Maize 9-lipoxygenase ZmLOX3 controls development, root-specific expression of defense genes, and resistance to root-knot nematodes

Cyclophilin 20-3 coordinates plant root hair growth and resistance against parasitic nematodes

Plant parasitic nematodes (PPN) are a major threat to agriculturally important crops, resulting in substantial yield losses and economic repercussions. However, the underlying modes of plant-PPN interactions remain largely elusive. Here, we describe a critical role of cyclophilin (CYP)20-3, a plastid dual enzyme [i.e., peptidyl-prolyl isomerase (PPIase) and reductase) in plant basal resistance against PPN attacks. Originally, in order to define a present working model of whether plant roots deploy hypersensitive response (HR) to restrict PPN infections, we co-imaged the 'real-time' interactions of a proposed HR system, cotton LONREN-1 vs. Rotylenchulus reniformis. The root imaginings, however, revealed no clear HR pattern, instead underpinning a negative relationship between PPN populations and extended root hair growth. The latter was then identified to couple with the spatial expression of PPIases, including homologs of CYP20-3, a known receptor of 12-oxophytodienoic acid (OPDA) signal. To elaborate these findings further, we employed a reverse generic approach using a model plant Arabidopsis, and illuminated that knockout cyp20-3 mutants i) abnormalize root hair formations and ii) enhance susceptibility to PPN, Meloidogyne hapla, challenges. Nevertheless, M. hapla infections did not induce OPDA synthesis and signaling marker gene expressions in Arabidopsis roots. In parallel, transgenic Arabidopsis plants overexpressing mutant CYP20-3s defective OPDA-binding/signaling (H140Q) or PPIase (F74L) could still improve plant PPN defenses, whereas the overexpression of CYP20-3C129S (-reductase) demonstrated WT-level galling formations. Thus, we conclude that OPDA-independent CYP20-3-reductase signaling plays a key role in the plant defense metabolic pathway, fortifying protective barriers and conferring innate resistance against PPN attacks.

Low temperature storage alleviates aging of paddy by reducing lipid degradation and peroxidation

The quality deterioration of paddy during storage is closely associated to lipid metabolism. To explore the effect of lipid metabolism on the texture of paddy, freshly harvested Nanjingxiangzhan (Indica rice) stored for 60 days at 15 °C and 25 °C for 60 days was investigated. Paddy stored at 15 °C showed higher contents of ATP, phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, triacylglycerols, diacylglycerols, monogalactosyldiacylglycerol and digalactosyldiacylglycerol but lower levels of lysophosphatidylcholine, lysophosphatidylethanolamine, phosphatidic acid, ceramides and free fatty acids. Storage at 15 °C inhibited lipase, phospholipase D (PLD), lipoxygenase (LOX) activities and the corresponding gene expressions. Moreover, 15 °C storage retarded the rise of hardness, cohesiveness and chewiness, while delayed the reduction of gumminess and springiness. These findings suggested that maintenance in glycerophospholipids, glycerolipids and saccharolipids abundance, reduction in lysophospholipids, phosphatidic acid, ceramides and free fatty acids accumulation could contribute to enhanced internal resistance to aging in freshly harvested paddy at low temperature storage.

Identification and Expression Analysis of Lipoxygenase Gene in Bitter Gourd (Momordica charantia)

BACKGROUND

Lipoxygenases (LOXs) are key enzymes in the unsaturated fatty acid oxidation reaction pathway and play an important regulatory role in the synthesis of fruit aroma volatiles.

METHODS

LOX gene family members were identified in the whole genome database of bitter gourd and analyzed bioinformatically. An RT-qPCR was used to analyze the expression differences in different tissues. Monoterpenes were determined by gas chromatography-mass spectrometry (GC-MS) technique.

RESULTS

A total of 12 LOX gene family members were identified in the genome. The expression of LOX genes varied significantly among the tissues of roots, stems, leaves, flowers, fruits, seeds and tendrils. A total of 29 monoterpenes were detected in the fruits of five different fruit colors of bitter gourd, mainly containing six types of alcohols, aldehydes, terpenes, ketones, esters and alkynes, with the highest relative content of alcohols.

CONCLUSIONS

The present study provides a reference for further elucidation of the biological functions of the LOX gene in the synthesis pathway of aroma volatiles in bitter gourd.

A Quantitative Trait Locus on Maize Chromosome 5 Is Associated with Root-Knot Nematode Resistance

This study provides the first report of a quantitative trait locus (QTL) in maize (Zea mays) for resistance to the southern root-knot nematode (SRKN) (Meloidogyne incognita). The SRKN can feed on the roots of maize in the U.S. Southern Coastal Plain region and can cause yield losses of 30% or more in heavily infested fields. Increases in SRKN density in the soil may reduce the yield for subsequently planted susceptible crops. The use of maize hybrids with resistance to SRKN could prevent an increase in SRKN density, yet no genetic regions have been identified that confer host resistance. In this study, a B73 (susceptible) × Ky21 (resistant) S5 recombinant inbred line (RIL) population was phenotyped for total number of eggs (TE) and root weight. This population had been genotyped using single-nucleotide polymorphisms (SNPs). By utilizing the SNP data with the phenotype data, a single QTL was identified on chromosome 5 that explained 15% of the phenotypic variation (PV) for the number of eggs and 11% of the PV for the number of eggs per gram of root (EGR). Plants that were homozygous for the Ky21 allele for the most associated marker PZA03172.3 had fewer eggs and fewer EGR than the plants that were homozygous or heterozygous for the B73 allele. Thus, the first QTL for SRKN resistance in maize has been identified and could be incorporated into maize hybrids.

Divergent colonization traits, convergent benefits: different species of arbuscular mycorrhizal fungi alleviate Meloidogyne incognita damage in tomato

Arbuscular mycorrhizal fungi (AMF) can increase plant tolerance and/or resistance to pests such as the root-knot nematode Meloidogyne incognita. However, the ameliorative effects may depend on AMF species. The aim of this work was therefore to evaluate whether four AMF species differentially affect plant performance in response to M. incognita infection. Tomato plants grown in greenhouse conditions were inoculated with four different AMF isolates (Claroideoglomus claroideum, Funneliformis mosseae, Gigaspora margarita, and Rhizophagus intraradices) and infected with 100 second stage juveniles of M. incognita at two different times: simultaneously or 2 weeks after the inoculation with AMF. After 60 days, the number of galls, egg masses, and reproduction factor of the nematodes were assessed along with plant biomass, phosphorus (P), and nitrogen concentrations in roots and shoots and root colonization by AMF. Only the simultaneous nematode inoculation without AMF caused a large reduction in plant shoot biomass, while all AMF species were able to ameliorate this effect and improve plant P uptake. The AMF isolates responded differently to the interaction with nematodes, either increasing the frequency of vesicles (C. claroideum) or reducing the number of arbuscules (F. mosseae and Gi. margarita). AMF inoculation did not decrease galls; however, it reduced the number of egg masses per gall in nematode simultaneous inoculation, except for C. claroideum. This work shows the importance of biotic stress alleviation associated with an improvement in P uptake and mediated by four different AMF species, irrespective of their fungal root colonization levels and specific interactions with the parasite.

Insights into the molecular mechanism of Trichoderma stimulating plant growth and immunity against phytopathogens

Trichoderma species have received significant interest as beneficial fungi for boosting plant growth and immunity against phytopathogens. By establishing a mutualistic relationship with plants, Trichoderma causes a series of intricate signaling events that eventually promote plant growth and improve disease resistance. The mechanisms contain the indirect or direct involvement of Trichoderma in enhancing plant growth by modulating phytohormones signaling pathways, improving uptake and accumulation of nutrients, and increasing soil bioavailability of nutrients. They contribute to plant resistance by stimulating systemic acquired resistance through salicylic acid, jasmonic acid, and ethylene signaling. A cascade of signal transduction processes initiated by the interaction of Trichoderma and plants regulate the expression of defense-related genes, resulting in the synthesis of defense hormones and pathogenesis-related proteins (PRPs), which collectively improve plant resistance. Additionally, advancements in omics technologies has led to the identification of key pathways, their regulating genes, and molecular interactions in the plant defense and growth promotion responses induced by Trichoderma. Deciphering the molecular mechanism behind Trichoderma's induction of plant defense and immunity is essential for harnessing the full plant beneficial potential of Trichoderma. This review article sheds light on the molecular mechanisms that underlie the positive effects of Trichoderma-induced plant immunity and growth and opens new opportunities for developing environmentally friendly and innovative approaches to improve plant immunity and growth.

