Highly effective mlo-based powdery mildew resistance in hexaploid wheat without pleiotropic effects

Application of the mlo-based resistance in barley against powdery mildew attacks is a major success in crop breeding, since it confers durable disease resistance. Resistance caused by mutations in the Mlo gene seems to be ubiquitous across a range of species. This work addresses the introduction of mlo-based resistance into hexaploid wheat, which is complicated by the occurrence of three homoeologous genes: Mlo-A1, Mlo-B1 and Mlo-D1. EMS-generated mutant plants were screened for mutations in the three homoeologues. We selected and combined 6, 8, and 4 mutations, respectively, to obtain triple homozygous mlo mutant lines. Twenty-four mutant lines showed highly effective resistance towards attack by the powdery mildew pathogen under field conditions. All 18 mutations appeared to contribute to resistance; however, they had different effects on the occurrence of symptoms such as chlorotic and necrotic spots, which are pleiotropic to the mlo-based powdery mildew resistance. We conclude that to obtain highly effective powdery mildew resistance in wheat and to avoid detrimental pleiotropic effects, all three Mlo homoeologues should be mutated; however, at least one of the mutations should be of the weaker type in order to alleviate strong pleiotropic effects from the other mutations.

Recent advances in the genetics underlying wheat grain protein content and grain protein deviation in hexaploid wheat

Wheat is one of the most important global crops and selection for better performance has been ongoing since ancient times. As a quantitative trait controlled by the interplay of several genomic loci and under the strong influence of the environment, grain protein content (GPC) is of major interest in breeding programs. Here, we review the most recent contributions to the genetics underlying wheat GPC and grain protein deviation (GPD, representing the relationship between grain protein content and yield), along with the performance of genomic prediction models characterizing these traits. A total of 364 significant loci related to GPC and GPD are located on the hexaploid wheat genome, highlighting genomic regions where significant independent QTL overlap, with special focus on two regions, located on chromosomes 3A and 5A. Some of the corresponding homoeologous sequences co-locate with significant independent QTL reported on B and D subgenomes. Overlapping independent QTL from different studies are indicative of genomic regions exhibiting stability across environments and genotypes, with promising candidates for improving grain quality.

Analysis of barley mutants ert-c.1 and ert-d.7 reveals two loci with additive effect on plant architecture

Both mutant ert-c.1 and ert-d.7 carry T2-T3 translocations in the Ert-c gene. Principal coordinate analyses revealed the translocation types and translocation breakpoints. Mutant ert-d.7 is an Ert-c Ert-d double mutant. Mutations in the Ert-c and Ert-d loci are among the most common barley mutations affecting plant architecture. The mutants have various degrees of erect and compact spikes, often accompanied with short and stiff culms. In the current study, complementation tests, linkage mapping, principal coordinate analyses and fine mapping were conducted. We conclude that the original ert-d.7 mutant does not only carry an ert-d mutation but also an ert-c mutation. Combined, mutations in Ert-c and Ert-d cause a pyramid-dense spike phenotype, whereas mutations in only Ert-c or Ert-d give a pyramid and dense phenotype, respectively. Associations between the Ert-c gene and T2-T3 translocations were detected in both mutant ert-c.1 and ert-d.7. Different genetic association patterns indicate different translocation breakpoints in these two mutants. Principal coordinate analysis based on genetic distance and screening of recombinants from all four ends of polymorphic regions was an efficient way to narrow down the region of interest in translocation-involved populations. The Ert-c gene was mapped to the marker interval of 2_0801to1_0224 on 3HL near the centromere. The results illuminate a complex connection between two single genes having additive effects on barley spike architecture and will facilitate the identification of the Ert-c and Ert-d genes.

Stepwise mass spectrometry-based approach for confirming the presence of benzoxazinoids in herbs and vegetables

INTRODUCTION

Benzoxazinoids (BXs) are plant phytochemicals that have both defensive properties in plants and therapeutic effects in humans. The presence of BXs has been largely studied in the Poaceae family (monocots). To study the presence or absence of BXs in dicotyledons and monocotyledons outside the Poaceae family, parts of 24 plant species at several growth stages were selected for analysis, some of which were already known to contain BXs.

OBJECTIVES

To devise a stepwise mass spectrometry-based approach for confirming the presence of BXs in plant samples, and to use the method to explore the status of BXs in selected plant species.

EXPERIMENTAL

Plant samples were extracted using accelerated solvent extraction and analysed using triple-quadrupole liquid chromatography-mass spectrometry.

RESULTS

The use of different columns, double mass transitions, and ion ratios proved to be a robust tool for confirming the presence of BXs in different plant species. By this method, the presence of BXs was confirmed in three of the 24 species. Double-hexose forms of BXs, which have not been reported before in dicotyledons, were confirmed to be present in the dicotyledon plants Acanthus mollis and Lamium galeobdolon, and the presence of BXs in the seeds of Consolida orientalis is reported for the first time here. High concentrations of BXs were found in the aerial parts of Acanthus mollis and Lamium galeobdolon, at 20 and 32 μmol/g plant dry weight, respectively.

CONCLUSIONS

The stepwise approach described in this work confirmed the presence of BXs in new samples.

