9-Lipoxygenase-Derived Oxylipins Activate Brassinosteroid Signaling to Promote Cell Wall-Based Defense and Limit Pathogen Infection

Osmotic stress in roots drives lipoxygenase-dependent plastid remodeling through singlet oxygen production

Osmotic stress, caused by the lack of water or by high salinity, is a common problem in plant roots. Osmotic stress can be reproducibly simulated with the application of solutions of the high-molecular-weight and impermeable polyethylene glycol. The accumulation of different reactive oxygen species, such as singlet oxygen, superoxide, and hydrogen peroxide, accompany this stress. Among them, singlet oxygen, produced as a byproduct of lipoxygenase activity, has been associated with limiting root growth. To better understand the source and effect of singlet oxygen, we followed its production at the cellular level in Arabidopsis (Arabidopsis thaliana). Osmotic stress initiated profound changes in plastid and vacuole structure. Confocal and electron microscopy showed that the plastids were a source of singlet oxygen accompanied by the appearance of multiple, small extraplastidic bodies that were also an intense source of singlet oxygen. A marker protein, CRUMPLED LEAF, indicated that these small bodies originated from the plastid outer membrane. Remarkably, LINOLEATE 9S-LIPOXYGENASE 5 (LOX5) was shown to change its distribution from uniformly cytoplasmic to a more clumped distribution together with plastids and the small bodies. In addition, oxylipin products of Type 9 lipoxygenase increased, while products of Type 13 lipoxygenases decreased. Inhibition of lipoxygenase by the salicylhydroxamic acid inhibitor or in downregulated lipoxygenase lines prevented cells from initiating the cellular responses, leading to cell death. In contrast, singlet oxygen scavenging halted terminal cell death. These findings underscore the reversible nature of osmotic stress-induced changes, emphasizing the pivotal roles of lipoxygenases and singlet oxygen in root stress physiology.

Integrated Metabolomic and Transcriptomic Profiling Reveals the Defense Response of Tea Plants (Camellia sinensis) to Toxoptera aurantii

The tea plant (Camellia sinensis) is a unique beverage crop worldwide, but its yield and quality are adversely affected by Toxoptera aurantii. However, the response mechanisms of tea plants to T. aurantii stress remain poorly known. Herein, we present the life table of T. aurantii on resistant (W016) and susceptible (HJY) tea cultivars, demonstrating that the fitness of T. aurantii on W016 was lower than that on HJY. Integrated metabolic and transcriptomic analyses revealed that T. aurantii feeding activated pathways associated with phenylpropanoid biosynthesis, plant hormone signal transduction, and ATP-binding cassette (ABC) transporters. Notably, T. aurantii feeding significantly upregulated the levels of brassinolide and p-coumaryl alcohol in W016 but not in HJY. Furthermore, in vitro enzymatic assays indicated that C. sinensis cinnamyl alcohol dehydrogenase (CsCAD1) catalyzes the formation of p-coumaryl alcohol participation in lignin synthesis. Our findings highlight the role of brassinolide-mediated lignin biosynthesis of the tea plant in response to T. aurantii feeding.

Deciphering Aphanomyces euteiches-pea-biocontrol bacterium interactions through untargeted metabolomics

Aphanomyces euteiches causes root rot in pea, leading to significant yield losses. However, the metabolites involved in this pathosystem have not been thoroughly studied. This study aimed to fill this gap and explore mechanisms of bacterial suppression of A. euteiches via untargeted metabolomics using pea grown in a controlled environment. Chemical isotope labeling (CIL), followed by liquid chromatography-mass spectrometry (LC-MS), was used for metabolite separation and detection. Univariate and multivariate analyses showed clear separation of metabolites from pathogen-treated pea roots and roots from other treatments. A three-tier approach positively or putatively identified 5249 peak pairs or metabolites. Of these, 403 were positively identified in tier 1; 940 were putatively identified with high confidence in tier 2. There were substantial changes in amino acid pool, and fatty acid and phenylpropanoid pathway products. More metabolites, including salicylic and jasmonic acids, were upregulated than downregulated in A. euteiches-infected roots. 1-aminocyclopropane-1-carboxylic acid and 12-oxophytodienoic acid were upregulated in A. euteiches + bacterium-treated roots compared to A. euteiches-infected roots. A great number of metabolites were up- or down-regulated in response to A. euteiches infection compared with the control and A. euteiches + bacterium-treated plants. The results of this study could facilitate improved disease management.

Characterization of infected process and primary mechanism in rice Acuce defense against rice blast fungus, Magnaporthe oryzae

Identification of infection process and defense response during M. oryzae infecting Acuce. Magnaporthe oryzae is a destructive rice pathogen. Recent studies have focused on the initial infectious stage, with a few studies conducted to elucidate the characteristics of the late infectious stages. This study aims to decipher the characteristics at different stages (biotrophic, biotrophy-necrotrophy switch (BNS), and necrotrophic) between the interaction of two M. oryzae-rice combinations and investigate the resistance mechanisms of rice to M. oryzae using cytological and molecular methods. The biotrophic phase of M. oryzae-LTH compatible interaction was found to be longer than that of M. oryzae-Acuce incompatible interaction. We also found that jasmonic acid (JA) signaling plays an important role in defense by regulating antimicrobial compound accumulation in infected Acuce via a synergistic interaction of JA-salicylic acid (SA) and JA-ethylene (ET). In infected LTH, JA-ET/JA-SA showed antagonistic interaction. Ibuprofen (IBU) is a JA inhibitor. Despite the above findings, we found that exogenous JA-Ile and IBU significantly alleviated blast symptoms in infected LTH at 36 hpi (biotrophic) and 72 hpi (BNS), indicating these two-time points may be critical for managing blast disease in the compatible interaction. Conversely, IBU significantly increased blast symptoms on the infected Acuce at 36 hpi, confirming that the JA signal plays a central role in the defense response in infected Acuce. According to transcriptional analysis, the number of genes enriched in the plant hormone signal pathway was significantly higher than in other pathways. Our findings suggested that JA-mediated defense mechanism is essential in regulating Acuce resistance, particularly during the biotrophic and BNS phases.

