Signaling in plant disease resistance and symbiosis

Uncovering the Mechanisms: The Role of Biotrophic Fungi in Activating or Suppressing Plant Defense Responses

This paper discusses the mechanisms by which fungi manipulate plant physiology and suppress plant defense responses by producing effectors that can target various host proteins. Effector-triggered immunity and effector-triggered susceptibility are pivotal elements in the complex molecular dialogue underlying plant-pathogen interactions. Pathogen-produced effector molecules possess the ability to mimic pathogen-associated molecular patterns or hinder the binding of pattern recognition receptors. Effectors can directly target nucleotide-binding domain, leucine-rich repeat receptors, or manipulate downstream signaling components to suppress plant defense. Interactions between these effectors and receptor-like kinases in host plants are critical in this process. Biotrophic fungi adeptly exploit the signaling networks of key plant hormones, including salicylic acid, jasmonic acid, abscisic acid, and ethylene, to establish a compatible interaction with their plant hosts. Overall, the paper highlights the importance of understanding the complex interplay between plant defense mechanisms and fungal effectors to develop effective strategies for plant disease management.

Quantitative Proteomics at Early Stages of the Symbiotic Interaction Between Oryza sativa and Nostoc punctiforme Reveals Novel Proteins Involved in the Symbiotic Crosstalk

Symbiosis between cyanobacteria and plants is considered pivotal for biological nitrogen deposition in terrestrial ecosystems. Despite extensive knowledge of the ecology of plant-cyanobacterium symbioses, little is known about the molecular mechanisms involved in recognition between partners. Here, we conducted a quantitative sequential window acquisition of all theoretical fragment ion spectra mass spectrometry pipeline to analyze protein changes in Oryza sativa and Nostoc punctiforme during early events of symbiosis. We found differentially expressed proteins in both organisms linked to several biological functions, including signal transduction, adhesion, defense-related proteins and cell wall modification. In N. punctiforme we found increased expression of 62 proteins that have been previously described in other Nostoc-plant symbioses, reinforcing the robustness of our study. Our findings reveal new proteins activated in the early stages of the Nostoc-Oryza symbiosis that might be important for the recognition between the plant and the host. Oryza mutants in genes in the common symbiosis signaling pathway (CSSP) show reduced colonization efficiency, providing first insights on the involvement of the CSSP for the accommodation of N. punctiforme inside the plant cells. This information may have long-term implications for a greater understanding of the symbiotic interaction between Nostoc and land plants.

Metabolomic effects of the colonization of Medicago truncatula by the facultative endophyte Arthrobacter agilis UMCV2 in a foliar inoculation system

Biofertilizer production and application for sustainable agriculture is already a reality. The methods for biofertilizers delivery in crop fields are diverse. Although foliar spray is gaining wide acceptance, little is known about the influence that the biochemical features of leaves have on the microbial colonization. Arthrobacter agilis UMCV2 is a rhizospheric and endophytic bacteria that promotes plant growth and health. In this study, we determined the capacity of the UMCV2 strain to colonize different leaves from Medicago truncatula in a foliar inoculation system. By using two powerful analytical methods based on mass spectrometry, we determined the chemical profile of the leaves in 15-d old plants. The metabolic signatures between the unifoliate leaf (m1) and the metameric units developing above (m2 and m3) were different, and interestingly, the highest colony forming units (CFU) was found in m1. The occurrence of the endophyte strongly affects the sugar composition in m1 and m2 leaves. Our results suggest that A. agilis UMCV2 colonize the leaves under a foliar inoculation system independently of the phenological age of the leaf and it is capable of modulating the carbohydrate metabolism without affecting the rest of the metabolome.

CaLRR-RLK1, a novel RD receptor-like kinase from Capsicum annuum and transcriptionally activated by CaHDZ27, act as positive regulator in Ralstonia solanacearum resistance

BACKGROUND

Bacterial wilt caused by Ralstonia solanacearum is one of the most important diseases in pepper worldwide, however, the molecular mechanism underlying pepper resistance to bacterial wilt remains poorly understood.