Combination of transcriptomic, proteomic, and degradomic profiling reveals common and distinct patterns of pathogen-induced cell death in maize

Regulated cell death (RCD) is crucial for plant development, as well as in decision-making in plant-microbe interactions. Previous studies revealed components of the molecular network controlling RCD, including different proteases. However, the identity, the proteolytic network as well as molecular components involved in the initiation and execution of distinct plant RCD processes, still remain largely elusive. In this study, we analyzed the transcriptome, proteome, and N-terminome of Zea mays leaves treated with the Xanthomonas effector avrRxo1, the mycotoxin Fumonisin B1 (FB1), or the phytohormone salicylic acid (SA) to dissect plant cellular processes related to cell death and plant immunity. We found highly distinct and time-dependent biological processes being activated on transcriptional and proteome levels in response to avrRxo1, FB1, and SA. Correlation analysis of the transcriptome and proteome identified general, as well as trigger-specific markers for cell death in Zea mays. We found that proteases, particularly papain-like cysteine proteases, are specifically regulated during RCD. Collectively, this study characterizes distinct RCD responses in Z. mays and provides a framework for the mechanistic exploration of components involved in the initiation and execution of cell death.

Cytokinin Promotes Jasmonic Acid Accumulation in the Control of Maize Leaf Growth

Plant organ growth results from the combined activity of cell division and cell expansion. The co-ordination of these two processes depends on the interplay between multiple hormones that determine the final organ size. Using the semidominant Hairy Sheath Frayed1 (Hsf1) maize mutant that hypersignals the perception of cytokinin (CK), we show that CK can reduce leaf size and growth rate by decreasing cell division. Linked to CK hypersignaling, the Hsf1 mutant has an increased jasmonic acid (JA) content, a hormone that can inhibit cell division. The treatment of wild-type seedlings with exogenous JA reduces maize leaf size and growth rate, while JA-deficient maize mutants have increased leaf size and growth rate. Expression analysis revealed the increased transcript accumulation of several JA pathway genes in the Hsf1 leaf growth zone. A transient treatment of growing wild-type maize shoots with exogenous CK also induced the expression of JA biosynthetic genes, although this effect was blocked by the co-treatment with cycloheximide. Together, our results suggest that CK can promote JA accumulation, possibly through the increased expression of specific JA pathway genes.

Hub Gene Mining and Co-Expression Network Construction of Low-Temperature Response in Maize of Seedling by WGCNA

Weighted gene co-expression network analysis (WGCNA) is a research method in systematic biology. It is widely used to identify gene modules related to target traits in multi-sample transcriptome data. In order to further explore the molecular mechanism of maize response to low-temperature stress at the seedling stage, B144 (cold stress tolerant) and Q319 (cold stress sensitive) provided by the Maize Research Institute of Heilongjiang Academy of Agricultural Sciences were used as experimental materials, and both inbred lines were treated with 5 °C for 0 h, 12 h, and 24 h, with the untreated material as a control. Eighteen leaf samples were used for transcriptome sequencing, with three biological replicates. Based on the above transcriptome data, co-expression networks of weighted genes associated with low-temperature-tolerance traits were constructed by WGCNA. Twelve gene modules significantly related to low-temperature tolerance at the seedling stage were obtained, and a number of hub genes involved in low-temperature stress regulation pathways were discovered from the four modules with the highest correlation with target traits. These results provide clues for further study on the molecular genetic mechanisms of low-temperature tolerance in maize at the seedling stage.

9,10-KODA, an α-ketol produced by the tonoplast-localized 9-lipoxygenase ZmLOX5, plays a signaling role in maize defense against insect herbivory

13-Lipoxygenases (LOXs) initiate the synthesis of jasmonic acid (JA), the best-understood oxylipin hormone in herbivory defense. However, the roles of 9-LOX-derived oxylipins in insect resistance remain unclear. Here, we report a novel anti-herbivory mechanism mediated by a tonoplast-localized 9-LOX, ZmLOX5, and its linolenic acid-derived product, 9-hydroxy-10-oxo-12(Z),15(Z)-octadecadienoic acid (9,10-KODA). Transposon-insertional disruption of ZmLOX5 resulted in the loss of resistance to insect herbivory. lox5 knockout mutants displayed greatly reduced wound-induced accumulation of multiple oxylipins and defense metabolites, including benzoxazinoids, abscisic acid (ABA), and JA-isoleucine (JA-Ile). However, exogenous JA-Ile failed to rescue insect defense in lox5 mutants, while applications of 1 μM 9,10-KODA or the JA precursor, 12-oxo-phytodienoic acid (12-OPDA), restored wild-type resistance levels. Metabolite profiling revealed that exogenous 9,10-KODA primed the plants for increased production of ABA and 12-OPDA, but not JA-Ile. While none of the 9-oxylipins were able to rescue JA-Ile induction, the lox5 mutant accumulated lower wound-induced levels of Ca2+, suggesting this as a potential explanation for lower wound-induced JA. Seedlings pretreated with 9,10-KODA exhibited rapid or more robust wound-induced defense gene expression. In addition, an artificial diet supplemented with 9,10-KODA arrested fall armyworm larvae growth. Finally, analysis of single and double lox5 and lox10 mutants showed that ZmLOX5 also contributed to insect defense by modulating ZmLOX10-mediated green leaf volatile signaling. Collectively, our study uncovered a previously unknown anti-herbivore defense and hormone-like signaling activity for a major 9-oxylipin α-ketol.

Integrative transcriptome and proteome analysis reveals maize responses to Fusarium verticillioides infection inside the stalks

Fusarium stalk rot caused by Fusarium verticillioides is one of the most devastating diseases of maize that causes significant yield losses and poses potential security concerns for foods worldwide. The underlying mechanisms of maize plants regulating defence against the disease remain poorly understood. Here, integrative proteomic and transcriptomic analyses were employed to identify pathogenesis-related protein genes by comparing differentially expressed proteins (DEPs) and differentially expressed genes (DEGs) in maize stalks after inoculation with F. verticillioides. Functional enrichment analysis showed that DEGs and DEPs were mainly enriched in glutathione metabolism, starch and sucrose metabolism, amino sugar and nucleotide sugar metabolism, linoleic acid metabolism, and phenylpropanoid biosynthesis. Fourteen DEGs and DEGs that were highly elevated after inoculation with F. verticillioides were confirmed with parallel reaction monitoring and reverse transcription-quantitative PCR, demonstrating the accountability and reliability of proteomic and transcriptomic data. We also assessed the potential roles of defence-related genes ZmCTA1, ZmWIP1, and ZmLOX2, identified from the multi-omics analysis, during the process of F. verticillioides infection through virus-induced gene silencing. The elevation of stalk rot symptomatic characteristics in the silenced plants revealed their contribution to resistance. We further functionally characterized the roles of ZmLOX2 expression in the defence response of maize plants conditioning fungal invasion via the salicylic acid-dependent pathway. Collectively, this study provides a comprehensive analysis of transcriptome and proteome of maize stalks following F. verticillioides inoculation, and defence-related genes that could inform selection of new genes as targets in breeding strategies.

Ketols Emerge as Potent Oxylipin Signals Regulating Diverse Physiological Processes in Plants

Plants produce an array of oxylipins implicated in defense responses against various stresses, with about 600 oxylipins identified in plants to date. Most known oxylipins are the products of lipoxygenase (LOX)-mediated oxygenation of polyunsaturated fatty acids. One of the most well-characterized oxylipins produced by plants is the hormone jasmonic acid (JA); however, the function of the vast majority of oxylipins remains a mystery. One of the lesser-studied groups of oxylipins is comprised of ketols produced by the sequential action of LOX, allene oxide synthase (AOS), followed by non-enzymatic hydrolysis. For decades, ketols were mostly considered mere by-products of JA biosynthesis. Recent accumulating evidence suggests that ketols exhibit hormone-like signaling activities in the regulation of diverse physiological processes, including flowering, germination, plant-symbiont interactions, and defense against biotic and abiotic stresses. To complement multiple reviews on jasmonate and overall oxylipin biology, this review focuses specifically on advancing our understanding of ketol biosynthesis, occurrence, and proposed functions in diverse physiological processes.