Fungal and bacterial oxylipins are signals for intra- and inter-cellular communication within plant disease

Lipids are central at various stages of host-pathogen interactions in determining virulence and modulating plant defense. Free fatty acids may act as substrates for oxidizing enzymes [e.g., lipoxygenases (LOXs) and dioxygenases (DOXs)] that synthesize oxylipins. Fatty acids and oxylipins function as modulators of several pathways in cell-to-cell communication; their structural similarity among plant, fungal, and bacterial taxa suggests potential in cross-kingdom communication. We provide a prospect of the known role of fatty acids and oxylipins in fungi and bacteria during plant-pathogen interactions. In the pathogens, oxylipin-mediated signaling pathways are crucial both in development and host infection. Here, we report on case studies suggesting that oxylipins derived from oleic, linoleic, and linolenic acids are crucial in modulating the pathogenic lifestyle in the host plant. Intriguingly, overlapping (fungi-plant/bacteria-plant) results suggest that different inter-kingdom pathosystems use similar lipid signals to reshape the lifestyle of the contenders and occasionally determine the outcome of the challenge.

WGCNA Identifies a Comprehensive and Dynamic Gene Co-Expression Network That Associates with Smut Resistance in Sugarcane

Sugarcane smut is a major fungal disease caused by Sporisorium scitamineum, which seriously reduces the yield and quality of sugarcane. In this study, 36 transcriptome data were collected from two sugarcane genotypes, YT93-159 (resistant) and ROC22 (susceptible) upon S. scitamineum infection. Data analysis revealed 20,273 (12,659 up-regulated and 7614 down-regulated) and 11,897 (7806 up-regulated and 4091 down-regulated) differentially expressed genes (DEGs) in YT93-159 and ROC22, respectively. A co-expression network was then constructed by weighted gene co-expression network analysis (WGCNA), which identified 5010 DEGs in 15 co-expressed gene modules. Four of the 15 modules, namely, Skyblue, Salmon, Darkorange, and Grey60, were significantly associated with smut resistance. The GO and KEGG enrichment analyses indicated that the DEGs involving in these four modules could be enriched in stress-related metabolic pathways, such as MAPK and hormone signal transduction, plant-pathogen interaction, amino acid metabolism, glutathione metabolism, and flavonoid, and phenylpropanoid biosynthesis. In total, 38 hub genes, including six from the Skyblue module, four from the Salmon module, 12 from the Darkorange module, and 16 from the Grey60 module, were screened as candidate hub genes by calculating gene connectivity in the corresponding network. Only 30 hub genes were amplifiable with RT-qPCR, of which 27 were up-regulated upon S. scitamineum infection. The results were consistent with the trend of gene expression in RNA-Seq, suggesting their positive roles in smut resistance. Interestingly, the expression levels of AOX, Cyb5, and LAC were higher in ROC22 than in YT93-159, indicating these three genes may act as negative regulators in response to S. scitamineum infection. This study revealed the transcriptome dynamics in sugarcane challenged by S. scitamineum infection and provided gene targets for smut resistance breeding in sugarcane.

Oxylipin signaling in salt-stressed soybean is modulated by ligand-dependent interaction of Class II acyl-CoA-binding proteins with lipoxygenase

Plant lipoxygenases (LOXs) oxygenate linoleic and linolenic acids, creating hydroperoxy derivatives, and from these, jasmonates and other oxylipins are derived. Despite the importance of oxylipin signaling, its activation mechanism remains largely unknown. Here, we show that soybean ACYL-COA-BINDING PROTEIN3 (ACBP3) and ACBP4, two Class II acyl-CoA-binding proteins, suppressed activity of the vegetative LOX homolog VLXB by sequestering it at the endoplasmic reticulum. The ACBP4-VLXB interaction was facilitated by linoleoyl-CoA and linolenoyl-CoA, which competed with phosphatidic acid (PA) for ACBP4 binding. In salt-stressed roots, alternative splicing produced ACBP variants incapable of VLXB interaction. Overexpression of the variants enhanced LOX activity and salt tolerance in Arabidopsis and soybean hairy roots, whereas overexpressors of the native forms exhibited reciprocal phenotypes. Consistently, the differential alternative splicing pattern in two soybean genotypes coincided with their difference in salt-induced lipid peroxidation. Salt-treated soybean roots were enriched in C32:0-PA species that showed high affinity to Class II ACBPs. We conclude that PA signaling and alternative splicing suppress ligand-dependent interaction of Class II ACBPs with VLXB, thereby triggering lipid peroxidation during salt stress. Hence, our findings unveil a dual mechanism that initiates the onset of oxylipin signaling in the salinity response.

