Genetic and physiological analysis of the relationship between partial resistance to clubroot and tolerance to trehalose in Arabidopsis thaliana

Environmental conditions modulate the effect of epigenetic factors controlling the response of Arabidopsis thaliana to Plasmodiophora brassicae

The resistance of Arabidopsis thaliana to clubroot, a major disease of Brassicaceae caused by the obligate protist Plasmodiophora brassicae, is controlled in part by epigenetic factors. The detection of some of these epigenetic quantitative trait loci (QTLepi) has been shown to depend on experimental conditions. The aim of the present study was to assess whether and how temperature and/or soil water availability influenced both the detection and the extent of the effect of response QTLepi. The epigenetic recombinant inbred line (epiRIL) population, derived from the cross between ddm1-2 and Col-0 (partially resistant and susceptible to clubroot, respectively), was phenotyped for response to P. brassicae under four abiotic conditions including standard conditions, a 5°C temperature increase, drought, and flooding. The abiotic constraints tested had a significant impact on both the leaf growth of the epiRIL population and the outcome of the epiRIL-pathogen interaction. Linkage analysis led to the detection of a total of 31 QTLepi, 18 of which were specific to one abiotic condition and 13 common to at least two environments. EpiRIL showed significant plasticity under epigenetic control, which appeared to be specific to the traits evaluated and to the abiotic conditions. These results highlight that the environment can affect the epigenetic architecture of plant growth and immune responses and advance our understanding of the epigenetic factors underlying plasticity in response to climate change.

Two adjacent NLR genes conferring quantitative resistance to clubroot disease in Arabidopsis are regulated by a stably inherited epiallelic variation

Clubroot caused by the protist Plasmodiophora brassicae is a major disease affecting cultivated Brassicaceae. Using a combination of quantitative trait locus (QTL) fine mapping, CRISPR-Cas9 validation, and extensive analyses of DNA sequence and methylation patterns, we revealed that the two adjacent neighboring NLR (nucleotide-binding and leucine-rich repeat) genes AT5G47260 and AT5G47280 cooperate in controlling broad-spectrum quantitative partial resistance to the root pathogen P. brassicae in Arabidopsis and that they are epigenetically regulated. The variation in DNA methylation is not associated with any nucleotide variation or any transposable element presence/absence variants and is stably inherited. Variations in DNA methylation at the Pb-At5.2 QTL are widespread across Arabidopsis accessions and correlate negatively with variations in expression of the two genes. Our study demonstrates that natural, stable, and transgenerationally inherited epigenetic variations can play an important role in shaping resistance to plant pathogens by modulating the expression of immune receptors.

Natural variation in Arabidopsis responses to Plasmodiophora brassicae reveals an essential role for Resistance to Plasmodiophora brasssicae 1 (RPB1)

Despite the identification of clubroot resistance genes in various Brassica crops our understanding of the genetic basis of immunity to Plasmodiophora brassicae infection in the model plant Arabidopsis thaliana remains limited. To address this issue, we performed a screen of 142 natural accessions and identified 11 clubroot-resistant Arabidopsis lines. Genome-wide association analysis identified several genetic loci significantly linked with resistance. Three genes from two of these loci were targeted for deletion by CRISPR/Cas9 mutation in resistant accessions Est-1 and Uod-1. Deletion of Resistance to Plasmodiophora brassicae 1 (RPB1) rendered both lines susceptible to the P. brassicae pathotype P1+. Further analysis of rpb1 knock-out Est-1 and Uod-1 lines showed that the RPB1 protein is required for activation of downstream defence responses, such as the expression of phytoalexin biosynthesis gene CYP71A13. RPB1 has recently been shown to encode a cation channel localised in the endoplasmic reticulum. The clubroot susceptible Arabidopsis accession Col-0 lacks a functional RPB1 gene; when Col-0 is transformed with RPB1 expression driven by its native promoter it is capable of activating RPB1 transcription in response to infection, but this is not sufficient to confer resistance. Transient expression of RPB1 in Nicotiana tabacum induced programmed cell death in leaves. We conclude that RPB1 is a critical component of the defence response to P. brassicae infection in Arabidopsis, acting downstream of pathogen recognition but required for the elaboration of effective resistance.

The trehalose 6-phosphate phosphatase family in plants

Trehalose 6-phosphate (Tre6P), the intermediate of trehalose biosynthesis, is an essential signalling metabolite linking plant growth and development to carbon metabolism. While recent work has focused predominantly on the enzymes that produce Tre6P, little is known about the proteins that catalyse its degradation, the trehalose 6-phosphate phosphatases (TPPs). Often occurring in large protein families, TPPs exhibit cell-, tissue- and developmental stage-specific expression patterns, suggesting important regulatory functions in controlling local levels of Tre6P and trehalose as well as Tre6P signalling. Furthermore, growing evidence through gene expression studies and transgenic approaches shows that TPPs play an important role in integrating environmental signals with plant metabolism. This review highlights the large diversity of TPP isoforms in model and crop plants and identifies how modulating Tre6P metabolism in certain cell types, tissues, and at different developmental stages may promote stress tolerance, resilience and increased crop yield.