Novel insights into maize (Zea mays) development and organogenesis for agricultural optimization

In maize, intrinsic hormone activities and sap fluxes facilitate organogenesis patterning and plant holistic development; these hormone movements should be a primary focus of developmental biology and agricultural optimization strategies. Maize (Zea mays) is an important crop plant with distinctive life history characteristics and structural features. Genetic studies have extended our knowledge of maize developmental processes, genetics, and molecular ecophysiology. In this review, the classical life cycle and life history strategies of maize are analyzed to identify spatiotemporal organogenesis properties and develop a definitive understanding of maize development. The actions of genes and hormones involved in maize organogenesis and sex determination, along with potential molecular mechanisms, are investigated, with findings suggesting central roles of auxin and cytokinins in regulating maize holistic development. Furthermore, investigation of morphological and structural characteristics of maize, particularly node ubiquity and the alternate attachment pattern of lateral organs, yields a novel regulatory model suggesting that maize organ initiation and subsequent development are derived from the stimulation and interaction of auxin and cytokinin fluxes. Propositions that hormone activities and sap flow pathways control organogenesis are thoroughly explored, and initiation and development processes of distinctive maize organs are discussed. Analysis of physiological factors driving hormone and sap movement implicates cues of whole-plant activity for hormone and sap fluxes to stimulate maize inflorescence initiation and organ identity determination. The physiological origins and biogenetic mechanisms underlying maize floral sex determination occurring at the tassel and ear spikelet are thoroughly investigated. The comprehensive outline of maize development and morphogenetic physiology developed in this review will enable farmers to optimize field management and will provide a reference for de novo crop domestication and germplasm improvement using genome editing biotechnologies, promoting agricultural optimization.

Oxylipins and Reactive Carbonyls as Regulators of the Plant Redox and Reactive Oxygen Species Network under Stress

Reactive oxygen species (ROS), and in particular H2O2, serve as essential second messengers at low concentrations. However, excessive ROS accumulation leads to severe and irreversible cell damage. Hence, control of ROS levels is needed, especially under non-optimal growth conditions caused by abiotic or biotic stresses, which at least initially stimulate ROS synthesis. A complex network of thiol-sensitive proteins is instrumental in realizing tight ROS control; this is called the redox regulatory network. It consists of sensors, input elements, transmitters, and targets. Recent evidence revealed that the interplay of the redox network and oxylipins–molecules derived from oxygenation of polyunsaturated fatty acids, especially under high ROS levels–plays a decisive role in coupling ROS generation and subsequent stress defense signaling pathways in plants. This review aims to provide a broad overview of the current knowledge on the interaction of distinct oxylipins generated enzymatically (12-OPDA, 4-HNE, phytoprostanes) or non-enzymatically (MDA, acrolein) and components of the redox network. Further, recent findings on the contribution of oxylipins to environmental acclimatization will be discussed using flooding, herbivory, and establishment of thermotolerance as prime examples of relevant biotic and abiotic stresses.

Cloning and functional analysis of the root-knot nematode resistance gene NtRk1 in tobacco

Tobacco (Nicotiana tabacum L.) is an economically important crop worldwide. Root-knot nematodes (RKNs) are responsible for yield losses in tobacco and other crops, such as tomato, potato, peanut, and soybean. Therefore, screening for resistance genes that can prevent RKN infestation and the associated damage is crucial. However, there is no report of cloning tobacco RKN resistance genes to date. Here, we cloned the tobacco RKN resistance gene NtRk1 from the resistant variety TI706, using rapid amplification of cDNA ends. NtRk1 has high homology with other RKN resistance genes (CaMi in pepper, Mi-1.1 and Mi-1.2 in tomato). Under normal conditions, NtRk1 was barely expressed in the roots; however, following RKN infection, its expression level rapidly increased. Overexpression of NtRk1 in the susceptible cultivar "Changbohuang" enhanced its resistance to Meloidogyne incognita, while RNA interference of NtRk1 in the resistant cultivar K326 resulted in its susceptibility to M. incognita. Moreover, compared with resistant variety K326, we found the salicylic acid and jasmonic acid contents of RNAi plants decreased after inoculation with M. incognita, and confirmed that the function of NtRk1 is related to these phytohormones. These findings indicate that NtRk1 is an RKN resistance gene, which is abundantly expressed in response to RKN infection and may enhance host defense responses by elevating salicylic acid and jasmonic acid levels.

Recognition and Response in Plant-Nematode Interactions

Plant-parasitic nematodes spend much of their lives inside or in contact with host tissue, and molecular interactions constantly occur and shape the outcome of parasitism. Eggs of these parasites generally hatch in the soil, and the juveniles must locate and infect an appropriate host before their stored energy is exhausted. Components of host exudate are evaluated by the nematode and direct its migration to its infection site. Host plants recognize approaching nematodes before physical contact through molecules released by the nematodes and launch a defense response. In turn, nematodes deploy numerous mechanisms to counteract plant defenses. This review focuses on these early stages of the interaction between plants and nematodes. We discuss how nematodes perceive and find suitable hosts, how plants perceive and mount a defense response against the approaching parasites, and how nematodes fight back against host defenses.

Effects of Arugula Vermicompost on the Root-Knot Nematode (Meloidogyne javanica) and the Promotion of Resistance Genes in Tomato Plants

Root-knot nematodes are the most important plantparasitic nematodes worldwide. Many efforts have been made to find non-chemical, risk-free, and environmentally friendly methods for nematode control. In this study, the effects of compost and vermicompost of arugula (Eruca sativa) on Meloidogyne javanica were investigated in three glasshouse experiments. In addition, the expression of the defense-related genes nonexpressor of pathogenesis-related 1 (NPR1) and lipoxygenase 1 (LOX1) was detected in tomato plants treated with vermicompost of arugula at 0, 2, 7, and 14 days after nematode inoculation. The result showed that the vermicompost of arugula significantly reduced the reproduction factor of the nematode by 54.4% to 70.5% in the three experiments and increased the dry weight of shoots of infected tomato plants. Gene expression analysis showed that LOX1 expression increased on the second and seventh day after nematode inoculation, while NPR1 expression decreased. The vermicompost of arugula showed stronger nematode inhibitory potential than the vermicompost of animal manure. The vermicompost of arugula is superior to arugula compost in suppressing the activity of M. javaniva and reducing its impact. It manipulates the expression of resistance genes and could induce systemic resistance against rootknot nematodes.

Effector-mediated relocalization of a maize lipoxygenase protein triggers susceptibility to Ustilago maydis

As the gall-inducing smut fungus Ustilago maydis colonizes maize (Zea mays) plants, it secretes a complex effector blend that suppresses host defense responses, including production of reactive oxygen species (ROS) and redirects host metabolism to facilitate colonization. We show that the U. maydis effector ROS burst interfering protein 1 (Rip1), which is involved in pathogen-associated molecular pattern (PAMP)-triggered suppression of host immunity, is functionally conserved in several other monocot-infecting smut fungi. We also have identified a conserved C-terminal motif essential for Rip1-mediated PAMP-triggered suppression of the ROS burst. The maize susceptibility factor lipoxygenase 3 (Zmlox3) bound by Rip1 was relocalized to the nucleus, leading to partial suppression of the ROS burst. Relocalization was independent of its enzymatic activity, revealing a distinct function for ZmLox3. Most importantly, whereas Zmlox3 maize mutant plants showed increased resistance to U. maydis wild-type strains, rip1 deletion strains infecting the Zmlox3 mutant overcame this effect. This could indicate that Rip1-triggered host resistance depends on ZmLox3 to be suppressed and that lox3 mutation-based resistance of maize to U. maydis requires functional Rip1. Together, our results reveal that Rip1 acts in several cellular compartments to suppress immunity and that targeting of ZmLox3 by Rip1 is responsible for the suppression of Rip1-dependent reduced susceptibility of maize to U. maydis.

