Cytokinin-Specific Glycosyltransferases Possess Different Roles in Cytokinin Homeostasis Maintenance

Systemic strategies for cytokinin biosynthesis and catabolism in Arabidopsis roots and leaves under prolonged ammonium nutrition

Cytokinins are growth-regulating plant hormones that are considered to adjust plant development under environmental stresses. During sole ammonium nutrition, a condition known to induce growth retardation of plants, altered cytokinin content can contribute to the characteristic ammonium toxicity syndrome. To understand the metabolic changes in cytokinin pools, cytokinin biosynthesis and degradation were analyzed in the leaves and roots of mature Arabidopsis plants. We found that in leaves of ammonium-grown plants, despite induction of biosynthesis on the expression level, there was no active cytokinin build-up because they were effectively routed toward their downstream catabolites. In roots, cytokinin conjugation was also induced, together with low expression of major synthetic enzymes, resulting in a decreased content of the trans-zeatin form under ammonium conditions. Based on these results, we hypothesized that in leaves and roots, cytokinin turnover is the major regulator of the cytokinin pool and does not allow active cytokinins to accumulate. A potent negative-regulator of root development is trans-zeatin, therefore its low level in mature root tissues of ammonium-grown plants may be responsible for occurrence of a wide root system. Additionally, specific cytokinin enhancement in apical root tips may evoke a short root phenotype in plants under ammonium conditions. The ability to flexibly regulate cytokinin metabolism and distribution in root and shoot tissues can contribute to adjusting plant development in response to ammonium stress.

Map-based cloning and functional analysis of a major quantitative trait locus, BolC.Pb9.1, controlling clubroot resistance in a wild Brassica relative (Brassica macrocarpa)

KEY MESSAGE

A causal gene BoUGT76C2, conferring clubroot resistance in wild Brassica oleracea, was identified and functionally characterized. Clubroot is a devastating soil-borne disease caused by the obligate biotrophic pathogen Plasmodiophora brassica (P. brassicae), which poses a great threat to Brassica oleracea (B. oleracea) production. Although several QTLs associated with clubroot resistance (CR) have been mapped in cultivated B. oleracea, none have been cloned in B. oleracea. Previously, we found that the wild B. oleracea B2013 showed high resistance to clubroot. In this study, we constructed populations using B2013 and broccoli line 90196. CR in B2013 is quantitatively inherited, and a major QTL, BolC.Pb9.1, was identified on C09 using QTL-seq and linkage analysis. The BolC.Pb9.1 was finely mapped to a 56 kb genomic region using F2:3 populations. From the target region, the candidate BoUGT76C2 showed nucleotide variations between the parents, and was inducible in response to P. brassicae infection. We generated BoUGT76C2 overexpression lines in the 90196 background, which showed significantly enhanced resistance to P. brassicae compared to the WT line, suggesting that BoUGT76C2 corresponds to the resistance gene BolC.Pb.9.1. This is the first report on the CR gene map-based cloning and functional analysis from wild relatives, which provides a theoretical basis to the understanding of the molecular mechanism of CR, and lays a foundation to improve the CR of cultivated B. oleracea.

Fluorescence-activated multi-organelle mapping of subcellular plant hormone distribution

Auxins and cytokinins are two major families of phytohormones that control most aspects of plant growth, development and plasticity. Their distribution in plants has been described, but the importance of cell- and subcellular-type specific phytohormone homeostasis remains undefined. Herein, we revealed auxin and cytokinin distribution maps showing their different organelle-specific allocations within the Arabidopsis plant cell. To do so, we have developed Fluorescence-Activated multi-Organelle Sorting (FAmOS), an innovative subcellular fractionation technique based on flow cytometric principles. FAmOS allows the simultaneous sorting of four differently labelled organelles based on their individual light scatter and fluorescence parameters while ensuring hormone metabolic stability. Our data showed different subcellular distribution of auxin and cytokinins, revealing the formation of phytohormone gradients that have been suggested by the subcellular localization of auxin and cytokinin transporters, receptors and metabolic enzymes. Both hormones showed enrichment in vacuoles, while cytokinins were also accumulated in the endoplasmic reticulum.

QT-GWAS: A novel method for unveiling biosynthetic loci affecting qualitative metabolic traits

Although the plant kingdom provides an enormous diversity of metabolites with potentially beneficial applications for humankind, a large fraction of these metabolites and their biosynthetic pathways remain unknown. Resolving metabolite structures and their biosynthetic pathways is key to gaining biological understanding and to allow metabolic engineering. In order to retrieve novel biosynthetic genes involved in specialized metabolism, we developed a novel untargeted method designated as qualitative trait GWAS (QT-GWAS) that subjects qualitative metabolic traits to a genome-wide association study, while the conventional metabolite GWAS (mGWAS) mainly considers the quantitative variation of metabolites. As a proof of the validity of QT-GWAS, 23 and 15 of the retrieved associations identified in Arabidopsis thaliana by QT-GWAS and mGWAS, respectively, were supported by previous research. Furthermore, seven gene-metabolite associations retrieved by QT-GWAS were confirmed in this study through reverse genetics combined with metabolomics and/or in vitro enzyme assays. As such, we established that CYTOCHROME P450 706A5 (CYP706A5) is involved in the biosynthesis of chroman derivatives, UDP-GLYCOSYLTRANSFERASE 76C3 (UGT76C3) is able to hexosylate guanine in vitro and in planta, and SULFOTRANSFERASE 202B1 (SULT202B1) catalyzes the sulfation of neolignans in vitro. Collectively, our study demonstrates that the untargeted QT-GWAS method can retrieve valid gene-metabolite associations at the level of enzyme-encoding genes, even new associations that cannot be found by the conventional mGWAS, providing a new approach for dissecting qualitative metabolic traits.

Oligonucleotide Catabolism-Derived Gluconucleosides in Caenorhabditis elegans

Nucleosides are essential cornerstones of life, and nucleoside derivatives and synthetic analogues have important biomedical applications. Correspondingly, production of non-canonical nucleoside derivatives in animal model systems is of particular interest. Here, we report the discovery of diverse glucose-based nucleosides in Caenorhabditis elegans and related nematodes. Using a mass spectrometric screen based on all-ion fragmentation in combination with total synthesis, we show that C. elegans selectively glucosylates a series of modified purines but not the canonical purine and pyrimidine bases. Analogous to ribonucleosides, the resulting gluconucleosides exist as phosphorylated and non-phosphorylated forms. The phosphorylated gluconucleosides can be additionally decorated with diverse acyl moieties from amino acid catabolism. Syntheses of representative variants, facilitated by a novel 2'-O- to 3'-O-dibenzyl phosphoryl transesterification reaction, demonstrated selective incorporation of different nucleobases and acyl moieties. Using stable-isotope labeling, we further show that gluconucleosides incorporate modified nucleobases derived from RNA and possibly DNA breakdown, revealing extensive recycling of oligonucleotide catabolites. Gluconucleosides are conserved in other nematodes, and biosynthesis of specific subsets is increased in germline mutants and during aging. Bioassays indicate that gluconucleosides may function in stress response pathways.

Cytokinin activity - transport and homeostasis at the whole plant, cell, and subcellular levels

Cytokinins (CKs) are important plant hormones that regulate a variety of biological processes implicated in plant development and stress responses. Here, we summarize the most recent advances in discovering and characterizing the membrane transporters involved in long- and short-distance translocation of CKs and their significance in CK signal activity. We highlight the discovery of PUP7 and PUP21 tonoplast-localized transporters and propose potential mechanisms for CK subcellular homeostasis. Finally, we discuss the importance of subcellular hormone transport in light of the localization of histidine kinase receptors of CKs at the endoplasmic reticulum and plasma membrane.

