A comprehensive view on organ-specific callose synthesis in wheat (Triticum aestivum L.): glucan synthase-like gene expression, callose synthase activity, callose quantification and deposition

Flagellin and mannitol modulate callose biosynthesis and deposition in soybean seedlings

Callose is a polymer deposited on the cell wall and is necessary for plant growth and development. Callose is synthesized by genes from the glucan synthase-like family (GSL) and dynamically responds to various types of stress. Callose can inhibit pathogenic infection, in the case of biotic stresses, and maintain cell turgor and stiffen the plant cell wall in abiotic stresses. Here, we report the identification of 23 GSL genes (GmGSL) in the soybean genome. We performed phylogenetic analyses, gene structure prediction, duplication patterns, and expression profiles on several RNA-Seq libraries. Our analyses show that WGD/Segmental duplication contributed to expanding this gene family in soybean. Next, we analyzed the callose responses in soybean under abiotic and biotic stresses. The data show that callose is induced by both osmotic stress and flagellin 22 (flg22) and is related to the activity of β-1,3-glucanases. By using RT-qPCR, we evaluated the expression of GSL genes during the treatment of soybean roots with mannitol and flg22. The GmGSL23 gene was upregulated in seedlings treated with osmotic stress or flg22, showing the essential role of this gene in the soybean defense response to pathogenic organisms and osmotic stress. Our results provide an important understanding of the role of callose deposition and regulation of GSL genes in response to osmotic stress and flg22 infection in soybean seedlings.

Combating powdery mildew: Advances in molecular interactions between Blumeria graminis f. sp. tritici and wheat

Wheat powdery mildew caused by a biotrophic fungus Blumeria graminis f. sp. tritici (Bgt), is a widespread airborne disease which continues to threaten global wheat production. One of the most chemical-free and cost-effective approaches for the management of wheat powdery mildew is the exploitation of resistant cultivars. Accumulating evidence has reported that more than 100 powdery mildew resistance genes or alleles mapping to 63 different loci (Pm1-Pm68) have been identified from common wheat and its wild relatives, and only a few of them have been cloned so far. However, continuous emergence of new pathogen races with novel degrees of virulence renders wheat resistance genes ineffective. An essential breeding strategy for achieving more durable resistance is the pyramiding of resistance genes into a single genotype. The genetics of host-pathogen interactions integrated with temperature conditions and the interaction between resistance genes and their corresponding pathogen a virulence genes or other resistance genes within the wheat genome determine the expression of resistance genes. Considerable progress has been made in revealing Bgt pathogenesis mechanisms, identification of resistance genes and breeding of wheat powdery mildew resistant cultivars. A detailed understanding of the molecular interactions between wheat and Bgt will facilitate the development of novel and effective approaches for controlling powdery mildew. This review gives a succinct overview of the molecular basis of interactions between wheat and Bgt, and wheat defense mechanisms against Bgt infection. It will also unleash the unsung roles of epigenetic processes, autophagy and silicon in wheat resistance to Bgt.

PbrCalS5, a callose synthase protein, is involved in pollen tube growth in Pyrus bretschneideri

Identification of CalS genes in seven Rosaceae species and functional characterization of PbrCalS5 in pear pollen tube growth by regulating callose deposition. Callose exists widely in angiosperms and has significant functions in a range of developmental processes. Callose is synthesized by callose synthase (CalS). However, the members of the callose synthase gene family and their evolutionary profiles, along with their biological functions, in species of the Rosaceae remain unknown. In this study, a total of 69 members of the CalS gene family in seven Rosaceae species (Fragaria vesca, Malus × domestica, Prunus avium, Pyrus bretschneideri, Prunus mume, Prunus persica and Rubus occidentalis) were identified and divided into six clades. Different types of gene duplication events contributed to the expansions of the CalS gene family in the seven species, with purifying selection playing a key role in the evolution of the CalS genes. Tissue-specific expression patterns analysis revealed that PbrCalS5 was highly expressed in the pear pollen tube and was selected for further functional analysis. Subcellular localization indicated that PbrCalS5 was localized in the plasma membrane and cell wall. Antisense oligodeoxynucleotide (AS-ODN) assays resulted in the inhibition of PbrCalS5 expression, leading to the decreased callose deposition in the pollen tube wall and subsequent inhibition of pear pollen tube growth. These results provide the theoretical basis for exploring the functional roles of CalS genes in pear pollen tube growth.