Development of mlo-based resistance in tetraploid wheat against wheat powdery mildew

Powdery mildew is a severe disease in wheat. In barley, durable resistance exists, based on non-functionality of the Mlo gene. As a model to analyse the effects of mutagenesis in the homoeologous Mlo genes of wheat, we developed mlo-based powdery mildew resistance in tetraploid durum wheat. To obtain Mlo mutations, we screened a TILLING population developed in tetraploid wheat "Kronos" for which the captured exome sequence of  > 1500 lines is available. This resulted in 23 mutants for Mlo-A1 and 26 non-redundant mutants for Mlo-B1. Two Mlo-A1 and four Mlo-B1 mutants were crossed to obtain eight F₂ mutant lines that showed a range of phenotypes from susceptibility to full resistance. Pot experiments under semi-field conditions confirmed the resistance levels for six of the mutants without any signs of adverse pleiotropic effects. Resistance ranking was similar across six powdery mildew isolates, indicating no isolate specificity of the mlo-based resistance. The effect of mutations in the Mlo-B1 gene was stronger than in the Mlo-A1 gene, probably reflecting differences in wild-type Mlo gene expression levels. Strong partial resistance effects were observed with single mlo-B1 mutations hence, revealing a dosage effect of mlo mutant alleles. Two of the four mlo-B1 mutations (W163* and P335L) were very strong; however, the highest combined effect was observed with the MloA-P335S/MloB-P335L combination, suggesting that non-functional, but full-length Mlo proteins might have the strongest effect compared with nonsense mutations. Our results show that mlo-based resistance might offer possibilities to introduce durable protection in tetraploid wheat against powdery mildew.

Expression analysis of the polyphenol oxidase gene in response to signaling molecules, herbivory and wounding in antisense transgenic tobacco plants

KEY MESSAGE

We provide evidence that the expression of the PPO gene was significantly reduced in response to wounding, MeJ and herbivory in transgenic tobacco under wound-inducible OsRGLP2 promoter in an anti-sense orientation.

ABSTRACT

Polyphenol oxidase (PPO) genes play an important role in plant defense mechanisms against biotic and abiotic stresses. In the present study, a 655 bp core sequence of the potato PPO gene was placed under the control of wound-inducible OsRGLP2 promoter in an anti-sense direction to evaluate its potential effects during biotic (Trialeurodes vaporariorum's infestation) and various abiotic (wounding, MeJ, ABA) stresses. Transcriptional profiling of PPO gene by real-time PCR (qRT-PCR) in transgenic tobacco revealed a significant suppression (3.5-fold) of PPO in response to wounding than control plants after 24 h. In response to MeJ at different concentrations (100 µM and 200 µM), the PPO expression was greatly down-regulated by 4.7-fold after 6 h at 100 µM MeJ, and a non-significant expression was observed with ABA treatment. Moreover, significant levels of PPO reduction (sixfolds) was found in whitefly feeding assay indicating that expression of potato PPO in an anti-sense orientation had down-regulated the PPO activity. This down-regulation of PPO by wounding, MeJ and whitefly infestation clearly links the specific expression of PPO in biotic and abiotic stresses. In the future, PPO gene suppression in transgenic plants using anti-sense potato PPO gene construct can be used to inhibit enzymatic browning in fruits and vegetables, e.g., potato.

De novo assembly of Agave sisalana transcriptome in response to drought stress provides insight into the tolerance mechanisms

Agave, monocotyledonous succulent plants, is endemic to arid regions of North America, exhibiting exceptional tolerance to their xeric environments. They employ various strategies to overcome environmental constraints, such as crassulacean acid metabolism, wax depositions, and protective leaf morphology. Genomic resources of Agave species have received little attention irrespective of their cultural, economic and ecological importance, which so far prevented the understanding of the molecular bases underlying their adaptations to the arid environment. In this study, we aimed to elucidate molecular mechanism(s) using transcriptome sequencing of A. sisalana. A de novo approach was applied to assemble paired-end reads. The expression study unveiled 3,095 differentially expressed unigenes between well-irrigated and drought-stressed leaf samples. Gene ontology and KEGG analysis specified a significant number of abiotic stress responsive genes and pathways involved in processes like hormonal responses, antioxidant activity, response to stress stimuli, wax biosynthesis, and ROS metabolism. We also identified transcripts belonging to several families harboring important drought-responsive genes. Our study provides the first insight into the genomic structure of A. sisalana underlying adaptations to drought stress, thus providing diverse genetic resources for drought tolerance breeding research.

Induced Genetic Variation in Crop Plants by Random or Targeted Mutagenesis: Convergence and Differences

New Breeding Techniques (NBTs) include several new technologies for introduction of new variation into crop plants for plant breeding, in particular the methods that aim to make targeted mutagenesis at specific sites in the plant genome (NBT mutagenesis). However, following that the French highest legislative body for administrative justice, the Conseil d'État, has sought advice from The Court of Justice of the European Union (CJEU) in interpreting the scope of the genetically modified organisms (GMO) Directive, CJEU in a decision from 2018, stated that organisms modified by these new techniques are not exempted from the current EU GMO legislation. The decision was based in a context of conventional plant breeding using mutagenesis of crop plants by physical or chemical treatments. These plants are explicitly exempted from the EU GMO legislation, based on the long-termed use of mutagenesis. Following its decision, the EU Court considers that the NBTs operate "at a rate out of all proportion to those resulting from the application of conventional methods of mutagenesis." In this paper, we argue that in fact this is not the case anymore; instead, a convergence has taken place between conventional mutagenesis and NBTs, in particular due to the possibilities of TILLING methods that allow the fast detection of mutations in any gene of a genome. Thus, by both strategies mutations in any gene across the genome can be obtained at a rather high speed. However, the differences between the strategies are 1) the precision of the exact site of mutation in a target gene, and 2) the number of off-target mutations affecting other genes than the target gene. Both aspects favour the NBT methods, which provide more precision and fewer off-target mutations. This is in stark contrast to the different status of the two technologies with respect to EU GMO legislation. In the future, this situation is not sustainable for the European plant breeding industry, since it is expected that restrictions on the use of NBTs will be weaker outside Europe. This calls for reconsiderations of the EU legislation of plants generated via NBT mutagenesis.