From synthesis to the biological effect of isoprenoid 2'-deoxyriboside and 2',3'-dideoxyriboside cytokinin analogues

Isoprenoid cytokinins are a class of naturally occurring plant signaling molecules. A series of prepared compounds derived from isoprenoid cytokinins (isopentenyladenine, trans-zeatin and cis-zeatin) with attached 2'-deoxy-d-ribose or 2',3'-dideoxy-d-ribose at the N⁹ position of the purine were prepared and their biological activities were examined. Different synthetic approaches were employed. The final compounds were characterized with variety of physicochemical methods (TLC, HPLC-MS, and NMR) and their cytokinin activity was determined in classical bioassays such as Amaranthus, tobacco callus, detached wheat leaf senescence and Arabidopsis thaliana root elongation inhibition assay. In addition, compounds were screened for activation of the cytokinin signaling pathway (bacterial receptor, competitive ligand binding and ARR5::GUS assay) to provide a detailed assessment of CK structure-activity relationship. The prepared compounds were found to be non-toxic to human cells and the majority of assays exhibited the highest activity of free bases while 2',3'-dideoxyribosides had very weak or no activity. In contrast to the free bases, all 2'-deoxyriboside derivatives were not toxic to tobacco callus even at the highest tested concentration (10^-4 moL/l) and compound 1 (iPdR) induced betacyanin synthesis at higher concentration even stronger than iP free base in the Amaranthus bioassay. The general cytokinin activity pattern base > riboside >2'-deoxyriboside > 2',3'-dideoxyriboside was distinguished.

Is auxin enough? Cytokinins and margin patterning in simple leaves

The interplay between auxin and cytokinins affects facets of plant development as different as ovule formation and lateral root initiation. Moreover, cytokinins favor complexity in the development of Solanum lycopersicum and Cardamine hirsuta compound leaves. Nevertheless, no role has been proposed for cytokinins in patterning the margins of the simple leaves of Arabidopsis thaliana, a process that is assumed to be sufficiently explained by auxin localization. Here, we discuss evidence supporting the hypothesis that cytokinins play a role in simple leaf margin morphogenesis via crosstalk with auxin, as occurs in other plant developmental events. Indeed, mutant or transgenic arabidopsis plants defective in cytokinin biosynthesis or signaling, or with increased cytokinin degradation have leaf margins less serrated than the wild type.

Enhanced proteostasis, lipid remodeling, and nitrogen remobilization define barley flag leaf senescence

Leaf senescence is a developmental process allowing nutrient remobilization to sink organs. We characterized flag leaf senescence at 7, 14, and 21 d past anthesis in two near-isogenic barley lines varying in the allelic state of the HvNAM1 transcription factor gene, which influences senescence timing. Metabolomics and microscopy indicated that, as senescence progressed, thylakoid lipids were transiently converted to neutral lipids accumulating in lipid droplets. Senescing leaves also exhibited an accumulation of sugars including glucose, while nitrogen compounds (nucleobases, nucleotides, and amino acids) decreased. RNA-Seq analysis suggested lipid catabolism via β-oxidation and the glyoxylate cycle, producing carbon skeletons and feeding respiration as a replacement of the diminished carbon supply from photosynthesis. Comparison of the two barley lines highlighted a more prominent up-regulation of heat stress transcription factor- and chaperone-encoding genes in the late-senescing line, suggesting a role for these genes in the control of leaf longevity. While numerous genes with putative roles in nitrogen remobilization were up-regulated in both lines, several peptidases, nucleases, and nitrogen transporters were more highly induced in the early-senescing line; this finding identifies processes and specific candidates which may affect nitrogen remobilization from senescing barley leaves, downstream of the HvNAM1 transcription factor.

Killing two birds with a single stone-genetic manipulation of cytokinin oxidase/dehydrogenase (CKX) genes for enhancing crop productivity and amelioration of drought stress response

The development of high-yielding, bio-fortified, stress-tolerant crop cultivars is the need of the hour in the wake of increasing global food insecurity, abrupt climate change, and continuous shrinking of resources and landmass suitable for agriculture. The cytokinin group of phytohormones positively regulates seed yield by simultaneous regulation of source capacity (leaf senescence) and sink strength (grain number and size). Cytokinins also regulate root-shoot architecture by promoting shoot growth and inhibiting root growth. Cytokinin oxidase/dehydrogenase (CKX) are the only enzymes that catalyze the irreversible degradation of active cytokinins and thus negatively regulate the endogenous cytokinin levels. Genetic manipulation of CKX genes is the key to improve seed yield and root-shoot architecture through direct manipulation of endogenous cytokinin levels. Downregulation of CKX genes expressed in sink tissues such as inflorescence meristem and developing seeds, through reverse genetics approaches such as RNAi and CRISPR/Cas9 resulted in increased yield marked by increased number and size of grains. On the other hand, root-specific expression of CKX genes resulted in decreased endogenous cytokinin levels in roots which in turn resulted in increased root growth indicated by increased root branching, root biomass, and root-shoot biomass ratio. Enhanced root growth provided enhanced tolerance to drought stress and improved micronutrient uptake efficiency. In this review, we have emphasized the role of CKX as a genetic factor determining yield, micronutrient uptake efficiency, and response to drought stress. We have summarised the efforts made to increase crop productivity and drought stress tolerance in different crop species through genetic manipulation of CKX family genes.

Transcriptome Analysis Reveals the Regulatory Networks of Cytokinin in Promoting Floral Feminization in Castanea henryi

Castanea henryi is a monoecious plant with a low female-to-male ratio, which limits its yield. The phytohormone cytokinin (CK) plays a crucial role in flower development, especially gynoecium development. Here, the feminizing effect of CK on the development of C. henryi was confirmed by the exogenous spraying of N-(2-chloro-4-pyridyl)-N'-phenylurea (CPPU). Spraying CPPU at 125 mg·L^-1 thrice changed the male catkin into a pure female catkin, whereas at 5 mg·L^-1 and 25 mg·L^-1, only a part of the male catkin was transformed into a female catkin. A comparative transcriptome analysis of male catkins subjected to CPPU was performed to study the mechanism of the role of CKs in sex differentiation. Using Pearson's correlation analysis between hormone content and hormone synthesis gene expression, four key genes, LOG1, LOG3, LOG7 and KO, were identified in the CK and GA synthesis pathways. Moreover, a hub gene in the crosstalk between JA and the other hormone signaling pathways, MYC2, was identified, and 15 flowering-related genes were significantly differentially expressed after CPPU treatment. These results suggest that CK interacts with other phytohormones to determine the sex of C. henryi, and CK may directly target floral organ recognition genes to control flower sex.

Integrative omics approaches revealed a crosstalk among phytohormones during tuberous root development in cassava

Integrative omics approaches revealed a crosstalk among phytohormones during tuberous root development in cassava. Tuberous root formation is a complex process consisting of phase changes as well as cell division and elongation for radial growth. We performed an integrated analysis to clarify the relationships among metabolites, phytohormones, and gene transcription during tuberous root formation in cassava (Manihot esculenta Crantz). We also confirmed the effects of the auxin (AUX), cytokinin (CK), abscisic acid (ABA), jasmonic acid (JA), gibberellin (GA), brassinosteroid (BR), salicylic acid, and indole-3-acetic acid conjugated with aspartic acid on tuberous root development. An integrated analysis of metabolites and gene expression indicated the expression levels of several genes encoding enzymes involved in starch biosynthesis and sucrose metabolism are up-regulated during tuberous root development, which is consistent with the accumulation of starch, sugar phosphates, and nucleotides. An integrated analysis of phytohormones and gene transcripts revealed a relationship among AUX signaling, CK signaling, and BR signaling, with AUX, CK, and BR inducing tuberous root development. In contrast, ABA and JA inhibited tuberous root development. These phenomena might represent the differences between stem tubers (e.g., potato) and root tubers (e.g., cassava). On the basis of these results, a phytohormonal regulatory model for tuberous root development was constructed. This model may be useful for future phytohormonal studies involving cassava.

