief overview of the latest root system phenotyping technologies and their potential impact on elucidating the genetic control of root development in plants.

Getting to the roots of it: Genetic and hormonal control of root architecture

Root system architecture (RSA) - the spatial configuration of a root system - is an important developmental and agronomic trait, with implications for overall plant architecture, growth rate and yield, abiotic stress resistance, nutrient uptake, and developmental plasticity in response to environmental changes. Root architecture is modulated by intrinsic, hormone-mediated pathways, intersecting with pathways that perceive and respond to external, environmental signals. The recent development of several non-invasive 2D and 3D root imaging systems has enhanced our ability to accurately observe and quantify architectural traits on complex whole-root systems. Coupled with the powerful marker-based genotyping and sequencing platforms currently available, these root phenotyping technologies lend themselves to large-scale genome-wide association studies, and can speed the identification and characterization of the genes and pathways involved in root system development. This capability provides the foundation for examining the contribution of root architectural traits to the performance of crop varieties in diverse environments. This review focuses on our current understanding of the genes and pathways involved in determining RSA in response to both intrinsic and extrinsic (environmental) response pathways, and provides a brief overview of the latest root system phenotyping technologies and their potential impact on elucidating the genetic control of root development in plants.

Physiological responses and endogenous cytokinin profiles of tissue-cultured 'Williams' bananas in relation to roscovitine and an inhibitor of cytokinin oxidase/dehydrogenase (INCYDE) treatments

The effect of supplementing either meta-topolin (mT) or N(6)-benzyladenine (BA) requiring cultures with roscovitine (6-benzylamino-2-[1(R)-(hydroxymethyl)propyl]amino-9-isopropylpurine), a cyclin-dependent kinase (CDK) and N-glucosylation inhibitor, and INCYDE (2-chloro-6-(3-methoxyphenyl)aminopurine), an inhibitor of cytokinin (CK) degradation, on the endogenous CK profiles and physiology of banana in vitro was investigated. Growth parameters including multiplication rate and biomass were recorded after 42 days. Endogenous CK levels were quantified using UPLC-MS/MS while the photosynthetic pigment and phenolic contents were evaluated spectrophotometrically. The highest regeneration rate (93 %) was observed in BA + roscovitine while mT + INCYDE plantlets produced most shoots. Treatment with BA + roscovitine had the highest shoot length and biomass. Although not significant, there was a higher proanthocyanidin level in BA + roscovitine treatments compared to the control (BA). The levels of total phenolics and flavonoids were significantly higher in mT + roscovitine treatment than in the mT-treated regenerants. The presence of roscovitine and/or INCYDE had no significant effect on the photosynthetic pigments of the banana plantlets. Forty-seven aromatic and isoprenoid CKs categorized into nine CK-types were detected at varying concentrations. The presence of mT + roscovitine and/or INCYDE increased the levels of O-glucosides while 9-glucosides were higher in the presence of BA. Generally, the underground parts had higher CK levels than the aerial parts; however, the presence of INCYDE increased the level of CK quantified in the aerial parts. From a practical perspective, the use of roscovitine and INCYDE in micropropagation could be crucial in the alleviation of commonly observed in vitro-induced physiological abnormalities.

The leaf reticulate mutant dov1 is impaired in the first step of purine metabolism

A series of reticulated Arabidopsis thaliana mutants were previously described. All mutants show a reticulate leaf pattern, namely green veins on a pale leaf lamina. They have an aberrant mesophyll structure but an intact layer of bundle sheath cells around the veins. Here, we unravel the function of the previously described reticulated EMS-mutant dov1 (differential development of vascular associated cells 1). By positional cloning, we identified the mutated gene, which encodes glutamine phosphoribosyl pyrophosphate aminotransferase 2 (ATase2), an enzyme catalyzing the first step of purine nucleotide biosynthesis. dov1 is allelic to the previously characterized cia1-2 mutant that was isolated in a screen for mutants with impaired chloroplast protein import. We show that purine-derived total cytokinins are lowered in dov1 and crosses with phytohormone reporter lines revealed differential reporter activity patterns in dov1. Metabolite profiling unraveled that amino acids that are involved in purine biosynthesis are increased in dov1. This study identified the molecular basis of an established mutant line, which has the potential for further investigation of the interaction between metabolism and leaf development.