RESULTS

Herein, a novel RD leucine-rich repeat receptor-like kinase, CaLRR-RLK1, was functionally characterized in immunity against R. solanacearum. CaLRR-RLK1 was targeted exclusively to plasma membrane and was up-regulated by R. solanacearum inoculation (RSI) as well as by the exogenous application of salicylic acid (SA), methyl jasmonate (MeJA) or ethephon (ETH). The silencing of CaLRR-RLK1 led to enhanced susceptibility of pepper plants to RSI, accompanied by down-regulation of immunity-related genes including CaACO1, CaHIR1, CaPR4 and CaPO2. In contrast, transient overexpression of CaLRR-RLK1 triggered hypersensitive response (HR)-like cell death and H2O2 accumulation in pepper leaves, manifested by darker trypan blue and DAB staining respectively. In addition, the ectopic overexpression of CaLRR-RLK1 in tobacco plants enhanced resistance R. solanacearum, accompanied with the immunity associated marker genes including NtPR2, NtPR2, NtHSR203 and NtHSR515. Furthermore, it was found that CaHDZ27, a positive regulator in pepper response to RSI in our previous study, transcriptionally activated CaLRR-RLK1 by direct targeting its promoter probably in a CAATTATTG dependent manner.

CONCLUSION

The study revealed that CaLRR-RLK1 confers pepper resistance to R. solanacearum as the direct targeting of CaHDZ27.

Global gene expression profiling of two switchgrass cultivars following inoculation with Burkholderia phytofirmans strain PsJN

Improvement and year-to-year stabilization of biomass yields are primary objectives for the development of a low-input switchgrass feedstock production system using microbial endophytes. An earlier investigation of the effect of Burkholderia phytofirmans strain PsJN on switchgrass germplasm demonstrated differential responses between genotypes. PsJN inoculation of cv. Alamo (lowland ecotype) increased the plant root system, shoot length, and biomass yields, whereas it had no beneficial effect on cv. Cave-in-Rock (upland ecotype). To understand the gene networks governing plant growth promotion responses triggered by PsJN, the gene expression profiles were analysed in these two hosts, following seedling inoculation. The Affymetrix platform switchgrass expressed sequence tag (EST) microarray chip representing 122 972 probe sets, developed by the DOE BioEnergy Science Center, was employed to assess transcript abundance at 0.5, 2, 4, and 8 DAI (days after PsJN inoculation). Approximately 20 000 switchgrass probe sets showed significant responses in either cultivar. Switchgrass identifiers were used to map 19 421 genes in MapMan software. There were apparent differences in gene expression profiling between responsive and non-responsive cultivars after PsJN inoculation. Overall, there were 14 984 and 9691 genes affected by PsJN inoculation in Alamo and Cave-in-Rock, respectively. Of these, 394 are annotated as pathogenesis-related genes. In the responsive cv. Alamo, 68 pathogenesis-related genes were affected, compared with only 10 in the non-responsive cv. Cave-in-Rock. At the very early stage at 0.5 DAI, both cultivars exhibited similar recognition and defence responses, such as genes in signalling and proteolysis, after which the defence reaction in the responsive cv. Alamo became weaker while it was sustained in non-responsive cv. Cave-in-Rock.

Characterization of culturable bacterial endophytes and their capacity to promote plant growth from plants grown using organic or conventional practices

Plants have a diverse internal microbial biota that has been shown to have an important influence on a range of plant health attributes. Although these endophytes have been found to be widely occurring, few studies have correlated agricultural production practices with endophyte community structure and function. One agricultural system that focuses on preserving and enhancing soil microbial abundance and biodiversity is organic farming, and numerous studies have shown that organically managed system have increased microbial community characteristics. Herein, the diversity and specificity of culturable bacterial endophytes were evaluated in four vegetable crops: corn, tomato, melon, and pepper grown under organic or conventional practices. Endophytic bacteria were isolated from surface-sterilized shoot, root, and seed tissues and sequence identified. A total of 336 bacterial isolates were identified, and grouped into 32 species and five phyla. Among these, 239 isolates were from organically grown plants and 97 from those grown conventionally. Although a diverse range of bacteria were documented, 186 were from the Phylum Firmicutes, representing 55% of all isolates. Using the Shannon diversity index, we observed a gradation of diversity in tissues, with shoots and roots having a similar value, and seeds having the least diversity. Importantly, endophytic microbial species abundance and diversity was significantly higher in the organically grown plants compared to those grown using conventional practices, potentially indicating that organic management practices may increase endophyte presence and diversity. The impact that these endophytes could have on plant growth and yield was evaluated by reintroducing them into tomato plants in a greenhouse environment. Of the bacterial isolates tested, 61% were found to promote tomato plant growth and 50-64% were shown to enhance biomass accumulation, illustrating their potential agroecosystem application.