Comparative Transcriptome Analysis in Taraxacum koksaghyz to Identify Genes that Determine Root Volume and Root Length

The Russian dandelion (Taraxacum koksaghyz, family Asteraceae) produces large amounts of natural rubber in the laticifers of its roots. This species has been proposed as an alternative source of natural rubber to augment or partly replace the rubber tree (Hevea brasiliensis) but domestication would require genetic improvement to increase rubber yields and agronomic optimization to facilitate harvesting and processing. Optimization has focused thus far on the size and shape of the roots, the primary storage organ for natural rubber and inulin. However, the corresponding genetic factors are poorly understood. Here we describe the comparative transcriptomic analysis of root tissues from T. koksaghyz plant sets featuring different root sizes and shapes, aiming to identify differentially expressed genes correlating with root length or root diameter in the upper root and root tip. The resulting datasets revealed multiple candidate genes for each trait and root part, including a glucan endo-1,3-β-d-glucosidase, an allene oxide synthase 3, and a TIFY10A/JAZ1 homolog. These three genes were tested by qRT-PCR in outdoor-grown plants with diverse root morphology, and the expression of two genes correlated with the appropriate root morphotype, confirming the effectiveness of our method. We evaluated the candidate genes to gain insight into their potential functions in root development. Such candidate genes could be suitable for marker-assisted breeding programs in the future.

Interaction With Fungi Promotes the Accumulation of Specific Defense Molecules in Orchid Tubers and May Increase the Value of Tubers for Biotechnological and Medicinal Applications: The Case Study of Interaction Between Dactylorhiza sp. and Tulasnella calospora

Terrestrial orchids can form tubers, organs modified to store energy reserves. Tubers are an attractive source of nutrients, and salep, a flour made from dried orchid tubers, is the source of traditional beverages. Tubers also contain valuable secondary metabolites and are used in traditional medicine. The extensive harvest of wild orchids is endangering their populations in nature; however, orchids can be cultivated and tubers mass-produced. This work illustrates the importance of plant-fungus interaction in shaping the content of orchid tubers in vitro. Orchid plants of Dactylorhiza sp. grown in asymbiotic culture were inoculated with a fungal isolate from Tulasnella calospora group and, after 3 months of co-cultivation, tubers were analyzed. The fungus adopted the saprotrophic mode of life, but no visible differences in the morphology and biomass of the tubers were detected compared to the mock-treated plants. To elucidate the mechanisms protecting the tubers against fungal infestation, proteome, metabolome, and lipidome of tubers were analyzed. In total, 1,526, 174, and 108 proteins, metabolites, and lipids were quantified, respectively, providing a detailed snapshot of the molecular process underlying plant-microbe interaction. The observed changes at the molecular level showed that the tubers of inoculated plants accumulated significantly higher amounts of antifungal compounds, including phenolics, alkaloid Calystegine B2, and dihydrophenanthrenes. The promoted antimicrobial effects were validated by observing transient inhibition of Phytophthora cactorum growth. The integration of omics data highlighted the promotion of flavonoid biosynthesis, the increase in the formation of lipid droplets and associated production of oxylipins, and the accumulation of auxin in response to T. calospora. Taken together, these results provide the first insights into the molecular mechanisms of defense priming in orchid tubers and highlight the possible use of fungal interactors in biotechnology for the production of orchid secondary metabolites.

Leaf herbivory counteracts nematode-triggered repression of jasmonate-related defenses in tomato roots

Shoot herbivores may influence the communities of herbivores associated with the roots via inducible defenses. However, the molecular mechanisms and hormonal signaling underpinning the systemic impact of leaf herbivory on root-induced responses against nematodes remain poorly understood. By using tomato (Solanum lycopersicum) as a model plant, we explored the impact of leaf herbivory by Manduca sexta on the performance of the root knot nematode Meloidogyne incognita. By performing glasshouse bioassays, we found that leaf herbivory reduced M. incognita performance in the roots. By analyzing the root expression profile of a set of oxylipin-related marker genes and jasmonate root content, we show that leaf herbivory systemically activates the 13-Lipoxigenase (LOX) and 9-LOX branches of the oxylipin pathway in roots and counteracts the M. incognita-triggered repression of the 13-LOX branch. By using untargeted metabolomics, we also found that leaf herbivory counteracts the M. incognita-mediated repression of putative root chemical defenses. To explore the signaling involved in this shoot-to-root interaction, we performed glasshouse bioassays with grafted plants compromised in jasmonate synthesis or perception, specifically in their shoots. We demonstrated the importance of an intact shoot jasmonate perception, whereas having an intact jasmonate biosynthesis pathway was not essential for this shoot-to-root interaction. Our results highlight the impact of leaf herbivory on the ability of M. incognita to manipulate root defenses and point to an important role for the jasmonate signaling pathway in shoot-to-root signaling.

Oxylipins From Different Pathways Trigger Mitochondrial Stress Signaling Through Respiratory Complex III

Plant oxylipins are signaling molecules produced from fatty acids by oxidative pathways, mainly initiated by 9- and 13-lipoxygenases (9-LOX and 13-LOX), alpha-dioxygenases or non-enzymatic oxidation. Oxylipins from the 9-LOX pathway induce oxidative stress and control root development and plant defense. These activities have been associated with mitochondrial processes, but precise cellular targets and pathways remain unknown. In order to study oxylipin signaling, we previously generated a collection of Arabidopsis thaliana mutants that were insensitive to the 9-LOX products 9(S)-hydroxy-10,12, 15-octadecatrienoic acid (9-HOT) and its ketone derivative 9-KOT (noxy mutants). Here, we describe noxy1, noxy3, noxy5, noxy23, and noxy54 mutants, all affected in nucleus-encoded mitochondrial proteins, and use them to study the role of mitochondria in oxylipin signaling. Functional and phenotypic analyses showed that noxy plants displayed mitochondrial aggregation, reduced respiration rates and resistance to the complex III inhibitor Antimycin A (AA), thus indicating a close similarity of the oxylipin signaling and mitochondrial stress. Application of 9-HOT and 9-KOT protected plants against subsequent mitochondrial stress, whereas they boosted root growth reduction when applied in combination with complex III inhibitors but did not with inhibitors of other respiratory complexes. A similar effect was caused by linear-chain oxylipins from 13-LOX or non-enzymatic pathways having α,β-unsaturated hydroxyl or keto groups in their structure. Studies to investigate 9-HOT and 9-KOT activity indicated that they do not reduce respiration rates, but their action is primarily associated with enhanced ROS responses. This was supported by the results showing that 9-HOT or 9-KOT combined with AA amplified the expression of oxylipin- and ROS-responding genes but not of the AA marker AOX1a, thus implying the activation of a specific mitochondria retrograde signaling pathway. Our results implicate mitochondrial complex III as a hub in the signaling activity of multiple oxylipin pathways and point at downstream ROS responses as components of oxylipin function.