Sandwich Enzyme-Linked Immunosorbent Assay for Quantification of Callose

The existing methods of callose quantification include epifluorescence microscopy and fluorescence spectrophotometry of aniline blue-stained callose particles, immuno-fluorescence microscopy and indirect assessment of both callose synthase and β-(1,3)-glucanase enzyme activities. Some of these methods are laborious, time consuming, not callose-specific, biased and require high technical skills. Here, we describe a method of callose quantification based on Sandwich Enzyme-Linked Immunosorbent Assay (S-ELISA). Tissue culture-derived banana plantlets were inoculated with Xanthomonas campestris pv. musacearum (Xcm) bacteria as a biotic stress factor inducing callose production. Banana leaf, pseudostem and corm tissue samples were collected at 14 days post-inoculation (dpi) for callose quantification. Callose levels were significantly different in banana tissues of Xcm-inoculated and control groups except in the pseudostems of both banana genotypes. The method described here could be applied for the quantification of callose in different plant species with satisfactory level of specificity to callose, and reproducibility. Additionally, the use of 96-well plate makes this method suitable for high throughput callose quantification studies with minimal sampling and analysis biases. We provide step-by-step detailed descriptions of the method.

MADS2 regulates priming defence in postharvest peach through combined salicylic acid and abscisic acid signaling

MADS-box genes play well-documented roles in plant development, but relatively little is known regarding their involvement in defence responses. In this study, pre-treatment of peach (Prunus persica) fruit with β-aminobutyric acid (BABA) activated resistance against Rhizopus stolonifer, leading to a significant delay in the symptomatic appearance of disease. This was associated with an integrated defence response that included a H2O2 burst, ABA accumulation, and callose deposition. cDNA library screening identified nucleus-localized MADS2 as an interacting partner with NPR1, and this was further confirmed by yeast two-hybrid, luciferase complementation imaging, and co-immunoprecipitation assays. The DNA-binding activity of NPR1 conferred by the NPR1-MADS2 complex was required for the transcription of SA-dependent pathogenesis-related (PR) and ABA-inducible CalS genes in order to gain the BABA-induced resistance, in which MAPK1-induced post-translational modification of MADS2 was also involved. In accordance with this, overexpression of PpMADS2 in Arabidopsis potentiated the transcription of a group of PR genes and conferred fungal resistance in the transgenic plants. Conversely, Arabidopsis mads2-knockout lines showed high sensitivity to the fungal pathogen. Our results indicate that MADS2 positively participates in BABA-elicited defence in peach through a combination of SA-dependent NPR1 activation and ABA signaling-induced callose accumulation, and that this defence is also related to the post-translational modification of MADS2 by MAPK1 for signal amplification.

Silencing of a Wheat Ortholog of Glucan Synthase-Like Gene Reduced Resistance to Blumeria graminis f. sp. tritici

Wheat powdery mildew, caused by the obligate biotrophic ascomycete fungal pathogen Blumeria graminis f. sp. tritici (Bgt), is a major threat to wheat production worldwide. It is known that Arabidopsis thaliana glucan synthase-like 5 (AtGSL5) improves the resistance of wheat to powdery mildew by increasing its anti-penetration abilities. However, the function of glucan synthase-like (GSL) orthologs in crop species remains largely unknown. In this study, TaGSL22, a novel functional ortholog of AtGSL5, was isolated as the only Bgt-induced GSL gene in wheat. Phylogenetic analysis indicated that TaGSL22 was conserved within the group of Gramineae and showed a closer relationship to GSL orthologs from monocots than to those from dicots. The TaGSL22 transcript was highest in the wheat leaves, followed by stems then roots. TaGSL22 was localized in the cell membrane and cytoplasm of wheat protoplasts, as predicted by transmembrane structure analysis. In addition, expression of TaGSL22 was induced by the plant hormones ethylene (ETH) and salicylic acid (SA), but down-regulated by jasmonate (JA) and abscisic acid (ABA). The transcript level of TaGSL22 was up-regulated in the incompatible interaction between Bgt and wheat, whereas it remained relatively unchanged in the compatible interaction. Knocking down of TaGSL22 by virus-induced gene silencing (VIGS) induced a higher infection type in the wheat-Bgt interaction. The TaGSL22-silenced plants exhibited reduced resistance to Bgt, accompanied by decreased callose accumulation. Our study shows a conserved function of GSL genes in plant immunity associated with penetration resistance, and it indicates that TaGSL22 can be used to improve papilla composition and enhance resistance to wheat powdery mildew.