Sugar Transporters in Plasmodiophora brassicae: Genome-Wide Identification and Functional Verification

Plasmodiophora brassicae, an obligate intracellular pathogen, can hijack the host’s carbohydrates for survival. When the host plant is infected by P. brassicae, a large amount of soluble sugar accumulates in the roots, especially glucose, which probably facilitates the development of this pathogen. Although a complete glycolytic and tricarboxylic acid cycle (TCA) cycle existed in P. brassicae, very little information about the hexose transport system has been reported. In this study, we screened 17 putative sugar transporters based on information about their typical domains. The structure of these transporters showed a lot of variation compared with that of other organisms, especially the number of transmembrane helices (TMHs). Phylogenetic analysis indicated that these sugar transporters were far from the evolutionary relationship of other organisms and were unique in P. brassicae. The hexose transport activity assay indicated that eight transporters transported glucose or fructose and could restore the growth of yeast strain EBY.VW4000, which was deficient in hexose transport. The expression level of these glucose transporters was significantly upregulated at the late inoculation time when resting spores and galls were developing and a large amount of energy was needed. Our study provides new insights into the mechanism of P. brassicae survival in host cells by hijacking and utilizing the carbohydrates of the host.

Multiple Metabolic Phenotypes as Screening Criteria Are Correlated With the Plant Growth-Promoting Ability of Rhizobacterial Isolates

Efficient screening method is the prerequisite for getting plant growth-promoting (PGP) rhizobacteria (PGPR) which may play an important role in sustainable agriculture from the natural environment. Many current traditional preliminary screening criteria based on knowledge of PGP mechanisms do not always work well due to complex plant-microbe interactions and may lead to the low screening efficiency. More new screening criteria should be evaluated to establish a more effective screening system. However, the studies focused on this issue were not enough, and few new screening criteria had been proposed. The aim of this study was to analyze the correlation between the metabolic phenotypes of rhizobacterial isolates and their PGP ability. The feasibility of using these phenotypes as preliminary screening criteria for PGPR was also evaluated. Twenty-one rhizobacterial isolates were screened for their PGP ability, traditional PGP traits, and multiple metabolic phenotypes that are not directly related to PGP mechanisms, but are possibly related to rhizosphere colonization. Correlations between the PGP traits or metabolic phenotypes and increases in plant agronomic parameters were analyzed to find the indicators that are most closely related to PGP ability. The utilization of 11 nutrient substrates commonly found in root exudates, such as D-salicin, β-methyl-D-glucoside, and D-cellobiose, was significantly positively correlated with the PGP ability of the rhizobacterial isolates. The utilization of one amino acid and two organic acids, namely L-aspartic acid, α-keto-glutaric acid, and formic acid, was negatively correlated with PGP ability. There were no significant correlations between four PGP traits tested in this study and the PGP ability. The ability of rhizobacterial isolates to metabolize nutrient substrates that are identical or similar to root exudate components may act as better criteria than PGP traits for the primary screening of PGPR, because rhizosphere colonization is a prerequisite for PGPR to affect plants.

Soil microbiota influences clubroot disease by modulating Plasmodiophora brassicae and Brassica napus transcriptomes

The contribution of surrounding plant microbiota to disease development has led to the 'pathobiome' concept, which represents the interaction between the pathogen, the host plant and the associated biotic microbial community, resulting or not in plant disease. The aim herein is to understand how the soil microbial environment may influence the functions of a pathogen and its pathogenesis, and the molecular response of the plant to the infection, with a dual-RNAseq transcriptomics approach. We address this question using Brassica napus and Plasmodiophora brassicae, the pathogen responsible for clubroot. A time-course experiment was conducted to study interactions between P. brassicae, two B. napus genotypes and three soils harbouring high, medium or low microbiota diversities and levels of richness. The soil microbial diversity levels had an impact on disease development (symptom levels and pathogen quantity). The P. brassicae and B. napus transcriptional patterns were modulated by these microbial diversities, these modulations being dependent on the host genotype plant and the kinetic time. The functional analysis of gene expressions allowed the identification of pathogen and plant host functions potentially involved in the change of plant disease level, such as pathogenicity-related genes (NUDIX effector) in P. brassicae and plant defence-related genes (glucosinolate metabolism) in B. napus.

Resolution of quantitative resistance to clubroot into QTL-specific metabolic modules

Plant disease resistance is often under quantitative genetic control. Thus, in a given interaction, plant cellular responses to infection are influenced by resistance or susceptibility alleles at different loci. In this study, a genetic linkage analysis was used to address the complexity of the metabolic responses of Brassica napus roots to infection by Plasmodiophora brassicae. Metabolome profiling and pathogen quantification in a segregating progeny allowed a comparative mapping of quantitative trait loci (QTLs) involved in resistance and in metabolic adjustments. Distinct metabolic modules were associated with each resistance QTL, suggesting the involvement of different underlying cellular mechanisms. This approach highlighted the possible role of gluconasturtiin and two unknown metabolites in the resistance conferred by two QTLs on chromosomes C03 and C09, respectively. Only two susceptibility biomarkers (glycine and glutathione) were simultaneously linked to the three main resistance QTLs, suggesting the central role of these compounds in the interaction. By contrast, several genotype-specific metabolic responses to infection were genetically unconnected to resistance or susceptibility. Likewise, variations of root sugar profiles, which might have influenced pathogen nutrition, were not found to be related to resistance QTLs. This work illustrates how genetic metabolomics can help to understand plant stress responses and their possible links with disease.