A multifaceted module of BRI1 ETHYLMETHANE SULFONATE SUPRESSOR1 (BES1)-MYB88 in growth and stress tolerance of apple

The R2R3 transcription factor MdMYB88 has previously been reported to function in biotic and abiotic stress responses. Here, we identify BRI1 ETHYLMETHANE SULFONATE SUPRESSOR1 (MdBES1), a vital component of brassinosteroid (BR) signaling in apple (Malus × domestica) that directly binds to the MdMYB88 promoter, regulating the expression of MdMYB88 in a dynamic and multifaceted mode. MdBES1 positively regulated expression of MdMYB88 under cold stress and pathogen attack, but negatively regulated its expression under control and drought conditions. Consistently, MdBES1 was a positive regulator for cold tolerance and disease resistance in apple, but a negative regulator for drought tolerance. In addition, MdMYB88 participated in BR biosynthesis by directly regulating the BR biosynthetic genes DE ETIOLATED 2 (MdDET2), DWARF 4 (MdDWF4), and BRASSINOSTEROID 6 OXIDASE 2 (MdBR6OX2). Applying exogenous BR partially rescued the erect leaf and dwarf phenotypes, as well as defects in stress tolerance in MdMYB88/124 RNAi plants. Moreover, knockdown of MdMYB88 in MdBES1 overexpression (OE) plants decreased resistance to a pathogen and C-REPEAT BINDING FACTOR1 expression, whereas overexpressing MdMYB88 in MdBES1 OE plants increased expression of SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 3 （MdSPL3） and BR biosynthetic genes, suggesting that MdMYB88 contributes to MdBES1 function during BR biosynthesis and the stress response. Taken together, our results reveal multifaceted regulation of MdBES1 on MdMYB88 in BR biosynthesis and stress tolerance.

Laminarin Induces Defense Responses and Efficiently Controls Olive Leaf Spot Disease in Olive

Olive leaf spot (OLS) caused by Fusicladiumoleagineum is mainly controlled using copper fungicides. However, the replacement of copper-based products with eco-friendly alternatives is a priority. The use of plant resistance-inducers (PRIs) or biological control agents (BCAs) could contribute in this direction. In this study we investigated the potential use of three PRIs (laminarin, acibenzolar-S-methyl, harpin) and a BCA (Bacillus amyloliquefaciens FZB24) for the management of OLS. The tested products provided control efficacy higher than 68%. In most cases, dual applications provided higher (p < 0.05) control efficacies compared to that achieved by single applications. The highest control efficacy of 100% was achieved by laminarin. Expression analysis of the selected genes by RT-qPCR revealed different kinetics of induction. In laminarin-treated plants, for most of the tested genes a higher induction rate (p < 0.05) was observed at 3 days post application. Pal, Lox, Cuao and Mpol were the genes with the higher inductions in laminarin-treated and artificially inoculated plants. The results of this study are expected to contribute towards a better understanding of PRIs in olive culture and the optimization of OLS control, while they provide evidence for potential contributions in the reduction of copper accumulation in the environment.

Expression of Castanea crenata Allene Oxide Synthase in Arabidopsis Improves the Defense to Phytophthora cinnamomi

Allene oxide synthase (AOS) is a key enzyme of the jasmonic acid (JA) signaling pathway. The AOS gene was previously found to be upregulated in an Asian chestnut species resistant to infection by the oomycete Phytophthora cinnamomi (Castanea crenata), while lower expression values were detected in the susceptible European chestnut (Castanea sativa). Here, we report a genetic and functional characterization of the C. crenata AOS (CcAOS) upon its heterologous gene expression in a susceptible ecotype of Arabidopsis thaliana, which contains a single AOS gene. It was found that Arabidopsis plants expressing CcAOS delay pathogen progression and exhibit more vigorous growth in its presence. They also show upregulation of jasmonic acid and salicylic acid-related genes. As in its native species, heterologous CcAOS localized to plastids, as revealed by confocal imaging of the CcAOS-eGFP fusion protein in transgenic Arabidopsis roots. This observation was confirmed upon transient expression in Nicotiana benthamiana leaf epidermal cells. To further confirm a specific role of CcAOS in the defense mechanism against the pathogen, we performed crosses between transgenic CcAOS plants and an infertile Arabidopsis AOS knockout mutant line. It was found that plants expressing CcAOS exhibit normal growth, remain infertile but are significantly more tolerant to the pathogen than wild type plants. Together, our results indicate that CcAOS is an important player in plant defense responses against oomycete infection and that its expression in susceptible varieties may be a valuable tool to mitigate biotic stress responses.

Modes of Brassinosteroid Activity in Cold Stress Tolerance

Cold stress is a significant environmental factor that negatively affects plant growth and development in particular when it occurs during the growth phase. Plants have evolved means to protect themselves from damage caused by chilling or freezing temperatures and some plant species, in particular those from temperate geographical zones, can increase their basal level of freezing tolerance in a process termed cold acclimation. Cold acclimation improves plant survival, but also represses growth, since it inhibits activity of the growth-promoting hormones gibberellins (GAs). In addition to GAs, the steroid hormones brassinosteroids (BRs) also take part in growth promotion and cold stress signaling; however, in contrast to Gas, BRs can improve cold stress tolerance with fewer trade-offs in terms of growth and yields. Here we summarize our current understanding of the roles of BRs in cold stress responses with a focus on freezing tolerance and cold acclimation pathways.