Polyamines - A New Metabolic Switch: Crosstalk With Networks Involving Senescence, Crop Improvement, and Mammalian Cancer Therapy

Polyamines (PAs) are low molecular weight organic cations comprising biogenic amines that play multiple roles in plant growth and senescence. PA metabolism was found to play a central role in metabolic and genetic reprogramming during dark-induced barley leaf senescence (DILS). Robust PA catabolism can impact the rate of senescence progression in plants. We opine that deciphering senescence-dependent polyamine-mediated multidirectional metabolic crosstalks is important to understand regulation and involvement of PAs in plant death and re-mobilization of nutrients during senescence. This will involve optimizing the use of PA biosynthesis inhibitors, robust transgenic approaches to modulate PA biosynthetic and catabolic genes, and developing novel germplasm enriched in pro- and anti-senescence traits to ensure sustained crop productivity. PA-mediated delay of senescence can extend the photosynthesis capacity, thereby increasing grain starch content in malting grains such as barley. On the other hand, accelerating the onset of senescence can lead to increases in mineral and nitrogen content in grains for animal feed. Unraveling the "polyamine metabolic switch" and delineating the roles of PAs in senescence should further our knowledge about autophagy mechanisms involved in plant senescence as well as mammalian systems. It is noteworthy that inhibitors of PA biosynthesis block cell viability in animal model systems (cell tumor lines) to control some cancers, in this instance, proliferative cancer cells were led toward cell death. Likewise, PA conjugates work as signal carriers for slow release of regulatory molecule nitric oxide in the targeted cells. Taken together, these and other outcomes provide examples for developing novel therapeutics for human health wellness as well as developing plant resistance/tolerance to stress stimuli.

Biosynthesis and chemical transformation of benzoxazinoids in rye during seed germination and the identification of a rye Bx6-like gene

Benzoxazinoids are secondary metabolites with plant defense properties and possible health-promoting effects in humans. In this study, the transcriptional activity of ScBx genes (ScBx1-ScBx5; ScBx6-like), involved in benzoxazinoid biosynthesis, was analyzed during germination and early seedling development in rye. Our results showed that ScBx genes had highest levels of expression at 24-30 h after germination, followed by a decrease at later stages. For ScBx1-ScBx5 genes expression was higher in shoots compared with root tissues and vice versa for ScBx6-like gene transcripts. Moreover, methylated forms of benzoxazinoids accumulated in roots rather than in shoots during seedling development, in particular reaching high levels of HMBOA-glc in roots. Chemical profiles of benzoxazinoid accumulation in the developing seedling reflected the combined effects of de novo biosynthesis of the compounds as well as the turnover of compounds either pre-stored in the embryo or de novo biosynthesized. Bioinformatic analysis, together with the differential distribution of ScBx6-like transcripts in root and shoot tissues, suggested the presence of a ZmBx6 homolog encoding a 2-oxoglutarate dependent dehydrogenase in rye. The ScBx6-like cDNA was expressed in E. coli for functional characterization in vitro. LC-MS/MS analysis showed that the purified enzyme was responsible for the oxidation of DIBOA-glc into TRIBOA-glc, strongly suggesting the ScBX6-like enzyme in rye to be a functional ortholog of maize ZmBX6.

Barley plants over-expressing the NAC transcription factor gene HvNAC005 show stunting and delay in development combined with early senescence

The plant-specific NAC transcription factors have attracted particular attention because of their involvement in stress responses, senescence, and nutrient remobilization. The HvNAC005 gene of barley encodes a protein belonging to subgroup NAC-a6 of the NAC family. This study shows that HvNAC005 is associated with developmental senescence. It was significantly up-regulated following ABA treatment, supported by ABA-responsive elements in its promoter, but it was not up-regulated during dark-induced senescence. The C-termini of proteins closely related to HvNAC005 showed overall high divergence but also contained conserved short motifs. A serine- and leucine-containing central motif was essential for transcriptional activity of the HvNAC005 C-terminus in yeast. Over-expression of HvNAC005 in barley resulted in a strong phenotype with delayed development combined with precocious senescence. The over-expressing plants showed up-regulation of genes involved with secondary metabolism, hormone metabolism, stress, signalling, development, and transport. Up-regulation of senescence markers and hormone metabolism and signalling genes supports a role of HvNAC005 in the cross field of different hormone and signalling pathways. Binding of HvNAC005 to promoter sequences of putative target genes containing the T[G/A]CGT core motif was shown by direct protein-DNA interactions of HvNAC005 with promoters for two of the up-regulated genes. In conclusion, HvNAC005 was shown to be a strong positive regulator of senescence and so is an obvious target for the fine-tuning of gene expression in future attempts to improve nutrient remobilization related to the senescence process in barley.

Correlation of Deoxynivalenol Accumulation in Fusarium-Infected Winter and Spring Wheat Cultivars with Secondary Metabolites at Different Growth Stages

Fusarium infection in wheat causes Fusarium head blight, resulting in yield losses and contamination of grains with trichothecenes. Some plant secondary metabolites inhibit accumulation of trichothecenes. Eighteen Fusarium infected wheat cultivars were harvested at five time points and analyzed for the trichothecene deoxynivalenol (DON) and 38 wheat secondary metabolites (benzoxazinoids, phenolic acids, carotenoids, and flavonoids). Multivariate analysis showed that harvest time strongly impacted the content of secondary metabolites, more distinctly for winter wheat than spring wheat. The benzoxazinoid 2-β-glucopyranoside-2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA-glc), α-tocopherol, and the flavonoids homoorientin and orientin were identified as potential inhibitors of DON accumulation. Several phenolic acids, lutein and β-carotene also affected DON accumulation, but the effect varied for the two wheat types. The results could form a basis for choosing wheat cultivars using metabolite profiling as a marker for selecting wheat cultivars with improved resistance against Fusarium head blight and accumulation of trichothecene toxins in wheat heads.