Genius Architect or Clever Thief-How Plasmodiophora brassicae Reprograms Host Development to Establish a Pathogen-Oriented Physiological Sink

When plants are infected by Plasmodiophora brassicae, their developmental programs are subjected to extensive changes and the resultant clubroot disease is associated with formation of large galls on underground tissue. The pathogen's need to build an efficient feeding site as the disease progresses drives these changes, ensuring successful production of resting spores. This developmental reprogramming is an outcome of interactions between the pathogen and the infected host. During disease progression, we can observe alteration of growth regulator dynamics, patterns of cell proliferation and differentiation, increased cell expansion, and eventual cell wall degradation as well as the redirection of nutrients toward the pathogen. Recently, detailed studies of anatomical changes occurring during infection and studies profiling transcriptional responses have come together to provide a clearer understanding of the sequence of events and processes underlying clubroot disease. Additionally, genome sequencing projects have revealed P. brassicae's potential for the production of signaling molecules and effectors as well as its requirements and capacities with respect to taking up host nutrients. Integration of these new findings together with physiological studies can significantly advance our understanding of how P. brassicae brings about reprogramming of host development. This article summarizes the current state of knowledge on cellular changes induced by P. brassicae infection and aims to explain their impact and importance for both the host and the pathogen.

Trehalose phosphate synthase 5-dependent trehalose metabolism modulates basal defense responses in Arabidopsis thaliana

Despite the recent discovery that trehalose synthesis is important for plant development and abiotic stress tolerance, the effects of trehalose on biotic stress responses remain relatively unknown. In this study, we demonstrate that TREHALOSE PHOSPHATE SYNTHASE 5 (TPS5)-dependent trehalose metabolism regulates Arabidopsis thaliana defenses against pathogens (necrotrophic Botrytis cinerea and biotrophic Pseudomonas syringae). Pathogen infection increased trehalose levels and upregulated TPS5 expression. Application of exogenous trehalose significantly improved plant defenses against B. cinerea, but increased the susceptibility of plants to P. syringae. We demonstrate that elevated trehalose biosynthesis, in transgenic plants over-expressing TPS5, also increased the susceptibility to P. syringae, but decreased the disease symptoms caused by B. cinerea. The knockout of TPS5 prevented the accumulation of trehalose and enhanced defense responses against P. syringae. Additionally, we observed that a TPS5-interacting protein (multiprotein bridging factor 1c) was required for induced expression of TPS5 during pathogen infections. Furthermore, we show that trehalose promotes P. syringae growth and disease development, via a mechanism involving suppression of the plant defense gene, Pathogenesis-Related Protein 1. These findings provide insight into the function of TPS5-dependent trehalose metabolism in plant basal defense responses.

Quantitative resistance to clubroot infection mediated by transgenerational epigenetic variation in Arabidopsis

Quantitative disease resistance, often influenced by environmental factors, is thought to be the result of DNA sequence variants segregating at multiple loci. However, heritable differences in DNA methylation, so-called transgenerational epigenetic variants, also could contribute to quantitative traits. Here, we tested this possibility using the well-characterized quantitative resistance of Arabidopsis to clubroot, a Brassica major disease caused by Plasmodiophora brassicae. For that, we used the epigenetic recombinant inbred lines (epiRIL) derived from the cross ddm1-2 × Col-0, which show extensive epigenetic variation but limited DNA sequence variation. Quantitative loci under epigenetic control (QTL^epi ) mapping was carried out on 123 epiRIL infected with P. brassicae and using various disease-related traits. EpiRIL displayed a wide range of continuous phenotypic responses. Twenty QTL^epi were detected across the five chromosomes, with a bona fide epigenetic origin for 16 of them. The effect of five QTL^epi was dependent on temperature conditions. Six QTL^epi co-localized with previously identified clubroot resistance genes and QTL in Arabidopsis. Co-localization of clubroot resistance QTL^epi with previously detected DNA-based QTL reveals a complex model in which a combination of allelic and epiallelic variations interacts with the environment to lead to variation in clubroot quantitative resistance.

Computational analysis of the Plasmodiophora brassicae genome: mitochondrial sequence description and metabolic pathway database design

Plasmodiophora brassicae is an obligate biotrophic pathogenic protist responsible for clubroot, a root gall disease of Brassicaceae species. In addition to the reference genome of the P. brassicae European e3 isolate and the draft genomes of Canadian or Chinese isolates, we present the genome of eH, a second European isolate. Refinement of the annotation of the eH genome led to the identification of the mitochondrial genome sequence, which was found to be bigger than that of Spongospora subterranea, another plant parasitic Plasmodiophorid phylogenetically related to P. brassicae. New pathways were also predicted, such as those for the synthesis of spermidine, a polyamine up-regulated in clubbed regions of roots. A P. brassicae pathway genome database was created to facilitate the functional study of metabolic pathways in transcriptomics approaches. These available tools can help in our understanding of the regulation of P. brassicae metabolism during infection and in response to diverse constraints.