Genome-Wide Identification, Structural, and Gene Expression Analysis of BRI1-EMS-Suppressor 1 Transcription Factor Family in Cucumis sativus

BRASSINOSTEROID INSENSITIVE1-EMS-suppressor 1 (BES1) is an essential regulator downstream of brassinosteroid signaling and plays important roles in plant stress response, growth, and development. To date, the regulation mechanisms of BES1 transcription factors have been identified and elucidated in model plants Arabidopsis and rice. However, little information is available regarding the BES1 family in Cucumis sativus. Therefore, this study conducted a genome-wide analysis of BES1 genes in cucumber. In cucumber, a total of six CsBES1 genes were identified, and their phylogenetic relationships, gene structures, and cis-elements in promoters were studied. CsBES1 genes were distributed on four of seven chromosomes. Gene structure analysis showed that the intron–exon model of CsBES1 genes was conserved and the CsBES1 protein contained a DUF822-conserved motif. Promoter cis-element prediction showed that plenty of developmental and stress- and hormone-related elements have been found in promoter regions of CsBES1 genes. Meanwhile, BES1 was divided into three groups (I, II, and III) on the basis of phylogenetic relationship analysis in six plant species. In addition, CsBES1 gene expression patterns were confirmed by transcription database and qRT-PCR analysis; the results showed that the expression of CsBES1 genes had not only tissue-specific expression but also different types of CsBES1 isoform which might respond to specific plant stresses. In summary, genome-wide identification, phylogeny, gene structure, and expression profile analysis of CsBES1 genes in cucumber provided a referable theoretical information for further functional study of CsBES1 genes and further facilitated the molecular breeding of cucumber.

Control of Phytophthora melonis damping-off treated with 24-epibrassinolide and a histological study of cucumber hypocotyl

The oomycete Phytophthora melonis causes a severe disease in cucumber plants in Asia. In this study, the diameter of cucumber hypocotyl in the resistant variety 'Shantou qing gua' was significantly larger than that of the susceptible variety 'Zhongnong No. 20'. The significantly lower incidence of disease and less invasive hyphae on the epidermis and transverse section of hypocotyls in P plants of the resistant variety than those in susceptible cultivars were also observed. Brassinosteroids are a class of phytohormones that affect plant growth and development and are involved in regulating plant resistance to a variety of biotic and abiotic stresses. 24-Epibrassinolide root drenching significantly enhanced the thickening of cucumber hypocotyl. Thick hypocotyls showed strong resistance to P. melonis, indicating that it significantly reduced the incidence of disease and retarded the hyphae extension for both resistant and susceptible cucumbers. 24-Epibrassinolide pretreatment had no significant effect on the elongation of cucumber hypocotyl. Further histological observation showed that under the condition of infection with P. melonis, exogenous 24-epibrassinolide could induce lignin deposition in external phloem and xylem vessel cell wall of the cucumber hypocotyl vascular bundle. There is also an accumulation of callose in the external phloem sieve plate, which activates the resistance responses in cell walls. It is worth mentioning that in both inoculated and uninoculated conditions, exogenous 24-epibrassinolide enhanced lignin formation in external phloem and xylem vessel cell wall of the vascular bundle. This increased the content of lignin in hypocotyl as well as the number of vascular bundles in the hypocotyl base. The above results show that 24-epibrassinolide constitutively regulates the thickening of cucumber hypocotyl and the development of vascular bundle, hence preventing phytophthora infection and inducing plant resistance to disease.

Role of Jasmonic Acid Pathway in Tomato Plant-Pseudomonas syringae Interaction

The jasmonic acid pathway has been considered as the backbone of the response against necrotrophic pathogens. However, a hemi-biotrophic pathogen, such as Pseudomonas syringae, has taken advantage of the crosstalk between the different plant hormones in order to manipulate the responses for its own interest. Despite that, the way in which Pseudomonas syringae releases coronatine to activate jasmonic acid-derived responses and block the activation of salicylic acid-mediated responses is widely known. However, the implication of the jasmonic intermediates in the plant-Pseudomonas interaction is not studied yet. In this work, we analyzed the response of both, plant and bacteria using SiOPR3 tomato plants. Interestingly, SiOPR3 plants are more resistant to infection with Pseudomonas. The gene expression of bacteria showed that, in SiOPR3 plants, the activation of pathogenicity is repressed in comparison to wild type plants, suggesting that the jasmonic acid pathway might play a role in the pathogenicity of the bacteria. Moreover, treatments with JA restore the susceptibility as well as activate the expression of bacterial pathogenicity genes. The observed results suggest that a complete jasmonic acid pathway is necessary for the susceptibility of tomato plants to Pseudomonas syringae.

Different regulations of cell-type transcription by UV-B in multicellular green alga Volvox carteri

There is a scarcity of research reports on the effect of ultraviolet (UV)-B radiation on genome-wide transcriptional regulation in the multicellular green microalga including Volvox carteri (V. carteri). This microalga possesses only two cell types including mortal and motile somatic cells, as well as immortal and immotile reproductive cells. Therefore, the present study evaluated the effect of low-dose UV-B radiation on the cell-type-specific gene expression pattern of reproductive and somatic cells in an asexual life cycle of V. carteri using RNA sequence method. To this end, the separated reproductive and somatic cells were treated for 1 hour at an intensity of 0.056 mW/cm^-2 UV-B radiation. Then, a transcriptome analysis was conducted between the UV-B and white light treated groups in either of the cell types. Based on differential gene expression analyses, no differentially expressed genes were found in reproductive cells under the treatment as compared to the control group. This type of cell maintained its steady state. However, treating the somatic cells with UV-B radiation led to at least 126 differentially expressed genes compared to the untreated control group. In addition, the results of a direct comparison demonstrated a restricted and wide response to UV-B radiation in somatic cells as compared to reproductive cells. Based on the results, UV-B radiation could be involved in cell-type-specific regulation of biological pathways.