A type-B response regulator drives the expression of the hydroxymethylbutenyl diphosphate synthase gene in periwinkle

In plant cytokinin (CK) signaling, type-B response regulators (RRs) act as major players in orchestrating the transcriptome changes in response to CK. However, their direct targets are poorly known. The identification of putative type-ARR1 motifs located within the promoter of the CK-responsive hydroxyl methyl butenyl diphosphate synthase (HDS) gene from the methyl erythritol phosphate (MEP) pathway prompted us to investigate the ability of a previously isolated periwinkle type-B RR (CrRR5) that presents high homologies with ARR1 to interact with the promoter. Electrophoretic mobility shift assays (EMSAs) demonstrated that the CrRR5 DNA-binding domain binds specifically type-ARR1 motifs within the HDS promoter. We also established through yellow fluorescent protein (YFP) imaging the targeting of CrRR5 into cell nucleus in accordance with its putative function of transcription factor. In transient assays performed on periwinkle cells cultivated with CK, overexpression of the full-length CrRR5 or a truncated CrRR5 engineering a constitutive active form (35S:ΔDDK) did not affect the HDS promoter activity that reached a threshold. By contrast, in absence of CK, overexpression of CrRR5ΔDDK enhanced promoter activity up to the threshold level observed in cells grown with CK. Our results strongly suggest that CrRR5 directly transactivates the HDS promoter. CrRR5 is the first identified transcription factor mediating the CK signaling that targets a gene from the MEP pathway involved in isoprenoid metabolism. Moreover, CrRR5 could play a role in a regulatory mechanism controlling CK homeostasis in periwinkle cells.

Concurrent profiling of indole-3-acetic acid, abscisic acid, and cytokinins and structurally related purines by high-performance-liquid-chromatography tandem electrospray mass spectrometry

BACKGROUND

Cytokinins (CKs) are a group of plant growth regulators that are involved in several plant developmental processes. Despite the breadth of knowledge surrounding CKs and their diverse functions, much remains to be discovered about the full potential of CKs, including their relationship with the purine salvage pathway, and other phytohormones. The most widely used approach to query unknown facets of CK biology utilized functional genomics coupled with CK metabolite assays and screening of CK associated phenotypes. There are numerous different types of assays for determining CK quantity, however, none of these methods screen for the compendium of metabolites that are necessary for elucidating all roles, including purine salvage pathway enzymes in CK metabolism, and CK cross-talk with other phytohormones. Furthermore, all published analytical methods have drawbacks ranging from the required use of radiolabelled compounds, or hazardous derivatization reagents, poor sensitivity, lack of resolution between CK isomers and lengthy run times.

RESULTS

In this paper, a method is described for the concurrent extraction, purification and analysis of several CKs (freebases, ribosides, glucosides, nucleotides), purines (adenosine monophosphate, inosine, adenosine, and adenine), indole-3-acetic acid, and abscisic acid from hundred-milligram (mg) quantities of Arabidopsis thaliana leaf tissue. This method utilizes conventional Bieleski solvents extraction, solid phase purification, and is unique because of its diverse range of detectable analytes, and implementation of a conventional HPLC system with a fused core column that enables good sensitivity without the requirement of a UHPLC system. Using this method we were able to resolve CKs about twice as fast as our previous method. Similarly, analysis of adenosine, indole-3-acetic acid, and abscisic acid, was comparatively rapid. A further enhancement of the method was the utilization of a QTRAP 5500 mass analyzer, which improved upon several aspects of our previous analytical method carried out on a Quattro mass analyzer. Notable improvements included much superior sensitivity, and number of analytes detectable within a single run. Limits of detection ranged from 2 pM for (9G)Z to almost 750 pM for indole-3-acetic acid.