Seed priming with calcium chloride enhances wheat resistance against wheat aphid Schizaphis graminum Rondani

BACKGROUND

Calcium is an essential macronutrient for plant growth. Although it has been shown that exogenous Ca application can increase plant resistance to abiotic stress, little is known about its potential to enhance plant tolerance to biotic stress. Here, we investigated whether pretreatment of wheat (Triticum aestivum L.) seeds with calcium chloride (CaCl₂ ) improves plant resistance against wheat aphid (Schizaphis graminum Rondani). The developmental time, population size, feeding behavior of aphids on plants grown from CaCl₂ - and water-pretreated seeds, and plant defense responses to aphid attack were investigated.

RESULTS

Seed pretreatment with CaCl₂ extended aphid development time and reduced aphid population size and feeding efficiency. In addition, the pretreatment significantly increased the concentration of Ca²⁺ in wheat leaves, and upregulated expression levels of TaCaM genes and callose synthase genes (TaGSL2, TaGSL8, TaGSL10, TaGSL12, TaGSL19, TaGSL22 and TaGSL23). Callose concentration in the leaves of plants grown from CaCl₂ -pretreated seeds increased significantly upon aphid attack. Further, callose deposition was observed mainly in the phloem.

CONCLUSION

These results suggest that seed pretreatment with CaCl₂ primes the plant response against wheat aphid attack, leading to modulation of callose deposition in the phloem in response to aphid attack. © 2021 Society of Chemical Industry.

Identification of Callose Synthases in Stinging Nettle and Analysis of Their Expression in Different Tissues

Callose is an important biopolymer of β-1,3-linked glucose units involved in different phases of plant development, reproduction and response to external stimuli. It is synthesized by glycosyltransferases (GTs) known as callose synthases (CalS) belonging to family 48 in the Carbohydrate-Active enZymes (CAZymes) database. These GTs are anchored to the plasma membrane via transmembrane domains. Several genes encoding CalS have been characterized in higher plants with 12 reported in the model organism Arabidopsis thaliana. Recently, the de novo transcriptome of a fibre-producing clone of stinging nettle (Urtica dioica L.) was published and here it is mined for CalS genes with the aim of identifying members differentially expressed in the core and cortical tissues of the stem. The goal is to understand whether specific CalS genes are associated with distinct developmental stages of the stem internodes (elongation, thickening). Nine genes, eight of which encoding full-length CalS, are identified in stinging nettle. The phylogenetic analysis with CalS proteins from other fibre crops, namely textile hemp and flax, reveals grouping into 6 clades. The expression profiles in nettle tissues (roots, leaves, stem internodes sampled at different heights) reveal differences that are most noteworthy in roots vs. leaves. Two CalS are differentially expressed in the internodes sampled at the top and middle of the stem. Implications of their role in nettle stem tissue development are discussed.

Evolutionary history of callose synthases in terrestrial plants with emphasis on proteins involved in male gametophyte development

