Degradation of cytokinins by maize cytokinin dehydrogenase is mediated by free radicals generated by enzymatic oxidation of natural benzoxazinones

Water Stress and Black Cutworm Feeding Modulate Plant Response in Maize Colonized by Metarhizium robertsii

Plants face many environmental challenges and have evolved different strategies to defend against stress. One strategy is the establishment of mutualistic associations with endophytic microorganisms which contribute to plant defense and promote plant growth. The fungal entomopathogen Metarhizium robertsii is also an endophyte that can provide plant-protective and growth-promoting benefits to the host plant. We conducted a greenhouse experiment in which we imposed stress from deficit and excess soil moisture and feeding by larval black cutworm (BCW), Agrotis ipsilon, to maize plants that were either inoculated or not inoculated with M. robertsii (Mr). We evaluated plant growth and defense indicators to determine the effects of the interaction between Mr, maize, BCW feeding, and water stress. There was a significant effect of water treatment, but no effect of Mr treatment, on plant chlorophyl, height, and dry biomass. There was no effect of water or Mr treatment on damage caused by BCW feeding. There was a significant effect of water treatment, but not Mr treatment, on the expression of bx7 and rip2 genes and on foliar content of abscisic acid (ABA), 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), and gibberellin 19 (GA19), whereas GA53 was modulated by Mr treatment. Foliar content of GA19 and cis-Zeatin (cZ) was modulated by BCW feeding. In a redundancy analysis, plant phenology, plant nutrient content, and foliar DIMBOA and ABA content were most closely associated with water treatments. This study contributes toward understanding the sophisticated stress response signaling and endophytic mutualisms in crops.

MODMS: a multi-omics database for facilitating biological studies on alfalfa (Medicago sativa L.)

Alfalfa (Medicago sativa L.) is a globally important forage crop. It also serves as a vegetable and medicinal herb because of its excellent nutritional quality and significant economic value. Multi-omics data on alfalfa continue to accumulate owing to recent advances in high-throughput techniques, and integrating this information holds great potential for expediting genetic research and facilitating advances in alfalfa agronomic traits. Therefore, we developed a comprehensive database named MODMS (multi-omics database of M. sativa) that incorporates multiple reference genomes, annotations, comparative genomics, transcriptomes, high-quality genomic variants, proteomics, and metabolomics. This report describes our continuously evolving database, which provides researchers with several convenient tools and extensive omics data resources, facilitating the expansion of alfalfa research. Further details regarding the MODMS database are available at https://modms.lzu.edu.cn/.

Novel insights into maize (Zea mays) development and organogenesis for agricultural optimization

In maize, intrinsic hormone activities and sap fluxes facilitate organogenesis patterning and plant holistic development; these hormone movements should be a primary focus of developmental biology and agricultural optimization strategies. Maize (Zea mays) is an important crop plant with distinctive life history characteristics and structural features. Genetic studies have extended our knowledge of maize developmental processes, genetics, and molecular ecophysiology. In this review, the classical life cycle and life history strategies of maize are analyzed to identify spatiotemporal organogenesis properties and develop a definitive understanding of maize development. The actions of genes and hormones involved in maize organogenesis and sex determination, along with potential molecular mechanisms, are investigated, with findings suggesting central roles of auxin and cytokinins in regulating maize holistic development. Furthermore, investigation of morphological and structural characteristics of maize, particularly node ubiquity and the alternate attachment pattern of lateral organs, yields a novel regulatory model suggesting that maize organ initiation and subsequent development are derived from the stimulation and interaction of auxin and cytokinin fluxes. Propositions that hormone activities and sap flow pathways control organogenesis are thoroughly explored, and initiation and development processes of distinctive maize organs are discussed. Analysis of physiological factors driving hormone and sap movement implicates cues of whole-plant activity for hormone and sap fluxes to stimulate maize inflorescence initiation and organ identity determination. The physiological origins and biogenetic mechanisms underlying maize floral sex determination occurring at the tassel and ear spikelet are thoroughly investigated. The comprehensive outline of maize development and morphogenetic physiology developed in this review will enable farmers to optimize field management and will provide a reference for de novo crop domestication and germplasm improvement using genome editing biotechnologies, promoting agricultural optimization.

Bioactive Nitrosylated and Nitrated N-(2-hydroxyphenyl)acetamides and Derived Oligomers: An Alternative Pathway to 2-Amidophenol-Derived Phytotoxic Metabolites