CONCLUSIONS

This method is well suited for functional genomics platforms tailored to understanding CK metabolism, CK interrelationships with purine recycling and associated hormonal cross-talk.

Vascular expression and C-terminal sequence divergence of cytokinin response factors in flowering plants

Cytokinin response factors (CRFs) are important transcription factors that form a side branch of the cytokinin signaling pathway and have been linked to cytokinin-regulated processes during development. CRF proteins are defined as belonging to a specific transcription factor family by the presence of an AP2/ERF DNA-binding domain and can be distinguished within this family by a group-specific CRF domain involved in protein-protein interactions. Here we further delimit CRFs into five distinct clades (I-V) represented across all major angiosperm lineages. Protein sequences within each clade contain a clade-specific C-terminal region distinct from other CRFs, suggesting ancient evolutionary divergence and specialization within this gene family. Conserved patterns of transcriptional regulation support these clade divisions. Despite these important differences, CRFs appear to show preferential localization or targeting to vascular tissue in quantitative real-time PCR and reporter line analyses of Arabidopsis thaliana and Solanum lycopersicum (tomato). Phloem tissue expression within the vasculature often appears the strongest in CRF reporter lines, and an analysis of CRF promoter sequences revealed conservation and significant enrichment of phloem targeting cis-elements, suggesting a potential role for CRFs in this tissue. An examination of CRF loss-of-function mutants from cytokinin-regulated clades revealed alterations in higher order vein patterning. This supports both the general link of CRFs to vascular tissue and clade-specific differences between CRFs, since alterations in vascular patterning appear to be clade specific. Together these findings indicate that CRFs are potential regulators of developmental processes associated with vascular tissues.

Effect of blue light on endogenous isopentenyladenine and endoreduplication during photomorphogenesis and de-etiolation of tomato (Solanum lycopersicum L.) seedlings

Light is one of the most important factor influencing plant growth and development all through their life cycle. One of the well-known light-regulated processes is de-etiolation, i.e. the switch from skotomorphogenesis to photomorphogenesis. The hormones cytokinins (CKs) play an important role during the establishment of photomorphogenesis as exogenous CKs induced photomorphogenesis of dark-grown seedlings. Most of the studies are conducted on the plant model Arabidopsis, but no or few information are available for important crop species, such as tomato (Solanum lycopersicum L.). In our study, we analyzed for the first time the endogenous CKs content in tomato hypocotyls during skotomorphogenesis, photomorphogenesis and de-etiolation. For this purpose, two tomato genotypes were used: cv. Rutgers (wild-type; WT) and its corresponding mutant (7B-1) affected in its responses to blue light (BL). Using physiological and molecular approaches, we identified that the skotomorphogenesis is characterized by an endoreduplication-mediated cell expansion, which is inhibited upon BL exposure as seen by the accumulation of trancripts encoding CycD3, key regulators of the cell cycle. Our study showed for the first time that iP (isopentenyladenine) is the CK accumulated in the tomato hypocotyl upon BL exposure, suggesting its specific role in photomorphogenesis. This result was supported by physiological experiments and gene expression data. We propose a common model to explain the role and the relationship between CKs, namely iP, and endoreduplication during de-etiolation and photomorphogenesis.