Callose is a plant-specific polysaccharide (β-1,3-glucan) playing an important role in angiosperms in many developmental processes and responses to biotic and abiotic stresses. Callose is synthesised at the plasma membrane of plant cells by callose synthase (CalS) and, among others, represents the main polysaccharide in the callose wall surrounding the tetrads of developing microspores and in the growing pollen tube wall. CalS proteins involvement in spore development is a plesiomorphic feature of terrestrial plants, but very little is known about their evolutionary origin and relationships amongst the members of this protein family. We performed thorough comparative analyses of callose synthase family proteins from major plant lineages to determine their evolutionary history across the plant kingdom. A total of 1211 candidate CalS sequences were identified and compared amongst diverse taxonomic groups of plants, from bryophytes to angiosperms. Phylogenetic analyses identified six main clades of CalS proteins and suggested duplications during the evolution of specialised functions. Twelve family members had previously been identified in Arabidopsis thaliana. We focused on five CalS subfamilies directly linked to pollen function and found that proteins expressed in pollen evolved twice. CalS9/10 and CalS11/12 formed well-defined clades, whereas pollen-specific CalS5 was found within subfamilies that mostly did not express in mature pollen vegetative cell, although were found in sperm cells. Expression of five out of seven mature pollen-expressed CalS genes was affected by mutations in bzip transcription factors. Only three subfamilies, CalS5, CalS10, and CalS11, however, formed monophyletic, mostly conserved clades. The pairs CalS9/CalS10, CalS11/CalS12 and CalS3 may have diverged after angiosperms diversified from lycophytes and bryophytes. Our analysis of fully sequenced plant proteins identified new evolutionary lineages of callose synthase subfamilies and has established a basis for understanding their functional evolution in terrestrial plants.

Glucanocellulosic ethanol: the undiscovered biofuel potential in energy crops and marine biomass

Converting biomass to biofuels is a key strategy in substituting fossil fuels to mitigate climate change. Conventional strategies to convert lignocellulosic biomass to ethanol address the fermentation of cellulose-derived glucose. Here we used super-resolution fluorescence microscopy to uncover the nanoscale structure of cell walls in the energy crops maize and Miscanthus where the typical polymer cellulose forms an unconventional layered architecture with the atypical (1, 3)-β-glucan polymer callose. This raised the question about an unused potential of (1, 3)-β-glucan in the fermentation of lignocellulosic biomass. Engineering biomass conversion for optimized (1, 3)-β-glucan utilization, we increased the ethanol yield from both energy crops. The generation of transgenic Miscanthus lines with an elevated (1, 3)-β-glucan content further increased ethanol yield providing a new strategy in energy crop breeding. Applying the (1, 3)-β-glucan-optimized conversion method on marine biomass from brown macroalgae with a naturally high (1, 3)-β-glucan content, we not only substantially increased ethanol yield but also demonstrated an effective co-fermentation of plant and marine biomass. This opens new perspectives in combining different kinds of feedstock for sustainable and efficient biofuel production, especially in coastal regions.

Reduced susceptibility to Fusarium head blight in Brachypodium distachyon through priming with the Fusarium mycotoxin deoxynivalenol

The fungal cereal pathogen Fusarium graminearum produces deoxynivalenol (DON) during infection. The mycotoxin DON is associated with Fusarium head blight (FHB), a disease that can cause vast grain losses. Whilst investigating the suitability of Brachypodium distachyon as a model for spreading resistance to F. graminearum, we unexpectedly discovered that DON pretreatment of spikelets could reduce susceptibility to FHB in this model grass. We started to analyse the cell wall changes in spikelets after infection with F. graminearum wild-type and defined mutants: the DON-deficient Δtri5 mutant and the DON-producing lipase disruption mutant Δfgl1, both infecting only directly inoculated florets, and the mitogen-activated protein (MAP) kinase disruption mutant Δgpmk1, with strongly decreased virulence but intact DON production. At 14 days post-inoculation, the glucose amounts in the non-cellulosic cell wall fraction were only increased in spikelets infected with the DON-producing strains wild-type, Δfgl1 and Δgpmk1. Hence, we tested for DON-induced cell wall changes in B. distachyon, which were most prominent at DON concentrations ranging from 1 to 100 ppb. To test the involvement of DON in defence priming, we pretreated spikelets with DON at a concentration of 1 ppm prior to F. graminearum wild-type infection, which significantly reduced FHB disease symptoms. The analysis of cell wall composition and plant defence-related gene expression after DON pretreatment and fungal infection suggested that DON-induced priming of the spikelet tissue contributed to the reduced susceptibility to FHB.