Incubation of Aminobacter aminovorans, Paenibacillus polymyxa, and Arthrobacter MPI764 with the microbial 2-benzoxazolinone (BOA)-degradation-product 2-acetamido-phenol, produced from 2-aminophenol, led to the recently identified N-(2-hydroxy-5-nitrophenyl) acetamide, to the hitherto unknown N-(2-hydroxy-5-nitrosophenyl)acetamide, and to N-(2-hydroxy-3-nitrophenyl)acetamide. As an alternative to the formation of phenoxazinone derived from aminophenol, dimers- and trimers-transformation products have been found. Identification of the compounds was carried out by LC/HRMS and MS/MS and, for the new structure N-(2-hydroxy-5-nitrosophenyl)acetamide, additionally by 1D- and 2D-NMR. Incubation of microorganisms, such as the soil bacteria Pseudomonas laurentiana, Arthrobacter MPI763, the yeast Papiliotrema baii and Pantoea ananatis, and the plants Brassica oleracea var. gongylodes L. (kohlrabi) and Arabidopsis thaliana Col-0, with N-(2-hydroxy-5-nitrophenyl) acetamide, led to its glucoside derivative as a prominent detoxification product; in the case of Pantoea ananatis, this was together with the corresponding glucoside succinic acid ester. In contrast, Actinomucor elegans consortium synthesized 2-acetamido-4-nitrophenyl sulfate. 1 mM bioactive N-(2-hydroxy-5-nitrophenyl) acetamide elicits alterations in the Arabidopsis thaliana expression profile of several genes. The most responsive upregulated gene was pathogen-inducible terpene synthase TPS04. The bioactivity of the compound is rapidly annihilated by glucosylation.

Overexpression of geranyl diphosphate synthase (PmGPPS1) boosts monoterpene and diterpene production involved in the response to pine wood nematode invasion

Outbreaks of pine wood nematode (PWN; Bursaphelenchus xylophilus) represent a severe biotic epidemic for the Pinus massoniana in China. When invaded by the PWN, the resistant P. massoniana might secret abundant oleoresin terpenoid to form certain defensive fronts for survival. However, the regulatory mechanisms of this process remain unclear. Here, the geranyl diphosphate synthase (PmGPPS1) gene was identified from resistant P. massoniana. Tissue-specific expression patterns of PmGPPS1 at transcript and protein level in resistant P. massoniana were determined by quantitative real-time polymerase chain reaction (qRT-PCR) and immunohistochemistry. Functional characteristics analysis of PmGPPS1 was performed on transgenic Nicotiana benthamiana by overexpression, as genetic transformation of P. massoniana is, so far, not possible. In summary, we identified and functionally characterized PmGPPS1 from the resistant P. massoniana following PWN inoculation. Tissue-specific expression patterns and localization of PmGPPS1 indicated that it may play a positive role involved in the metabolic and defensive processes of oleoresin terpenes production in response to PWN attack. Furthermore, overexpression of PmGPPS1 may enhance the production of monoterpene, among which limonene reduced the survival of PWN in vitro. In addition, PmGPPS1 upregulated the expression level of key genes involved in mevalonic acid (MVA) pathway, the methylerythritol phosphate (MEP) pathway and gibberellins (GAs) biosynthesis to boost the growth and development of tobacco through a feedback regulation mechanism. Our results offered new insights into the pivotal role of the PmGPPS1 involved in terpene-based defense mechanisms responding to the PWN invasion in resistant P. massoniana and provided a new metabolic engineering scenario to improve monoterpene production in tobacco.

The Effectiveness of Physical and Chemical Defense Responses of Wild Emmer Wheat Against Aphids Depends on Leaf Position and Genotype

The bird cherry-oat aphid (Rhopalosiphum padi) is one of the most destructive insect pests in wheat production. To reduce aphid damage, wheat plants have evolved various chemical and physical defense mechanisms. Although these mechanisms have been frequently reported, much less is known about their effectiveness. The tetraploid wild emmer wheat (WEW; Triticum turgidum ssp. dicoccoides), one of the progenitors of domesticated wheat, possesses untapped resources from its numerous desirable traits, including insect resistance. The goal of this research was to determine the effectiveness of trichomes (physical defense) and benzoxazinoids (BXDs; chemical defense) in aphid resistance by exploiting the natural diversity of WEW. We integrated a large dataset composed of trichome density and BXD abundance across wheat genotypes, different leaf positions, conditions (constitutive and aphid-induced), and tissues (whole leaf and phloem sap). First, we evaluated aphid reproduction on 203 wheat accessions and found large variation in this trait. Then, we chose eight WEW genotypes and one domesticated durum wheat cultivar for detailed quantification of the defense mechanisms across three leaves. We discovered that these defense mechanisms are influenced by both leaf position and genotype, where aphid reproduction was the highest on leaf-1 (the oldest), and trichome density was the lowest. We compared the changes in trichome density and BXD levels upon aphid infestation and found only minor changes relative to untreated plants. This suggests that the defense mechanisms in the whole leaf are primarily anticipatory and unlikely to contribute to aphid-induced defense. Next, we quantified BXD levels in the phloem sap and detected a significant induction of two compounds upon aphid infestation. Moreover, evaluating aphid feeding patterns showed that aphids prefer to feed on the oldest leaf. These findings revealed the dynamic response at the whole leaf and phloem levels that altered aphid feeding and reproduction. Overall, they suggested that trichomes and the BXD 2,4-dihydroxy-7- methoxy-1,4-benzoxazin-3-one (DIMBOA) levels are the main factors determining aphid resistance, while trichomes are more effective than BXDs. Accessions from the WEW germplasm, rich with trichomes and BXDs, can be used as new genetic sources to improve the resistance of elite wheat cultivars.