Cytokinin activity of cis-zeatin and phenotypic alterations induced by overexpression of putative cis-Zeatin-O-glucosyltransferase in rice

cis-Zeatin (cZ) is generally regarded as a cytokinin with little or no activity, compared with the highly active trans-zeatin (tZ). Although recent studies suggested possible roles for cZ, its physiological significance remains unclear. In our studies with rice (Oryza sativa), cZ inhibited seminal root elongation and up-regulated cytokinin-inducible genes, and its activities were comparable to those of tZ. Tracer experiments showed that exogenously supplied cZ-riboside was mainly converted into cZ derivatives but scarcely into tZ derivatives, indicating that isomerizations of cZ derivatives into tZ derivatives are a minor pathway in rice cytokinin metabolism. We identified three putative cZ-O-glucosyltransferases (cZOGT1, cZOGT2, and cZOGT3) in rice. The cZOGTs preferentially catalyzed O-glucosylation of cZ and cZ-riboside rather than tZ and tZ-riboside in vitro. Transgenic rice lines ectopically overexpressing the cZOGT1 and cZOGT2 genes exhibited short-shoot phenotypes, delay of leaf senescence, and decrease in crown root number, while cZOGT3 overexpressor lines did not show shortened shoots. These results propose that cZ activity has a physiological impact on the growth and development of rice.

Roles and regulation of cytokinins in tomato fruit development

Cytokinins (CKs) are thought to play important roles in fruit development, especially cell division. However, the mechanisms and regulation of CK activity have not been well investigated. This study analysed CK concentrations and expression of genes involved in CK metabolism in developing tomato (Solanum lycopersicum) ovaries. The concentrations of CK ribosides and isopentenyladenine and the transcript levels of the CK biosynthetic genes SlIPT3, SlIPT4, SlLOG6, and SlLOG8 were high at anthesis and decreased immediately afterward. In contrast, trans-zeatin concentration and the transcript levels of the CK biosynthetic genes SlIPT1, SlIPT2, SlCYP735A1, SlCYP735A2, and SlLOG2 increased after anthesis. The expression of type-A response regulator genes was high in tomato ovaries from pre-anthesis to early post-anthesis stages. These results suggest that the CK signal transduction pathway is active in the cell division phase of fruit development. This study also investigated the effect of CK application on fruit set and development. Application of a synthetic CK, N-(2-chloro-pyridin-4-yl)-N'-phenylurea (CPPU), to unpollinated tomato ovaries induced parthenocarpic fruit development. The CPPU-induced parthenocarpic fruits were smaller than pollinated fruits, because of reduction of pericarp cell size rather than reduced cell number. Thus, CPPU-induced parthenocarpy was attributable to the promotion of cell division, not cell expansion. Overall, the results provide evidence that CKs are involved in cell division during development of tomato fruit.

Overlapping and lineage-specific roles for the type-B response regulators of monocots and dicots

Cytokinins are plant hormones with profound roles in growth and development. Cytokinin signaling is mediated through a 'two-component' signaling system composed of histidine kinases, histidine-containing phosphotransfer proteins, and response regulators. Phylogenetic analysis of two-component signaling elements from the monocot rice and the dicot Arabidopsis reveals lineage-specific expansions of the type-B response regulators, transcription factors that act as positive regulators for the cytokinin signal. We recently reported in Plant Physiology on a functional analysis of rice type-B response regulators. A type-B response regulator from a subfamily comprised of both monocot and dicot type-B response regulators complemented an Arabidopsis type-B response regulator mutant, but a type-B response regulator from a monocot-specific subfamily generally did not. Here, we extend this analysis to demonstrate that the promoter of an Arabidopsis cytokinin primary response gene is induced by type-B response regulators from a shared subfamily, but not by one from a lineage-specific subfamily. These results support a model in which the type-B response regulators of monocots and dicots share conserved roles in the cytokinin signaling pathway but have also diverged to take on lineage-specific roles.