Transgenic Arabidopsis thaliana plants expressing a β-1,3-glucanase from sweet sorghum (Sorghum bicolor L.) show reduced callose deposition and increased tolerance to aluminium toxicity

Seventy-one cultivars of sweet sorghum (Sorghum bicolor L.) were screened for aluminium (Al) tolerance by measuring relative root growth (RRG). Two contrasting cultivars, ROMA (Al tolerant) and POTCHETSTRM (Al sensitive), were selected to study shorter term responses to Al stress. POTCHETSTRM had higher callose synthase activity, lower β-1,3-glucanase activity and more callose deposition in the root apices during Al treatment compared with ROMA. We monitored the expression of 12 genes involved in callose synthesis and degradation and found that one of these, SbGlu1 (Sb03g045630.1), which encodes a β-1,3-glucanase enzyme, best explained the contrasting deposition of callose in ROMA and POTCHETSTRM during Al treatment. Full-length cDNAs of SbGlu1 was prepared from ROMA and POTCHETSTRM and expressed in Arabidopsis thaliana using the constitutive cauliflower mosaic virus (CaMV) 35S promoter. Independent transgenic lines displayed significantly greater Al tolerance than wild-type plants and vector-only controls. This phenotype was associated with greater total β-1,3-glucanase activity, less Al accumulation and reduced callose deposition in the roots. These results suggest that callose production is not just an early indicator of Al stress in plants but likely to be part of the toxicity pathway that leads to the inhibition of root growth.

Interaction of the Arabidopsis GTPase RabA4c with its effector PMR4 results in complete penetration resistance to powdery mildew

The (1,3)-β-glucan callose is a major component of cell wall thickenings in response to pathogen attack in plants. GTPases have been suggested to regulate pathogen-induced callose biosynthesis. To elucidate the regulation of callose biosynthesis in Arabidopsis thaliana, we screened microarray data and identified transcriptional upregulation of the GTPase RabA4c after biotic stress. We studied the function of RabA4c in its native and dominant negative (dn) isoform in RabA4c overexpression lines. RabA4c overexpression caused complete penetration resistance to the virulent powdery mildew Golovinomyces cichoracearum due to enhanced callose deposition at early time points of infection, which prevented fungal ingress into epidermal cells. By contrast, RabA4c(dn) overexpression did not increase callose deposition or penetration resistance. A cross of the resistant line with the pmr4 disruption mutant lacking the stress-induced callose synthase PMR4 revealed that enhanced callose deposition and penetration resistance were PMR4-dependent. In live-cell imaging, tagged RabA4c was shown to localize at the plasma membrane prior to infection, which was broken in the pmr4 disruption mutant background, with callose deposits at the site of attempted fungal penetration. Together with our interactions studies including yeast two-hybrid, pull-down, and in planta fluorescence resonance energy transfer assays, we concluded that RabA4c directly interacts with PMR4, which can be seen as an effector of this GTPase.

Secreted fungal effector lipase releases free fatty acids to inhibit innate immunity-related callose formation during wheat head infection

The deposition of the (1,3)-β-glucan cell wall polymer callose at sites of attempted penetration is a common plant defense response to intruding pathogens and part of the plant's innate immunity. Infection of the Fusarium graminearum disruption mutant Δfgl1, which lacks the effector lipase FGL1, is restricted to inoculated wheat (Triticum aestivum) spikelets, whereas the wild-type strain colonized the whole wheat spike. Our studies here were aimed at analyzing the role of FGL1 in establishing full F. graminearum virulence. Confocal laser-scanning microscopy revealed that the Δfgl1 mutant strongly induced the deposition of spot-like callose patches in vascular bundles of directly inoculated spikelets, while these callose deposits were not observed in infections by the wild type. Elevated concentrations of the polyunsaturated free fatty acids (FFAs) linoleic and α-linolenic acid, which we detected in F. graminearum wild type-infected wheat spike tissue compared with Δfgl1-infected tissue, provided clear evidence for a suggested function of FGL1 in suppressing callose biosynthesis. These FFAs not only inhibited plant callose biosynthesis in vitro and in planta but also partially restored virulence to the Δfgl1 mutant when applied during infection of wheat spikelets. Additional FFA analysis confirmed that the purified effector lipase FGL1 was sufficient to release linoleic and α-linolenic acids from wheat spike tissue. We concluded that these two FFAs have a major function in the suppression of the innate immunity-related callose biosynthesis and, hence, the progress of F. graminearum wheat infection.