Arabidopsis thaliana AtHRS1 gene is involved in the response to Heterodera schachtii infection and its overexpression hampers development of syncytia and involves a jasmonic acid-dependent mechanism

Sedentary plant parasitic nematodes have developed competences to reprogram host plant cell metabolism via sophisticated manipulation of gene expression, leading to the formation of permanent feeding sites for an unlimited source of food. Arabidopsis thaliana and the beet cyst nematode Heterodera schachtii is a good model for studying the mechanisms of compatible plant-nematode interactions and basic plant responses to nematode infection. Transcription factors are proteins that modulate plant reactions during regular development and under different biotic and abiotic stresses via direct binding to promoter regions of genes. Here, we report on the AtHRS1 gene encoding a MYB-related transcription factor belonging to the GARP family, whose expression is downregulated in syncytia, as confirmed by gene expression analysis. Constitutive overexpression of AtHRS1 disturbed the development of nematode-induced syncytia and led to a reduction in the number of developed females in transgenic A. thaliana roots. In contrast, the hrs1 mutant with decreased expression of AtHRS1 was more susceptible to cyst nematode infection. The influence of AtHRS1 on selected elements of the JA-dependent defence pathway suggests its mode of action in plant response to nematode attack. Based on these results, we suggest that the downregulation of AtHRS1 expression by nematode is important for its successful development.

Hypoxia-Induced Aquaporins and Regulation of Redox Homeostasis by a Trans-Plasma Membrane Electron Transport System in Maize Roots

In plants, flooding-induced oxygen deficiency causes severe stress, leading to growth reduction and yield loss. It is therefore important to understand the molecular mechanisms for adaptation to hypoxia. Aquaporins at the plasma membrane play a crucial role in water uptake. However, their role during hypoxia and membrane redox changes is still not fully understood. The influence of 24 h hypoxia induction on hydroponically grown maize (Zea mays L.) was investigated using an oil-based setup. Analyses of physiological parameters revealed typical flooding symptoms such as increased ethylene and H₂O₂ levels, an increased alcohol dehydrogenase activity, and an increased redox activity at the plasma membrane along with decreased oxygen of the medium. Transcriptomic analysis and shotgun proteomics of plasma membranes and soluble fractions were performed to determine alterations in maize roots. RNA-sequencing data confirmed the upregulation of genes involved in anaerobic metabolism, biosynthesis of the phytohormone ethylene, and its receptors. Transcripts of several antioxidative systems and other oxidoreductases were regulated. Mass spectrometry analysis of the plasma membrane proteome revealed alterations in redox systems and an increased abundance of aquaporins. Here, we discuss the importance of plasma membrane aquaporins and redox systems in hypoxia stress response, including the regulation of plant growth and redox homeostasis.

Identification of the LOX Gene Family in Peanut and Functional Characterization of AhLOX29 in Drought Tolerance

Lipoxygenases (LOXs) are a gene family of nonheme iron-containing dioxygenases that play important roles in plant development and defense responses. To date, a comprehensive analysis of LOX genes and their biological functions in response to abiotic stresses in peanut has not been performed. In this study, a total of 72 putative LOX genes were identified in cultivated (Arachis hypogaea) and wild-type peanut (Arachis duranensis and Arachis ipaensis) and classified into three subfamilies: 9-LOX, type I 13-LOX and type II 13-LOX. The gene structures and protein motifs of these peanut LOX genes were highly conserved among most LOXs. We found that the chromosomal distribution of peanut LOXs was not random and that gene duplication played a crucial role in the expansion of the LOX gene family. Cis-acting elements related to development, hormones, and biotic and abiotic stresses were identified in the promoters of peanut LOX genes. The expression patterns of peanut LOX genes were tissue-specific and stress-inducible. Quantitative real-time PCR results further confirmed that peanut LOX gene expression could be induced by drought, salt, methyl jasmonate and abscisic acid treatments, and these genes exhibited diverse expression patterns. Furthermore, overexpression of AhLOX29 in Arabidopsis enhanced the resistance to drought stress. Compared with wide-type, AhLOX29-overexpressing plants showed significantly decreased malondialdehyde contents, as well as increased chlorophyll degradation, proline accumulation and superoxide dismutase activity, suggesting that the transgenic plants exhibit strengthened capacity to scavenge reactive oxygen species and prevent membrane damage. This systematic study provides valuable information about the functional characteristics of AhLOXs in the regulation of abiotic stress responses of peanut.

Jasmonic acid biosynthetic genes TgLOX4 and TgLOX5 are involved in daughter bulb development in tulip (Tulipa gesneriana)

The tulip bulbs are modified underground stems which originate from axillary meristems of mother bulb scales. Hormones including jasmonic acids (JAs) play key roles in regulating tulip bulb development. Here, we compared variations of daughter bulb development through transcriptomic profiling analysis and characterized the functions of JA biosynthesis related genes during daughter bulb enlargement. The results showed that tulip varieties exhibited contrasting bulb size variations. Transcriptomic analyses revealed that genes involved in plant hormones and development were significantly changed following tulip bulb growth, including two lipoxygenase genes TgLOX4 and TgLOX5. Ectopic overexpression of TgLOX4 and TgLOX5 in Arabidopsis enhanced endogenous JA content, improved plant growth and increased lateral root numbers. Silencing of these two genes in tulip repressed the growth of daughter bulbs. Furthermore, exogenous JA treatment promoted tulip bulb growth, whereas JA biosynthesis inhibitor sodium diethyldithiocarbamate (DIECA) inhibited this process. This study offers supporting evidence for the involvement of tulip TgLOX4 and TgLOX5 in the regulation of daughter bulb growth and development.

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.

Fusarium verticillioides Induces Maize-Derived Ethylene to Promote Virulence by Engaging Fungal G-Protein Signaling

Seed maceration and contamination with mycotoxin fumonisin inflicted by Fusarium verticillioides is a major disease concern for maize producers worldwide. Meta-analyses of quantitative trait loci for Fusarium ear rot resistance uncovered several ethylene (ET) biosynthesis and signaling genes within them, implicating ET in maize interactions with F. verticillioides. We tested this hypothesis using maize knockout mutants of the 1-aminocyclopropane-1-carboxylate (ACC) synthases ZmACS2 and ZmACS6. Infected wild-type seed emitted five-fold higher ET levels compared with controls, whereas ET was abolished in the acs2 and acs6 single and double mutants. The mutants supported reduced fungal biomass, conidia, and fumonisin content. Normal susceptibility was restored in the acs6 mutant with exogenous treatment of ET precursor ACC. Subsequently, we showed that fungal G-protein signaling is required for virulence via induction of maize-produced ET. F. verticillioides G_β subunit and two regulators of G-protein signaling mutants displayed reduced seed colonization and decreased ET levels. These defects were rescued by exogenous application of ACC. We concluded that pathogen-induced ET facilitates F. verticillioides colonization of seed, and, in turn, host ET production is manipulated via G-protein signaling of F. verticillioides to facilitate pathogenesis.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Evolution and functional diversity of lipoxygenase (LOX) genes in allotetraploid rapeseed (Brassica napus L.)

Lipoxygenase (LOX, EC 1.13.11.12) is a non-haeme iron-containing dioxygenase family that catalyzes the oxygenation of polyunsaturated fatty acids into bio-functionally fatty acid diverse (oxylipins) and plays vital role in plant growth and development and responses to abiotic and biotic stresses. Though LOX genes have been studied in many plant species, their roles in Brassicaceae species are still unknown. Here, a set of 14, 18, and 33 putative LOX genes were identified in Brassica rapa, Brassica oleracea and Brassica napus (allotetraploid rapeseed), respectively, which could be divided into 9-LOX (LOX1/5), 13-LOX type I (LOX3/4/6), and type II (LOX2) subgroups. There was an expansion of LOX2 orthologous genes in Brassicaceae. Most of the LOX genes are intron rich and conserved in gene structure, and the LOX proteins all have the conserved lipoxygenase and PLAT/LH2 domain. Ka/Ks ratio revealed that the majority of LOXs underwent purifying selection in Brassicaceae. The light-, ABA-, MeJA-related cis-elements and MYB-binding sites in the promoters of BnaLOXs were the most abundant. BnaLOXs displayed different spatiotemporal expression patterns and various abiotic/biotic stress responsive expression patterns. BnaLOX1/5 were slightly or no response to phytohormones and abiotic stresses. BnaLOX3/4/6 predominantly express in roots and were strongly up-regulated by salinity and PEG treatments, and BnaLOX3/4 were the methyl jasmonate (MeJA) and salicylic acid (SA) early response genes and strongly induced by infection of Sclerotinia sclerotiorum; while the BnaLOX2 members predominantly express in stamens, were MeJA and SA continuous response genes and strongly repressed by cold, heat and waterlogging treatments in leaves. Our results are useful for understanding the biological functions of the BnaLOX genes in allotetraploid rapeseed.