NAC Transcription Factors in Senescence: From Molecular Structure to Function in Crops

Within the last decade, NAC transcription factors have been shown to play essential roles in senescence, which is the focus of this review. Transcriptome analyses associate approximately one third of Arabidopsis NAC genes and many crop NAC genes with senescence, thereby implicating NAC genes as important regulators of the senescence process. The consensus DNA binding site of the NAC domain is used to predict NAC target genes, and protein interaction sites can be predicted for the intrinsically disordered transcription regulatory domains of NAC proteins. The molecular characteristics of these domains determine the interactions in gene regulatory networks. Emerging local NAC-centered gene regulatory networks reveal complex molecular mechanisms of stress- and hormone-regulated senescence and basic physiological steps of the senescence process. For example, through molecular interactions involving the hormone abscisic acid, Arabidopsis NAP promotes chlorophyll degradation, a hallmark of senescence. Furthermore, studies of the functional rice ortholog, OsNAP, suggest that NAC genes can be targeted to obtain specific changes in lifespan control and nutrient remobilization in crop plants. This is also exemplified by the wheat NAM1 genes which promote senescence and increase grain zinc, iron, and protein content. Thus, NAC genes are promising targets for fine-tuning senescence for increased yield and quality.

Members of the barley NAC transcription factor gene family show differential co-regulation with senescence-associated genes during senescence of flag leaves

The senescence process of plants is important for the completion of their life cycle, particularly for crop plants, it is essential for efficient nutrient remobilization during seed filling. It is a highly regulated process, and in order to address the regulatory aspect, the role of genes in the NAC transcription factor family during senescence of barley flag leaves was studied. Several members of the NAC transcription factor gene family were up-regulated during senescence in a microarray experiment, together with a large range of senescence-associated genes, reflecting the coordinated activation of degradation processes in senescing barley leaf tissues. This picture was confirmed in a detailed quantitative reverse transcription-PCR (qRT-PCR) experiment, which also showed distinct gene expression patterns for different members of the NAC gene family, suggesting a group of ~15 out of the 47 studied NAC genes to be important for signalling processes and for the execution of degradation processes during leaf senescence in barley. Seven models for DNA-binding motifs for NAC transcription factors were designed based on published motifs, and available promoter sequences of barley genes were screened for the motifs. Genes up-regulated during senescence showed a significant over-representation of the motifs, suggesting regulation by the NAC transcription factors. Furthermore, co-regulation studies showed that genes possessing the motifs in the promoter in general were highly co-expressed with members of the NAC gene family. In conclusion, a list of up to 15 NAC genes from barley that are strong candidates for being regulatory factors of importance for senescence and biotic stress-related traits affecting the productivity of cereal crop plants has been generated. Furthermore, a list of 71 senescence-associated genes that are potential target genes for these NAC transcription factors is presented.

Identification of predominant genes involved in regulation and execution of senescence-associated nitrogen remobilization in flag leaves of field grown barley

The transcriptomes of senescing flag leaves collected from barley field plots with standard or high nitrogen supply were compared to identify genes specifically associated with nitrogen remobilization during leaf senescence under agronomically relevant conditions. In flag leaves collected in field plots with high nitrogen supply, the decline in chlorophyll content was delayed. By comparing changes in gene expression for the two nitrogen levels, it was possible to discriminate genes related to nitrogen remobilization during senescence and genes involved in other processes associated with the late development of leaves under field conditions. Predominant genes that were more strongly upregulated during senescence of flag leaves from plants with standard nitrogen supply included genes encoding the transcription factor HvNAC026, serine type protease SCPL51, and the autophagy factors APG7 and ATG18F. Elevated expression of these genes in senescing leaves from plants with standard nitrogen supply indicates important roles of the corresponding proteins in nitrogen remobilization. In comparison, the genes upregulated in both flag leaf samples might have roles in general senescence processes associated with late leaf development. Among these genes were the transcription factor genes HvNAC001, HvNAC005, HvNAC013, HvWRKY12 and MYB, genes encoding the papain-like cysteine peptidases HvPAP14 and HvPAP20, as well as a subtilase gene.

The barley HvNAC6 transcription factor affects ABA accumulation and promotes basal resistance against powdery mildew

Barley HvNAC6 is a member of the plant-specific NAC (NAM, ATAF1,2, CUC2) transcription factor family and we have shown previously that it acts as a positive regulator of basal resistance in barley against the biotrophic pathogen Blumeria graminis f. sp. hordei (Bgh). In this study, we use a transgenic approach to constitutively silence HvNAC6 expression, using RNA interference (RNAi), to investigate the in vivo functions of HvNAC6 in basal resistance responses in barley in relation to the phytohormone ABA. The HvNAC6 RNAi plants displayed reduced HvNAC6 transcript levels and were more susceptible to Bgh than wild-type plants. Application of exogenous ABA increased basal resistance against Bgh in wild-type plants, but not in HvNAC6 RNAi plants, suggesting that ABA is a positive regulator of basal resistance which depends on HvNAC6. Silencing of HvNAC6 expression altered the light/dark rhythm of ABA levels which were, however, not influenced by Bgh inoculation. The expression of the two ABA biosynthetic genes HvNCED1 and HvNCED2 was compromised, and transcript levels of the ABA conjugating HvBG7 enzyme were elevated in the HvNAC6 RNAi lines, but this effect was not clearly associated with transgene-mediated resistance. Together, these data support a function of HvNAC6 as a regulator of ABA-mediated defence responses for maintenance of effective basal resistance against Bgh.