Comparative Transcriptome Analysis between Broccoli (Brassica oleracea var. italica) and Wild Cabbage (Brassica macrocarpa Guss.) in Response to Plasmodiophora brassicae during Different Infection Stages

Clubroot, one of the most devastating diseases to the Brassicaceae family, is caused by the obligate biotrophic pathogen Plasmodiophora brassicae. However, studies of the molecular basis of disease resistance are still poor especially in quantitative resistance. In the present paper, two previously identified genotypes, a clubroot-resistant genotype (wild cabbage, B2013) and a clubroot-susceptible genotype (broccoli, 90196) were inoculated by P. brassicae for 0 (T0), 7 (T7), and 14 (T14) day after inoculation (DAI). Gene expression pattern analysis suggested that response changes in transcript level of two genotypes under P. brassicae infection were mainly activated at the primary stage (T7). Based on the results of DEGs functional enrichments from two infection stages, genes associated with cell wall biosynthesis, glucosinolate biosynthesis, and plant hormone signal transduction showed down-regulated at T14 compared to T7, indicating that defense responses to P. brassicae were induced earlier, and related pathways were repressed at T14. In addition, the genes related to NBS-LRR proteins, SA signal transduction, cell wall and phytoalexins biosynthesis, chitinase, Ca2+ signals and RBOH proteins were mainly up-regulated in B2013 by comparing those of 90196, indicating the pathways of response defense to clubroot were activated in the resistant genotype. This is the first report about comparative transcriptome analysis for broccoli and its wild relative during the different stages of P. brassicae infection and the results should be useful for molecular assisted screening and breeding of clubroot-resistant genotypes.

Comparative Transcriptome Analysis between Broccoli (Brassica oleracea var. italica) and Wild Cabbage (Brassica macrocarpa Guss.) in Response to Plasmodiophora brassicae during Different Infection Stages

Clubroot, one of the most devastating diseases to the Brassicaceae family, is caused by the obligate biotrophic pathogen Plasmodiophora brassicae. However, studies of the molecular basis of disease resistance are still poor especially in quantitative resistance. In the present paper, two previously identified genotypes, a clubroot-resistant genotype (wild cabbage, B2013) and a clubroot-susceptible genotype (broccoli, 90196) were inoculated by P. brassicae for 0 (T0), 7 (T7), and 14 (T14) day after inoculation (DAI). Gene expression pattern analysis suggested that response changes in transcript level of two genotypes under P. brassicae infection were mainly activated at the primary stage (T7). Based on the results of DEGs functional enrichments from two infection stages, genes associated with cell wall biosynthesis, glucosinolate biosynthesis, and plant hormone signal transduction showed down-regulated at T14 compared to T7, indicating that defense responses to P. brassicae were induced earlier, and related pathways were repressed at T14. In addition, the genes related to NBS-LRR proteins, SA signal transduction, cell wall and phytoalexins biosynthesis, chitinase, Ca²⁺ signals and RBOH proteins were mainly up-regulated in B2013 by comparing those of 90196, indicating the pathways of response defense to clubroot were activated in the resistant genotype. This is the first report about comparative transcriptome analysis for broccoli and its wild relative during the different stages of P. brassicae infection and the results should be useful for molecular assisted screening and breeding of clubroot-resistant genotypes.

Virus-Induced Gene Silencing-Based Functional Analyses Revealed the Involvement of Several Putative Trehalose-6-Phosphate Synthase/Phosphatase Genes in Disease Resistance against Botrytis cinerea and Pseudomonas syringae pv. tomato DC3000 in Tomato

Trehalose and its metabolism have been demonstrated to play important roles in control of plant growth, development, and stress responses. However, direct genetic evidence supporting the functions of trehalose and its metabolism in defense response against pathogens is lacking. In the present study, genome-wide characterization of putative trehalose-related genes identified 11 SlTPSs for trehalose-6-phosphate synthase, 8 SlTPPs for trehalose-6-phosphate phosphatase and one SlTRE1 for trehalase in tomato genome. Nine SlTPSs, 4 SlTPPs, and SlTRE1 were selected for functional analyses to explore their involvement in tomato disease resistance. Some selected SlTPSs, SlTPPs, and SlTRE1 responded with distinct expression induction patterns to Botrytis cinerea and Pseudomonas syringae pv. tomato (Pst) DC3000 as well as to defense signaling hormones (e.g., salicylic acid, jasmonic acid, and a precursor of ethylene). Virus-induced gene silencing-mediated silencing of SlTPS3, SlTPS4, or SlTPS7 led to deregulation of ROS accumulation and attenuated the expression of defense-related genes upon pathogen infection and thus deteriorated the resistance against B. cinerea or Pst DC3000. By contrast, silencing of SlTPS5 or SlTPP2 led to an increased expression of the defense-related genes upon pathogen infection and conferred an increased resistance against Pst DC3000. Silencing of SlTPS3, SlTPS4, SlTPS5, SlTPS7, or SlTPP2 affected trehalose level in tomato plants with or without infection of B. cinerea or Pst DC3000. These results demonstrate that SlTPS3, SlTPS4, SlTPS5, SlTPS7, and SlTPP2 play roles in resistance against B. cinerea and Pst DC3000, implying the importance of trehalose and tis metabolism in regulation of defense response against pathogens in tomato.