Characterization of full-length transcriptome in Saccharum officinarum and molecular insights into tiller development

BACKGROUND

Although extensive breeding efforts are ongoing in sugarcane (Saccharum officinarum L.), the average yield is far below the theoretical potential. Tillering is an important component of sugarcane yield, however, the molecular mechanism underlying tiller development is still elusive. The limited genomic data in sugarcane, particularly due to its complex and large genome, has hindered in-depth molecular studies.

RESULTS

Herein, we generated full-length (FL) transcriptome from developing leaf and tiller bud samples based on PacBio Iso-Seq. In addition, we performed RNA-seq from tiller bud samples at three developmental stages (T0, T1 and T2) to uncover key genes and biological pathways involved in sugarcane tiller development. In total, 30,360 and 20,088 high-quality non-redundant isoforms were identified in leaf and tiller bud samples, respectively, representing 41,109 unique isoforms in sugarcane. Likewise, we identified 1063 and 1037 alternative splicing events identified in leaf and tiller bud samples, respectively. We predicted the presence of coding sequence for 40,343 isoforms, 98% of which was successfully annotated. Comparison with previous FL transcriptomes in sugarcane revealed 2963 unreported isoforms. In addition, we characterized 14,946 SSRs from 11,700 transcripts and 310 lncRNAs. By integrating RNA-seq with the FL transcriptome, 468 and 57 differentially expressed genes (DEG) were identified in T1vsT0 and T2vsT0, respectively. Strong up-regulation of several pyruvate phosphate dikinase and phosphoenolpyruvate carboxylase genes suggests enhanced carbon fixation and protein synthesis to facilitate tiller growth. Similarly, up-regulation of linoleate 9S-lipoxygenase and lipoxygenase genes in the linoleic acid metabolism pathway suggests high synthesis of key oxylipins involved in tiller growth and development.

CONCLUSIONS

Collectively, we have enriched the genomic data available in sugarcane and provided candidate genes for manipulating tiller formation and development, towards productivity enhancement in sugarcane.

Transcriptome dynamic landscape underlying the improvement of maize lodging resistance under coronatine treatment

BACKGROUND

Lodging is one of the important factors causing maize yield. Plant height is an important factor in determining plant architecture in maize (Zea mays L.), which is closely related to lodging resistance under high planting density. Coronatine (COR), which is a phytotoxin and produced by the pathogen Pseudomonas syringae, is a functional and structural analogue of jasmonic acid (JA).

RESULTS

In this study, we found COR, as a new plant growth regulator, could effectively reduce plant height and ear height of both hybrids (ZD958 and XY335) and inbred (B73) maize by inhibiting internode growth during elongation, thus improve maize lodging resistance. To study gene expression changes in internode after COR treatment, we collected spatio-temporal transcriptome of inbred B73 internode under normal condition and COR treatment, including the three different regions of internode (fixed, meristem and elongation regions) at three different developmental stages. The gene expression levels of the three regions at normal condition were described and then compared with that upon COR treatment. In total, 8605 COR-responsive genes (COR-RGs) were found, consist of 802 genes specifically expressed in internode. For these COR-RGs, 614, 870, 2123 of which showed expression changes in only fixed, meristem and elongation region, respectively. Both the number and function were significantly changed for COR-RGs identified in different regions, indicating genes with different functions were regulated at the three regions. Besides, we found more than 80% genes of gibberellin and jasmonic acid were changed under COR treatment.

CONCLUSIONS

These data provide a gene expression profiling in different regions of internode development and molecular mechanism of COR affecting internode elongation. A putative schematic of the internode response to COR treatment is proposed which shows the basic process of COR affecting internode elongation. This research provides a useful resource for studying maize internode development and improves our understanding of the COR regulation mechanism based on plant height.

Loss of ZmLIPOXYGENASE4 Decreases Fusarium verticillioides Resistance in Maize Seedlings

Fusarium verticillioides is one of the most relevant fungal species in maize responsible for ear, stalk and seedling rot, as well as the fumonisin contamination of kernels. Plant lipoxygenases (LOX) synthesize oxylipins that play a crucial role in the regulation of defense mechanisms against pathogens and influence the outcome of pathogenesis. To better uncover the role of these signaling molecules in maize resistance against F. verticillioides, the functional characterization of the 9-LOX gene, ZmLOX4, was carried out in this study by employing mutants carrying Mu insertions in this gene (named as UFMulox4). In this regard, the genotyping of five UFMulox4 identified the mutant UFMu10924 as the only one having an insertion in the coding region of the gene. The impact of ZmLOX4 mutagenesis on kernel defense against F. verticillioides and fumonisin accumulation were investigated, resulting in an increased fungal susceptibility compared to the inbred lines W22 and Tzi18. Moreover, the expression of most of the genes involved in the LOX, jasmonic acid (JA) and green leaf volatiles (GLV) pathways, as well as LOX enzymatic activity, decreased or were unaffected by fungal inoculation in the mutant UFMu10924. These results confirm the strategic role of ZmLOX4 in controlling defense against F. verticillioides and its influence on the expression of several LOX, JA and GLV genes.

A 1232 bp upstream sequence of glutamine synthetase 1b from Eichhornia crassipes is a root-preferential promoter sequence

BACKGROUND

Glutamine synthetase (GS) acts as a key enzyme in plant nitrogen (N) metabolism. It is important to understand the regulation of GS expression in plant. Promoters can initiate the transcription of its downstream gene. Eichhornia crassipes is a most prominent aquatic invasive plant, which has negative effects on environment and economic development. It also can be used in the bioremediation of pollutants present in water and the production of feeding and energy fuel. So identification and characterization of GS promoter in E. crassipes can help to elucidate its regulation mechanism of GS expression and further to control its N metabolism.

RESULTS

A 1232 bp genomic fragment upstream of EcGS1b sequence from E. crassipes (EcGS1b-P) has been cloned, analyzed and functionally characterized. TSSP-TCM software and PlantCARE analysis showed a TATA-box core element, a CAAT-box, root specific expression element, light regulation elements including chs-CMA1a, Box I, and Sp1 and other cis-acting elements in the sequence. Three 5'-deletion fragments of EcGS1b upstream sequence with 400 bp, 600 bp and 900 bp length and the 1232 bp fragment were used to drive the expression of β-glucuronidase (GUS) in tobacco. The quantitative test revealed that GUS activity decreased with the decreasing of the promoter length, which indicated that there were no negative regulated elements in the EcGS1-P. The GUS expressions of EcGS1b-P in roots were significantly higher than those in leaves and stems, indicating EcGS1b-P to be a root-preferential promoter. Real-time Quantitative Reverse Transcription-Polymerase Chain Reaction (qRT-PCR) analysis of EcGS1b gene also showed higher expression in the roots of E.crassipes than in stems and leaves.

CONCLUSIONS

EcGS1b-P is a root-preferential promoter sequence. It can specifically drive the transcription of its downstream gene in root. This study will help to elucidate the regulatory mechanisms of EcGS1b tissue-specific expression and further study its other regulatory mechanisms in order to utilize E.crassipes in remediation of eutrophic water and control its overgrowth from the point of nutrient metabolism.

Chorismate mutase and isochorismatase, two potential effectors of the migratory nematode Hirschmanniella oryzae, increase host susceptibility by manipulating secondary metabolite content of rice

Hirschmanniella oryzae is one of the most devastating nematodes on rice, leading to substantial yield losses. Effector proteins aid the nematode during the infection process by subduing plant defence responses. In this research we characterized two potential H. oryzae effector proteins, chorismate mutase (HoCM) and isochorismatase (HoICM), and investigated their enzymatic activity and their role in plant immunity. Both HoCM and HoICM proved to be enzymatically active in complementation tests in mutant Escherichia coli strains. Infection success by the migratory nematode H. oryzae was significantly higher in transgenic rice lines constitutively expressing HoCM or HoICM. Expression of HoCM, but not HoICM, increased rice susceptibility against the sedentary nematode Meloidogyne graminicola also. Transcriptome and metabolome analyses indicated reductions in secondary metabolites in the transgenic rice plants expressing the potential nematode effectors. The results presented here demonstrate that both HoCM and HoICM suppress the host immune system and that this may be accomplished by lowering secondary metabolite levels in the plant.