Knockdown of LjALD1, AGD2-like defense response protein 1, influences plant growth and nodulation in Lotus japonicus

The discovery of the enzyme L,L-diaminopimelate aminotransferase (LL-DAP-AT, EC 2.6.1.83) uncovered a unique step in the L-lysine biosynthesis pathway in plants. In Arabidopsis thaliana, LL-DAP-AT has been shown to play a key role in plant-pathogen interactions by regulation of the salicylic acid (SA) signaling pathway. Here, a full-length cDNA of LL-DAP-AT named as LjALD1 from Lotus japonicus (Regel) Larsen was isolated. The deduced amino acid sequence shares 67% identity with the Arabidopsis aminotransferase AGD2-LIKE DEFENSE RESPONSE PROTEIN1 (AtALD1) and is predicted to contain the same key elements: a conserved aminotransferase domain and a pyridoxal-5'-phosphate cofactor binding site. Quantitative real-time PCR analysis showed that LjALD1 was expressed in all L. japonicus tissues tested, being strongest in nodules. Expression was induced in roots that had been infected with the symbiotic rhizobium Mesorhizobium loti or treated with SA agonist benzo-(1, 2, 3)-thiadiazole-7-carbothioic acid. LjALD1 Knockdown exhibited a lower SA content, an increased number of infection threads and nodules, and a slight reduction in nodule size. In addition, compared with wild-type, root growth was increased and shoot growth was suppressed in LjALD1 RNAi plant lines. These results indicate that LjALD1 may play important roles in plant development and nodulation via SA signaling in L. japonicus.

Chitosan and a fungal elicitor inhibit tracheary element differentiation and promote accumulation of stress lignin-like substance in Zinnia elegans xylogenic culture

We investigated the effect of elicitors on xylem differentiation and lignification using a Zinnia elegans xylogenic culture system. Water-soluble chitosan and a fungal elicitor derived from Botrytis cinerea were used as elicitors. Elicitor addition at the start of culturing inhibited tracheary element (TE) differentiation in a concentration-dependent manner, and 30 μg mL(-1) of chitosan or 16.7 μg mL(-1) of the fungal elicitor strikingly inhibited TE differentiation and lignification. Addition of chitosan (at 50 μg mL(-1)) or the fungal elicitor (at 16.7 μg mL(-1)) during the culturing period also inhibited TE differentiation without inhibiting cell division, except for immature TEs undergoing secondary wall thickening. Elicitor addition after immature TE appearance also caused the accumulation of an extracellular lignin-like substance. It appears that elicitor addition at the start of culturing inhibits the process by which dedifferentiated cells differentiate into xylem cell precursors. Elicitor addition during culturing also appears to inhibit the transition from xylem cell precursors to immature TEs, and induces xylem cell precursors or xylem parenchyma cells to produce an extracellular stress lignin-like substance.

A new set of differentially expressed signaling genes is early expressed in coffee leaf rust race II incompatible interaction