TaCKX6-D1, the ortholog of rice OsCKX2, is associated with grain weight in hexaploid wheat

The cytokinin oxidase/dehydrogenase (CKX) gene plays a principal role in controlling cytokinin levels and has been shown to be a major quantitative trait locus (QTL) affecting grain number in rice. However, the function and evaluation of the haplotypes of the wheat CKX gene have yet to be illustrated. In this study, TaCKX6-D1, a wheat ortholog of rice OsCKX2, was cloned and its haplotype variants were determined to be significantly associated with the 1000-grain weight on the basis of linkage mapping, association analysis and gene expression analysis. Five TaCKX6-D1 haplotypes, designated a-e, were identified. An indel marker was developed to identify haplotype a, which was associated with higher grain weight. Haplotype a showed decreased expression relative to haplotype b in seeds at 8 d after pollination. Sequence variations among modern cultivars, landraces and wild species suggest a significant domestication signature at the TaCKX6-D1 locus in Chinese wheat germplasm. TaCKX6-D1 may serve as a useful gene for the breeding of high-yielding wheat. A strategy for allele mining and utilization of TaCKX6-D1 was proposed. Our study also sheds light on the mechanisms of grain development and domestication of wheat, as well as the functional divergence of orthologs in comparative genomics.

Remodeling of cytokinin metabolism at infection sites of Colletotrichum graminicola on maize leaves

When inoculated onto maize leaves at the onset of senescence, the hemibiotroph Colletotrichum graminicola causes green islands that are surrounded by senescing tissue. Taking advantage of green islands as indicators of sites of the establishment of successful infection and of advanced high-performance liquid chromatography tandem mass spectrometry methodology, we analyzed changes in the patterns and levels of cytokinins (CK) at high spatial and analytical resolution. Twenty individual CK were detected in green islands. Levels of cis-zeatin-9-riboside and cis-zeatin-9-riboside-5'-monophosphate increased drastically, whereas that of the most prominent CK, cis-zeatin-O-glucoside, decreased. The fungus likely performed these conversions because corresponding activities were also detected in in vitro cultures amended with CK. We found no evidence that C. graminicola is able to synthesize CK entirely de novo in minimal medium but, after adding dimethylallyl diphosphate, a precursor of CK biosynthesis occurring in plants, a series of trans-zeatin isoforms (i.e., trans-zeatin-9-riboside-5'-monophosphate, trans-zeatin-9-riboside, and trans-zeatin) was formed. After applying CK onto uninfected leaves, transcripts of marker genes for senescence, photosynthesis, and assimilate distribution were measured by quantitative reverse-transcribed polymerase chain reaction; furthermore, pulse-amplitude modulation chlorophyll fluorometry and single-photon avalanche diode analyses were conducted. These experiments suggested that modulation of CK metabolism at the infection site affects host physiology.

Genetic dissection of growth, wood basic density and gene expression in interspecific backcrosses of Eucalyptus grandis and E. urophylla

BACKGROUND

F1 hybrid clones of Eucalyptus grandis and E. urophylla are widely grown for pulp and paper production in tropical and subtropical regions. Volume growth and wood quality are priority objectives in Eucalyptus tree improvement. The molecular basis of quantitative variation and trait expression in eucalypt hybrids, however, remains largely unknown. The recent availability of a draft genome sequence (http://www.phytozome.net) and genome-wide genotyping platforms, combined with high levels of genetic variation and high linkage disequilibrium in hybrid crosses, greatly facilitate the detection of quantitative trait loci (QTLs) as well as underlying candidate genes for growth and wood property traits. In this study, we used Diversity Arrays Technology markers to assess the genetic architecture of volume growth (diameter at breast height, DBH) and wood basic density in four-year-old progeny of an interspecific backcross pedigree of E. grandis and E. urophylla. In addition, we used Illumina RNA-Seq expression profiling in the E. urophylla backcross family to identify cis- and trans-acting polymorphisms (eQTLs) affecting transcript abundance of genes underlying QTLs for wood basic density.

RESULTS

A total of five QTLs for DBH and 12 for wood basic density were identified in the two backcross families. Individual QTLs for DBH and wood basic density explained 3.1 to 12.2% of phenotypic variation. Candidate genes underlying QTLs for wood basic density on linkage groups 8 and 9 were found to share trans-acting eQTLs located on linkage groups 4 and 10, which in turn coincided with QTLs for wood basic density suggesting that these QTLs represent segregating components of an underlying transcriptional network.