Engineering Smut Resistance in Maize by Site-Directed Mutagenesis of LIPOXYGENASE 3

Biotic stresses caused by microbial pathogens impair crop yield and quality if not restricted by expensive and often ecologically problematic pesticides. For a sustainable agriculture of tomorrow, breeding or engineering of pathogen-resistant crop varieties is therefore a major cornerstone. Maize is one of the four most important cereal crops in the world. The biotrophic fungal pathogen Ustilago maydis causes galls on all aerial parts of the maize plant. Biotrophic pathogens like U. maydis co-evolved with their host plant and depend during their life cycle on successful manipulation of the host's cellular machinery. Therefore, removing or altering plant susceptibility genes is an effective and usually durable way to obtain resistance in plants. Transcriptional time course experiments in U. maydis-infected maize revealed numerous maize genes being upregulated upon establishment of biotrophy. Among these genes is the maize LIPOXYGENASE 3 (LOX3) previously shown to be a susceptibility factor for other fungal genera as well. Aiming to engineer durable resistance in maize against U. maydis and possibly other pathogens, we took a Cas endonuclease technology approach to generate loss of function mutations in LOX3. lox3 maize mutant plants react with an enhanced PAMP-triggered ROS burst implicating an enhanced defense response. Based on visual assessment of disease symptoms and quantification of relative fungal biomass, homozygous lox3 mutant plants exposed to U. maydis show significantly decreased susceptibility. U. maydis infection assays using a transposon mutant lox3 maize line further substantiated that LOX3 is a susceptibility factor for this important maize pathogen.

Transcriptome Profiling of Potato (Solanum tuberosum L.) Responses to Root-Knot Nematode (Meloidogyne javanica) Infestation during A Compatible Interaction

Root-knot nematode (RKN) Meloidogyne javanica presents a great challenge to Solanaceae crops, including potato. In this study, we investigated transcriptional responses of potato roots during a compatible interaction with M. javanica. In this respect, differential gene expression of Solanum tuberosum cultivar (cv.) Mondial challenged with M. javanica at 0, 3 and 7 days post-inoculation (dpi) was profiled. In total, 4948 and 4484 genes were detected, respectively, as differentially expressed genes (DEGs) at 3 and 7 dpi. Functional annotation revealed that genes associated with metabolic processes were enriched, suggesting they might have an important role in M. javanica disease development. MapMan analysis revealed down-regulation of genes associated with pathogen perception and signaling suggesting interference with plant immunity system. Notably, delayed activation of pathogenesis-related genes, down-regulation of disease resistance genes, and activation of host antioxidant system contributed to a susceptible response. Nematode infestation suppressed ethylene (ET) and jasmonic acid (JA) signaling pathway hindering JA/ET responsive genes associated with defense. Genes related to cell wall modification were differentially regulated while transport-related genes were up-regulated, facilitating the formation of nematode feeding sites (NFSs). Several families of transcription factors (TFs) were differentially regulated by M. javanica infestation. Suggesting that TFs play an indispensable role in physiological adaptation for successful M. javanica disease development. This genome-wide analysis reveals the molecular regulatory networks in potato roots which are potentially manipulated by M. javanica. Being the first study analyzing transcriptome profiling of M. javanica-diseased potato, it provides unparalleled insight into the mechanism underlying disease development.

The Molecular and Functional Characterization of the Durum Wheat Lipoxygenase TdLOX2 Suggests Its Role in Hyperosmotic Stress Response

In plants, lipoxygenases (LOXs) are involved in various processes, such as growth, development, and response to stress cues. In the present study, the expression pattern of six durum wheat LOX-encoding genes (TdLpx-B1.1, TdLpx-B1.2, TdLpx-A2, TdLpx-B2, TdLpx-A3 and TdLpx-B3) under hyperosmotic stress was investigated. With osmotic (0.42 M mannitol) and salt (0.21 M NaCl) stress imposed at the early stages of seedling growth, a strong induction of the TdLpx-A2 gene expression in the shoots paralleled an equally strong increase in the LOX activity. Enhanced levels of malondialdehyde (MDA) and increased rates of superoxide anion generation were also observed as a result of the stress imposition. Sequence analysis of the TdLOX2 encoded by the TdLpx-A2 gene revealed that it belonged to the type-1 9-LOX group. When overexpressed in E. coli, TdLOX2 exhibited normal enzyme activity, high sensitivity to specific LOX inhibitors, with 76% and 99% inhibition by salicylhydroxamic and propyl gallate, respectively, and a preference for linoleic acid as substrate, which was converted exclusively to its corresponding 13-hydroperoxide. This unexpected positional specificity could be related to the unusual TV/K motif that in TdLOX2 replaces the canonical TV/R motif of 9-LOXs. Treatment of seedlings with propyl gallate strongly suppressed the increase in LOX activity induced by the hyperosmotic stress; the MDA accumulation was also reduced but less markedly, whereas the rate of superoxide anion generation was even more increased. Overall, our findings suggest that the up-regulation of the TdLpx-A2 gene is a component of the durum wheat response to hyperosmotic stress and that TdLOX2 may act by counteracting the excessive generation of harmful reactive oxygen species responsible for the oxidative damages that occur in plants under stress.

Trichoderma virens colonization of maize roots triggers rapid accumulation of 12-oxophytodienoate and two ᵧ-ketols in leaves as priming agents of induced systemic resistance

Two oxylipins 12-OPDA (12-Oxo-10(Z),15(Z)-phytodienoic acid) and an ᵧ-ketol, 9,10-KODA (10-oxo-9-hydroxy-12(Z), 15(Z)-octadecadienoic acid) were recently identified as important long-distance-induced systemic resistance (ISR) signals in Trichoderma virens-treated maize. On the other hand, jasmonic acid (JA), long believed to be a major signal of ISR, was not involved, as the JA-deficient mutant, opr7 opr8, retained the capacity for T. virens-triggered ISR. In order to further understand the biochemical basis for ISR priming in maize leaves, diverse oxylipins and phytohormones in the leaves of wild-type maize or ISR-deficient lox10-3 mutants treated with T. virens were quantified. This analysis revealed that 12-OPDA and two novel ᵧ-ketols, 9,12-KOMA (12-Oxo-9-hydroxy-10(E)-octadecenoic acid) and 9,12-KODA (12-Oxo-9-hydroxy-10(E),15(Z)-octadecadienoic acid), accumulated at high levels in ISR-positive plants. In support of the notion that 12-OPDA serves as a priming agent for ISR in addition to being a xylem-mobile signal, leaf pretreatment with this JA precursor resulted in increased resistance to Colletotrichum graminicola. Furthermore, the injection of 9,12-KODA or 9,12-KOMA in wild-type plants enhanced resistance against C. graminicola infection, suggesting that they play roles in ISR priming.

The Aspergilli and Their Mycotoxins: Metabolic Interactions With Plants and the Soil Biota

Species of the highly diverse fungal genus Aspergillus are well-known agricultural pests, and, most importantly, producers of various mycotoxins threatening food safety worldwide. Mycotoxins are studied predominantly from the perspectives of human and livestock health. Meanwhile, their roles are far less known in nature. However, to understand the factors behind mycotoxin production, the roles of the toxins of Aspergilli must be understood from a complex ecological perspective, taking mold-plant, mold-microbe, and mold-animal interactions into account. The Aspergilli may switch between saprophytic and pathogenic lifestyles, and the production of secondary metabolites, such as mycotoxins, may vary according to these fungal ways of life. Recent studies highlighted the complex ecological network of soil microbiotas determining the niches that Aspergilli can fill in. Interactions with the soil microbiota and soil macro-organisms determine the role of secondary metabolite production to a great extent. While, upon infection of plants, metabolic communication including fungal secondary metabolites like aflatoxins, gliotoxin, patulin, cyclopiazonic acid, and ochratoxin, influences the fate of both the invader and the host. In this review, the role of mycotoxin producing Aspergillus species and their interactions in the ecosystem are discussed. We intend to highlight the complexity of the roles of the main toxic secondary metabolites as well as their fate in natural environments and agriculture, a field that still has important knowledge gaps.