Plant-specific small peptide AtZSP1 interacts with ROCK1 to regulate organ size in Arabidopsis

Organ size is an important agronomic trait. Small peptides function in various stages of plant growth, but their regulatory mechanisms in organ growth remain poorly understood. Here, we characterize a novel small peptide, AtZSP1, which positively regulates organ size in Arabidopsis. Loss-of-function mutant atzsp1-1 exhibited small organs, whereas AtZSP1 overexpression plants (p35S:AtZSP1#1) produced larger organs. Differentially expressed genes in the shoots of atzsp1-1 and p35S:AtZSP1#1 were enriched in the cytokinin pathway. Further analysis on shoots of atzsp1-1 showed that endogenous cytokinin levels were significantly reduced, consistent with reduced expression of the cytokinin response genes ARR5/6/7 and a decrease in pARR5:GUS activity. By contrast, cytokinin levels were elevated in p35S:AtZSP1#1. These results indicate that AtZSP1 affects shoot size via changes in cytokinin levels. AtZSP1 is ubiquitously expressed and encodes a 57-amino acid endomembrane-associated protein that is highly conserved among plant species. AtZSP1 interacts with ROCK1 at the endomembrane. Genetic analysis confirmed that the small organs and low cytokinin levels in atzsp1-1 shoots are partially suppressed by the rock1-4 mutation, suggesting that AtZSP1 may function in a common pathway with ROCK1 to antagonistically regulate organ growth. Our study identified an unknown small peptide, AtZSP1, and defined its function in regulating organ size in Arabidopsis.

Genome-Wide Identification and Characterisation of Cytokinin-O-Glucosyltransferase (CGT) Genes of Rice Specific to Potential Pathogens

Cytokinin glucosyltransferases (CGTs) are key enzymes of plants for regulating the level and function of cytokinins. In a genomic identification of rice CGTs, 41 genes with the plant secondary product glycosyltransferases (PSPG) motif of 44-amino-acid consensus sequence characteristic of plant uridine diphosphate (UDP)-glycosyltransferases (UGTs) were identified. In-silico physicochemical characterisation revealed that, though the CGTs belong to the same subfamily, they display varying molecular weights, ranging from 19.6 kDa to 59.7 kDa. The proteins were primarily acidic (87.8%) and hydrophilic (58.6%) and were observed to be distributed in the plastids (16), plasma membrane (13), mitochondria (5), and cytosol (4). Phylogenetic analysis of the CGTs revealed that their evolutionary relatedness ranged from 70-100%, and they aligned themselves into two major clusters. In a comprehensive analysis of the available transcriptomics data of rice samples representing different growth stages only the CGT, Os04g25440.1 was significantly expressed at the vegetative stage, whereas 16 other genes were highly expressed only at the reproductive growth stage. On the contrary, six genes, LOC_Os07g30610.1, LOC_Os04g25440.1, LOC_Os07g30620.1, LOC_Os04g25490.1, LOC_Os04g37820.1, and LOC_Os04g25800.1, were significantly upregulated in rice plants inoculated with Rhizoctonia solani (RS), Xoo (Xanthomonas oryzae pv. oryzae) and Mor (Magnaporthe oryzae). In a qRT-PCR analysis of rice sheath tissue susceptible to Rhizoctonia solani, Mor, and Xoo pathogens, compared to the sterile distilled water control, at 24 h post-infection only two genes displayed significant upregulation in response to all the three pathogens: LOC_Os07g30620.1 and LOC_Os04g25820.1. On the other hand, the expression of genes LOC_Os07g30610.1, LOC_Os04g25440, LOC_Os04g25490, and LOC_Os04g25800 were observed to be pathogen-specific. These genes were identified as the candidate-responsive CGT genes and could serve as potential susceptibility genes for facilitating pathogen infection.

Glycosyltransferases: Mining, engineering and applications in biosynthesis of glycosylated plant natural products

UDP-Glycosyltransferases (UGTs) catalyze the transfer of nucleotide-activated sugars to specific acceptors, among which the GT1 family enzymes are well-known for their function in biosynthesis of natural product glycosides. Elucidating GT function represents necessary step in metabolic engineering of aglycone glycosylation to produce drug leads, cosmetics, nutrients and sweeteners. In this review, we systematically summarize the phylogenetic distribution and catalytic diversity of plant GTs. We also discuss recent progress in the identification of novel GT candidates for synthesis of plant natural products (PNPs) using multi-omics technology and deep learning predicted models. We also highlight recent advances in rational design and directed evolution engineering strategies for new or improved GT functions. Finally, we cover recent breakthroughs in the application of GTs for microbial biosynthesis of some representative glycosylated PNPs, including flavonoid glycosides (fisetin 3-O-glycosides, astragalin, scutellarein 7-O-glucoside), terpenoid glycosides (rebaudioside A, ginsenosides) and polyketide glycosides (salidroside, polydatin).

CRISPR/Cas9-Mediated Targeted Mutagenesis of GmUGT Enhanced Soybean Resistance Against Leaf-Chewing Insects Through Flavonoids Biosynthesis

Leaf-chewing insects are important pests that cause yield loss and reduce seed quality in soybeans (Glycine max). Breeding soybean varieties that are resistant to leaf-chewing insects can minimize the need for insecticide use and reduce yield loss. The marker gene for QTL-M, Glyma.07g110300 (LOC100775351) that encodes a UDP-glycosyltransferase (UGT) is the major determinant of resistance against leaf-chewing insects in soybean; it exhibits a loss of function in insect-resistant soybean germplasms. In this study, Agrobacterium-mediated transformation introduced the CRISPR/Cas9 expression vector into the soybean cultivar Tianlong No. 1 to generate Glyma.07g110300-gene mutants. We obtained two novel types of mutations, a 33-bp deletion and a single-bp insertion in the GmUGT coding region, which resulted in an enhanced resistance to Helicoverpa armigera and Spodoptera litura. Additionally, overexpressing GmUGT produced soybean varieties that were more sensitive to H. armigera and S. litura. Both mutant and overexpressing lines exhibited no obvious phenotypic changes. The difference in metabolites and gene expression suggested that GmUGT is involved in imparting resistance to leaf-chewing insects by altering the flavonoid content and expression patterns of genes related to flavonoid biosynthesis and defense. Furthermore, ectopic expression of the GmUGT gene in the ugt72b1 mutant of Arabidopsis substantially rescued the phenotype of H. armigera resistance in the atugt72b1 mutant. Our study presents a strategy for increasing resistance against leaf-chewing insects in soybean through CRISPR/Cas9-mediated targeted mutagenesis of the UGT genes.

On the biological activity of cytokinin free bases and their ribosides

The free bases of cytokinins are the biologically active forms of the hormone while cytokinin ribosides become active only upon removal of the ribose residue. Cytokinins (CKs) belong to the classical plant hormones. They were discovered more than 65 years ago, but which molecular forms possess genuine CK activity is still matter of debate. Numerous studies support the view that only the free bases are the biologically active molecules. This standpoint has been challenged in a recent review (Nguyen et al. in Planta 254: 45, 2021) proposing that also CK ribosides may have genuine own CK activity. Here we critically discuss the pros and cons of this viewpoint considering the results of biological assays, CK binding studies, 3D structural data of CK-receptor interaction and mutant analyses. It is concluded that all types of study provide clear and convincing evidence only for biological activity of free bases and not ribosides; the latter are rather a transport form of the hormone without their own biological activity.

Dynamic Hormone Gradients Regulate Wound-Induced de novo Organ Formation in Tomato Hypocotyl Explants

Plants have a remarkable regenerative capacity, which allows them to survive tissue damage after biotic and abiotic stresses. In this study, we use Solanum lycopersicum 'Micro-Tom' explants as a model to investigate wound-induced de novo organ formation, as these explants can regenerate the missing structures without the exogenous application of plant hormones. Here, we performed simultaneous targeted profiling of 22 phytohormone-related metabolites during de novo organ formation and found that endogenous hormone levels dynamically changed after root and shoot excision, according to region-specific patterns. Our results indicate that a defined temporal window of high auxin-to-cytokinin accumulation in the basal region of the explants was required for adventitious root formation and that was dependent on a concerted regulation of polar auxin transport through the hypocotyl, of local induction of auxin biosynthesis, and of local inhibition of auxin degradation. In the apical region, though, a minimum of auxin-to-cytokinin ratio is established shortly after wounding both by decreasing active auxin levels and by draining auxin via its basipetal transport and internalization. Cross-validation with transcriptomic data highlighted the main hormonal gradients involved in wound-induced de novo organ formation in tomato hypocotyl explants.