The Hulks and the Deadpools of the Cytokinin Universe: A Dual Strategy for Cytokinin Production, Translocation, and Signal Transduction

Cytokinins are plant hormones, derivatives of adenine with a side chain at the N⁶-position. They are involved in many physiological processes. While the metabolism of trans-zeatin and isopentenyladenine, which are considered to be highly active cytokinins, has been extensively studied, there are others with less obvious functions, such as cis-zeatin, dihydrozeatin, and aromatic cytokinins, which have been comparatively neglected. To help explain this duality, we present a novel hypothesis metaphorically comparing various cytokinin forms, enzymes of CK metabolism, and their signalling and transporter functions to the comics superheroes Hulk and Deadpool. Hulk is a powerful but short-lived creation, whilst Deadpool presents a more subtle and enduring force. With this dual framework in mind, this review compares different cytokinin metabolites, and their biosynthesis, translocation, and sensing to illustrate the different mechanisms behind the two CK strategies. This is put together and applied to a plant developmental scale and, beyond plants, to interactions with organisms of other kingdoms, to highlight where future study can benefit the understanding of plant fitness and productivity.

Genome-wide identification and characterization of cytokinin oxidase/dehydrogenase family genes in Medicago truncatula

Cytokinin oxidase/dehydrogenases (CKXs) play a key role in the irreversible degradation of phytohormone cytokinin that is necessary for various plant growth and development processes. However, thus far, detailed investigations of the CKX gene family in the model legume Medicago truncatula are limited. In this study, we identified 9 putative CKX homologues with conserved FAD- and cytokinin-binding domains in the M. truncatula genome. We analyzed their phylogenetic relationship, gene structure, conserved domain, expression pattern, protein subcellular locations and other properties. The tissue-specific expression profiles of the MtCKX genes are different among different members and these MtCKXs also displayed different patterns in response to synthetic cytokinin 6-benzylaminopurine (6-BA) and indole-3-acetic acid (IAA), suggesting their diverse roles in M. truncatula development. To further understand the biological function of MtCKXs, we identified and characterized mutants of each MtCKX by taking advantage of the Tnt1 mutant population in M. truncatula. Results indicated that M. truncatula plants harboring Tnt1 insertions in each single MtCKX genes showed no morphological changes in aerial parts, suggesting functional redundancy of MtCKXs in M. truncatula shoot development. However, disruption of Medtr4g126160, which is predominantly expressed in roots, leads to an obvious reduced primary root length and increased lateral root number, indicating the specific roles of cytokinin in regulating root architecture. We systematically analyzed the MtCKX gene family at the genome-wide level and revealed their possible roles in M. truncatula shoot and root development, which shed lights on understanding the biological function of CKX family genes in related legume plants.

Nitrate deficiency decreased photosynthesis and oxidation-reduction processes, but increased cellular transport, lignin biosynthesis and flavonoid metabolism revealed by RNA-Seq in Oryza sativa leaves

Rice cultivar "Weiyou916" (Oryza sativa L. ssp. Indica) were cultured with control (10 mM NO3-) and nitrate deficient solution (0 mM NO3-) for four weeks. Nitrogen (N) deficiency significantly decreased the content of N and P, dry weight (DW) of the shoots and roots, but increased the ratio of root to shoot in O. sativa. N deficiency decreased the photosynthesis rate and the maximum quantum yield of primary photochemistry (Fv/Fm), however, increased the intercellular CO2 concentration and primary fluorescence (Fo). N deficiency significantly increased the production of H2O2 and membrane lipid peroxidation revealed as increased MDA content in O. sativa leaves. N deficiency significantly increased the contents of starch, sucrose, fructose, and malate, but did not change that of glucose and total soluble protein in O. sativa leaves. The accumulated carbohydrates and H2O2 might further accelerate biosynthesis of lignin in O. sativa leaves under N limitation. A total of 1635 genes showed differential expression in response to N deficiency revealed by Illumina sequencing. Gene Ontology (GO) analysis showed that 195 DEGs were found to highly enrich in nine GO terms. Most of DEGs involved in photosynthesis, biosynthesis of ethylene and gibberellins were downregulated, whereas most of DEGs involved in cellular transport, lignin biosynthesis and flavonoid metabolism were upregulated by N deficiency in O. sativa leaves. Results of real-time quantitative PCR (RT-qPCR) further verified the RNA-Seq data. For the first time, DEGs involved oxygen-evolving complex, phosphorus response and lignin biosynthesis were identified in rice leaves. Our RNA-Seq data provided a global view of transcriptomic profile of principal processes implicated in the adaptation of N deficiency in O. sativa and shed light on the candidate direction in rice breeding for green and sustainable agriculture.

Transcriptome analysis reveals rapid defence responses in wheat induced by phytotoxic aphid Schizaphis graminum feeding

BACKGROUND

Schizaphis graminum is one of the most important and devastating cereal aphids worldwide, and its feeding can cause chlorosis and necrosis in wheat. However, little information is available on the wheat defence responses triggered by S. graminum feeding at the molecular level.