The dual nature of trehalose in citrus canker disease: a virulence factor for Xanthomonas citri subsp. citri and a trigger for plant defence responses

Xanthomonas citri subsp. citri (Xcc) is a bacterial pathogen that causes citrus canker in susceptible Citrus spp. The Xcc genome contains genes encoding enzymes from three separate pathways of trehalose biosynthesis. Expression of genes encoding trehalose-6-phosphate synthase (otsA) and trehalose phosphatase (otsB) was highly induced during canker development, suggesting that the two-step pathway of trehalose biosynthesis via trehalose-6-phosphate has a function in pathogenesis. This pathway was eliminated from the bacterium by deletion of the otsA gene. The resulting XccΔotsA mutant produced less trehalose than the wild-type strain, was less resistant to salt and oxidative stresses, and was less able to colonize plant tissues. Gene expression and proteomic analyses of infected leaves showed that infection with XccΔotsA triggered only weak defence responses in the plant compared with infection with Xcc, and had less impact on the host plant's metabolism than the wild-type strain. These results suggested that trehalose of bacterial origin, synthesized via the otsA-otsB pathway, in Xcc, plays a role in modifying the host plant's metabolism to its own advantage but is also perceived by the plant as a sign of pathogen attack. Thus, trehalose biosynthesis has both positive and negative consequences for Xcc. On the one hand, it enables this bacterial pathogen to survive in the inhospitable environment of the leaf surface before infection and exploit the host plant's resources after infection, but on the other hand, it is a tell-tale sign of the pathogen's presence that triggers the plant to defend itself against infection.

Camalexin contributes to the partial resistance of Arabidopsis thaliana to the biotrophic soilborne protist Plasmodiophora brassicae

Camalexin has been reported to play defensive functions against several pathogens in Arabidopsis. In this study, we investigated the possible role of camalexin accumulation in two Arabidopsis genotypes with different levels of basal resistance to the compatible eH strain of the clubroot agent Plasmodiophora brassicae. Camalexin biosynthesis was induced in infected roots of both Col-0 (susceptible) and Bur-0 (partially resistant) accessions during the secondary phase of infection. However, the level of accumulation was four-to-seven times higher in Bur-0 than Col-0. This was associated with the enhanced transcription of a set of camalexin biosynthetic P450 genes in Bur-0: CYP71A13, CYP71A12, and CYP79B2. This induction correlated with slower P. brassicae growth in Bur-0 compared to Col-0, thus suggesting a relationship between the levels of camalexin biosynthesis and the different levels of resistance. Clubroot-triggered biosynthesis of camalexin may also participate in basal defense in Col-0, as gall symptoms and pathogen development were enhanced in the pad3 mutant (Col-0 genetic background), which is defective in camalexin biosynthesis. Clubroot and camalexin responses were then studied in Heterogeneous Inbred Families (HIF) lines derived from a cross between Bur-0 and Col-0. The Bur/Col allelic substitution in the region of the previously identified clubroot resistance QTL PbAt5.2 (Chromosome 5) was associated with both the enhanced clubroot-triggered induction of camalexin biosynthesis and the reduced P. brassicae development. Altogether, our results suggest that high levels of clubroot-triggered camalexin biosynthesis play a role in the quantitative control of partial resistance of Arabidopsis to clubroot.

Fine mapping of Rcr1 and analyses of its effect on transcriptome patterns during infection by Plasmodiophora brassicae

BACKGROUND

The protist Plasmodiophora brassicae is a biotrophic soil-borne pathogen that causes clubroot on Brassica crops worldwide. Clubroot disease is a serious threat to the 8 M ha of canola (Brassica napus) grown annually in western Canada. While host resistance is the key to clubroot management, sources of resistance are limited.

RESULTS

To identify new sources of clubroot resistance (CR), we fine mapped a CR gene (Rcr1) from B. rapa ssp. chinensis to the region between 24.26 Mb and 24.50 Mb on the linkage group A03, with several closely linked markers identified. Transcriptome analysis was conducted using RNA sequencing on a segregating F1 population inoculated with P. brassicae, with 2,212 differentially expressed genes (DEGs) identified between plants carrying and not carrying Rcr1. Functional annotation of these DEGs showed that several defense-related biological processes, including signaling and metabolism of jasmonate and ethylene, defensive deposition of callose and biosynthesis of indole-containing compounds, were up-regulated significantly in plants carrying Rcr1 while genes involved in salicylic acid metabolic and signaling pathways were generally not elevated. Several DEGs involved in metabolism potentially related to clubroot symptom development, including auxin biosynthesis and cell growth/development, showed significantly lower expression in plants carrying Rcr1.