Plant senescence and crop productivity

Senescence is a developmental process which in annual crop plants overlaps with the reproductive phase. Senescence might reduce crop yield when it is induced prematurely under adverse environmental conditions. This review covers the role of senescence for the productivity of crop plants. With the aim to enhance productivity, a number of functional stay-green cultivars have been selected by conventional breeding, in particular of sorghum and maize. In many cases, a positive correlation between leaf area duration and yield has been observed, although in a number of other cases, stay-green cultivars do not display significant effects with regards to productivity. In several crops, the stay-green phenotype is observed to be associated with a higher drought resistance and a better performance under low nitrogen conditions. Among the approaches used to achieve stay-green phenotypes in transgenic plants, the expression of the IPT gene under control of senescence-associated promoters has been the most successful. The promoters employed for senescence-regulated expression contain cis-elements for binding of WRKY transcription factors and factors controlled by abscisic acid. In most crops transformed with such constructs the stay-green character has led to increased biomass, but only in few cases to increased seed yield. A coincidence of drought stress resistance and stay-green trait is observed in many transgenic plants.

Interaction of barley powdery mildew effector candidate CSEP0055 with the defence protein PR17c

A large number of effector candidates have been identified recently in powdery mildew fungi. However, their roles and how they perform their functions remain unresolved. In this study, we made use of host-induced gene silencing and confirmed that the secreted barley powdery mildew effector candidate, CSEP0055, contributes to the aggressiveness of the fungus. This result suggests that CSEP0055 is involved in the suppression of plant defence. A yeast two-hybrid screen indicated that CSEP0055 interacts with members of the barley pathogenesis-related protein families, PR1 and PR17. Interaction with PR17c was confirmed by bimolecular fluorescence complementation analyses. Down-regulation and over-expression of PR17c in epidermal cells of barley confirmed that this protein is important for penetration resistance against the powdery mildew fungus. In line with this, PR17c was found to be apoplastic, localizing to the papillae formed in response to this fungus. The CSEP0055 transcript did not start to accumulate until 24 h after inoculation. This suggests that this gene is expressed too late to influence primary penetration events, but rather sustains the fungus at sites of secondary penetration, where PR17c appears to be able to accumulate.

Regulation of basal resistance by a powdery mildew-induced cysteine-rich receptor-like protein kinase in barley

The receptor-like protein kinases (RLKs) constitute a large and diverse group of proteins controlling numerous plant physiological processes, including development, hormone perception and stress responses. The cysteine-rich RLKs (CRKs) represent a prominent subfamily of transmembrane-anchored RLKs. We have identified a putative barley (Hordeum vulgare) CRK gene family member, designated HvCRK1. The mature putative protein comprises 645 amino acids, and includes a putative receptor domain containing two characteristic 'domain 26 of unknown function' (duf26) domains in the N-terminal region, followed by a rather short 17-amino-acid transmembrane domain, which includes an AAA motif, two features characteristic of endoplasmic reticulum (ER)-targeted proteins and, finally, a characteristic putative protein kinase domain in the C-terminus. The HvCRK1 transcript was isolated from leaves inoculated with the biotrophic fungal pathogen Blumeria graminis f.sp. hordei (Bgh). HvCRK1 transcripts were observed to accumulate transiently following Bgh inoculation of susceptible barley. Transient silencing of HvCRK1 expression in bombarded epidermal cells led to enhanced resistance to Bgh, but did not affect R-gene-mediated resistance. Silencing of HvCRK1 phenocopied the effective penetration resistance found in mlo-resistant barley plants, and the possible link between HvCRK1 and MLO was substantiated by the fact that HvCRK1 induction on Bgh inoculation was dependent on Mlo. Finally, using both experimental and in silico approaches, we demonstrated that HvCRK1 localizes to the ER of barley cells. The negative effect on basal resistance against Bgh and the functional aspects of MLO- and ER-localized HvCRK1 signalling on Bgh inoculation are discussed.

Characterization of barley (Hordeum vulgare L.) NAC transcription factors suggests conserved functions compared to both monocots and dicots

BACKGROUND

The NAC transcription factor family is involved in the regulation of traits in both monocots and dicots of high agronomic importance. Understanding the precise functions of the NAC genes can be of utmost importance for the improvement of cereal crop plants through plant breeding. For the cereal crop plant barley (Hordeum vulgare L.) only a few NAC genes have so far been investigated.

RESULTS

Through searches in publicly available barley sequence databases we have obtained a list of 48 barley NAC genes (HvNACs) with 43 of them representing full-length coding sequences. Phylogenetic comparisons to Brachypodium, rice, and Arabidopsis NAC proteins indicate that the barley NAC family includes members from all of the eight NAC subfamilies, although by comparison to these species a number of HvNACs still remains to be identified. Using qRT-PCR we investigated the expression profiles of 46 HvNACs across eight barley tissues (young flag leaf, senescing flag leaf, young ear, old ear, milk grain, late dough grain, roots, and developing stem) and two hormone treatments (abscisic acid and methyl jasmonate).

CONCLUSIONS

Comparisons of expression profiles of selected barley NAC genes with the published functions of closely related NAC genes from other plant species, including both monocots and dicots, suggest conserved functions in the areas of secondary cell wall biosynthesis, leaf senescence, root development, seed development, and hormone regulated stress responses.

A roadmap for zinc trafficking in the developing barley grain based on laser capture microdissection and gene expression profiling

Nutrients destined for the developing cereal grain encounter several restricting barriers on their path towards their final storage sites in the grain. In order to identify transporters and chelating agents that may be involved in transport and deposition of zinc in the barley grain, expression profiles have been generated of four different tissue types: the transfer cells, the aleurone layer, the endosperm, and the embryo. Cells from these tissues were isolated with the 'laser capture microdissection' technology and the extracted RNA was subjected to three rounds of T7-based amplification. The amplified RNA was subsequently hybridized to Affymetrix 22K Barley GeneChips. Due to the short average length of the amplified transcripts and the positioning of numerous probe sets at locations more than 400 base pairs (bp) from the poly(A)-tail, a normalization approach was used where the probe positions were taken into account. On the basis of the expression levels of a number of metal homeostasis genes, a working model is proposed for the translocation of zinc from the phloem to the storage sites in the developing grain.