12-Oxo-Phytodienoic Acid Acts as a Regulator of Maize Defense against Corn Leaf Aphid

The corn leaf aphid (CLA; Rhopalosiphum maidis) is a phloem sap-sucking insect that attacks many cereal crops, including maize (Zea mays). We previously showed that the maize inbred line Mp708, which was developed by classical plant breeding, provides enhanced resistance to CLA. Here, using electrophysiological monitoring of aphid feeding behavior, we demonstrate that Mp708 provides phloem-mediated resistance to CLA. Furthermore, feeding by CLA on Mp708 plants enhanced callose deposition, a potential defense mechanism utilized by plants to limit aphid feeding and subsequent colonization. In maize, benzoxazinoids (BX) or BX-derived metabolites contribute to enhanced callose deposition by providing heightened resistance to CLA. However, BX and BX-derived metabolites were not significantly altered in CLA-infested Mp708 plants, indicating BX-independent defense against CLA. Evidence presented here suggests that the constitutively higher levels of 12-oxo-phytodienoic acid (OPDA) in Mp708 plants contributed to enhanced callose accumulation and heightened CLA resistance. OPDA enhanced the expression of ethylene biosynthesis and receptor genes, and the synergistic interactions of OPDA and CLA feeding significantly induced the expression of the transcripts encoding Maize insect resistance1-Cysteine Protease, a key defensive protein against insect pests, in Mp708 plants. Furthermore, exogenous application of OPDA on maize jasmonic acid-deficient plants caused enhanced callose accumulation and heightened resistance to CLA, suggesting that the OPDA-mediated resistance to CLA is independent of the jasmonic acid pathway. We further demonstrate that the signaling function of OPDA, rather than a direct toxic effect, contributes to enhanced CLA resistance in Mp708.

Crosstalk of the Brassinosteroid Signalosome with Phytohormonal and Stress Signaling Components Maintains a Balance between the Processes of Growth and Stress Tolerance

Brassinosteroids (BRs) are a class of phytohormones, which regulate various processes during plant life cycle. Intensive studies conducted with genetic, physiological and molecular approaches allowed identification of various components participating in the BR signaling—from the ligand perception, through cytoplasmic signal transduction, up to the BR-dependent gene expression, which is regulated by transcription factors and chromatin modifying enzymes. The identification of new components of the BR signaling is an ongoing process, however an emerging view of the BR signalosome indicates that this process is interconnected at various stages with other metabolic pathways. The signaling crosstalk is mediated by the BR signaling proteins, which function as components of the transmembrane BR receptor, by a cytoplasmic kinase playing a role of the major negative regulator of the BR signaling, and by the transcription factors, which regulate the BR-dependent gene expression and form a complicated regulatory system. This molecular network of interdependencies allows a balance in homeostasis of various phytohormones to be maintained. Moreover, the components of the BR signalosome interact with factors regulating plant reactions to environmental cues and stress conditions. This intricate network of interactions enables a rapid adaptation of plant metabolism to constantly changing environmental conditions.

Identification of a dioxin-responsive oxylipin signature in roots of date palm: involvement of a 9-hydroperoxide fatty acid reductase, caleosin/peroxygenase PdPXG2

Dioxins are highly hazardous pollutants that have well characterized impacts on both animal and human health. However, the biological effects of dioxins on plants have yet to be described in detail. Here we describe a dioxin-inducible caleosin/peroxygenase isoform, PdPXG2, that is mainly expressed in the apical zone of date palm roots and specifically reduces 9-hydroperoxide fatty acids. A characteristic spectrum of 18 dioxin-responsive oxylipin (DROXYL) congeners was also detected in date palm roots after exposure to dioxin. Of particular interest, six oxylipins, mostly hydroxy fatty acids, were exclusively formed in response to TCDD. The DROXYL signature was evaluated in planta and validated in vitro using a specific inhibitor of PdPXG2 in a root-protoplast system. Comparative analysis of root suberin showed that levels of certain monomers, especially the mono-epoxides and tri-hydroxides of C16:3 and C18:3, were significantly increased after exposure to TCDD. Specific inhibition of PdPXG2 activity revealed a positive linear relationship between deposition of suberin in roots and their permeability to TCDD. The results highlight the involvement of this peroxygenase in the plant response to dioxin and suggest the use of dioxin-responsive oxylipin signatures as biomarkers for plant exposure to this important class of xenobiotic contaminants.