CONCLUSION

The CR gene Rcr1 and closely linked markers will be highly useful for breeding new resistant canola cultivars. The identification of DEGs between inoculated plants carrying and not carrying Rcr1 is an important step towards understanding of specific metabolic/signaling pathways in clubroot resistance mediated by Rcr1. This information may help judicious use of CR genes with complementary resistance mechanisms for durable clubroot resistance.

Trehalose metabolism in plants

Trehalose is a quantitatively important compatible solute and stress protectant in many organisms, including green algae and primitive plants. These functions have largely been replaced by sucrose in vascular plants, and trehalose metabolism has taken on new roles. Trehalose is a potential signal metabolite in plant interactions with pathogenic or symbiotic micro-organisms and herbivorous insects. It is also implicated in responses to cold and salinity, and in regulation of stomatal conductance and water-use efficiency. In plants, as in other eukaryotes and many prokaryotes, trehalose is synthesized via a phosphorylated intermediate, trehalose 6-phosphate (Tre6P). A meta-analysis revealed that the levels of Tre6P change in parallel with sucrose, which is the major product of photosynthesis and the main transport sugar in plants. We propose the existence of a bi-directional network, in which Tre6P is a signal of sucrose availability and acts to maintain sucrose concentrations within an appropriate range. Tre6P influences the relative amounts of sucrose and starch that accumulate in leaves during the day, and regulates the rate of starch degradation at night to match the demand for sucrose. Mutants in Tre6P metabolism have highly pleiotropic phenotypes, showing defects in embryogenesis, leaf growth, flowering, inflorescence branching and seed set. It has been proposed that Tre6P influences plant growth and development via inhibition of the SNF1-related protein kinase (SnRK1). However, current models conflict with some experimental data, and do not completely explain the pleiotropic phenotypes exhibited by mutants in Tre6P metabolism. Additional explanations for the diverse effects of alterations in Tre6P metabolism are discussed.

Evolution of the biochemistry of the photorespiratory C2 cycle

Oxygenic photosynthesis would not be possible without photorespiration in the present day O2 -rich atmosphere. It is now generally accepted that cyanobacteria-like prokaryotes first evolved oxygenic photosynthesis, which was later conveyed via endosymbiosis into a eukaryotic host, which then gave rise to the different groups of algae and streptophytes. For photosynthetic CO2 fixation, all these organisms use RubisCO, which catalyses both the carboxylation and the oxygenation of ribulose 1,5-bisphosphate. One of the reaction products of the oxygenase reaction, 2-phosphoglycolate (2PG), represents the starting point of the photorespiratory C2 cycle, which is considered largely responsible for recapturing organic carbon via conversion to the Calvin-Benson cycle (CBC) intermediate 3-phosphoglycerate, thereby detoxifying critical intermediates. Here we discuss possible scenarios for the evolution of this process toward the well-defined 2PG metabolism in extant plants. While the origin of the C2 cycle core enzymes can be clearly dated back towards the different endosymbiotic events, the evolutionary scenario that allowed the compartmentalised high flux photorespiratory cycle is uncertain, but probably occurred early during the algal radiation. The change in atmospheric CO2 /O2 ratios promoting the acquisition of different modes for inorganic carbon concentration mechanisms, as well as the evolutionary specialisation of peroxisomes, clearly had a dramatic impact on further aspects of land plant photorespiration.

A fluorometric assay for trehalose in the picomole range

BACKGROUND

Trehalose is a non-reducing disaccharide that is used as an osmolyte, transport sugar, carbon reserve and stress protectant in a wide range of organisms. In plants, trehalose 6-phosphate (Tre6P), the intermediate of trehalose biosynthesis, is thought to be a signal of sucrose status. Trehalose itself may play a role in pathogenic and symbiotic plant-microbe interactions, in responses to abiotic stress and in developmental signalling, but its precise functions are unknown. A major obstacle to investigating its function is the technical difficulty of measuring the very low levels of trehalose usually found in plant tissues, as most of the established trehalose assays lack sufficient specificity and/or sensitivity.

RESULTS

A kinetic assay for trehalose was established using recombinant Escherichia coli cytoplasmic trehalase (treF), which was shown to be highly specific for trehalose. Hydrolysis of trehalose to glucose is monitored fluorometrically and the trehalose content of the tissue extract is determined from an internal calibration curve. The assay is linear for 0.2-40 pmol trehalose, and recoveries of trehalose were ≥88%. A. thaliana Col-0 rosettes contain about 20-30 nmol g-1FW of trehalose, increasing to about 50-60 nmol g-1FW in plants grown at 8°C. Trehalose is not correlated with sucrose content, whereas a strong correlation between Tre6P and sucrose was confirmed. The trehalose contents of ear inflorescence primordia from the maize ramosa3 mutant and wild type plants were 6.6±2.6 nmol g-1FW and 19.0±12.7 nmol g-1FW, respectively. The trehalose:Tre6P ratios in the ramosa3 and wild-type primordia were 2.43±0.85 and 6.16±3.45, respectively.