New races of coffee rust are overcoming resistance genes available in germplasm and cultivated cultivars and bringing recently some coffee-producing countries in severe economic challenge. The objective of this study was to identify the genes that are linked to host resistance to the major coffee rust race II. In our study, we have identified and studied a segregating population that has a single monogenic resistant gene to coffee rust. Coffee leaves of parents, resistant, and susceptible genotypes of the F2 generation plants were inoculated with pathogen spores. A differential analysis was performed by combined cDNA-AFLP and bulk segregant analysis (BSA) in pooled samples collected 48 and 72 h postinoculation, increasing the selectiveness for differential gene expression. Of 108 differential expressed genes, between 33,000 gene fragments analyzed, 108 differential expressed genes were identified in resistant plants. About 20 and 22 % of these resistant-correlated genes are related to signaling and defense genes, respectively. Between signaling genes, the major subclass corresponds to receptor and resistant homolog genes, like nucleotide-binding site leucine-rich repeat (NBS-LRR), Pto-like, RLKs, Bger, and RGH1A, all not previously described in coffee rust responses. The second major subclass included kinases, where two mitogen-activated kinases (MAPK) are identified. Further gene expression analysis was performed for 21 selected genes by real-time PCR gene expression analysis at 0, 12, 24, 48, and 72 h postinoculation. The expression of genes involved in signaling and defense was higher at 24 and 72 h after inoculation, respectively. The NBS-LRR was the more differentially expressed gene between the signaling genes (four times more expressed in the resistant genotype), and thraumatin (PR5) was the more expressed between all genes (six times more expressed). Multivariate analysis reinforces the significance of the temporal separation of identified signaling and defense genes: early expression of signaling genes support the hypothesis that higher expression of the signaling components up regulates the defense genes identified. Additionally the increased gene expression of these two gene sets is associated with a single monogenic resistance trait to to leaf coffee rust in the interaction characterized here.

Salicylate degradation by the fungal plant pathogen Sclerotinia sclerotiorum

The fungal plant pathogen Sclerotinia sclerotiorum was studied to determine its ability to degrade salicylate, an important defense-signaling molecule in plants. S. sclerotiorum D-E7 was grown at 25 °C in an undefined medium (50 ml) containing minerals, 0.1% soytone, 50 mM MES buffer (pH 6.5), 25 mM glucose, and 1 mM salicylate. Glucose, oxalate, and salicylate concentrations were monitored by HPLC. S. sclerotiorum D-E7 was found to be active in salicylate degradation. However, salicylate alone was not growth supportive and, at higher levels (10 mM), inhibited glucose-dependent growth. Biomass formation (130 mg [dry wt] of mycelium per 50 ml of undefined medium), oxalate concentrations (~10 mM), and culture acidification (final culture pH approximated 5) were essentially the same in cultures grown with or without salicylate (1 mM). Time-course analyses revealed that salicylate degradation and glucose consumption were complete after 7 days of incubation and was concomitant with growth. Trace amounts of catechol, a known intermediate of salicylate metabolism, were detected during salicylate degradation. Overall, these results indicated that S. sclerotiorum has the ability to degrade salicylate and that the presence of low levels of salicylate did not affect growth or oxalate production by S. sclerotiorum.

Impact of ectomycorrhizal colonization and rust infection on the secondary metabolism of poplar (Populus trichocarpa x deltoides)

Fungal colonization can significantly affect the secondary metabolism of the host plants. We tested the impact of a common below-ground symbiosis, i.e., ectomycorrhiza formation, on poplar leaf chemical components that are involved in the defence against a common disease, i.e., rust fungi, in N-deficient soil. A rust-susceptible poplar clone (Populus trichocarpa × deltoides 'Beaupré') was (a) non-associated with ectomycorrhizal fungus (EM) Hebeloma mesophaeum (Pers.) Quélet MÜN and non-infected with rust fungus Melampsora larici-populina Kleb. (isolate 98AG31), (b) associated with EM, (c) inoculated with rust fungus and (d) associated with EM and inoculated with rust fungus. Poplar leaves were analysed by photometric and mass spectrometric techniques (liquid chromatography-tandem mass spectrometry (LC-MS/MS), pyrolysis-field ionization mass spectrometry (Py-FIMS)). Both rust infection and mycorrhiza formation led to increased proportions of condensed tannins in relation to total phenolics (13% in the control, 18-19% in the fungal treatments). In contrast, salicylic acid concentration (6.8 µg g(-1) in the control) was higher only in the rust treatments (17.9 and 25.4 µg g(-1) with rust infection). The Py-FIMS analysis revealed that the rust-infected treatments were significantly separated from the non-rust-infected treatments on the basis of six flavonoids and one lipid. The relative abundance of these components, which have known functions in plant defence, was decreased after rust infection of non-mycorrhizal plants, but not in mycorrhizal plants. The results indicate that the ectomycorrhizal formation compensated the rust infection by a decrease in the flavonoid syntheses. The study provides new evidence for an interactive response of mycorrhizal colonization and infection with rust fungi in the metabolism of poplar.