CONCLUSION

This is the first demonstration of the use of next-generation expression profiling to quantify transcript abundance in a segregating tree population and identify candidate genes potentially affecting wood property variation. The QTLs identified in this study provide a resource for identifying candidate genes and developing molecular markers for marker-assisted breeding of volume growth and wood basic density. Our results suggest that integrated analysis of transcript and trait variation in eucalypt hybrids can be used to dissect the molecular basis of quantitative variation in wood property traits.

Agrobacterium tumefaciens tumor morphology root plastid localization and preferential usage of hydroxylated prenyl donor is important for efficient gall formation

Upon Agrobacterium tumefaciens infection of a host plant, Tumor morphology root (Tmr) a bacterial adenosine phosphate-isopentenyltransferase (IPT), creates a metabolic bypass in the plastid for direct synthesis of trans-zeatin (tZ) with 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate as the prenyl donor. To understand the biological importance of Tmr function for gall formation, we compared Tmr and Trans-zeatin secretion (Tzs) another agrobacterial IPT that functions within the bacterial cell. Although there is no significant difference in their substrate specificities in vitro, ectopic overexpression of Tzs in Arabidopsis (Arabidopsis thaliana) resulted in the accumulation of comparable amounts of tZ- and N⁶-(Δ²-isopentenyl)adenine (iP)-type cytokinins, whereas overexpression of Tmr resulted exclusively in the accumulation of tZ-type cytokinins. Ectopic expression of Tzs in plant cells yields only small amounts of the polypeptide in plastid-enriched fractions. Obligatory localization of Tzs into Arabidopsis plastid stroma by translational fusions with ferredoxin transit peptide (TP-Tzs) increased the accumulation of both tZ- and iP-type cytokinins. Replacement of tmr on the Ti plasmid with tzs, TP-tzs, or an Arabidopsis plastidic IPT induced the formation of smaller galls than wild-type A. tumefaciens, and they were accompanied by the accumulation of iP-type cytokinins. Tmr is thus specialized for plastid localization and preferential usage of 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate in vivo and is important for efficient gall formation.

Retargeting a maize β-glucosidase to the vacuole--evidence from intact plants that zeatin-O-glucoside is stored in the vacuole

Cytokinin (CK) activity is regulated by the complex interplay of their metabolism, transport, stability and cellular/tissue localization. O-glucosides of zeatin-type CKs are postulated to be storage and/or transport forms. Active CK levels are determined in part by their differential distribution of CK metabolites across different subcellular compartments. We have previously shown that overexpressing chloroplast-localized Zm-p60.1, a maize β-glucosidase capable of releasing active cytokinins from their O- and N3-glucosides, perturbs CK homeostasis in transgenic tobacco. We obtained tobacco (Nicotiana tabacum L., cv Petit Havana SR1) plants overexpressing a recombinant Zm-p60.1 that is targeted to the vacuole. The protein is correctly processed and localized to the vacuole. When grown on medium containing exogenous zeatin, transgenic seedlings rapidly accumulate fresh weight due to ectopic growths at the base of the hypocotyl. The presence of the enzyme in these ectopic structures is shown by histochemical staining. CK quantification reveals that these transgenic seedlings are unable to accumulate zeatin-O-glucoside to levels similar to those observed in the wild type. When crossed with tobacco overexpressing the zeatin-O-glucosyltransferase gene from Phaseolus, the vacuolar variant shows an almost complete reversion in the root elongation assay. This is the first evidence from intact plants that the vacuole is the storage organelle for CK O-glucosides and that they are available to attack by Zm-p60.1. We propose the use of Zm-p60.1 as a robust molecular tool that exploits the reversibility of O-glucosylation and enables delicate manipulations of active CK content at the cellular level.