Broadening the roles of UDP-glycosyltransferases in auxin homeostasis and plant development

The levels of the important plant growth regulator indole-3-acetic acid (IAA) are tightly controlled within plant tissues to spatiotemporally orchestrate concentration gradients that drive plant growth and development. Metabolic inactivation of bioactive IAA is known to participate in the modulation of IAA maxima and minima. IAA can be irreversibly inactivated by oxidation and conjugation to aspartate and glutamate. Usually overlooked because of its reversible nature, the most abundant inactive IAA form is the IAA-glucose (IAA-glc) conjugate. Glycosylation of IAA in Arabidopsis thaliana is reported to be carried out by UDP-glycosyltransferase 84B1 (UGT84B1), while UGT74D1 has been implicated in the glycosylation of the irreversibly formed IAA catabolite oxIAA. Here we demonstrated that both UGT84B1 and UGT74D1 modulate IAA levels throughout plant development by dual IAA and oxIAA glycosylation. Moreover, we identified a novel UGT subfamily whose members redundantly mediate the glycosylation of oxIAA and modulate skotomorphogenic growth.

Two Auxinic Herbicides Affect Brassica napus Plant Hormone Levels and Induce Molecular Changes in Transcription

With the introduction of the new auxinic herbicide halauxifen-methyl into the oilseed rape (Brassica napus) market, there is a need to understand how this new molecule interacts with indigenous plant hormones (e.g., IAA) in terms of crop response. The aim of this study was to investigate the molecular background by using different growth conditions under which three different auxinic herbicides were administered. These were halauxifen-methyl (Hal), alone and together with aminopyralid (AP) as well as picloram (Pic). Three different hormone classes were determined, free and conjugated indole-3-acetic acid (IAA), aminocyclopropane carboxylic acid (ACC) as a precursor for ethylene, and abscisic acid (ABA) at two different temperatures and growth stages as well as over time (2–168 h after treatment). At 15 °C growth temperature, the effect was more pronounced than at 9 °C, and generally, the younger leaves independent of the developmental stage showed a larger effect on the alterations of hormones. IAA and ACC showed reproducible alterations after auxinic herbicide treatments over time, while ABA did not. Finally, a transcriptome analysis after treatment with two auxinic herbicides, Hal and Pic, showed different expression patterns. Hal treatment leads to the upregulation of auxin and hormone responses at 48 h and 96 h. Pic treatment induced the hormone/auxin response already after 2 h, and this continued for the other time points. The more detailed analysis of the auxin response in the datasets indicate a role for GH3 genes and genes encoding auxin efflux proteins. The upregulation of the GH3 genes correlates with the increase in conjugated IAA at the same time points and treatments. Also, genes for were found that confirm the upregulation of the ethylene pathway.

Isopentenyltransferases as master regulators of crop performance: their function, manipulation, and genetic potential for stress adaptation and yield improvement

Isopentenyltransferase (IPT) in plants regulates a rate-limiting step of cytokinin (CTK) biosynthesis. IPTs are recognized as key regulators of CTK homeostasis and phytohormone crosstalk in both biotic and abiotic stress responses. Recent research has revealed the regulatory function of IPTs in gene expression and metabolite profiles including source-sink modifications, energy metabolism, nutrient allocation and storage, stress defence and signalling pathways, protein synthesis and transport, and membrane transport. This suggests that IPTs play a crucial role in plant growth and adaptation. In planta studies of IPT-driven modifications indicate that, at a physiological level, IPTs improve stay-green characteristics, delay senescence, reduce stress-induced oxidative damage and protect photosynthetic machinery. Subsequently, these improvements often manifest as enhanced or stabilized crop yields and this is especially apparent under environmental stress. These mechanisms merit consideration of the IPTs as 'master regulators' of core cellular metabolic pathways, thus adjusting plant homeostasis/adaptive responses to altered environmental stresses, to maximize yield potential. If their expression can be adequately controlled, both spatially and temporally, IPTs can be a key driver for seed yield. In this review, we give a comprehensive overview of recent findings on how IPTs influence plant stress physiology and yield, and we highlight areas for future research.

Cytokinin glucosyl transferases, key regulators of cytokinin homeostasis, have potential value for wheat improvement

The cytokinins, which are N⁶ -substituted adenine derivatives, control key aspects of crop productivity. Cytokinin levels are controlled via biosynthesis by isopentenyl transferase (IPT), destruction by cytokinin oxidase/dehydrogenase (CKX), and inactivation via glucosylation by cytokinin glucosyl transferases (CGTs). While both yield components and tolerance to drought and related abiotic stressors have been positively addressed via manipulation of IPT and/or CKX expression, much less attention has been paid to the CGTs. As naming of the CGTs has been unclear, we suggest COGT, CNGT, CONGT and CNOGT to describe the O-, N- and dual function CGTs. As specific CGT mutants of both rice and arabidopsis showed impacts on yield components, we interrogated the wheat genome database, IWGSC RefSeq v1.0 & v2.0, to investigate wheat CGTs. Besides providing unambiguous names for the 53 wheat CGTs, we show their expression patterns in 70 developmental tissues and their response characteristics to various stress conditions by reviewing more than 1000 RNA-seq data sets. These revealed various patterns of responses and showed expression generally being more limited in reproductive tissues than in vegetative tissues. Multiple cis-regulatory elements are present in the 3 kb upstream of the start codons of the 53 CGTs. Elements associated with abscisic acid, light and methyl jasmonate are particularly over-represented, indicative of the responsiveness of CGTs to the environment. These data sets indicate that CGTs have potential value for wheat improvement and that these could be targeted in TILLING or gene editing wheat breeding programmes.

CsLOB1 regulates susceptibility to citrus canker through promoting cell proliferation in citrus

Citrus sinensis lateral organ boundary 1 (CsLOB1) was previously identified as a critical disease susceptibility gene for citrus bacterial canker, which is caused by Xanthomonas citri subsp. citri (Xcc). However, the molecular mechanisms of CsLOB1 in citrus response to Xcc are still elusive. Here, we constructed transgenic plants overexpressing and RNAi-silencing of CsLOB1 using the canker-disease susceptible 'wanjincheng' orange (C. sinensis Osbeck) as explants. CsLOB1-overexpressing plants exhibited dwarf phenotypes with smaller and thicker leaf, increased branches and adventitious buds clustered on stems. These phenotypes were followed by a process of pustule- and canker-like development that exhibited enhanced cell proliferation. Pectin depolymerization and expansin accumulation were enhanced by CsLOB1 overexpression, while cellulose and hemicellulose synthesis were increased by CsLOB1 silence. Whilst overexpression of CsLOB1 increased susceptibility, RNAi-silencing of CsLOB1 enhanced resistance to canker disease without impairing pathogen entry. Transcriptome analysis revealed that CsLOB1 positively regulated cell wall degradation and modification processes, cytokinin metabolism, and cell division. Additionally, 565 CsLOB1-targeted genes were identified in chromatin immunoprecipitation-sequencing (ChIP-seq) experiments. Motif discovery analysis revealed that the most highly overrepresented binding sites had a conserved 6-bp 'GCGGCG' consensus DNA motif. RNA-seq and ChIP-seq data suggested that CsLOB1 directly activates the expression of four genes involved in cell wall remodeling, and three genes that participate in cytokinin and brassinosteroid hormone pathways. Our findings indicate that CsLOB1 promotes cell proliferation by mechanisms depending on cell wall remodeling and phytohormone signaling, which may be critical to citrus canker development and bacterial growth in citrus.