Transgenic expression of a functional fragment of harpin protein Hpa1 in wheat induces the phloem-based defence against English grain aphid

The harpin protein Hpa1 has multiple beneficial effects in plants, promoting plant growth and development, increasing crop yield, and inducing resistance to pathogens and insect pests. For these effects, the 10-40 residue fragment (Hpa1₁₀₋₄₂) isolated from the Hpa1 sequence is 1.3- to 7.5-fold more effective than the full-length protein. Here it is reported that the expression of Hpa1₁₀₋₄₂ under the direction of an insect-induced promoter induces the phloem-based defence to English grain aphid, a dominant species of wheat aphids. The expression of Hpa1₁₀₋₄₂ was found to compromise the colonization preference of aphids on the plant and further inhibit aphid reproduction in leaf colonies. In Hpa1₁₀₋₄₂-expressing wheat lines, moreover, aphid feeding from the phloem was repressed in correlation with the phloem-based defence. This defensive mechanism was shown as enhanced expression of wheat genes encoding phloem lectin proteins (PP2-A1 and PP2-A2) and β-1,3-glucan synthase-like enzymes (GSL2, GSL10, and GSL12). Both PP2-A and β-1,3-glucan formed high molecular mass polymers to block phloem sieve plate pores and therefore impede aphid feeding from the phloem. However, the phloem-based defence was impaired by treating plants with ethylene signalling inhibitors, suggesting the requirement for the ethylene signalling pathway. In addition, if Hpa1₁₀₋₄₂-expressing plants were subjected to attack by a small number of aphids, they newly acquired agriculturally beneficial characters, such as enhanced vegetative growth and increased tiller numbers and grain output values. These results suggest that the defensive and developmental roles of Hpa1₁₀₋₄₂ can be integrated into the germplasm of this agriculturally significant crop.

Resistance of callose synthase activity to free fatty acid inhibition as an indicator of Fusarium head blight resistance in wheat

The fungal pathogen Fusarium graminearum is the causal agent of Fusarium head blight (FHB); a devastating crop disease resulting in heavy yield losses and grain contamination with mycotoxins. We recently showed that the secreted lipase FGL1, a virulence factor of F. graminearum, targets plant defense-related callose biosynthesis during wheat head infection. This effector-like function is based on a FGL1-mediated release of polyunsaturated free fatty acids (FFA) that can inhibit callose synthase activity. The importance of FGL1 in successful wheat head colonization was demonstrated in FGL1 disruption mutants (Δfgl1), where infection was restricted to directly inoculated spikelets and accompanied by strong callose deposition in the spikelet's phloem. The application of polyunsaturated FFA to Δfgl1-infected spikelets prevented callose deposition in the phloem and partially restored wheat head colonization.   The comparative analysis of 3 wheat cultivars revealed that the level of resistance to FHB correlated with resistance to FFA-dependent inhibition of callose biosynthesis. Therefore, resistance of callose biosynthesis to FFA inhibition might be used as marker and/or direct target in the breeding of FHB-resistant wheat cultivars.

Elevated early callose deposition results in complete penetration resistance to powdery mildew in Arabidopsis

A common response by plants to fungal attack is deposition of callose, a (1,3)-β-glucan polymer, in the form of cell wall thickenings called papillae, at site of wall penetration. While it has been generally believed that the papillae provide a structural barrier to slow fungal penetration, this idea has been challenged in recent studies of Arabidopsis (Arabidopsis thaliana), where fungal resistance was found to be independent of callose deposition. To the contrary, we show that callose can strongly support penetration resistance when deposited in elevated amounts at early time points of infection. We generated transgenic Arabidopsis lines that express POWDERY MILDEW RESISTANT4 (PMR4), which encodes a stress-induced callose synthase, under the control of the constitutive 35S promoter. In these lines, we detected callose synthase activity that was four times higher than that in wild-type plants 6 h post inoculation with the virulent powdery mildew Golovinomyces cichoracearum. The callose synthase activity was correlated with enlarged callose deposits and the focal accumulation of green fluorescent protein-tagged PMR4 at sites of attempted fungal penetration. We observed similar results from infection studies with the nonadapted powdery mildew Blumeria graminis f. sp. hordei. Haustoria formation was prevented in resistant transgenic lines during both types of powdery mildew infection, and neither the salicylic acid-dependent nor jasmonate-dependent pathways were induced. We present a schematic model that highlights the differences in callose deposition between the resistant transgenic lines and the susceptible wild-type plants during compatible and incompatible interactions between Arabidopsis and powdery mildew.