RESULTS

Here, we collected and analysed transcriptome sequencing data from leaf tissues of wheat infested with S. graminum at 2, 6, 12, 24 and 48 hpi (hours post infestation). A total of 44,835 genes were either up- or downregulated and differed significantly in response to aphid feeding. The expression levels of a number of genes (9761 genes) were significantly altered within 2 hpi and continued to change during the entire 48 h experiment. Gene Ontology analysis showed that the downregulated DEGs were mainly enriched in photosynthesis and light harvesting, and the total chlorophyll content in wheat leaves was also significantly reduced after S. graminum infestation at 24 and 48 hpi. However, a number of related genes of the salicylic acid (SA)-mediated defence signalling pathway and MAPK-WRKY pathway were significantly upregulated at early feeding time points (2 and 6 hpi). In addition, the gene expression and activity of antioxidant enzymes, such as peroxidase and superoxide dismutase, were rapidly increased at 2, 6 and 12 hpi. DAB staining results showed that S. graminum feeding induced hydrogen peroxide (H2O2) accumulation at the feeding sites at 2 hpi, and increased H2O2 production was detected with the increases in aphid feeding time. Pretreatment with diphenylene iodonium, an NADPH oxidase inhibitor, repressed the H2O2 accumulation and expression levels of SA-associated defence genes in wheat.

CONCLUSIONS

Our transcriptomic analysis revealed that defence-related pathways and oxidative stress in wheat were rapidly induced within hours after the initiation of aphid feeding. Additionally, NADPH oxidase plays an important role in aphid-induced defence responses and H2O2 accumulation in wheat. These results provide valuable insight into the dynamic transcriptomic responses of wheat leaves to phytotoxic aphid feeding and the molecular mechanisms of aphid-plant interactions.

Morphological and stage-specific transcriptome analyses reveal distinct regulatory programs underlying yam (Dioscorea alata L.) bulbil growth

In yam (Dioscorea spp) species, bulbils at leaf axils are the most striking species-specific axillary structure and exhibit important ecological niches. Genetic regulation underlying bulbil growth remains largely unclear so far. Here, we characterize yam (Dioscorea alata L.) bulbil development using histological analysis, and perform full transcriptional profiling on key developmental stages together with phytohormone analyses. Using the stage-specific scoring algorithm, we have identified 3451 stage-specifically expressed genes that exhibit a tight link between major transcriptional changes and stages. Co-expressed gene clusters revealed an obvious over-representation of genes associated with cell division and expansion at the initiation stage of bulbils (T1). Transcriptional changes of hormone-related genes highly coincided with hormone levels, indicating that bulbil initiation and growth are coordinately controlled by multiple phytohormones. In particular, localized auxin is transiently required to trigger bulbil initiation, and be further depleted or exported from bulbils to promote growth by up-regulation of genes involved in auxinconjugation and efflux. The sharp increase in supply of sucrose and an enhanced trehalose-6-phophate pathway at T1 were observed, suggesting that sucrose probably functions as a key signal and promotes bulbil initiation. Analysis of the expression of transcription factors (TFs) predicated 149 TFs as stage-specifically expressed; several T1-specific TFs (from Aux/IAA, E2F, MYB, and bHLH families) have been shown to play key roles in triggering bulbil formation. Together, our work provides a crucial angle for in-depth understanding of the molecular programs underlying yam's unique bulbil development processes. Stage-specific gene sets can be queried to obtain key candidates regulating bulbil growth, serving as valuable resources for further functional research.

Computational approach towards understanding structural and functional role of cytokinin oxidase/dehydrogenase 2 (CKX2) in enhancing grain yield in rice plant

Cytokinin oxidase/dehydrogenase (CKX) is the only known enzyme associated with irreversible degradation of cytokinins in plants. CKX2 contains flavin adenine dinucleotide (FAD) domain. Earlier studies utilising antisense & hpRNAi suppression techniques in mutant/transgenic rice plants revealed that when CKX2 binds with FAD, CKX2 expression reduces, which in turn causes cytokinin aggregation in inflorescence meristem that subsequently enhances both branches and grain number resulting in increased grain yield. Owing to the non-existence of complete three-dimensional structure of CKX2, insight into the structure and function of CKX2 and its relationship with its cofactor FAD is still a topic of debate. In the present study, computational approach was employed to estimate the three-dimensional structure of CKX2 through comparative modelling approach. Later, CKX2 and FAD interaction study was performed to understand the underlying mechanism involved with reduced expression of CKX2. Molecular dynamic simulation studies of both CKX2 and CKX-FAD complex revealed that after binding with FAD, CKX2 experienced increased pressure and reduced RMSD, potential energy and free energy landscape energy, which in turn lessen anti-correlation between almost all α and β strands and random motion of C-α, subsequently reducing CKX2 expression. In near future, these information can be utilised for increasing rice yield under irrigated field condition by introgression of Gn1a gene through marker assisted back-crossing breeding. AbbreviationsGROMACSGROningen MAchine for Chemical SimulationsNPTConstant Number of Particles, Volume and TemperatureRMSDRoot Mean Square DeviationRMSFRoot Mean Square FluctuationsQTLquantitative trait lociFADflavin adenine dinucleotideNVTConstant Number of Particles, Pressure and TemperatureLINCSLinear Constraint SolverCKX2Cytokinin oxidase/dehydrogenase 2MM/PBSAMolecular Mechanics/Poisson-Boltzmann surface areaSDFStructure Data FileCommunicated by Ramaswamy H. Sarma.