Evolutionary diversification of cytokinin-specific glucosyltransferases in angiosperms and enigma of missing cis-zeatin O-glucosyltransferase gene in Brassicaceae

In the complex process of homeostasis of phytohormones cytokinins (CKs), O-glucosylation catalyzed by specific O-glucosyltransferases represents one of important mechanisms of their reversible inactivation. The CK O-glucosyltransferases belong to a highly divergent and polyphyletic multigene superfamily of glycosyltransferases, of which subfamily 1 containing UDP-glycosyltransferases (UGTs) is the largest in the plant kingdom. It contains recently discovered O and P subfamilies present in higher plant species but not in Arabidopsis thaliana. The cis-zeatin O-glucosyltransferase (cisZOG) genes belong to the O subfamily encoding a stereo-specific O-glucosylation of cis-zeatin-type CKs. We studied different homologous genes, their domains and motifs, and performed a phylogenetic reconstruction to elucidate the plant evolution of the cisZOG gene. We found that the cisZOG homologs do not form a clear separate clade, indicating that diversification of the cisZOG gene took place after the diversification of the main angiosperm families, probably within genera or closely related groups. We confirmed that the gene(s) from group O is(are) not present in A. thaliana and is(are) also missing in the family Brassicaceae. However, cisZOG or its metabolites are found among Brassicaceae clade, indicating that remaining genes from other groups (UGT73-group D and UGT85-group G) are able, at least in part, to substitute the function of group O lost during evolution. This study is the first detailed evolutionary evaluation of relationships among different plant ZOGs within angiosperms.

Differential Subcellular Distribution of Cytokinins: How Does Membrane Transport Fit into the Big Picture?

Cytokinins are a class of phytohormones, signalling molecules specific to plants. They act as regulators of diverse physiological processes in complex signalling pathways. It is necessary for plants to continuously regulate cytokinin distribution among different organs, tissues, cells, and compartments. Such regulatory mechanisms include cytokinin biosynthesis, metabolic conversions and degradation, as well as cytokinin membrane transport. In our review, we aim to provide a thorough picture of the latter. We begin by summarizing cytokinin structures and physicochemical properties. Then, we revise the elementary thermodynamic and kinetic aspects of cytokinin membrane transport. Next, we review which membrane-bound carrier proteins and protein families recognize cytokinins as their substrates. Namely, we discuss the families of "equilibrative nucleoside transporters" and "purine permeases", which translocate diverse purine-related compounds, and proteins AtPUP14, AtABCG14, AtAZG1, and AtAZG2, which are specific to cytokinins. We also address long-distance cytokinin transport. Putting all these pieces together, we finally discuss cytokinin distribution as a net result of these processes, diverse in their physicochemical nature but acting together to promote plant fitness.

A Pipeline towards the Biochemical Characterization of the Arabidopsis GT14 Family

Glycosyltransferases (GTs) catalyze the synthesis of glycosidic linkages and are essential in the biosynthesis of glycans, glycoconjugates (glycolipids and glycoproteins), and glycosides. Plant genomes generally encode many more GTs than animal genomes due to the synthesis of a cell wall and a wide variety of glycosylated secondary metabolites. The Arabidopsis thaliana genome is predicted to encode over 573 GTs that are currently classified into 42 diverse families. The biochemical functions of most of these GTs are still unknown. In this study, we updated the JBEI Arabidopsis GT clone collection by cloning an additional 105 GT cDNAs, 508 in total (89%), into Gateway-compatible vectors for downstream characterization. We further established a functional analysis pipeline using transient expression in tobacco (Nicotiana benthamiana) followed by enzymatic assays, fractionation of enzymatic products by reversed-phase HPLC (RP-HPLC) and characterization by mass spectrometry (MS). Using the GT14 family as an exemplar, we outline a strategy for identifying effective substrates of GT enzymes. By addition of UDP-GlcA as donor and the synthetic acceptors galactose-nitrobenzodiazole (Gal-NBD), β-1,6-galactotetraose (β-1,6-Gal₄) and β-1,3-galactopentose (β-1,3-Gal₅) to microsomes expressing individual GT14 enzymes, we verified the β-glucuronosyltransferase (GlcAT) activity of three members of this family (AtGlcAT14A, B, and E). In addition, a new family member (AT4G27480, 248) was shown to possess significantly higher activity than other GT14 enzymes. Our data indicate a likely role in arabinogalactan-protein (AGP) biosynthesis for these GT14 members. Together, the updated Arabidopsis GT clone collection and the biochemical analysis pipeline present an efficient means to identify and characterize novel GT catalytic activities.

Cytokinin N-glucosides: Occurrence, Metabolism and Biological Activities in Plants

Cytokinins (CKs) are a class of phytohormones affecting many aspects of plant growth and development. In the complex process of CK homeostasis in plants, N-glucosylation represents one of the essential metabolic pathways. Its products, CK N7- and N9-glucosides, have been largely overlooked in the past as irreversible and inactive CK products lacking any relevant physiological impact. In this work, we report a widespread distribution of CK N-glucosides across the plant kingdom proceeding from evolutionary older to younger plants with different proportions between N7- and N9-glucosides in the total CK pool. We show dramatic changes in their profiles as well as in expression levels of the UGT76C1 and UGT76C2 genes during Arabidopsis ontogenesis. We also demonstrate specific physiological effects of CK N-glucosides in CK bioassays including their antisenescent activities, inhibitory effects on root development, and activation of the CK signaling pathway visualized by the CK-responsive YFP reporter line, TCSv2::3XVENUS. Last but not least, we present the considerable impact of CK N7- and N9-glucosides on the expression of CK-related genes in maize and their stimulatory effects on CK oxidase/dehydrogenase activity in oats. Our findings revise the apparent irreversibility and inactivity of CK N7- and N9-glucosides and indicate their involvement in CK evolution while suggesting their unique function(s) in plants.

Endoplasmic Reticulum-Localized PURINE PERMEASE1 Regulates Plant Height and Grain Weight by Modulating Cytokinin Distribution in Rice

Cytokinins (CKs) are a class of phytohormones playing essential roles in various biological processes. However, the mechanisms underlying CK transport as well as its function in plant growth and development are far from being fully elucidated. Here, we characterize the function of PURINE PERMEASE1 (OsPUP1) in rice (Oryza sativa L.). OsPUP1 was predominantly expressed in the root, particularly in vascular cells, and CK treatment can induce its expression. Subcellular localization analysis showed that OsPUP1 was predominantly localized to the endoplasmic reticulum (ER). Overexpression of OsPUP1 resulted in growth defect of various aerial tissues, including decreased leaf length, plant height, grain weight, panicle length, and grain number. Hormone profiling revealed that the CK content was decreased in the shoot of OsPUP1-overexpressing seedling, but increased in the root, compared with the wild type. The CK content in the panicle was also decreased. Quantitative reverse transcription-PCR (qRT-PCR) analysis using several CK type-A response regulators (OsRRs) as the marker genes suggested that the CK response in the shoot of OsPUP1-overexpressing seedling is decreased compared to the wild type when CKs are applied to the root. Genetic analysis revealed that BG3/OsPUP4, a putative plasma membrane-localized CK transporter, overcomes the function of OsPUP1. We hypothesize that OsPUP1 might be involved in importing CKs into ER to unload CKs from the vascular tissues by cell-to-cell transport.

The Importance of Cytokinins during Reproductive Development in Arabidopsis and Beyond

Fertilization and seed formation are fundamental events in the life cycle of flowering plants. The seed is a functional unit whose main purpose is to propagate the plant. The first step in seed development is the formation of male and female gametophytes and subsequent steps culminate in successful fertilization. The detailed study of this process is highly relevant because it directly impacts human needs, such as protecting biodiversity and ensuring sustainable agriculture to feed the increasing world population. Cytokinins comprise a class of phytohormones that play many important roles during plant growth and development and in recent years, the role of this class of phytohormones during reproduction has become clear. Here, we review the role of cytokinins during ovule, pollen and seed formation at the genetic and molecular levels. The expansion of knowledge concerning the molecular mechanisms that control plant reproduction is extremely important to optimise seed production.