Setaria viridis as a model for translational genetic studies of jasmonic acid-related insect defenses in Zea mays

Little is known regarding insect defense pathways in Setaria viridis (setaria), a model system for panicoid grasses, including Zea mays (maize). It is thus of interest to compare insect herbivory responses of setaria and maize. Here we use metabolic, phylogenetic, and gene expression analyses to measure a subset of jasmonic acid (JA)-related defense responses to leaf-chewing caterpillars. Phylogenetic comparisons of known defense-related maize genes were used to identify putative orthologs in setaria, and candidates were tested by quantitative PCR to determine transcriptional responses to insect challenge. Our findings show that while much of the core JA-related metabolic and genetic responses appear conserved between setaria and maize, production of downstream secondary metabolites such as benzoxazinoids and herbivore-induced plant volatiles are dissimilar. This diversity of chemical defenses and gene families involved in secondary metabolism among grasses presents new opportunities for cross species engineering. The high degree of genetic similarity and ease of orthologous gene identification between setaria and maize make setaria an excellent species for translational genetic studies, but the species specificity of downstream insect defense chemistry makes some pathways unamenable to cross-species comparisons.

Genome-scale mining of root-preferential genes from maize and characterization of their promoter activity

BACKGROUND

Modification of root architecture and improvement of root resistance to stresses can increase crop productivity. Functional analyses of root-specific genes are necessary for root system improvement, and root-specific promoters enable research into the regulation of root development and genetic manipulation of root traits. Maize is an important crop species; however, little systematic mining of root-specific genes and promoters has been performed to date.

RESULTS

Genomic-scale mining based on microarray data sets followed by transcript detection resulted in the identification of 222 root-specific genes. Gene Ontology enrichment analyses revealed that these 222 root-specific genes were mainly involved in responses to chemical, biotic, and abiotic stresses. Of the 222 genes, 33 were verified by quantitative reverse transcription polymerase chain reaction, and 31 showed root-preferential activity. About 2 kb upstream 5 of the 31 identified root-preferential genes were cloned from the maize genome as putative promoters and named p8463, p5023, p1534, p8531 and p6629. GUS staining of transgenic maize-derived promoter-GUS constructs revealed that the five promoters drove GUS expression in a root-preferential manner.

CONCLUSIONS

We mined root-preferential genes and their promoters in maize and verified p8463, p5023, p1534, p8531 and p6629 as root-preferential promoters. Our research enables the identification of other tissue-specific genes and promoters in maize and other species. In addition, the five promoters may enable enhancement of target gene(s) of maize in a root-preferential manner to generate novel maize cultivars with resistance to water, fertilizer constraints, or biotic stresses.

Toward Mass Spectrometry Imaging in the Metabolomics Scale: Increasing Metabolic Coverage Through Multiple On-Tissue Chemical Modifications

Exploring the metabolic differences directly on tissues is essential for the comprehensive understanding of how multicellular organisms function. Mass spectrometry imaging (MSI) is an attractive technique toward this goal; however, MSI in metabolomics scale has been hindered by multiple limitations. This is most notable for single cell level high-spatial resolution imaging because of the limited number of molecules in small sampling size and the low ionization yields of many metabolites. Several on-tissue chemical derivatization approaches have been reported to increase MSI signals of targeted compounds, especially in matrix-assisted laser desorption/ionization (MALDI)-MSI. Herein, we adopt a combination of chemical derivatization reactions, to selectively enhance metabolite signals of a specific functional group for each consecutive tissue section. Three well-known on-tissue derivatization methods were used as a proof of concept experiment: coniferyl aldehyde for primary amines, Girard's reagent T for carbonyl groups, and 2-picolylamine for carboxylic acids. This strategy was applied to the cross-sections of leaves and roots from two different maize genotypes (B73 and Mo17), and enabled the detection of over six hundred new unique metabolite features compared to without modification. Statistical analysis indicated quantitative variation between metabolites in the tissue sections, while MS images revealed differences in localization of these metabolites. Combined, this untargeted approach facilitated the visualization of various classes of compounds, demonstrating the potential for untargeted MSI in the metabolomics scale.

Loss of cytosolic glucose-6-phosphate dehydrogenase increases the susceptibility of Arabidopsis thaliana to root-knot nematode infection

BACKGROUND AND AIMS

Root knot nematodes (RKNs, Meloidogyne spp.) are microscopic roundworms with a wide host range causing great economic losses worldwide. Understanding how metabolic pathways function within the plant upon RKN infection will provide insight into the molecular aspects of plant-RKN interactions. Glucose-6-phosphate dehydrogenase (G6PDH), the key regulatory enzyme of the oxidative pentose phosphate pathway (OPPP), is involved in plant responses to abiotic stresses and pathogenesis. In this study, the roles of Arabidopsis cytosolic G6PDH in plant-RKN interactions were investigated.

METHODS

Enzyme assays and western blotting were used to characterize changes in total G6PDH activity and protein abundance in wild-type Arabidopsis in response to RKN infection. The susceptibility of wild-type plants and the double mutant g6pd5/6 to RKNs was analysed and the expression of genes associated with the basal defence response was tested after RKN infection using quantitative reverse transcription PCR.

KEY RESULTS

RKN infection caused a marked increase in total G6PDH activity and protein abundance in wild-type Arabidopsis roots. However, the transcript levels of G6PDH genes except G6PD6 were not significantly induced following RKN infection, suggesting that the increase in G6PDH activity may occur at the post-transcriptional level. The double mutant g6pd5/6 with loss-of-function of the two cytosolic isoforms G6PD5 and G6PD6 displayed enhanced susceptibility to RKNs. Moreover, reactive oxygen species (ROS) production and gene expression involved in the defence response including jasmonic acid and salicylic acid pathways were suppressed in the g6pd5/6 mutant at the early stage of RKN infection when compared to the wild-type plants.

CONCLUSIONS

The results demonstrated that the G6PDH-mediated OPPP plays an important role in the plant-RKN interaction. In addition, a new aspect of G6PDH activity involving NADPH production by the OPPP in plant basal defence against RKNs is defined, which may be involved in ROS signalling.

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.

Aboveground herbivory induced jasmonates disproportionately reduce plant reproductive potential by facilitating root nematode infestation

Different plant feeders, including insects and parasitic nematodes, can influence each other by triggering systemic changes in their shared host plants. In most cases, however, the underlying mechanisms are unclear, and the consequences for plant fitness are not well understood. We studied the interaction between leaf feeding Manduca sexta caterpillars and root parasitic nematodes in Nicotiana attenuata. Simulated M. sexta attack increased the abundance of root parasitic nematodes in the field and facilitated Meloidogyne incognita reproduction in the glasshouse. Intact jasmonate biosynthesis was found to be required for both effects. Flower counts revealed that the jasmonate-dependent facilitation of nematode infestation following simulated leaf attack reduces the plant's reproductive potential to a greater degree than would be expected from the additive effects of the individual stresses. This work reveals that jasmonates mediate the interaction between a leaf herbivore and root parasitic nematodes and illustrates how plant-mediated interactions can alter plant's reproductive potential. The selection pressure resulting from the demonstrated fitness effects is likely to influence the evolution of plant defense traits in nature.

Insight into Genes Regulating Postharvest Aflatoxin Contamination of Tetraploid Peanut from Transcriptional Profiling

Postharvest aflatoxin contamination is a challenging issue that affects peanut quality. Aflatoxin is produced by fungi belonging to the Aspergilli group, and is known as an acutely toxic, carcinogenic, and immune-suppressing class of mycotoxins. Evidence for several host genetic factors that may impact aflatoxin contamination has been reported, e.g., genes for lipoxygenase (PnLOX1 and PnLOX2/PnLOX3 that showed either positive or negative regulation with Aspergillus infection), reactive oxygen species, and WRKY (highly associated with or differentially expressed upon infection of maize with Aspergillus flavus); however, their roles remain unclear. Therefore, we conducted an RNA-sequencing experiment to differentiate gene response to the infection by A. flavus between resistant (ICG 1471) and susceptible (Florida-07) cultivated peanut genotypes. The gene expression profiling analysis was designed to reveal differentially expressed genes in response to the infection (infected vs. mock-treated seeds). In addition, the differential expression of the fungal genes was profiled. The study revealed the complexity of the interaction between the fungus and peanut seeds as the expression of a large number of genes was altered, including some in the process of plant defense to aflatoxin accumulation. Analysis of the experimental data with "keggseq," a novel designed tool for Kyoto Encyclopedia of Genes and Genomes enrichment analysis, showed the importance of α-linolenic acid metabolism, protein processing in the endoplasmic reticulum, spliceosome, and carbon fixation and metabolism pathways in conditioning resistance to aflatoxin accumulation. In addition, coexpression network analysis was carried out to reveal the correlation of gene expression among peanut and fungal genes. The results showed the importance of WRKY, toll/Interleukin1 receptor-nucleotide binding site leucine-rich repeat (TIR-NBS-LRR), ethylene, and heat shock proteins in the resistance mechanism.