Production and function of jasmonates in nodulated roots of soybean plants inoculated with Bradyrhizobium japonicum

Little is known regarding production and function of endogenous jasmonates (JAs) in root nodules of soybean plants inoculated with Bradyrhizobium japonicum. We investigated (1) production of jasmonic acid (JA) and 12-oxophytodienoic acid (OPDA) in roots of control and inoculated plants and in isolated nodules; (2) correlations between JAs levels, nodule number, and plant growth during the symbiotic process; and (3) effects of exogenous JA and OPDA on nodule cell number and size. In roots of control plants, JA and OPDA levels reached a maximum at day 18 after inoculation; OPDA level was 1.24 times that of JA. In roots of inoculated plants, OPDA peaked at day 15, whereas JA level did not change appreciably. Shoot dry matter of inoculated plants was higher than that of control at day 21. Chlorophyll a decreased more abruptly in control plants than in inoculated plants, whereas b decreased gradually in both cases. Exogenous JA or OPDA changed number and size of nodule central cells and peripheral cells. Findings from this and previous studies suggest that increased levels of JA and OPDA in control plants are related to senescence induced by nutritional stress. OPDA accumulation in nodulated roots suggests its involvement in "autoregulation of nodulation."

HR4 gene is induced in the Arabidopsis-Trichoderma atroviride beneficial interaction

Plants are constantly exposed to microbes, for this reason they have evolved sophisticated strategies to perceive and identify biotic interactions. Thus, plants have large collections of so-called resistance (R) proteins that recognize specific microbe factors as signals of invasion. One of these proteins is codified by the Arabidopsis thaliana HR4 gene in the Col-0 ecotype that is homologous to RPW8 genes present in the Ms-0 ecotype. In this study, we investigated the expression patterns of the HR4 gene in Arabidopsis seedlings interacting with the beneficial fungus Trichoderma atroviride. We observed the induction of the HR4 gene mainly at 96 hpi when the fungus interaction was established. Furthermore, we found that the HR4 gene was differentially regulated in interactions with the beneficial bacterium Pseudomonas fluorescens and the pathogenic bacterium P. syringae. When hormone treatments were applied to A. thaliana (Col-0), each hormone treatment induced changes in HR4 gene expression. On the other hand, the expression of the RPW8.1 and RPW8.2 genes of Arabidopsis ecotype Ms-0 in interaction with T. atroviride was assessed. Interestingly, these genes are interaction-responsive; in particular, the RPW8.1 gene shows a very high level of expression in the later stages of interaction. These results indicate that HR4 and RPW8 genes could play a role in the establishment of Arabidopsis interactions with beneficial microbes.

The hidden function of photosynthesis: a sensing system for environmental conditions that regulates plant acclimation responses

Plants convert light energy from the sun into chemical energy by photosynthesis. Since they are sessile, they have to deal with a wide range of conditions in their immediate environment. Many abiotic and biotic parameters exhibit considerable fluctuations which can have detrimental effects especially on the efficiency of photosynthetic light harvesting. During evolution, plants, therefore, evolved a number of acclimation processes which help them to adapt photosynthesis to such environmental changes. This includes protective mechanisms such as excess energy dissipation and processes supporting energy redistribution, e.g. state transitions or photosystem stoichiometry adjustment. Intriguingly, all these responses are triggered by photosynthesis itself via the interplay of its light reaction and the Calvin-Benson cycle with the residing environmental condition. Thus, besides its primary function in harnessing and converting light energy, photosynthesis acts as a sensing system for environmental changes that controls molecular acclimation responses which adapt the photosynthetic function to the environmental change. Important signalling parameters directly or indirectly affected by the environment are the pH gradient across the thylakoid membrane and the redox states of components of the photosynthetic electron transport chain and/or electron end acceptors coupled to it. Recent advances demonstrate that these signals control post-translational modifications of the photosynthetic protein complexes and also affect plastid and nuclear gene expression machineries as well as metabolic pathways providing a regulatory framework for an integrated response of the plant to the environment at all cellular levels.