Development of screening methods for functional characterization of UGTs from Stevia rebaudiana

Glycosylation is a key modification that contributes to determine bioactivity and bioavailability of plant natural products, including that of terpenoids and steviol glycosides (SVglys). It is mediated by uridine-diphosphate glycosyltransferases (UGTs), that achieve their activity by transferring sugars on small molecules. Thus, the diversity of SVglys is due to the number, the position and the nature of glycosylations on the hydroxyl groups in C-13 and C-19 of steviol. Despite the intense sweetener property of SVglys and the numerous studies conducted, the SVglys biosynthetic pathway remains largely unknown. More than 60 SVglys and 68 putative UGTs have been identified in Stevia rebaudiana. This study aims to provide methods to characterize UGTs putatively involved in SVglys biosynthesis. After agroinfiltration-based transient gene expression in Nicotiana benthamiana, functionality of the recombinant UGT can be tested simply and directly in plants expressing it or from a crude extract. The combined use of binary vectors from pGWBs series to produce expression vectors containing the stevia's UGT, enables functionality testing with many substrates as well as other applications for further analysis, including subcellular localization.

flasher, a novel mutation in a glucosinolate modifying enzyme, conditions changes in plant architecture and hormone homeostasis

Meristem function is underpinned by numerous genes that affect hormone levels, ultimately controlling phyllotaxy, the transition to flowering and general growth properties. Class I KNOX genes are major contributors to this process, promoting cytokinin biosynthesis but repressing gibberellin production to condition a replication competent state. We identified a suppressor mutant of the KNOX1 mutant brevipedicellus (bp) that we termed flasher (fsh), which promotes stem and pedicel elongation, suppresses early senescence, and negatively affects reproductive development. Map-based cloning and complementation tests revealed that fsh is due to an E40K change in the flavin monooxygenase GS-OX5, a gene encoding a glucosinolate (GSL) modifying enzyme. In vitro enzymatic assays revealed that fsh poorly converts substrate to product, yet the levels of several GSLs are higher in the suppressor line, implicating FSH in feedback control of GSL flux. FSH is expressed predominantly in the vasculature in patterns that do not significantly overlap those of BP, implying a non-cell autonomous mode of meristem control via one or more GSL metabolites. Hormone analyses revealed that cytokinin levels are low in bp, but fsh restores cytokinin levels to near normal by activating cytokinin biosynthesis genes. In addition, jasmonate levels in the fsh suppressor are significantly lower than in bp, which is likely due to elevated expression of JA inactivating genes. These observations suggest the involvement of the GSL pathway in generating one or more negative effectors of growth that influence inflorescence architecture and fecundity by altering the balance of hormonal regulators.

Applications of Cytokinins in Horticultural Fruit Crops: Trends and Future Prospects

Cytokinins (CKs) are a chemically diverse class of plant growth regulators, exhibiting wide-ranging actions on plant growth and development, hence their exploitation in agriculture for crop improvement and management. Their coordinated regulatory effects and cross-talk interactions with other phytohormones and signaling networks are highly sophisticated, eliciting and controlling varied biological processes at the cellular to organismal levels. In this review, we briefly introduce the mode of action and general molecular biological effects of naturally occurring CKs before highlighting the great variability in the response of fruit crops to CK-based innovations. We present a comprehensive compilation of research linked to the application of CKs in non-model crop species in different phases of fruit production and management. By doing so, it is clear that the effects of CKs on fruit set, development, maturation, and ripening are not necessarily generic, even for cultivars within the same species, illustrating the magnitude of yet unknown intricate biochemical and genetic mechanisms regulating these processes in different fruit crops. Current approaches using genomic-to-metabolomic analysis are providing new insights into the in planta mechanisms of CKs, pinpointing the underlying CK-derived actions that may serve as potential targets for improving crop-specific traits and the development of new solutions for the preharvest and postharvest management of fruit crops. Where information is available, CK molecular biology is discussed in the context of its present and future implications in the applications of CKs to fruits of horticultural significance.

Survival in the Tropics despite isolation, inbreeding and asexual reproduction: insights from the genome of the world's southernmost poplar (Populus ilicifolia)

Species are becoming extinct at unprecedented rates as a consequence of human activity. Hence it is important to understand the evolutionary dynamics of species with already small population sizes. Populus ilicifolia is a vulnerable poplar species that is isolated from other poplar species and is uniquely adapted to the Tropics. It has a very limited size, reproduces partly clonally and is therefore an excellent case study for conservation genomics. We present here the first annotated draft genome of P. ilicifolia, characterize genome-wide patterns of polymorphisms and compare those to other poplar species with larger natural ranges. P. ilicifolia experienced a more prolonged and severe decline of effective population size (N_e ) and signs of genetic erosion than any other poplar species with which it was compared. At present, the species has the lowest genome-wide genetic diversity, the highest abundance of long runs of homozygosity, high inbreeding levels as well as a high overall accumulation of deleterious variants. However, more effective purging of severely deleterious variants and adaptation to the Tropics may have contributed to its survival. Hence, in spite of its limited genetic variation, it is certainly worth pursuing the conservation efforts of this unique species.

Cytokinins in Dictyostelia - A Unique Model for Studying the Functions of Signaling Agents From Species to Kingdoms

Cytokinins (CKs) are a diverse group of evolutionarily significant growth-regulating molecules. While the CK biosynthesis and signal transduction pathways are the most well-understood in plant systems, these molecules have been identified in all kingdoms of life. This review follows the recent discovery of an expanded CK profile in the social amoeba, Dictyostelium discoideum. A comprehensive review on the present knowledge of CK biosynthesis, signal transduction, and CK-small molecule interactions within members of Dictyostelia will be summarized. In doing so, the utility of social amoebae will be highlighted as a model system for studying the evolution of these hormone-like signaling agents, which will set the stage for future research in this area.

Naturally Occurring and Artificial N9-Cytokinin Conjugates: From Synthesis to Biological Activity and Back

Cytokinins and their sugar or non-sugar conjugates are very active growth-promoting factors in plants, although they occur at very low concentrations. These compounds have been identified in numerous plant species. This review predominantly focuses on 9-substituted adenine-based cytokinin conjugates, both artificial and endogenous, sugar and non-sugar, and their roles in plants. Acquired information about their biological activities, interconversions, and metabolism improves understanding of their mechanisms of action and functions in planta. Although a number of 9-substituted cytokinins occur endogenously, many have also been prepared in laboratories to facilitate the clarification of their physiological roles and the determination of their biological properties. Here, we chart advances in knowledge of 9-substituted cytokinin conjugates from their discovery to current understanding and reciprocal interactions between biological properties and associated structural motifs. Current organic chemistry enables preparation of derivatives with better biological properties, such as improved anti-senescence, strong cell division stimulation, shoot forming, or more persistent stress tolerance compared to endogenous or canonical cytokinins. Many artificial cytokinin conjugates stimulate higher mass production than naturally occurring cytokinins, improve rooting, or simply have high stability or bioavailability. Thus, knowledge of the biosynthesis, metabolism, and activity of 9-substituted cytokinins in various plant species extends the scope for exploiting both natural and artificially prepared cytokinins in plant biotechnology, tissue culture, and agriculture.