Exploiting co-linearity among grass species to map the Aegilops ventricosa-derived Pch1 eyespot resistance in wheat and establish its relationship to Pch2

Introgressions into wheat from related species have been widely used as a source of agronomically beneficial traits. One such example is the introduction of the potent eyespot resistance gene Pch1 from the wild relative Aegilops ventricosa onto chromosome 7DL of wheat. In common with genes carried on many other such introgressions, the use of Pch1 in commercial wheat varieties has been hindered by linkage drag with yield-limiting traits. Attempts to break this linkage have been frustrated by a lack of co-dominant PCR markers suitable for identifying heterozygotes in F(2) populations. We developed conserved orthologous sequence (COS) markers, utilising the Brachypodium distachyon (Brachypodium) genome sequence, to provide co-dominant markers in the Pch1 region. These were supplemented with previously developed sequence-tagged site (STS) markers and simple sequence repeat (SSR) markers. Markers were applied to a panel of varieties and to a BC(6) F(2) population, segregating between wheat and Ae. ventricosa over the distal portion of 7DL, to identify recombinants in the region of Pch1. By exploiting co-linearity between wheat chromosome 7D, Brachypodium chromosome 1, rice chromosome 6 and sorghum chromosome 10, Pch1 was located to an interval between the flanking markers Xwg7S and Xcos7-9. Furthermore candidate gene regions were identified in Brachypodium (364 Kb), rice (178 Kb) and sorghum (315 Kb) as a prelude to the map-based cloning of the gene. In addition, using homoeologue transferable markers, we obtained evidence that the eyespot resistances Pch1 and Pch2 on chromosomes 7D and 7A, respectively, are potentially homoeoloci. It is anticipated that the COS marker methodology could be used for the identification of recombinants in other introgressions into wheat from wild relatives. This would assist the mapping of genes of interest and the breaking of deleterious linkages to enable greater use of these introgressions in commercial varieties.

Callose synthase GSL7 is necessary for normal phloem transport and inflorescence growth in Arabidopsis

One isoform of callose synthase, Glucan Synthase-Like7 (GSL7), is tightly coexpressed with two isoforms of sucrose synthase (SUS5 and SUS6) known to be confined to phloem sieve elements in Arabidopsis (Arabidopsis thaliana). Investigation of the phenotype of gsl7 mutants of Arabidopsis revealed that the sieve plate pores of stems and roots lack the callose lining seen in wild-type plants. Callose synthesis in other tissues of the plant appears to be unaffected. Although gsl7 plants show only minor phenotypic alterations during vegetative growth, flowering stems are reduced in height and all floral parts are smaller than those of wild-type plants. Several lines of evidence suggest that the reduced growth of the inflorescence is a result of carbohydrate starvation. Levels of sucrose, hexoses, and starch are lower in the terminal bud clusters of gsl7 than in those of wild-type plants. Transcript levels of "starvation" genes expressed in response to low sugars are elevated in the terminal bud clusters of gsl7 plants, at the end of the night, and during an extended night. Pulse-chase experiments with (14)CO(2) show that transport of assimilate in the flowering stem is much slower in gsl7 mutants than in wild-type plants. We suggest that the callose lining of sieve plate pores is essential for normal phloem transport because it confers favorable flow characteristics on the pores.

Plant-eriophyoid mite interactions: cellular biochemistry and metabolic responses induced in mite-injured plants. Part I

This review is a comprehensive study of recent advances related to cytological, biochemical and physiological changes induced in plants in response to eriophyoid mite attack. It has been shown that responses of host plants to eriophyoids are variable. Most of the variability is due to individual eriophyoid mite-plant interactions. Usually, the direction and intensity of changes in eriophyoid-infested plant organs depend on mite genotype, density, or the feeding period, and are strongly differentiated relative to host plant species, cultivar, age and location. Although the mechanisms of changes elicited by eriophyoid mites within plants are not fully understood, in many cases the qualitative and quantitative biochemical status of mite-infested plants are known to affect the performance of consecutive herbivorous arthropods. In future, elucidation of the pathways from eriophyoid mite damage to plant gene activation will be necessary to clarify plant responses and to explain variation in plant tissue damage at the feeding and adjacent sites.