Global changes in gene expression of grapefruit peel tissue in response to the yeast biocontrol agent Metschnikowia fructicola

To gain a better understanding of the molecular changes taking place in citrus fruit tissue following the application of the yeast biocontrol agent Metschnikowia fructicola, microarray analysis was performed on grapefruit surface wounds using an Affymetrix Citrus GeneChip. Using a cut-off of P < 0.05 and a 1.5-fold change difference as biologically significant, the data indicated that 1007 putative unigenes showed significant expression changes following wounding and yeast application relative to wounded controls. Microarray results of selected genes were validated by reverse transcription-quantitative real-time polymerase chain reaction (RT-qPCR). The data indicated that yeast application induced the expression of the genes encoding Respiratory burst oxidase (Rbo), mitogen-activated protein kinase (MAPK) and mitogen-activated protein kinase kinase (MAPKK), G-proteins, chitinase (CHI), phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS) and 4-coumarate-CoA ligase (4CL). In contrast, three genes, peroxidase (POD), superoxide dismutase (SOD) and catalase (CAT), were down-regulated in grapefruit peel tissue treated with yeast cells. Moreover, suppression was correlated with significantly higher levels of hydrogen peroxide, superoxide anion and hydroxyl radical production in yeast-treated surface wounds. Interestingly, large amounts of hydrogen peroxide were detected inside yeast cells recovered from wounded fruit tissue, indicating the ability of the yeast to activate reactive oxygen species when it is in contact with plant tissue. This study provides the first global picture of gene expression changes in grapefruit in response to the yeast antagonist M. fructicola.

Co-inoculation with rhizobia and AMF inhibited soybean red crown rot: from field study to plant defense-related gene expression analysis

BACKGROUND

Soybean red crown rot is a major soil-borne disease all over the world, which severely affects soybean production. Efficient and sustainable methods are strongly desired to control the soil-borne diseases.

PRINCIPAL FINDINGS

We firstly investigated the disease incidence and index of soybean red crown rot under different phosphorus (P) additions in field and found that the natural inoculation of rhizobia and arbuscular mycorrhizal fungi (AMF) could affect soybean red crown rot, particularly without P addition. Further studies in sand culture experiments showed that inoculation with rhizobia or AMF significantly decreased severity and incidence of soybean red crown rot, especially for co-inoculation with rhizobia and AMF at low P. The root colony forming unit (CFU) decreased over 50% when inoculated by rhizobia and/or AMF at low P. However, P addition only enhanced CFU when inoculated with AMF. Furthermore, root exudates of soybean inoculated with rhizobia and/or AMF significantly inhibited pathogen growth and reproduction. Quantitative RT-PCR results indicated that the transcripts of the most tested pathogen defense-related (PR) genes in roots were significantly increased by rhizobium and/or AMF inoculation. Among them, PR2, PR3, PR4 and PR10 reached the highest level with co-inoculation of rhizobium and AMF.

CONCLUSIONS

Our results indicated that inoculation with rhizobia and AMF could directly inhibit pathogen growth and reproduction, and activate the plant overall defense system through increasing PR gene expressions. Combined with optimal P fertilization, inoculation with rhizobia and AMF could be considered as an efficient method to control soybean red crown rot in acid soils.

Comparative genomics yields insights into niche adaptation of plant vascular wilt pathogens

The vascular wilt fungi Verticillium dahliae and V. albo-atrum infect over 200 plant species, causing billions of dollars in annual crop losses. The characteristic wilt symptoms are a result of colonization and proliferation of the pathogens in the xylem vessels, which undergo fluctuations in osmolarity. To gain insights into the mechanisms that confer the organisms' pathogenicity and enable them to proliferate in the unique ecological niche of the plant vascular system, we sequenced the genomes of V. dahliae and V. albo-atrum and compared them to each other, and to the genome of Fusarium oxysporum, another fungal wilt pathogen. Our analyses identified a set of proteins that are shared among all three wilt pathogens, and present in few other fungal species. One of these is a homolog of a bacterial glucosyltransferase that synthesizes virulence-related osmoregulated periplasmic glucans in bacteria. Pathogenicity tests of the corresponding V. dahliae glucosyltransferase gene deletion mutants indicate that the gene is required for full virulence in the Australian tobacco species Nicotiana benthamiana. Compared to other fungi, the two sequenced Verticillium genomes encode more pectin-degrading enzymes and other carbohydrate-active enzymes, suggesting an extraordinary capacity to degrade plant pectin barricades. The high level of synteny between the two Verticillium assemblies highlighted four flexible genomic islands in V. dahliae that are enriched for transposable elements, and contain duplicated genes and genes that are important in signaling/transcriptional regulation and iron/lipid metabolism. Coupled with an enhanced capacity to degrade plant materials, these genomic islands may contribute to the expanded genetic diversity and virulence of V. dahliae, the primary causal agent of Verticillium wilts. Significantly, our study reveals insights into the genetic mechanisms of niche adaptation of fungal wilt pathogens, advances our understanding of the evolution and development of their pathogenesis, and sheds light on potential avenues for the development of novel disease management strategies to combat destructive wilt diseases.