Down-regulation of the CrHPT1 histidine phosphotransfer protein prevents cytokinin-mediated up-regulation of CrDXR, and CrG10H transcript levels in periwinkle cell cultures

In Catharanthus roseus cell cultures, cytokinins (CK) improve monoterpenoid indole alkaloids (MIAs) accumulation. This metabolite production is correlated with an increase of transcripts corresponding to genes encoding both elements of the CK-signaling pathway and enzymes implicated in MIAs biosynthesis. However, it has not been demonstrated that the CK signal, leading to MIAs accumulation, comes through components identified as belonging to the CK-signaling pathway. In this work, we addressed this question, by transgenesis, using an inducible RNAi system targeting element of CK-signaling. In transgenic lines, the up-regulation by CK of two genes involved in MIA biosynthesis was abolished. These results demonstrate a relationship between the CK-signaling and the MIAs biosynthetic pathways.

When cytokinin, a plant hormone, meets the adenosine A2A receptor: a novel neuroprotectant and lead for treating neurodegenerative disorders?

It is well known that cytokinins are a class of phytohormones that promote cell division in plant roots and shoots. However, their targets, biological functions, and implications in mammalian systems have rarely been examined. In this study, we show that one cytokinin, zeatin riboside, can prevent pheochromocytoma (PC12) cells from serum deprivation-induced apoptosis by acting on the adenosine A(2A) receptor (A(2A)-R), which was blocked by an A(2A)-R antagonist and a protein kinase A (PKA) inhibitor, demonstrating the functional ability of zeatin riboside by mediating through A(2A)-R signaling event. Since the A(2A)-R was implicated as a therapeutic target in treating Huntington's disease (HD), a cellular model of HD was applied by transfecting mutant huntingtin in PC12 cells. By using filter retardation assay and confocal microscopy we found that zeatin riboside reversed mutant huntingtin (Htt)-induced protein aggregations and proteasome deactivation through A(2A)-R signaling. PKA inhibitor blocked zeatin riboside-induced suppression of mutant Htt aggregations. In addition, PKA activated proteasome activity and reduced mutant Htt protein aggregations. However, a proteasome inhibitor blocked both zeatin riboside-and PKA activator-mediated suppression of mutant Htt aggregations, confirming mediation of the A(2A)-R/PKA/proteasome pathway. Taken together, zeatin riboside might have therapeutic potential as a novel neuroprotectant and a lead for treating neurodegenerative disorders.

Co-ordinate regulation of cytokinin gene family members during flag leaf and reproductive development in wheat

BACKGROUND

As the global population continues to expand, increasing yield in bread wheat is of critical importance as 20% of the world's food supply is sourced from this cereal. Several recent studies of the molecular basis of grain yield indicate that the cytokinins are a key factor in determining grain yield. In this study, cytokinin gene family members in bread wheat were isolated from four multigene families which regulate cytokinin synthesis and metabolism, the isopentenyl transferases (IPT), cytokinin oxidases (CKX), zeatin O-glucosyltransferases (ZOG), and β-glucosidases (GLU). As bread wheat is hexaploid, each gene family is also likely to be represented on the A, B and D genomes. By using a novel strategy of qRT-PCR with locus-specific primers shared among the three homoeologues of each family member, detailed expression profiles are provided of family members of these multigene families expressed during leaf, spike and seed development.

RESULTS

The expression patterns of individual members of the IPT, CKX, ZOG, and GLU multigene families in wheat are shown to be tissue- and developmentally-specific. For instance, TaIPT2 and TaCKX1 were the most highly expressed family members during early seed development, with relative expression levels of up to 90- and 900-fold higher, respectively, than those in the lowest expressed samples. The expression of two cis-ZOG genes was sharply increased in older leaves, while an extremely high mRNA level of TaGLU1-1 was detected in young leaves.

CONCLUSIONS

Key genes with tissue- and developmentally-specific expression have been identified which would be prime targets for genetic manipulation towards yield improvement in bread wheat breeding programmes, utilising TILLING and MAS strategies.