Analysis of the (1,3)-beta-D-glucan synthase gene family of barley

Callose consists mostly of (1,3)-beta-D-glucan and is synthesised in many plant tissues during growth and development, where it is believed to play a fundamental role in cell plate formation during cell division. Callose deposition also represents an important response to pathogen attack, wounding and to various abiotic stresses. Here, the transcription patterns of members of the callose synthase gene family from barley (Hordeum vulgare) were defined. Thus, fragments of six barley (1,3)-beta-D-glucan synthase-like (GSL) cDNAs were obtained by PCR amplification using primers designed to barley expressed sequence tag (EST) sequences. The HvGSL genes, designated HvGSL2 to HvGSL7, were mapped to individual loci that were distributed across the barley genome on chromosomes 3H, 4H, 6H and 7H. The HvGSL1 gene has been isolated and characterised previously. Transcript levels for all the genes were analysed by quantitative real-time PCR in a range of barley tissues and organs, at various developmental stages. High levels of transcript for many of the HvGSL genes were detected in endosperm during the early stages of grain development, when cellularisation of the endosperm was occurring and it is likely that many of the genes participate in this process. Transcripts of HvGSL1 and HvGSL5 mRNAs were significantly more abundant than other GSL mRNAs in the roots of young seedlings, while HvGSL7 mRNA was detected at relatively high levels along the length of two week old shoots.

Novel localization of callose in the spores of Physcomitrella patens and phylogenomics of the callose synthase gene family

BACKGROUND AND AIMS

Callose involvement in spore development is a plesiomorphic feature of land plants. Correlated light, fluorescence and immuno-electron microscopy was conducted on the developing spores of Physcomitrella patens to probe for callose. Using a bioinformatic approach, the callose synthase (PpCalS) genes were annotated and PpCalS and AtCalS gene families compared, testing the hypothesis that an exine development orthologue is present in P. patens based on deduced polypeptide similarity with AtCalS5, a known exine development gene.

METHODS

Spores were stained with aniline blue fluorescent dye. Capsules were prepared for immuno-light and immuno-electron microscopy by gold labelling callose epitopes with monoclonal antibody. BLAST searches were conducted using the AtCalS5 sequence as a query against the P. patens genome. Phylogenomic analysis of the CalS gene family was conducted using PAUP (v.4.1b10).

KEY RESULTS

Callose is briefly present in the aperture of developing P. patens spores. The PpCalS gene family consists of 12 copies that fall into three distinct clades with AtCalS genes. PpCalS5 is an orthologue to AtCalS5 with highly conserved domains and 64 % similarity of their deduced polypeptides.

CONCLUSIONS

This is the first study to identify the presence of callose in moss spores. AtCalS5 was previously shown to be involved in pollen exine development, thus making PpCalS5 a suspect gene involved in moss spore exine development.

Integrating genes and phenotype: a wheat-Arabidopsis-rice glycosyltransferase database for candidate gene analyses

Glycosyltransferases (GTs) constitute a very large multi-gene superfamily, containing several thousand members identified in sequenced organisms especially in plants. GTs are key enzymes involved in various biological processes such as cell wall formation, storage polysaccharides biosynthesis, and glycosylation of various metabolites. GTs have been identified in rice (Oryza sativa) and Arabidopsis thaliana, but their precise function has been demonstrated biochemically for only a few. In this work we have established a repertoire of virtually all the wheat (Triticum aestivum) GT sequences, using the large publicly available banks of expressed sequences. Based on sequence similarity with Arabidopsis and rice GTs compiled in the carbohydrate active enzyme database (CAZY), we have identified and classified these wheat sequences. The results were used to feed a searchable database available on the web ( http://wwwappli.nantes.inra.fr:8180/GTIDB ) that can be used for initiating an exhaustive candidate gene survey in wheat applied to a particular biological process. This is illustrated through the identification of GT families which are expressed during cell wall formation in wheat grain maturation.