Response of cytokinins and nitrogen metabolism in the fronds of Pteris sp. under arsenic stress

Given the close relationship between cytokinins (CKs), photosynthesis and nitrogen metabolism, this study assessed the effect of arsenic (As) contamination on these metabolic components in the As-hyperaccumulators Pteris cretica L. var. Albo-lineata (Pc-A) and var. Parkerii (Pc-P) as well as the As-non-hyperaccumulator Pteris straminea Mett. ex Baker (Ps). The ferns were cultivated in a pot experiment for 23 weeks in soil spiked with As at the levels 20 and 100 mg·kg-1. For the purpose of this study, the CKs were placed into five functionally different groups according to their structure and physiological roles: bioactive forms (bCKs; CK free bases); inactive or weakly active forms (dCKs; CK N-glucosides); transport forms (tCKs; CK ribosides); storage forms (sCKs; O-glucosides); and primary products of CK biosynthesis (ppbCKs; CK nucleotides). An important finding was higher CKs total content, accumulation of sCKs and reduction of dCKs in As-hyperaccumulators in contrast to non-hyperaccumulator ferns. A significant depletion of C resources was confirmed in ferns, especially Ps, which was determined by measuring the photosynthetic rate and chlorophyll fluorescence. A fluorescence decrease signified a reduction in the C/N ratio, inducing an increase of bioactive CKs forms in Pc-P and Ps. The impact of As on N utilization was significant in As-hyperaccumulators. The glutamic acid/glutamine ratio, an indicator of primary N assimilation, diminished in all ferns with increased As level in the soil. In conclusion, the results indicate a large phenotypic diversity of Pteris species to As and suggest that the CKs composition and the glutamic acid/glutamine ratio can be used as a tool to diagnose As stress in plants.

Distinct metabolism of N-glucosides of isopentenyladenine and trans-zeatin determines cytokinin metabolic spectrum in Arabidopsis

The diversity of cytokinin (CK) metabolites suggests their interconversions are the predominant regulatory mechanism of CK action. Nevertheless, little is known about their directionality and kinetics in planta. CK metabolite levels were measured in 2-wk-old Arabidopsis thaliana plants at several time points up to 100 min following exogenous application of selected CKs. The data were then evaluated qualitatively and by mathematical modeling. Apart from elevated levels of trans-zeatin (tZ) metabolites upon application of N⁶ -(Δ² -isopentenyl)adenine (iP), we observed no conversions between the individual CK-types - iP, tZ, dihydrozeatin (DHZ) and cis-zeatin (cZ). In particular, there was no sign of isomerization between tZ and cZ families. Also, no increase of DHZ-type CKs was observed after application of tZ, suggesting low baseline activity of zeatin reductase. Among N-glucosides, those of iP were not converted back to iP while tZ N-glucosides were cleaved to tZ bases, thus affecting the whole metabolic spectrum. We present the first large-scale study of short-term CK metabolism kinetics and show that tZ N7- and N9-glucosides are metabolized in vivo. We thus refute the generally accepted hypothesis that N-glucosylation irreversibly inactivates CKs. The subsequently constructed mathematical model provides estimates of the metabolic conversion rates.

Cytokinin Signaling Downstream of the His-Asp Phosphorelay Network: Cytokinin-Regulated Genes and Their Functions

The plant hormone cytokinin, existing in several molecular forms, is perceived by membrane-localized histidine kinases. The signal is transduced to transcription factors of the type-B response regulator family localized in the nucleus by a multi-step histidine-aspartate phosphorelay network employing histidine phosphotransmitters as shuttle proteins across the nuclear envelope. The type-B response regulators activate a number of primary response genes, some of which trigger in turn further signaling events and the expression of secondary response genes. Most genes activated in both rounds of transcription were identified with high confidence using different transcriptomic toolkits and meta analyses of multiple individual published datasets. In this review, we attempt to summarize the existing knowledge about the primary and secondary cytokinin response genes in order to try connecting gene expression with the multitude of effects that cytokinin exerts within the plant body and throughout the lifespan of a plant.

New Insights Into the Metabolism and Role of Cytokinin N-Glucosides in Plants

Cytokinin (CK) N-glucosides are the most abundant group of CK metabolites in many species; however, their physiological role in planta was for a long time perceived as irreversible storage CK forms only. Recently, a comprehensive screen showed that only vascular plants form CK N-glucosides in contrast to mosses, algae, and fungi. The formation of CK N-glucosides as biologically inactive CK conjugates thus represents an evolutionarily young mechanism for deactivation of CK bases. Even though CK N-glucosides are not biologically active themselves due to their inability to activate the CK perception system, new data on CK N-glucoside metabolism show that trans-zeatin (tZ) N7- and N9-glucosides are metabolized in vivo, efficiently releasing free CK bases that are most probably responsible for the biological activities observed in a number of bioassays. Moreover, CK N-glucosides' subcellular localization as well as their abundance in xylem both point to their possible plasma membrane transport and indicate a role also as CK transport forms. Identification of the enzyme(s) responsible for the hydrolysis of tZ N7- and N9-glucosides, as well as the discovery of putative CK N-glucoside plasma membrane transporter, would unveil important parts of the overall picture of CK metabolic interconversions and their physiological importance.

Physiological response of North China red elder container seedlings to inoculation with plant growth-promoting rhizobacteria under drought stress

The issue of how to alleviate the negative effects imposed by water stress is an interesting problem. Plant growth-promoting rhizobacteria (PGPR) colonize the rhizosphere of plants and are known to promote the growth of crops. However, there are few studies characterizing the physiological response of plants to drought stress after PGPR inoculation. The aim of this study was to investigate the effectiveness of different PGPRs in arid environments and then investigated the effects of PGPR inoculation under drought stress on the physiological characteristics and growth of North China red elder (Sambucus williamsii) nursery container seedlings. The viable count of different PGPRs under drought stress varies widely, and the drought tolerance of Acinetobacter calcoaceticus X128 was significantly higher than that of other PGPRs. In comparison with non-inoculation, inoculation with X128 in an arid environment significantly increased stomatal conductance and mitigated the inhibition of net photosynthetic rate caused by drought stress; this mitigating effect of inoculation is enhanced as the level of drought stress increases. Relative to non-inoculated seedlings, cytokinin levels in the leaves increased by 91.17% under severe drought stress conditions in inoculated seedlings. However, X128 inoculation decreased this deficit to only 44.54%. Compared with non-inoculated seedlings, the relative water content of inoculated seedlings under severe drought stress increased by 15.06%, however the relative conductivity decreased by 12.48%. Consequently, X128 could increase dry matter accumulation of S. williamsii regardless of watering status, indicative of the greater benefits of PGPR on shoot growth than root. Therefore, inoculation of A. calcoaceticus X128 under drought conditions play a significant role for alleviating the negative effects imposed by water stress and promoting plant growth.

Auxin and cytokinin coordinate the dormancy and outgrowth of axillary bud in strawberry runner

BACKGROUND

Axillary buds allow the production of either vegetative or reproductive shoots, which display a plastic developmental potential of the plant to suit the prevailing environmental changes. Strawberry represents one of many plant species which displays horizontal above-ground growth of shoot development for asexual reproduction. Two distinct runner growth patterns exist in different strawberry species: one is called sympodial type such as Fragaria vesca, and the other one is called monopodial type such as Fragaria pentaphylla. Despite the runner growth morphology of these strawberry species have been well known, the mechanisms that determine the distinct patterns have rarely been reported.

RESULTS

In this study, we used Fragaria vesca Hawaii-4 and Fragaria pentaphylla as model species, and captured the initiated dormant bud and non-dormant bud as materials to compare their transcriptome profiles and phytohormone content. Comparisons revealed that relatively higher auxin activity is present in the dormant bud and relatively higher cytokinin activity is in the non-dormant bud. Decapitation and pharmacological experiments on dormant buds showed that the reduction of auxin accumulation triggers the regeneration of vegetative shoots in dormant buds, and exogenous cytokinin application triggers cell fate turnover and generation of reproductive shoots.

CONCLUSION

Here, we uncover a mechanism by which auxin and cytokinin coordinate the dormancy and outgrowth of axillary bud in strawberry runner. Our results suggest a contrasting behavior of auxin and cytokinin in control of axillary bud development, facilitating a preliminary understanding of shoot architecture formation in strawberry.