Biotic stress globally downregulates photosynthesis genes

To determine if damage to foliage by biotic agents, including arthropods, fungi, bacteria and viral pathogens, universally downregulates the expression of genes involved in photosynthesis, we compared transcriptome data from microarray experiments after twenty two different forms of biotic damage on eight different plant species. Transcript levels of photosynthesis light reaction, carbon reduction cycle and pigment synthesis genes decreased regardless of the type of biotic attack. The corresponding upregulation of genes coding for the synthesis of jasmonic acid and those involved in the responses to salicylic acid and ethylene suggest that the downregulation of photosynthesis-related genes was part of a defence response. Analysis of the sub-cellular targeting of co-expressed gene clusters revealed that the transcript levels of 84% of the genes that carry a chloroplast targeting peptide sequence decreased. The majority of these downregulated genes shared common regulatory elements, such as G-box (CACGTG), T-box (ACTTTG) and SORLIP (GCCAC) motifs. Strong convergence in the response of transcription suggests that the universal downregulation of photosynthesis-related gene expression is an adaptive response to biotic attack. We hypothesize that slow turnover of many photosynthetic proteins allows plants to invest resources in immediate defence needs without debilitating near term losses in photosynthetic capacity.

The phenotypic expression of QTLs for partial resistance to barley leaf rust during plant development

Partial resistance is generally considered to be a durable form of resistance. In barley, Rphq2, Rphq3 and Rphq4 have been identified as consistent quantitative trait loci (QTLs) for partial resistance to the barley leaf rust pathogen Puccinia hordei. These QTLs have been incorporated separately into the susceptible L94 and the partially resistant Vada barley genetic backgrounds to obtain two sets of near isogenic lines (NILs). Previous studies have shown that these QTLs are not effective at conferring disease resistance in all stages of plant development. In the present study, the two sets of QTL-NILs and the two recurrent parents, L94 and Vada, were evaluated for resistance to P. hordei isolate 1.2.1 simultaneously under greenhouse conditions from the first leaf to the flag leaf stage. Effect of the QTLs on resistance was measured by development rate of the pathogen, expressed as latency period (LP). The data show that Rphq2 prolongs LP at the seedling stage (the first and second leaf stages) but has almost no effect on disease resistance in adult plants. Rphq4 showed no effect on LP until the third leaf stage, whereas Rphq3 is consistently effective at prolonging LP from the first leaf to the flag leaf. The changes in the effectiveness of Rphq2 and Rphq4 happen at the barley tillering stage (the third to fourth leaf stages). These results indicate that multiple disease evaluations of a single plant by repeated inoculations of the fourth leaf to the flag leaf should be conducted to precisely estimate the effect of Rphq4. The present study confirms and describes in detail the plant development-dependent effectiveness of partial resistance genes and, consequently, will enable a more precise evaluation of partial resistance regulation during barley development.

Creation of a genome-wide metabolic pathway database for Populus trichocarpa using a new approach for reconstruction and curation of metabolic pathways for plants