Interactions Among Plants, Insects, and Microbes: Elucidation of Inter-Organismal Chemical Communications in Agricultural Ecology

The last 2 decades have witnessed a sustained increase in the study of plant-emitted volatiles and their role in plant-insect, plant-microbe, and plant-plant interactions. While each of these binary systems involves complex chemical and biochemical processes between two organisms, the progression of increasing complexity of a ternary system (i.e., plant-insect-microbe), and the study of a ternary system requires nontrivial planning. This planning can include an experimental design that factors in potential overarching ecological interactions regarding the binary or ternary system, correctly identifying and understanding unexpected observations that may occur during the experiment and thorough interpretation of the resultant data. This challenge of planning, performing, and interpreting a plant's defensive response to multiple biotic stressors will be even greater when abiotic stressors (i.e., temperature or water) are factored into the system. To fully understand the system, we need to not only continue to investigate and understand the volatile profiles but also include and understand the biochemistry of the plant's response to these stressors. In this review, we provide examples and discuss interaction considerations with respect to how readers and future authors of the Journal of Agricultural and Food Chemistry can contribute their expertise toward the extraction and interpretation of chemical information exchanged between agricultural commodities and their associated pests. This holistic, multidisciplinary, and thoughtful approach to interactions of plants, insects, and microbes, and the resultant response of the plants can lead to a better understanding of agricultural ecology, in turn leading to practical and viable solutions to agricultural problems.

Rapid defense responses in maize leaves induced by Spodoptera exigua caterpillar feeding

Insects such as the beet armyworm (Spodoptera exigua) cause extensive damage to maize (Zea mays). Maize plants respond by triggering defense signaling, changes in gene expression, and biosynthesis of specialized metabolites. Leaves of maize inbred line B73, which has an available genome sequence, were infested with S. exigua for 1 to 24 h, followed by comparisons of the transcript and metabolite profiles with those of uninfested controls. The most extensive gene expression responses occurred rapidly, within 4-6 h after caterpillar infestation. However, both gene expression and metabolite profiles were altered within 1 h and continued to change during the entire 24 h experiment. The defensive functions of three caterpillar-induced genes were examined using available Dissociation transposon insertions in maize inbred line W22. Whereas mutations in the benzoxazinoid biosynthesis pathway (Bx1 and Bx2) significantly improved caterpillar growth, the knockout of a 13-lipoxygenase (Lox8) involved in jasmonic acid biosynthesis did not. Interestingly, 9-lipoxygenases, which lead to the production of maize death acids, were more strongly induced by caterpillar feeding than 13-lipoxygenases, suggesting an as yet unknown function in maize defense against herbivory. Together, these results provide a comprehensive view of the dynamic transcriptomic and metabolomic responses of maize leaves to caterpillar feeding.

The family of berberine bridge enzyme-like enzymes: A treasure-trove of oxidative reactions

Biological oxidations form the basis of life on earth by utilizing organic compounds as electron donors to drive the generation of metabolic energy carriers, such as ATP. Oxidative reactions are also important for the biosynthesis of complex compounds, i.e. natural products such as alkaloids that provide vital benefits for organisms in all kingdoms of life. The vitamin B₂-derived cofactors flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) enable an astonishingly diverse array of oxidative reactions that is based on the versatility of the redox-active isoalloxazine ring. The family of FAD-linked oxidases can be divided into subgroups depending on specific sequence features in an otherwise very similar structural context. The sub-family of berberine bridge enzyme (BBE)-like enzymes has recently attracted a lot of attention due to the challenging chemistry catalyzed by its members and the unique and unusual bi-covalent attachment of the FAD cofactor. This family is the focus of the present review highlighting recent advancements into the structural and functional aspects of members from bacteria, fungi and plants. In view of the unprecedented reaction catalyzed by the family's namesake, BBE from the California poppy, recent studies have provided further insights into nature's treasure chest of oxidative reactions.

Maize cytokinin dehydrogenase isozymes are localized predominantly to the vacuoles

The maize genome encompasses 13 genes encoding for cytokinin dehydrogenase isozymes (CKXs). These enzymes are responsible for irreversible degradation of cytokinin plant hormones and thus, contribute regulating their levels. Here, we focus on the unique aspect of CKXs: their diverse subcellular distribution, important in regulating cytokinin homeostasis. Maize CKXs were tagged with green fluorescent protein (GFP) and transiently expressed in maize protoplasts. Most of the isoforms, namely ZmCKX1, ZmCKX2, ZmCKX4a, ZmCKX5, ZmCKX6, ZmCKX8, ZmCKX9, and ZmCKX12, were associated with endoplasmic reticulum (ER) several hours after transformation. GFP-fused CKXs were observed to accumulate in putative prevacuolar compartments. To gain more information about the spatiotemporal localization of the above isoforms, we prepared stable expression lines of all ZmCKX-GFP fusions in Arabidopsis thaliana Ler suspension culture. All the ER-associated isoforms except ZmCKX1 and ZmCKX9 were found to be targeted primarily to vacuoles, suggesting that ER-localization is a transition point in the intracellular secretory pathway and vacuoles serve as these isoforms' final destination. ZmCKX9 showed an ER-like localization pattern similar to those observed in the transient maize assay. Apoplastic localization of ZmCKX1 was further confirmed and ZmCKX10 showed cytosolic/nuclear localization due to the absence of the signal peptide sequence as previously reported. Additionally, we prepared GFP-fused N-terminal signal deletion mutants of ZmCKX2 and ZmCKX9 and clearly demonstrated that the localization pattern of these mutant forms was cytosolic/nuclear. This study provides the first complex model for spatiotemporal localization of the key enzymes of the cytokinin degradation/catabolism in monocotyledonous plants.