CONCLUSION

The fluorometric assay is highly specific for trehalose and sensitive enough to measure the trehalose content of very small amounts of plant tissue. Chilling induced a 2-fold accumulation of trehalose in A. thaliana rosettes, but the levels were too low to make a substantial quantitative contribution to osmoregulation. Trehalose is unlikely to function as a signal of sucrose status. The abnormal inflorescence branching phenotype of the maize ramosa3 mutant might be linked to a decrease in trehalose levels in the inflorescence primordia or a downward shift in the trehalose:Tre6P ratio.

Partial resistance to clubroot in Arabidopsis is based on changes in the host primary metabolism and targeted cell division and expansion capacity

To date, studies of the molecular basis of disease resistance mainly focused on qualitative resistance. However, deciphering mechanisms underlying quantitative resistance could lead to insights into the relationship between qualitative and quantitative resistance and guide the utilization of these two types of resistance to produce durably resistant cultivars. A functional genomics approach, using the CATMA whole-genome microarray, was used to detect changes in gene expression associated with partial quantitative resistance in the Arabidopsis thaliana-Plasmodiophora brassicae pathosystem. The time course of transcript abundance during partial clubroot resistance response was monitored at the whole plant level, and direct comparisons between partial resistance and susceptibility responses were made using the same host genotype. An increasingly complex host response was revealed, as was the differential influence of P. brassicae infection on the transcription of Arabidopsis genes according to the isolate used. We observed, at the transcriptomic level, that metabolic diversion by the pathogen was reduced or delayed, classical plant defense responses were induced earlier and/or more strongly, and cell enlargement and proliferation were actively inhibited in the partial quantitative resistance response compared to the susceptible one.

Aphid-induced accumulation of trehalose in Arabidopsis thaliana is systemic and dependent upon aphid density

Trehalose is a disaccharide sugar that is now considered to be widely distributed among higher plants. Trehalose has been attributed a number of roles, including control of basic plant processes, such as photosynthesis, and conferring tolerance to abiotic stresses, such as desiccation and high salinity. Trehalose is also a common storage sugar used by insects. In this study, we used laboratory investigations to examine various aspects of trehalose dynamics in an aphid-host plant system (Arabidopsis and the peach potato aphid, Myzus persicae). Trehalose concentrations were measured by [1-H]-NMR. Myzus persicae reared on Arabidopsis, but not on black mustard or spring cabbage, contained considerable quantities of trehalose (5 % w/w dry matter). In Arabidopsis foliage, feeding by aphids induced a density-dependent accumulation of trehalose up to 5 mg g(-1) dry weight. Leaves that were not challenged directly by aphids also exhibited increased trehalose concentrations, indicating that this accumulation was systemic. Trehalose was measured at high concentrations in the phloem sap of plants challenged by aphids, suggesting that aphid feeding induced the plant to produce significant quantities of trehalose, which moved through the plant and into the aphids via the phloem sap. Trehalose was also excreted in the aphid honeydew. Further work is required to clarify whether this trehalose accumulation in Arabidopsis has a direct role or a signalling function in plant tolerance of, or resistance to, aphid feeding, and if a similar accumulation of this sugar occurs when other species or genotypes of aphids are reared on this host plant.

Trehalose biosynthesis promotes Pseudomonas aeruginosa pathogenicity in plants

Pseudomonas aeruginosa strain PA14 is a multi-host pathogen that infects plants, nematodes, insects, and vertebrates. Many PA14 factors are required for virulence in more than one of these hosts. Noting that plants have a fundamentally different cellular architecture from animals, we sought to identify PA14 factors that are specifically required for plant pathogenesis. We show that synthesis by PA14 of the disaccharide trehalose is required for pathogenesis in Arabidopsis, but not in nematodes, insects, or mice. In-frame deletion of two closely-linked predicted trehalose biosynthetic operons, treYZ and treS, decreased growth in Arabidopsis leaves about 50 fold. Exogenously co-inoculated trehalose, ammonium, or nitrate, but not glucose, sulfate, or phosphate suppressed the phenotype of the double ΔtreYZΔtreS mutant. Exogenous trehalose or ammonium nitrate does not suppress the growth defect of the double ΔtreYZΔtreS mutant by suppressing the plant defense response. Trehalose also does not function intracellularly in P. aeruginosa to ameliorate a variety of stresses, but most likely functions extracellularly, because wild-type PA14 rescued the in vivo growth defect of the ΔtreYZΔtreS in trans. Surprisingly, the growth defect of the double ΔtreYZΔtreS double mutant was suppressed by various Arabidopsis cell wall mutants that affect xyloglucan synthesis, including an xxt1xxt2 double mutant that completely lacks xyloglucan, even though xyloglucan mutants are not more susceptible to pathogens and respond like wild-type plants to immune elicitors. An explanation of our data is that trehalose functions to promote the acquisition of nitrogen-containing nutrients in a process that involves the xyloglucan component of the plant cell wall, thereby allowing P. aeruginosa to replicate in the intercellular spaces in a leaf. This work shows how P. aeruginosa, a multi-host opportunistic pathogen, has repurposed a highly conserved "house-keeping" anabolic pathway (trehalose biosynthesis) as a potent virulence factor that allows it to replicate in the intercellular environment of a leaf.