UGT709G1: a novel uridine diphosphate glycosyltransferase involved in the biosynthesis of picrocrocin, the precursor of safranal in saffron (Crocus sativus)

Saffron, a spice derived from the dried red stigmas of Crocus sativus, is one of the oldest natural food additives. The flowers have long red stigmas, which store significant quantities of the glycosylated apocarotenoids crocins and picrocrocin. The apocarotenoid biosynthetic pathway in saffron starts with the oxidative cleavage of zeaxanthin, from which crocins and picrocrocin are derived. In the processed stigmas, picrocrocin is converted to safranal, giving saffron its typical aroma. By a targeted search for differentially expressed uridine diphosphate glycosyltransferases (UGTs) in Crocus transcriptomes, a novel apocarotenoid glucosyltransferase (UGT709G1) from saffron was identified. Biochemical analyses revealed that UGT709G1 showed a high catalytic efficiency toward 2,6,6-trimethyl-4-hydroxy-1-carboxaldehyde-1-cyclohexene (HTCC), making it suited for the biosynthesis of picrocrocin, the precursor of safranal. The role of UGT709G1 in picrocrocin/safranal biosynthesis was supported by the absence or presence of gene expression in a screening for HTCC and picrocrocin production in different Crocus species and by a combined transient expression assay with CsCCD2L in Nicotiana benthamiana leaves. The identification of UGT709G1 completes one of the most highly valued specialized metabolic biosynthetic pathways in plants and provides novel perspectives on the industrial production of picrocrocin to be used as a flavor additive or as a pharmacological constituent.

Quantitative trait loci analysis of hormone levels in Arabidopsis roots

Quantitative trait loci (QTL) analyses for five groups of hormones, including cytokinins in Arabidopsis roots were performed using recombinant inbred lines (Ler×Cvi). Significant QTLs were detected for cytokinins, jasmonic acid and salicylic acid. Separate analysis of two sub-populations, viz., vegetative and flowering plants revealed that many of the QTLs were development-specific. Using near-isogenic lines, several significant QTLs were confirmed; three co-localized QTL regions were responsible for determining several cytokinin metabolites. Using a knock-out plant, a functional role of zeatin N-glucosyltransferase gene (UGT76C2) underlying a large-effect QTL for levels of tZ-N-glucosides and tZRMP was evaluated in the metabolism of cytokinins. Pleotropic effects of this gene were found for cytokinin levels in both roots and leaves, but significant changes of morphological traits were observed only in roots. Hormone QTL analysis reveals development-specific and organ-dependent aspects of the regulation of plant hormone content and metabolism.

Identification and characterization of the first cytokinin glycosyltransferase from rice

BACKGROUND

Cytokinins are one of the five major hormones families in plants and are important for their normal growth and environmental adaptability. In plants, cytokinins are mostly present as glycosides in plants, and their glycosylation modifications are catalyzed by family 1 glycosyltransferases. Current research on cytokinin glycosylation has focused on the biochemical identification of enzymes and the analysis of metabolites in Arabidopsis. There are few studies that examine how cytokinin glycosylation affects its synthesis and accumulation in plants. It is particularly important to understand these processes in food crops such as rice (Oryza sativa); however, to date, cytokinin glycosyltransferase genes in rice have not been reported.

RESULTS

In this study, we identified eight rice genes that were functionally homologous to an Arabidopsis cytokinin glycosyltransferase gene. These genes were cloned and expressed in a prokaryotic system to obtain their purified proteins. Through enzymatic analysis and liquid chromatography-mass spectrometry, a single rice glycosyltransferase, Os6, was identified that glycosylated cytokinin in vitro. Os6 was overexpressed in Arabidopsis, and the extraction of cytokinin glycosides showed that Os6 is functionally active in planta.

CONCLUSIONS

The identification and characterization of the first cytokinin glycosyltransferase from rice is important for future studies on the cytokinin metabolic pathway in rice. An improved understanding of rice cytokinin glycosyltransferases may facilitate genetic improvements in rice quality.

Expression of Genes Related to Sugar and Amino Acid Transport and Cytokinin Metabolism during Leaf Development and Senescence in Pisum sativum L

Gene editing is becoming the plant breeding tool of choice, but prior to targeting a gene for editing, a knowledge of the gene family members (GFMs) controlling yield is required in the specific crop plant. Critical to yield are components from senescing leaves. We targeted genes controlling senescence in Pisum sativum and the release and transport of carbohydrates and amino acids from the source leaves to the pods and seeds. The expression of GFMs for cytokinin biosynthesis (IPT) and destruction (CKX), sucrose transporters (SUT), Sugar Will Eventually be Exported Transporters (SWEET), amino acid permeases (AAP), and cell wall invertases, was determined using RT-qPCR. GFMs were differentially expressed in leaves of different ages. The expression of many gene family members was lower in the expanding sink leaves compared with the senescing leaves, with the exception of two PsAAP GFMs and PsCKX5, which were highly expressed. Expression of specific PsSWEETs, SUTs, and AAPs increased in the mature and/or senescing leaves. Expression of PsIPTs was least in the mature source leaves, but as strong in the senescing leaves as in the young source leaves. PsCKX7 was expressed in source and senescing leaves. We discuss the potential impact of the targeted reduction of specific PsCKX GFMs on source-sink relationships.

CUP-SHAPED COTYLEDON1 (CUC1) and CUC2 regulate cytokinin homeostasis to determine ovule number in Arabidopsis

Seeds derive from ovules upon fertilization and therefore the total number of ovules determines the final seed yield, a fundamental trait in crop plants. Among the factors that co-ordinate the process of ovule formation, the transcription factors CUP-SHAPED COTYLEDON 1 (CUC1) and CUC2 and the hormone cytokinin (CK) have a particularly prominent role. Indeed, the absence of both CUC1 and CUC2 causes a severe reduction in ovule number, a phenotype that can be rescued by CK treatment. In this study, we combined CK quantification with an integrative genome-wide target identification approach to select Arabidopsis genes regulated by CUCs that are also involved in CK metabolism. We focused our attention on the functional characterization of UDP-GLUCOSYL TRANSFERASE 85A3 (UGT85A3) and UGT73C1, which are up-regulated in the absence of CUC1 and CUC2 and encode enzymes able to catalyse CK inactivation by O-glucosylation. Our results demonstrate a role for these UGTs as a link between CUCs and CK homeostasis, and highlight the importance of CUCs and CKs in the determination of seed yield.

Auxins and Cytokinins-The Role of Subcellular Organization on Homeostasis

Plant hormones are master regulators of plant growth and development. Better knowledge of their spatial signaling and homeostasis (transport and metabolism) on the lowest structural levels (cellular and subcellular) is therefore crucial to a better understanding of developmental processes in plants. Recent progress in phytohormone analysis at the cellular and subcellular levels has greatly improved the effectiveness of isolation protocols and the sensitivity of analytical methods. This review is mainly focused on homeostasis of two plant hormone groups, auxins and cytokinins. It will summarize and discuss their tissue- and cell-type specific distributions at the cellular and subcellular levels.

A 22-bp deletion in OsPLS3 gene encoding a DUF266-containing protein is implicated in rice leaf senescence

Key message The OsPLS3 locus was isolated by map-based cloning that encodes a DUF266-containing protein. OsPLS3 regulates the onset of leaf senescence in rice. Glycosyltransferases (GTs) are one of the most important enzyme groups required for the modification of plant secondary metabolites and play a crucial role in plant growth and development, however the biological functions of most GTs remain elusive. We reported here the identification and characterization of a novel Oryza sativa premature leaf senescence mutant (ospls3). Through map-based cloning strategy, we determined that 22-bp deletion in the OsPLS3 gene encoding a domain of unknown function 266 (DUF266)-containing protein, a member of GT14-like, underlies the premature leaf senescence phenotype in the ospls3 mutant. The OsPLS3 mRNA levels progressively declined with the age-dependent leaf senescence in wild-type rice, implying a negative role of OsPLS3 in regulating leaf senescence. Physiological analysis, and histochemical staining and transmission electron microscopy assays indicated that the ospls3 mutant accumulated higher levels of ethylene and reactive oxygen species than its wild type. Furthermore, the ospls3 mutant showed hypersensitivity to exogenous 1-aminocyclopropane-1-carboxylic acid, H₂O₂ and high level of cytokinins. Our results indicated that the DUF266-containing gene OsPLS3 plays an important role in the onset of leaf senescence, in part through cytokinin and ethylene signaling in rice.