Plant Lipid Droplets and Their Associated Proteins: Potential for Rapid Advances

Cytoplasmic lipid droplets (LDs) of neutral lipids (triacylglycerols [TAGs], sterylesters, etc.) are reserves of high-energy metabolites and other constituents for future needs. They are present in diverse cells of eukaryotes and prokaryotes. An LD has a core of neutral lipids enclosed with a monolayer of phospholipids and proteins, which play structural and/or metabolic roles. During the past 3 decades, studies of LDs in diverse organisms have blossomed after they were found to be involved in prevalent human diseases and industrial uses. LDs in plant seeds were studied before those in mammals and microbes, and the latter studies have since moved forward. Plant LDs carry a hallmark protein called oleosin, which has a long hydrophobic hairpin penetrating the TAG core and stabilizing the LD. The oleosin gene first appeared in green algae and has evolved in enhancing promoter strength, tandem repeats, and/or expression specificity, leading to the appearance of new LD organelles, such as tapetosomes in Brassicaceae. The synthesis of LDs occurs with TAG-synthesizing enzymes on the endoplasmic reticulum (ER), and nascent TAGs are sequestered in the acyl moiety region between the bilayers of phospholipids, which results in ER-LD swelling. Oleosin is synthesized on the cytosol side of the ER and extracts the LD from the ER-LD to cytosol. This extraction of LD to the cytosol is controlled solely by the innate properties of oleosin, and modified oleosin can redirect the LD to the ER lumen and then vacuoles. The breakdown of LDs requires lipase associating with core retromer and binding to peroxisomes, which then send the enzyme to LDs via tubular extensions. Two groups of LD-associated proteins, caleosin/dioxygenase/steroleosin and LD/oil body-associated proteins, participate in cellular stress defenses via enzymic activities and binding, respectively. The surface of LDs in all plant cells may be an inert refuge for these and other proteins, which exert functions on diverse cell components. Oleosin-LDs have been explored for commercial applications; successes in their uses will rely on overcoming conceptual and technical difficulties.

High-Speed Counter-Current Chromatography (HSCCC) Purification of Antifungal Hydroxy Unsaturated Fatty Acids from Plant-Seed Oil and Lactobacillus Cultures

Hydroxy unsaturated fatty acids (HUFA) can function as antifungal agents. To investigate the antifungal spectrum, that is, the scope of the in vitro fungal-inhibition activities of HUFA and their potential applications, three HUFA were produced by microbial transformation or extracted from plant-seed oils; these compounds included coriolic acid (13-hydroxy-9,11-octadecadienoic acid) from Coriaria seed oil, 10-hydroxy-12-octadecenoic acid from cultures of Lactobacillus hammesii, and 13-hydroxy-9-octadecenoic acid from cultures of Lactobacillus plantarum TMW1.460Δlah. HUFA were purified by high-speed counter-current chromatography (HSCCC), characterized by LC-MS and MS/MS, and their antifungal activities were evaluated with 15 indicator fungal strains. The HUFA had different antifungal spectra when compared with unsaturated fatty acids with comparable structures but without hydroxy groups. The inhibitory effects of HUFA specifically targeted filamentous fungi, including Aspergillus niger and Penicillium roqueforti, whereas yeasts, including Candida spp. and Saccharomyces spp., were resistant to HUFA. The findings here support the development of food applications for antifungal HUFA.

Genomic and phenotypic analysis of Vavilov's historic landraces reveals the impact of environment and genomic islands of agronomic traits

The Vavilov Institute of Plant Genetic Resources (VIR), in St. Petersburg, Russia, houses a unique genebank, with historical collections of landraces. When they were collected, the geographical distribution and genetic diversity of most crops closely reflected their historical patterns of cultivation established over the preceding millennia. We employed a combination of genomics, computational biology and phenotyping to characterize VIR's 147 chickpea accessions from Turkey and Ethiopia, representing chickpea's center of origin and a major location of secondary diversity. Genotyping by sequencing identified 14,059 segregating polymorphisms and genome-wide association studies revealed 28 GWAS hits in potential candidate genes likely to affect traits of agricultural importance. The proportion of polymorphisms shared among accessions is a strong predictor of phenotypic resemblance, and of environmental similarity between historical sampling sites. We found that 20 out of 28 polymorphisms, associated with multiple traits, including days to maturity, plant phenology, and yield-related traits such as pod number, localized to chromosome 4. We hypothesize that selection and introgression via inadvertent hybridization between more and less advanced morphotypes might have resulted in agricultural improvement genes being aggregated to genomic 'agro islands', and in genotype-to-phenotype relationships resembling widespread pleiotropy.

Plant cell wall signalling and receptor-like kinases

Communication between the extracellular matrix and the cell interior is essential for all organisms as intrinsic and extrinsic cues have to be integrated to co-ordinate development, growth, and behaviour. This applies in particular to plants, the growth and shape of which is governed by deposition and remodelling of the cell wall, a rigid, yet dynamic, extracellular network. It is thus generally assumed that cell wall surveillance pathways exist to monitor the state of the wall and, if needed, elicit compensatory responses such as altered expression of cell wall remodelling and biosynthesis genes. Here, I highlight recent advances in the field of cell wall signalling in plants, with emphasis on the role of plasma membrane receptor-like kinase complexes. In addition, possible roles for cell wall-mediated signalling beyond the maintenance of cell wall integrity are discussed.

Divergent and convergent modes of interaction between wheat and Puccinia graminis f. sp. tritici isolates revealed by the comparative gene co-expression network and genome analyses

BACKGROUND

Two opposing evolutionary constraints exert pressure on plant pathogens: one to diversify virulence factors in order to evade plant defenses, and the other to retain virulence factors critical for maintaining a compatible interaction with the plant host. To better understand how the diversified arsenals of fungal genes promote interaction with the same compatible wheat line, we performed a comparative genomic analysis of two North American isolates of Puccinia graminis f. sp. tritici (Pgt).

RESULTS

The patterns of inter-isolate divergence in the secreted candidate effector genes were compared with the levels of conservation and divergence of plant-pathogen gene co-expression networks (GCN) developed for each isolate. Comprative genomic analyses revealed substantial level of interisolate divergence in effector gene complement and sequence divergence. Gene Ontology (GO) analyses of the conserved and unique parts of the isolate-specific GCNs identified a number of conserved host pathways targeted by both isolates. Interestingly, the degree of inter-isolate sub-network conservation varied widely for the different host pathways and was positively associated with the proportion of conserved effector candidates associated with each sub-network. While different Pgt isolates tended to exploit similar wheat pathways for infection, the mode of plant-pathogen interaction varied for different pathways with some pathways being associated with the conserved set of effectors and others being linked with the diverged or isolate-specific effectors.