Leaf senescence and nutrient remobilisation in barley and wheat

Extensive studies have been undertaken on senescence processes in barley and wheat and their importance for the nitrogen use efficiency of these crop plants. During the senescence processes, proteins are degraded and nutrients are re-mobilised from senescing leaves to other organs, especially the developing grain. Most of the proteins degraded reside in the chloroplasts, with Rubisco constituting the most dominant protein fraction. Despite intensive studies, the proteases responsible for Rubisco degradation have not yet been identified. Evidence for degradation of stromal proteins outside of chloroplasts is summarised. Rubisco is thought to be released from chloroplasts into vesicles containing stroma material (RCB = Rubisco-containing bodies). These vesicles may then take different routes for their degradation. Transcriptome analyses on barley and wheat senescence have identified genes involved in degradative, metabolic and regulatory processes that could be used in future strategies aimed at modifying the senescence process. The breeding of crops for characters related to senescence processes, e.g. higher yields and better nutrient use efficiency, is complex. Such breeding has to cope with the dilemma that delayed senescence, which could lead to higher yields, is correlated with a decrease in nutrient use efficiency. Pinpointing regulatory genes involved in senescence might lead to tools that could effectively overcome this dilemma.

Leaf senescence and nutrient remobilisation in barley and wheat

Extensive studies have been undertaken on senescence processes in barley and wheat and their importance for the nitrogen use efficiency of these crop plants. During the senescence processes, proteins are degraded and nutrients are re-mobilised from senescing leaves to other organs, especially the developing grain. Most of the proteins degraded reside in the chloroplasts, with Rubisco constituting the most dominant protein fraction. Despite intensive studies, the proteases responsible for Rubisco degradation have not yet been identified. Evidence for degradation of stromal proteins outside of chloroplasts is summarised. Rubisco is thought to be released from chloroplasts into vesicles containing stroma material (RCB = Rubisco-containing bodies). These vesicles may then take different routes for their degradation. Transcriptome analyses on barley and wheat senescence have identified genes involved in degradative, metabolic and regulatory processes that could be used in future strategies aimed at modifying the senescence process. The breeding of crops for characters related to senescence processes, e.g. higher yields and better nutrient use efficiency, is complex. Such breeding has to cope with the dilemma that delayed senescence, which could lead to higher yields, is correlated with a decrease in nutrient use efficiency. Pinpointing regulatory genes involved in senescence might lead to tools that could effectively overcome this dilemma.

Transcriptome analysis of senescence in the flag leaf of wheat (Triticum aestivum L.)

The senescence process in wheat flag leaves was investigated over a time course from ear emergence until 50% yellowing of harvested leaf samples using an in-house fabricated cDNA microarray based on a 9K wheat unigene set. The top 1000 ranked differentially expressed probes were subjected to a cluster analysis and, from these, we selected 140 up-regulated genes with informative annotations. There was a considerable overlap between this list of genes and genes previously observed to be associated with senescence in other species, covering several functional categories involved in the degradation of macromolecules and nutrient remobilization, notably of nitrogen via the metabolism of carboxylic and amino acids. The up-regulation of a number of genes in this metabolism was confirmed by real-time polymerase chain reaction experiments. The data suggest a role for cytosolic/peroxisomal routes in the integration of the degradation of carbohydrates, fatty acids and proteins, leading to the remobilization of nitrogen. Illustrative examples of up-regulated genes comprise cytoplasmic aconitate hydratase and peroxisomal citrate synthase. The data support a protective role of the mitochondria towards oxidative cell damage via the up-regulation of the alternative oxidase, and possibly also involving the up-regulated succinate dehydrogenase. A number of up-regulated regulatory genes were also identified, notably NAC-domain and WRKY transcription factors. These factors have previously been identified as being associated with senescence in other species. The data support the notion that a generic senescence programme exists across monocot and dicot plant species. However, notable differences can also be recognized. We thus found transcriptional up-regulation of the biosynthetic pathway for benzoxazinoids, a group of graminaceous-specific secondary metabolites.

A microarray-based comparative analysis of gene expression profiles during grain development in transgenic and wild type wheat

Global, comparative gene expression analysis is potentially a very powerful tool in the safety assessment of transgenic plants since it allows for the detection of differences in gene expression patterns between a transgenic line and the mother variety. In the present study, we compared the gene expression profile in developing seeds of wild type wheat and wheat transformed for endosperm-specific expression of an Aspergillus fumigatus phytase. High-level expression of the phytase gene was ensured by codon modification towards the prevalent codon usage of wheat genes and by using the wheat 1DX5HMW glutenin promoter for driving transgene expression. A 9K wheat unigene cDNA microarray was produced from cDNA libraries prepared mainly from developing wheat seed. The arrays were hybridised to flourescently labelled cDNA prepared from developing seeds of the transgenic wheat line and the mother variety, Bobwhite, at three developmental stages. Comparisons and statistical analyses of the gene expression profiles of the transgenic line vs. that of the mother line revealed only slight differences at the three developmental stages. In the few cases where differential expression was indicated by the statistical analysis it was primarily genes that were strongly expressed over a shorter interval of seed development such as genes encoding storage proteins. Accordingly, we interpret these differences in gene expression levels to result from minor asynchrony in seed development between the transgenic line and the mother line. In support of this, real time PCR validation of results from selected genes at the late developmental stage could not confirm differential expression of these genes. We conclude that the expression of the codon-modified A. fumigatus phytase gene in the wheat seed had no significant effects on the overall gene expression patterns in the developing seed.