Benzoxazolinone detoxification by N-Glucosylation: The multi-compartment-network of Zea mays L

The major detoxification product in maize roots after 24 h benzoxazolin-2(3H)-one (BOA) exposure was identified as glucoside carbamate resulting from rearrangement of BOA-N-glucoside, but the pathway of N-glucosylation, enzymes involved and the site of synthesis were previously unknown. Assaying whole cell proteins revealed the necessity of H2O2 and Fe(2+) ions for glucoside carbamate production. Peroxidase produced BOA radicals are apparently formed within the extraplastic space of the young maize root. Radicals seem to be the preferred substrate for N-glucosylation, either by direct reaction with glucose or, more likely, the N-glucoside is released by glucanase/glucosidase catalyzed hydrolysis from cell wall components harboring fixed BOA. The processes are accompanied by alterations of cell wall polymers. Glucoside carbamate accumulation could be suppressed by the oxireductase inhibitor 2-bromo-4´-nitroacetophenone and by peroxidase inhibitor 2,3-butanedione. Alternatively, activated BOA molecules with an open heterocycle may be produced by microorganisms (e.g., endophyte Fusarium verticillioides) and channeled for enzymatic N-glucosylation. Experiments with transgenic Arabidopsis lines indicate a role of maize glucosyltransferase BX9 in BOA-N-glycosylation. Western blots with BX9 antibody demonstrate the presence of BX9 in the extraplastic space. Proteomic analyses verified a high BOA responsiveness of multiple peroxidases in the apoplast/cell wall. BOA incubations led to shifting, altered abundances and identities of the apoplast and cell wall located peroxidases, glucanases, glucosidases and glutathione transferases (GSTs). GSTs could function as glucoside carbamate transporters. The highly complex, compartment spanning and redox-regulated glucoside carbamate pathway seems to be mainly realized in Poaceae. In maize, carbamate production is independent from benzoxazinone synthesis.

Kinetic and structural investigation of the cytokinin oxidase/dehydrogenase active site

Cytokinins are hormones that regulate plant development and their environmental responses. Their levels are mainly controlled by the cytokinin oxidase/dehydrogenase (CKO), which oxidatively cleaves cytokinins using redox-active electron acceptors. CKO belongs to the group of flavoproteins with an 8α-N1-histidyl FAD covalent linkage. Here, we investigated the role of seven active site residues, H105, D169, E288, V378, E381, P427 and L492, in substrate binding and catalysis of the CKO1 from maize (Zea mays, ZmCKO1) combining site-directed mutagenesis with kinetics and X-ray crystallography. We identify E381 as a key residue for enzyme specificity that restricts substrate binding as well as quinone electron acceptor binding. We show that D169 is important for catalysis and that H105 covalently linked to FAD maintains the enzyme's structural integrity, stability and high rates with electron acceptors. The L492A mutation significantly modulates the cleavage of aromatic cytokinins and zeatin isomers. The high resolution X-ray structures of ZmCKO1 and the E381S variant in complex with N6-(2-isopentenyl)adenosine reveal the binding mode of cytokinin ribosides. Those of ZmCKO2 and ZmCKO4a contain a mobile domain, which might contribute to binding of the N9 substituted cytokinins.

Nitrate induction triggers different transcriptional changes in a high and a low nitrogen use efficiency maize inbred line

In higher plants, NO3(-) can induce its own uptake and the magnitude of this induction is positively related to the external anion concentration. This phenomenon has been characterized in both herbaceous and woody plants. Here, different adaptation strategies of roots from two maize (Zea mays L., ZmAGOs) inbred lines differing in nitrogen use efficiency (NUE) and exhibiting different timing of induction were discussed by investigating NO3(-) -induced changes in their transcriptome. Lo5 line (high NUE) showing the maximum rate of NO3(-) uptake 4 h after the provision of 200 μmol/L NO3(-) treatment modulated a higher number of transcripts relative to T250 (low NUE) that peaked after 12 h. The two inbred lines share only 368 transcripts that are modulated by the treatment with NO3(-) and behaved differently when transcripts involved in anion uptake and assimilation were analyzed. T250 line responded to the NO3(-) induction modulating this group of genes as reported for several plant species. On the contrary, the Lo5 line did not exhibit during the induction changes in this set of genes. Obtained data suggest the importance of exploring the physiological and molecular variations among different maize genotypes in response to environmental clues like NO3(-) provision, in order to understand mechanisms underlying NUE.