How do sugars regulate plant growth and development? New insight into the role of trehalose-6-phosphate

Plant growth and development are tightly controlled in response to environmental conditions that influence the availability of photosynthetic carbon in the form of sucrose. Trehalose-6-phosphate (T6P), the precursor of trehalose in the biosynthetic pathway, is an important signaling metabolite that is involved in the regulation of plant growth and development in response to carbon availability. In addition to the plant's own pathway for trehalose synthesis, formation of T6P or trehalose by pathogens can result in the reprogramming of plant metabolism and development. Developmental processes that are regulated by T6P range from embryo development to leaf senescence. Some of these processes are regulated in interaction with phytohormones, such as auxin. A key interacting factor of T6P signaling in response to the environment is the protein kinase sucrose non-fermenting related kinase-1 (SnRK1), whose catalytic activity is inhibited by T6P. SnRK1 is most likely involved in the adjustment of metabolism and growth in response to starvation. The transcription factor bZIP11 has recently been identified as a new player in the T6P/SnRK1 regulatory pathway. By inhibiting SnRK1, T6P promotes biosynthetic reactions. This regulation has important consequences for crop production, for example, in the developing wheat grain and during the growth of potato tubers.

Metabotyping: a new approach to investigate rapeseed (Brassica napus L.) genetic diversity in the metabolic response to clubroot infection

Clubroot disease affects all Brassicaceae spp. and is caused by the obligate biotroph pathogen Plasmodiophora brassicae. The development of galls on the root system is associated with the establishment of a new carbon metabolic sink. Here, we aimed to deepen our knowledge of the involvement of primary metabolism in the Brassica napus response to clubroot infection. We studied the dynamics and the diversity of the metabolic responses to the infection. Root system metabotyping was carried out for 18 rapeseed genotypes displaying different degrees of symptom severity, under inoculated and noninoculated conditions at 42 days postinoculation (dpi). Clubroot susceptibility was positively correlated with clubroot-induced accumulation of several amino acids. Although glucose and fructose accumulated in some genotypes with minor symptoms, their levels were negatively correlated to the disease index across the whole set of genotypes. The dynamics of the metabolic response were studied for the susceptible genotype 'Yudal,' which allowed an "early" metabolic response (established from 14 to 28 dpi) to be differentiated from a "late" response (from 35 dpi). We discuss the early accumulation of amino acids in the context of the establishment of a nitrogen metabolic sink and the hypothetical biological role of the accumulation of glutathione and S-methylcysteine.

Trehalose metabolism is activated upon chilling in grapevine and might participate in Burkholderia phytofirmans induced chilling tolerance

During the last decade, there has been growing interest in the role of trehalose metabolism in tolerance to abiotic stress in higher plants, especially cold stress. So far, this metabolism has not yet been studied in Vitis vinifera L., despite the economic importance of this crop. The goal of this paper was to investigate the involvement of trehalose metabolism in the response of grapevine to chilling stress, and to compare the response in plants bacterised with Burkholderia phytofirmans strain PsJN, a plant growth-promoting rhizobacterium that confers grapevine chilling tolerance, with mock-inoculated plants. In silico analysis revealed that the V. vinifera L. genome contains genes encoding the enzymes responsible for trehalose synthesis and degradation. Transcript analysis showed that these genes were differentially expressed in various plant organs, and we also characterised their response to chilling. Both trehalose and trehalose 6-phosphate (T6P) were present in grapevine tissues and showed a distinct pattern of accumulation upon chilling. Our results suggest a role for T6P as the main active molecule in the metabolism upon chilling, with a possible link with sucrose metabolism. Furthermore, plants colonised by B. phytofirmans and cultivated at 26°C accumulated T6P and trehalose in stems and leaves at concentrations similar to non-bacterised plants exposed to chilling temperatures for 1 day. Overall, our data suggest that T6P and trehalose accumulate upon chilling stress in grapevine and might participate in the resistance to chilling stress conferred by B. phytofirmans.

Arginase induction represses gall development during clubroot infection in Arabidopsis

Arginase induction can play a defensive role through the reduction of arginine availability for phytophageous insects. Arginase activity is also induced during gall growth caused by Plasmodiophora brassicae infection in roots of Arabidopsis thaliana; however, its possible role in this context has been unclear. We report here that the mutation of the arginase-encoding gene ARGAH2 abrogates clubroot-induced arginase activity and results in enhanced gall size in infected roots, suggesting that arginase plays a defensive role. Induction of arginase activity in infected roots was impaired in the jar1 mutant, highlighting a link between the arginase response to clubroot and jasmonate signaling. Clubroot-induced accumulation of the principal amino acids in galls was not affected by the argah2 mutation. Because ARGAH2 was previously reported to control auxin response, we investigated the role of ARGAH2 in callus induction. ARGAH2 was found to be highly induced in auxin/cytokinin-triggered aseptic plant calli, and callus development was enhanced in argah2 in the absence of the pathogen. We hypothesized that arginase contributes to a negative control over clubroot symptoms, by reducing hormone-triggered cellular proliferation.