The molecular characterization of two barley proteins establishes the novel PR-17 family of pathogenesis-related proteins

Summary Two barley (Hordeum vulgare L.) cDNA clones (pBH6-12 and pBH6-17) were isolated from a cDNA library prepared from leaves 6 h after inoculation with Blumeria graminis f.sp. hordei (Bgh). The two transcripts accumulate strongly in response to Bgh, peaking around 6, 15-24 and 48-96 h after inoculation, concomitant with fungal penetration attempts, hypersensitive response and fungal growth. The encoded proteins, HvPR-17a and HvPR-17b, belong to a new family of plant pathogenesis-related proteins, designated 'PR-17'. The family also include NtPRp27 from tobacco (Okushima et al., 2000, Plant Mol. Biol.42, 479-488) and WCI-5 from wheat (Görlach et al., 1996, Plant Cell8, 629-643), responsive to viral and fungal infection, respectively. Antisera were raised to HvPR-17a and HvPR-17b, and the proteins exhibit apparent molecular weights of 26 and 24 kDa, respectively. They accumulate in the mesophyll apoplast following Bgh-inoculation, as well as in the leaf epidermis, the only tissue to be invaded by the fungus. Several homologous plant proteins exist, and a highly conserved part of the members of this new protein family show similarity to the active site and to the peptide-binding groove of the exopeptidase 'aminopeptidase N' from eukaryotes and the endopeptidase 'thermolysin' from bacteria.

Up-regulation of the ascorbate-dependent antioxidative system in barley leaves during powdery mildew infection

Abstract The ascorbate-dependent antioxidative system was studied in leaves of barley infected with the barley powdery mildew fungus Blumeria graminis f.sp. hordei (Bgh). Increased ascorbate peroxidase (APX) activity was detected upon infection, especially in the compatible interaction. APX activity was determined in epidermal and total leaf tissues. A relatively higher increase in APX activity was found in the epidermis compared to total leaf 72 h after inoculation in the compatible interaction, but the increase was not restricted to the epidermis. Activity assays in native gels and Northern blot hybridization indicated that the increase in APX activity was caused by a cytosolic APX isoform. 'Inverse Northern blot' hybridization results with the cDNA of a cytosolic APX supported the relatively higher increase in epidermal APX activity compared to total leaf activity. In the compatible interaction, monodehydroascorbate reductase (MDHAR) activity increased in temporal and spatial patterns similar to that of APX activity. In contrast to this, dehydroascorbate reductase and glutathione reductase activities either decreased or were unaffected by Bgh infection. The increase in APX and MDHAR activities in the compatible interaction continued until severe infection of the leaves. Thus, an up-regulation of the antioxidative system of the host cells could play a role for maintenance of the biotrophic relationship between Bgh and the barley leaf by preventing proliferating oxidative processes, which would otherwise be harmful to the living plant cell on which the biotrophic powdery mildew fungus depends.

A chalcone synthase with an unusual substrate preference is expressed in barley leaves in response to UV light and pathogen attack

A cDNA clone was isolated by differential hybridization from a library prepared from barley leaves inoculated with the fungus Blumeria graminis f.sp. hordei (Bgh). The open reading frame of the insert (designated HvCHS2) encoded a polypeptide with 72-79% identity to chalcone synthases (CHS) and 65-68% identity to stilbene synthases. Alignments of the amino acid sequence of HvCHS2 with the consensus sequence of naringenin-CHS (EC 2.3.1.74) reveals significant differences between HvCHS2 and naringenin-CHS. HvCHS2 transcripts accumulate strongly in barley leaves in response to inoculation with Bgh, whereas only insignificant accumulation of barley naringenin-CHS (CHS1) transcripts is seen upon the inoculation. The accumulation of HvCHS2 transcripts is also elicited by UV light. To compare the activity of HvCHS2 with the activity of CHS1, the two enzymes were expressed in Escherichia coli. Both HvCHS2 and CHS1 catalyse the formation of chalcones. However, HvCHS2 and CHS1 differ in their substrate requirements. CHS1 uses cinnamoyl-CoA and 4-coumaroyl-CoA at comparable rates whereas feruloyl-CoA is a poor substrate for this enzyme. In contrast, HvCHS2 converts feruloyl-CoA and caffeoyl-CoA at the highest rate whereas cinnamoyl-CoA is a poor substrate. Thus, HvCHS2 is a novel pathogen and UV light induces homoeriodictyol/eriodictyol CHS involved in the direct production of flavonoids possessing multi-substituted B-rings.

Molecular characterization of the oxalate oxidase involved in the response of barley to the powdery mildew fungus

Previously we reported that oxalate oxidase activity increases in extracts of barley (Hordeum vulgare) leaves in response to the powdery mildew fungus (Blumeria [syn. Erysiphe] graminis f.sp. hordei) and proposed this as a source of H2O2 during plant-pathogen interactions. In this paper we show that the N terminus of the major pathogen-response oxalate oxidase has a high degree of sequence identity to previously characterized germin-like oxalate oxidases. Two cDNAs were isolated, pHvOxOa, which represents this major enzyme, and pHvOxOb', representing a closely related enzyme. Our data suggest the presence of only two oxalate oxidase genes in the barley genome, i.e. a gene encoding HvOxOa, which possibly exists in several copies, and a single-copy gene encoding HvOxOb. The use of 3' end gene-specific probes has allowed us to demonstrate that the HvOxOa transcript accumulates to 6 times the level of the HvOxOb transcript in response to the powdery mildew fungus. The transcripts were detected in both compatible and incompatible interactions with a similar accumulation pattern. The oxalate oxidase is found exclusively in the leaf mesophyll, where it is cell wall located. A model for a signal transduction pathway in which oxalate oxidase plays a central role is proposed for the regulation of the hypersensitive response.