Biochemical characterization of the maize cytokinin dehydrogenase family and cytokinin profiling in developing maize plantlets in relation to the expression of cytokinin dehydrogenase genes

The cytokinin dehydrogenases (CKX; EC 1.5.99.12) are a protein family that maintains the endogenous levels of cytokinins in plants by catalyzing their oxidative degradation. The CKX family in maize (Zea mays L.) has thirteen members, only two of which--ZmCKX1 and ZmCKX10--have previously been characterized in detail. In this study, nine further maize CKX isoforms were heterologously expressed in Escherichia coli, purified by affinity and ion-exchange chromatography and biochemically characterized. ZmCKX6 and ZmCKX9 could only be expressed successfully after the removal of putative sequence-specific vacuolar sorting signals (LLPT and LPTS, respectively), suggesting that these proteins are localized to the vacuole. Substrate specificity analyses revealed that the CKX isoforms can be grouped into two subfamilies: members of the first strongly prefer cytokinin free bases while members of the second degrade a broad range of substrates. The most active isoform was found to be ZmCKX1. One of the studied isoforms, ZmCKX6, seemed to encode a nonfunctional enzyme due to a mutation in a conserved HFG protein domain at the C-terminus. Site-directed mutagenesis experiments revealed that this domain is essential for CKX activity. The roles of the maize CKX enzymes in the development of maize seedlings during the two weeks immediately after radicle emergence were also investigated. It appears that ZmCKX1 is a key regulator of active cytokinin levels in developing maize roots. However, the expression of individual CKX isoforms in the shoots varied and none of them seemed to have strong effects on the cytokinin pool.

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.

Metabolomics reveals herbivore-induced metabolites of resistance and susceptibility in maize leaves and roots

Plants respond to herbivory by reprogramming their metabolism. Most research in this context has focused on locally induced compounds that function as toxins or feeding deterrents. We developed an ultra-high-pressure liquid chromatography time-of-flight mass spectrometry (UHPLC-TOF-MS)-based metabolomics approach to evaluate local and systemic herbivore-induced changes in maize leaves, sap, roots and root exudates without any prior assumptions about their function. Thirty-two differentially regulated compounds were identified from Spodoptera littoralis-infested maize seedlings and isolated for structure assignment by microflow nuclear magnetic resonance (CapNMR). Nine compounds were quantified by a high throughput direct nano-infusion tandem mass spectrometry/mass spectrometry (MS/MS) method. Leaf infestation led to a marked local increase of 1,3-benzoxazin-4-ones, phospholipids, N-hydroxycinnamoyltyramines, azealic acid and tryptophan. Only few changes were found in the root metabolome, but 1,3-benzoxazin-4-ones increased in the vascular sap and root exudates. The role of N-hydroxycinnamoyltyramines in plant-herbivore interactions is unknown, and we therefore tested the effect of the dominating p-coumaroyltyramine on S. littoralis. Unexpectedly, p-coumaroyltyramine was metabolized by the larvae and increased larval growth, possibly by providing additional nitrogen to the insect. Taken together, this study illustrates that herbivore attack leads to the induction of metabolites that can have contrasting effects on herbivore resistance in the leaves and roots.

Cytokinin dehydrogenase differentially regulates cytokinin and indirectly affects hydrogen peroxide accumulation in tomato leaf

Cytokinin dehydrogenase (CKX) catalyzes the irreversible degradation of cytokinins (CKs). CKs play a role in the regulation of hydrogen peroxide (H(2)O(2)) accumulation while H(2)O(2) is involved in chlorophyll degradation. Here, we elucidated how CKX differentially regulates representative CK levels and indirectly influence H(2)O(2) accumulation in tomato leaves. We induced drought, salt and iron-deficiency stresses in tomato plants and found that chlorosis-inducing stresses (salt and iron-deficiency) induced lower chlorophyll levels as compared to drought stress and control. Protein gel blot analysis detected two CKXs in tomato leaves: a 35 kDa protein (CKX35) found in chlorotic leaves associated with a reduction in representative CK levels and high H(2)O(2) concentrations, and a 37 kDa protein (CKX37) found in green leaves associated with representative CKs and H(2)O(2) at normal levels. In summary, CKX isoforms in the tomato leaf appear to have distinct roles in differentially regulating CK levels and indirectly influencing H(2)O(2) accumulation.

Cytokinins - recent news and views of evolutionally old molecules

Cytokinins (CKs) are evolutionally old and highly conserved low-mass molecules that have been identified in almost all known organisms. In plants, they evolved into an important group of plant hormones controlling many physiological and developmental processes throughout the whole lifespan of the plant. CKs and their functions are, however, not unique to plants. In this review, the strategies and mechanisms of plants - and phylogenetically distinct plant-interacting organisms such as bacteria, fungi, nematodes and insects employing CKs or regulation of CK status in plants - are described and put into their evolutionary context. The major breakthroughs made in the last decade in the fields of CK biosynthesis, degradation and signalling are also summarised.

Genetic engineering of cytokinin metabolism: prospective way to improve agricultural traits of crop plants

Cytokinins (CKs) are ubiquitous phytohormones that participate in development, morphogenesis and many physiological processes throughout plant kingdom. In higher plants, mutants and transgenic cells and tissues with altered activity of CK metabolic enzymes or perception machinery, have highlighted their crucial involvement in different agriculturally important traits, such as productivity, increased tolerance to various stresses and overall plant morphology. Furthermore, recent precise metabolomic analyses have elucidated the specific occurrence and distinct functions of different CK types in various plant species. Thus, smooth manipulation of active CK levels in a spatial and temporal way could be a very potent tool for plant biotechnology in the future. This review summarises recent advances in cytokinin research ranging from transgenic alteration of CK biosynthetic, degradation and glucosylation activities and CK perception to detailed elucidation of molecular processes, in which CKs work as a trigger in model plants. The first attempts to improve the quality of crop plants, focused on cereals are discussed, together with proposed mechanism of action of the responses involved.