A role for 9-lipoxygenases in maize defense against insect herbivory

Feeding by Spodoptera exigua (beet armyworm) larvae on Zea mays (maize) induces expression of 9-lipoxygenases to a greater extent than 13-lipoxygenases. Whereas 13-lipoxygenases have an established role in the synthesis of jasmonates that serve as defense signaling molecules in many plant species, relatively little is known about the role of 9-lipoxygenases in herbivore defense. Phylogenetic analysis of lipoxygenases from maize inbred lines B73 and W22 shows that, although most Lox genes are present in both lines, Lox12, a 9-lipoxygenase that has been implicated in fungal defense, is truncated and unlikely to encode a functional protein in W22. Two independent Mutator transposon insertions in another 9-lipoxygenase, Lox4, caused improved S. exigua growth on the mutant lines relative to wildtype W22. This observation suggests a function in herbivore defense for metabolic products downstream of maize Lox4, either through direct toxicity or a perhaps an as yet unknown signaling function.

Global transcriptome analysis of the maize (Zea mays L.) inbred line 08LF during leaf senescence initiated by pollination-prevention

In maize (Zea mays), leaf senescence acts as a nutrient recycling process involved in proteins, lipids, and nucleic acids degradation and transport to the developing sink. However, the molecular mechanisms of pre-maturation associated with pollination-prevention remain unclear in maize. To explore global gene expression changes during the onset and progression of senescence in maize, the inbred line 08LF, with severe early senescence caused by pollination prevention, was selected. Phenotypic observation showed that the onset of leaf senescence of 08LF plants occurred approximately 14 days after silking (DAS) by pollination prevention. Transcriptional profiling analysis of the leaf at six developmental stages during induced senescence revealed that a total of 5,432 differentially expressed genes (DEGs) were identified, including 2314 up-regulated genes and 1925 down-regulated genes. Functional annotation showed that the up-regulated genes were mainly enriched in multi-organism process and nitrogen compound transport, whereas down-regulated genes were involved in photosynthesis. Expression patterns and pathway enrichment analyses of early-senescence related genes indicated that these DEGs are involved in complex regulatory networks, especially in the jasmonic acid pathway. In addition, transcription factors from several families were detected, particularly the CO-like, NAC, ERF, GRAS, WRKY and ZF-HD families, suggesting that these transcription factors might play important roles in driving leaf senescence in maize as a result of pollination-prevention.

A gene encoding maize caffeoyl-CoA O-methyltransferase confers quantitative resistance to multiple pathogens

Alleles that confer multiple disease resistance (MDR) are valuable in crop improvement, although the molecular mechanisms underlying their functions remain largely unknown. A quantitative trait locus, qMdr_9.02, associated with resistance to three important foliar maize diseases-southern leaf blight, gray leaf spot and northern leaf blight-has been identified on maize chromosome 9. Through fine-mapping, association analysis, expression analysis, insertional mutagenesis and transgenic validation, we demonstrate that ZmCCoAOMT2, which encodes a caffeoyl-CoA O-methyltransferase associated with the phenylpropanoid pathway and lignin production, is the gene within qMdr_9.02 conferring quantitative resistance to both southern leaf blight and gray leaf spot. We suggest that resistance might be caused by allelic variation at the level of both gene expression and amino acid sequence, thus resulting in differences in levels of lignin and other metabolites of the phenylpropanoid pathway and regulation of programmed cell death.

Characterization of the maize lipoxygenase gene family in relation to aflatoxin accumulation resistance

Maize (Zea mays L.) is a globally important staple food crop prone to contamination by aflatoxin, a carcinogenic secondary metabolite produced by the fungus Aspergillus flavus. An efficient approach to reduce accumulation of aflatoxin is the development of germplasm resistant to colonization and toxin production by A. flavus. Lipoxygenases (LOXs) are a group of non-heme iron containing dioxygenase enzymes that catalyze oxygenation of polyunsaturated fatty acids (PUFAs). LOX derived oxylipins play critical roles in plant defense against pathogens including A. flavus. The objectives of this study were to summarize sequence diversity and expression patterns for all LOX genes in the maize genome, and map their effect on aflatoxin accumulation via linkage and association mapping. In total, 13 LOX genes were identified, characterized, and mapped. The sequence of one gene, ZmLOX10, is reported from 5 inbred lines. Genes ZmLOX1/2, 5, 8, 9, 10 and 12 (GRMZM2G156861, or V4 numbers ZM00001D042541 and Zm00001D042540, GRMZM2G102760, GRMZM2G104843, GRMZM2G017616, GRMZM2G015419, and GRMZM2G106748, respectively) fell under previously published QTL in one or more mapping populations and are linked to a measurable reduction of aflatoxin in maize grains. Association mapping results found 28 of the 726 SNPs tested were associated with reduced aflatoxin levels at p ≤ 9.71 x 10-4 according to association statistics. These fell within or near nine of the ZmLOX genes. This work confirms the importance of some lipoxygenases for resistance to aflatoxin accumulation and may be used to direct future genetic selection in maize.

Specific Caleosin/Peroxygenase and Lipoxygenase Activities Are Tissue-Differentially Expressed in Date Palm (Phoenix dactylifera L.) Seedlings and Are Further Induced Following Exposure to the Toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin

Two caleosin/peroxygenase isoforms from date palm, Phoenix dactylifera L., PdCLO2 and PdCLO4, were characterized with respect to their tissue expression, subcellular localization, and oxylipin pathway substrate specificities in developing seedlings. Both PdCLO2 and PdCLO4 had peroxygenase activities that peaked at the mid-stage (radicle length of 2.5 cm) of seedling growth and were associated with the lipid droplet (LD) and microsomal fractions. Recombinant PdCLO2 and PdCLO4 proteins heterologously expressed in yeast cells were localized in both LD and microsomal fractions. Each of the purified recombinant proteins exhibited peroxygenase activity but they were catalytically distinct with respect to their specificity and product formation from fatty acid epoxide and hydroxide substrates. We recently showed that date palm CLO genes were upregulated following exposure to the potent toxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (Hanano et al., 2016), and we show here that transcripts of 9- and 13-lipoxygenase (LOX) genes were also induced by TCDD exposure. At the enzyme level, 9-LOX and 13-LOX activities were present in a range of seedling tissues and responded differently to TCDD exposure, as did the 9- and 13-fatty acid hydroperoxide reductase activities. This demonstrates that at least two branches of the oxylipin pathway are involved in responses to the environmental organic toxin, TCDD in date palm.

Seed-Derived Ethylene Facilitates Colonization but Not Aflatoxin Production by Aspergillus flavus in Maize

Ethylene (ET) emitted by plant tissues has been broadly reported to play important roles in plant development, response to environmental stresses and defense against certain pathogens. Recent evidence obtained from using in vitro fungal cultures exposed to ET suggested that exogenous ET may regulate the production of aflatoxin by Aspergilli. However, the function of endogenous, seed-derived ET has not been explored. In this study, we found that the maize lipoxygenase lox3 mutant, previously reported to be susceptible to Aspergillus spp., emitted greater levels of ET upon A. flavus infection, suggesting the potential involvement of endogenous ET in the susceptibility of maize to A. flavus. Supporting this idea, both colonization and conidiation of A. flavus were reduced in wild-type (WT) kernels treated with AgNO₃, an ET synthesis inhibitor. There was no ET emission from non-viable kernels colonized by A. flavus, suggesting that living seed but not the fungus itself was the primary source of ET released upon infection with A. flavus. The kernels of acs2 and acs6, two ET biosynthetic mutants carrying Mutator transposons in the ACC synthase genes, ACS2 and ACS6, respectively, displayed enhanced seed colonization and conidiation, but not the levels of aflatoxin, upon infection with A. flavus. Surprisingly, both acs2 and acs6 mutant kernels emitted greater levels of ET in response to infection by A. flavus as compared with WT seed. The increased ET in single mutants was found to be due to overexpression of functional ACS genes in response to A. flavus infection. Collectively, these findings suggested that ET emitted by infected seed facilitates colonization by A. flavus but not aflatoxin production.