A flavonoid 7-O-methyltransferase is expressed in barley leaves in response to pathogen attack

We have shown previously that transcripts corresponding to the cDNA clone pBH72-F1, with similarities to O-methyltransferases (OMT), accumulated in barley leaves in response to attack by the pathogenic fungus Blumeria graminis (Plant Mol Biol 26 (1994) 1797). To investigate the accumulation pattern in the defence response and the organ localization of the pBH72-F1-encoded polypeptide (F1-OMT), an antiserum was raised against Escherichia coli expressed F1-OMT. The 43 kDa protein was absent in normal leaves but accumulated strongly in response to pathogen attack. The F1-OMT protein accumulated faster in barley lines inoculated with an avirulent B. graminis isolates compared to a virulent isolate. Additionally, F1-OMT related proteins were detected in developing kernels. F1-OMT was expressed as a functional enzyme in E. coli and the substrate specificity was investigated. The enzyme exhibited OMT activity towards flavonoid aglycones with the highest activity against apigenin (4',5,7-trihydroxyflavone). In contrast, caffeic acid did not serve as substrate for F1-OMT. The product of F1-OMT was analyzed by HPLC and GC-MS and found to be genkwanin (4',5-dihydroxy-7-methoxyflavone). Initial velocity data were best represented by a sequential bi-bi mechanism, and kinetic parameters of KSAM = 10.9 microM, Kapigenin = 4.6 microM and a specific activity of 0.45 mukat/g were obtained. Barley F1-OMT, apigenin 7-O-methyltransferase, is suggested to be involved in the production of a methylated flavonoid phytoalexin.

An epidermis/papilla-specific oxalate oxidase-like protein in the defence response of barley attacked by the powdery mildew fungus

A cDNA clone of a defence response transcript was isolated from a library prepared from barley leaves expressing papilla resistance towards the powdery mildew fungus, Blumeria (syn. Erysiphe) graminis f.sp. hordei (Bgh). The 904 bp sequence encodes a 229 amino acid polypeptide with a putative signal peptide of 23 amino acids. After cleavage, the protein has a mass of 22.3 kDa and exhibits up to 60% amino acid identity to certain dicot proteins, and 46% amino acid identity to barley oxalate oxidase; therefore we designated it HvOxOLP (for Hordeum vulgare oxalate oxidase-like protein). Single-base substitutions among several cDNA and RACE clones demonstrate a gene of many copies. Both the transcript and protein accumulate from 3 h after inoculation with Bgh. The transcript level peaks at 18-24 h and subsequently decreases, whereas the protein level is stable from 24 h after inoculation. The accumulation patterns are independent of the outcome of the barley/powdery mildew interaction, unlike that of PR proteins, for example. The transcript accumulates specifically in the inoculated epidermal tissue. This temporal and spatial expression pattern suggests a very close relationship to papilla formation. Immunoblot analyses have facilitated a demonstration that HvOxOLP, like oxalate oxidase, is a water-soluble 100 kDa oligomeric protein. The oligomer is heat-stable and SDS-tolerant, and it can be denatured into a 25 kDa monomer. Attempts to demonstrate oxalate oxidase activity for this protein have failed. However, the relationships to oxalate oxidase suggests that HvOxOLP may be involved in H2O2 generation necessary for, for example, cross-linking of cell wall components during formation of papillae.

Purification, characterization, and molecular cloning of basic PR-1-type pathogenesis-related proteins from barley

Partial amino acid sequences of two proteins, purified from barley leaves reacting hypersensitively to the powdery mildew fungus, showed a high degree of amino acid identity to the PR-1 proteins originally described in tobacco. The proteins, subsequently designated HvPR-1a and HvPR-1b, show apparent pI values of approximately 10.5 and 11, respectively and apparent M(r) 15,000. Independently, differential screening of a cDNA library prepared from barley leaves, exhibiting a compatible interaction with the powdery mildew fungus, resulted in isolation of cDNA species representing two PR-1 homologs. With the exception of one amino acid, the partial amino acid sequences of HvPR-1a and HvPR-1b are identical to internal sequences of the polypeptides derived from the two cDNA species. These derived polypeptides are each 164 amino acids long and both have putative N-terminal leader sequences of 24 amino acids. That these proposed leader sequences are functional is indicated by the observed occurrence of both proteins in the intercellular fluid. The proposed mature proteins (calculated M(r) 14,490 and 15,204) share 91% identical amino acids with each other and 56 to 74% with other PR-1 proteins. Northern blot hybridization and immunoblotting, respectively, show that both transcripts and both proteins accumulate following inoculation of susceptible and hypersensitivity resistant barley leaves with the powdery mildew fungus.

The effect of indomethacin 1% ophthalmic suspension on the pupil during extracapsular cataract surgery

Fifty-six patients who underwent extracapsular cataract surgery were randomized in two groups with 28 patients in each group. Group 1 was treated with standard preoperative dilation regime and group 2 received in addition indomethacin 1% ophthalmic solution. Horizontal and vertical pupil diameter measurements were taken before capsulotomy, before exprimation of the lens nucleus and before lens implantation. The size of the pupil at the time of capsulotomy and at the time just before lens exprimation was greatest in the non-indomethacin treated group, but at the time of lens implantation the pupil of the indomethacin treated group was greatest. It is concluded that even though indomethacin seems to be able to inhibit surgically induced miosis, probably through its inhibitory effect on prostaglandin synthesis, the effect is only marginal and from a clinical point of view it cannot be recommended.

[Treatment of corneal abrasion]

Forty patients with corneal epithelial defects were randomized into two groups. Group 1 (twenty patients) were treated with chloramphenicol ointment and an eye patch for six hours followed by chloramphenicol eyedrops six time a day for six days. Group 2 (twenty patients) were treated with chloramphenicol ointment and double eye patches for twenty-four hours as the only treatment. The patients were examined the next day, and again after a week. Two patients defected in group 1 and two patients from group 1 were excluded from the experiment because they failed to improve after three and four days, respectively. Five patients from group 2 defected. Corneal epithelial wound healing was found to be significantly faster in the group treated with double eye patches and the patients' subjective problems in this group were significantly less. (Significant with a probability less than 1%).