Benzoxazinoid metabolites regulate innate immunity against aphids and fungi in maize

Benzoxazinoids (BXs), such as 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one (DIMBOA), are secondary metabolites in grasses. The first step in BX biosynthesis converts indole-3-glycerol phosphate into indole. In maize (Zea mays), this reaction is catalyzed by either BENZOXAZINELESS1 (BX1) or INDOLE GLYCEROL PHOSPHATE LYASE (IGL). The Bx1 gene is under developmental control and is mainly responsible for BX production, whereas the Igl gene is inducible by stress signals, such as wounding, herbivory, or jasmonates. To determine the role of BXs in defense against aphids and fungi, we compared basal resistance between Bx1 wild-type and bx1 mutant lines in the igl mutant background, thereby preventing BX production from IGL. Compared to Bx1 wild-type plants, BX-deficient bx1 mutant plants allowed better development of the cereal aphid Rhopalosiphum padi, and were affected in penetration resistance against the fungus Setosphaeria turtica. At stages preceding major tissue disruption, R. padi and S. turtica elicited increased accumulation of DIMBOA-glucoside, DIMBOA, and 2-hydroxy-4,7-dimethoxy-1,4-benzoxazin-3-one-glucoside (HDMBOA-glc), which was most pronounced in apoplastic leaf extracts. Treatment with the defense elicitor chitosan similarly enhanced apoplastic accumulation of DIMBOA and HDMBOA-glc, but repressed transcription of genes controlling BX biosynthesis downstream of BX1. This repression was also obtained after treatment with the BX precursor indole and DIMBOA, but not with HDMBOA-glc. Furthermore, BX-deficient bx1 mutant lines deposited less chitosan-induced callose than Bx1 wild-type lines, whereas apoplast infiltration with DIMBOA, but not HDMBOA-glc, mimicked chitosan-induced callose. Hence, DIMBOA functions as a defense regulatory signal in maize innate immunity, which acts in addition to its well-characterized activity as a biocidal defense metabolite.

Evolution of cytokinin biosynthesis and degradation

Cytokinin hormones are important regulators of development and environmental responses of plants that execute their action via the molecular machinery of signal perception and transduction. The limiting step of the whole process is the availability of the hormone in suitable concentrations in the right place and at the right time to interact with the specific receptor. Hence, the hormone concentrations in individual tissues, cells, and organelles must be properly maintained by biosynthetic and metabolic enzymes. Although there are merely two active cytokinins, isopentenyladenine and its hydroxylated derivative zeatin, a variety of conjugates they may form and the number of enzymes/isozymes with varying substrate specificity involved in their biosynthesis and conversion gives the plant a variety of tools for fine tuning of the hormone level. Recent genome-wide studies revealed the existence of the respective coding genes and gene families in plants and in some bacteria. This review summarizes present knowledge on the enzymes that synthesize cytokinins, form cytokinin conjugates, and carry out irreversible elimination of the hormones, including their phylogenetic analysis and possible variations in different organisms.

Vacuolar and cytosolic cytokinin dehydrogenases of Arabidopsis thaliana: heterologous expression, purification and properties

The catabolism of cytokinins is a vital component of hormonal regulation, contributing to the control of active forms of cytokinins and their cellular distribution. The enzyme catalyzing the irreversible cleavage of N(6)-side chains from cytokinins is a flavoprotein classified as cytokinin dehydrogenase (CKX, EC 1.5.99.12). CKXs also show low cytokinin oxidase activity, but molecular oxygen is a comparatively poor electron acceptor. The CKX gene family of Arabidopsis thaliana comprises seven members. Four code for proteins secreted to the apoplast, the remainder are not secreted. Two are targeted to the vacuoles and one is restricted to the cytosol. This study presents the purification and characterization of each of these non-secreted CKX enzymes and substrate specificities are discussed with respect to their compartmentation. Vacuolar enzymes AtCKX1 and AtCKX3 were produced in Pichia pastoris and cytosolic enzyme AtCKX7 was expressed in Escherichia coli. The recombinant proteins were purified by column chromatography. All enzymes preferred synthetic electron acceptors over oxygen, namely potassium ferricyanide and 2,3-dimetoxy-5-methyl-1,4-benzoquinone (Q(0)). In slightly acidic conditions (pH 5.0), N(6)-(2-isopentenyl)adenine 9-glucoside (iP9G) was the best substrate for AtCKX1 and AtCKX7, whereas AtCKX3 preferentially degraded N(6)-(2-isopentenyl)adenine 9-riboside-5'-monophosphate (iPMP). Moreover, vacuolar AtCKX enzymes in certain conditions degraded N(6)-(2-isopentenyl)adenine di- and triphosphates two to five times more effectively than its monophosphate.