Metabolic networks reconstructed from sequenced genomes or transcriptomes can help visualize and analyze large-scale experimental data, predict metabolic phenotypes, discover enzymes, engineer metabolic pathways, and study metabolic pathway evolution. We developed a general approach for reconstructing metabolic pathway complements of plant genomes. Two new reference databases were created and added to the core of the infrastructure: a comprehensive, all-plant reference pathway database, PlantCyc, and a reference enzyme sequence database, RESD, for annotating metabolic functions of protein sequences. PlantCyc (version 3.0) includes 714 metabolic pathways and 2,619 reactions from over 300 species. RESD (version 1.0) contains 14,187 literature-supported enzyme sequences from across all kingdoms. We used RESD, PlantCyc, and MetaCyc (an all-species reference metabolic pathway database), in conjunction with the pathway prediction software Pathway Tools, to reconstruct a metabolic pathway database, PoplarCyc, from the recently sequenced genome of Populus trichocarpa. PoplarCyc (version 1.0) contains 321 pathways with 1,807 assigned enzymes. Comparing PoplarCyc (version 1.0) with AraCyc (version 6.0, Arabidopsis [Arabidopsis thaliana]) showed comparable numbers of pathways distributed across all domains of metabolism in both databases, except for a higher number of AraCyc pathways in secondary metabolism and a 1.5-fold increase in carbohydrate metabolic enzymes in PoplarCyc. Here, we introduce these new resources and demonstrate the feasibility of using them to identify candidate enzymes for specific pathways and to analyze metabolite profiling data through concrete examples. These resources can be searched by text or BLAST, browsed, and downloaded from our project Web site (http://plantcyc.org).

Trehalose and plant stress responses: friend or foe?

The disaccharide trehalose is involved in stress response in many organisms. However, in plants, its precise role remains unclear, although some data indicate that trehalose has a protective role during abiotic stresses. By contrast, some trehalose metabolism mutants exhibit growth aberrations, revealing potential negative effects on plant physiology. Contradictory effects also appear under biotic stress conditions. Specifically, trehalose is essential for the infectivity of several pathogens but at the same time elicits plant defense. Here, we argue that trehalose should not be regarded only as a protective sugar but rather like a double-faced molecule and that further investigation is required to elucidate its exact role in stress tolerance in plants.

Cuscuta reflexa invasion induces Ca release in its host

Cuscuta reflexa induces a variety of reaction in its hosts. Some of these are visual reactions, and it is clear that these morphological changes are preceded by events at the molecular level, where signal transduction is one of the early processes. Calcium (Ca(2+)) release is the major second messenger during signal transduction, and we therefore studied Ca(2+) spiking in tomato during infection with C. reflexa. Bioluminescence in aequorin-expressing tomato was monitored for 48 h after the onset of Cuscuta infestation. Signals at the attachment sites were observed from 30 to 48 h. Treatment of aequorin-expressing tomato leaf disks with Cuscuta plant extracts suggested that the substance that induced Ca(2+) release from the host was closely linked to parasite haustoria.

Synergistic action of exogenous salicylic acid and arbuscular mycorrhizal fungus colonization in Avena nuda seedlings in response to NO(2) exposure

Colonization of arbuscular mycorrhizal fungi Glomus mosseae or exogenous salicylic acid (SA) treatment can increase Avena nuda plant tolerance to elevated NO(2) exposure. The combination of the two factors, namely application of SA to the mycorrhizal plants, further promoted NO(2) tolerance, as indicated by an alleviated plant biomass decrease compared to the respective treatment. The analysis of antioxidant capacity, redox status and photon energy utilization showed that the increased NO(2) tolerance in the treated plants may be associated, at least in part, with scavenging reactive oxygen species, maintaining CO(2) assimilated rate and reducing conditions in cells.

The role of jasmonates in mutualistic symbioses between plants and soil-born microorganisms

Many plants are able to develop mutualistic interactions with arbuscular mycorrhizal fungi and/or nitrogen-fixing bacteria. Whereas the former is widely distributed among most of the land plants, the latter is restricted to species of ten plant families, including the legumes. The establishment of both associations is based on mutual recognition and a high degree of coordination at the morphological and physiological level. This requires the activity of a number of signals, including jasmonates. Here, recent knowledge on the putative roles of jasmonates in both mutualistic symbioses will be reviewed. Firstly, the action of jasmonates will be discussed in terms of the initial signal exchange between symbionts and in the resulting plant signaling cascade common for nodulation and mycorrhization. Secondly, the putative role of jasmonates in the autoregulation of the endosymbioses will be outlined. Finally, aspects of function of jasmonates in the fully established symbioses will be presented. Various processes will be discussed that are possibly mediated by jasmonates, including the redox status of nodules and the carbohydrate partitioning of mycorrhizal roots.