CONCLUSIONS

Our data suggest that at the intra-species level pathogen populations likely maintain divergent sets of effectors capable of targeting the same plant host pathways. This functional redundancy may play an important role in the dynamic of the "arms-race" between host and pathogen serving as the basis for diverse virulence strategies and creating conditions where mutations in certain effector groups will not have a major effect on the pathogen's ability to infect the host.

Lipidomic Analysis of Oxidized Fatty Acids in Plant and Algae Oils

Linoleic acid (LA) and α-linolenic acid (ALA) in plant or algae oils are precursors to oxidized fatty acid metabolites known as oxylipins. Liquid chromatography tandem mass spectrometry was used to quantify oxylipins in soybean, corn, olive, canola, and four high-oleic acid algae oils at room temperature or after heating for 10 min at 100 °C. Flaxseed oil oxylipin concentrations were determined in a follow-up experiment that compared it to soybean, canola, corn, and olive oil. Published consumption data for soybean, canola, corn, and olive oil were used to estimate daily oxylipin intake. The LA and ALA fatty acid composition of the oils was generally related to their respective oxylipin metabolites, except for olive and flaxseed oil, which had higher LA derived monohydroxy and ketone oxylipins than other oils, despite their low LA content. Algae oils had the least amount of oxylipins. The change in oxylipin concentrations was not significantly different among the oils after short-term heating. The estimated oxylipin intake from nonheated soybean, canola, corn, and olive oil was 1.1 mg per person per day. These findings suggest that oils represent a dietary source of LA and ALA derived oxylipins and that the response of oils to short-term heating does not differ among the various oils.

Targeted and Untargeted Approaches Unravel Novel Candidate Genes and Diagnostic SNPs for Quantitative Resistance of the Potato (Solanum tuberosum L.) to Phytophthora infestans Causing the Late Blight Disease

The oomycete Phytophthora infestans causes late blight of potato, which can completely destroy the crop. Therefore, for the past 160 years, late blight has been the most important potato disease worldwide. The identification of cultivars with high and durable field resistance to P. infestans is an objective of most potato breeding programs. This type of resistance is polygenic and therefore quantitative. Its evaluation requires multi-year and location trials. Furthermore, quantitative resistance to late blight correlates with late plant maturity, a negative agricultural trait. Knowledge of the molecular genetic basis of quantitative resistance to late blight not compromised by late maturity is very limited. It is however essential for developing diagnostic DNA markers that facilitate the efficient combination of superior resistance alleles in improved cultivars. We used association genetics in a population of 184 tetraploid potato cultivars in order to identify single nucleotide polymorphisms (SNPs) that are associated with maturity corrected resistance (MCR) to late blight. The population was genotyped for almost 9000 SNPs from three different sources. The first source was candidate genes specifically selected for their function in the jasmonate pathway. The second source was novel candidate genes selected based on comparative transcript profiling (RNA-Seq) of groups of genotypes with contrasting levels of quantitative resistance to P. infestans. The third source was the first generation 8.3k SolCAP SNP genotyping array available in potato for genome wide association studies (GWAS). Twenty seven SNPs from all three sources showed robust association with MCR. Some of those were located in genes that are strong candidates for directly controlling quantitative resistance, based on functional annotation. Most important were: a lipoxygenase (jasmonate pathway), a 3-hydroxy-3-methylglutaryl coenzyme A reductase (mevalonate pathway), a P450 protein (terpene biosynthesis), a transcription factor and a homolog of a major gene for resistance to P. infestans from the wild potato species Solanum venturii. The candidate gene approach and GWAS complemented each other as they identified different genes. The results of this study provide new insight in the molecular genetic basis of quantitative resistance in potato and a toolbox of diagnostic SNP markers for breeding applications.

Screen Identifying Arabidopsis Transcription Factors Involved in the Response to 9-Lipoxygenase-Derived Oxylipins

13-Lipoxygenase-derived oxylipins, such as jasmonates act as potent signaling molecules in plants. Although experimental evidence supports the impact of oxylipins generated by the 9-Lipoxygenase (9-LOX) pathway in root development and pathogen defense, their signaling function in plants remains largely elusive. Based on the root growth inhibiting properties of the 9-LOX-oxylipin 9-HOT (9-hydroxy-10,12,15-octadecatrienoic acid), we established a screening approach aiming at identifying transcription factors (TFs) involved in signaling and/or metabolism of this oxylipin. Making use of the AtTORF-Ex (ArabidopsisthalianaTranscription Factor Open Reading Frame Expression) collection of plant lines overexpressing TF genes, we screened for those TFs which restore root growth on 9-HOT. Out of 6,000 lines, eight TFs were recovered at least three times and were therefore selected for detailed analysis. Overexpression of the basic leucine Zipper (bZIP) TF TGA5 and its target, the monoxygenase CYP81D11 reduced the effect of added 9-HOT, presumably due to activation of a detoxification pathway. The highly related ETHYLENE RESPONSE FACTORs ERF106 and ERF107 induce a broad detoxification response towards 9-LOX-oxylipins and xenobiotic compounds. From a set of 18 related group S-bZIP factors isolated in the screen, bZIP11 is known to participate in auxin-mediated root growth and may connect oxylipins to root meristem function. The TF candidates isolated in this screen provide starting points for further attempts to dissect putative signaling pathways involving 9-LOX-derived oxylipins.