Involvement of mitogen-activated protein kinase signalling in pearl millet-downy mildew interaction

The Genome of the Korean Island-Originated Perilla citriodora 'Jeju17' Sheds Light on Its Environmental Adaptation and Fatty Acid and Lipid Production Pathways

Perilla is a key component of Korean food. It contains several plant-specialized metabolites that provide medical benefits. In response to an increased interest in healthy supplement food from the public, people are focusing on the properties of Perilla. Nevertheless, unlike rice and soybeans, there are few studies based on molecular genetics on Perilla, so it is difficult to systematically study the molecular breed. The wild Perilla, Perilla citriodora 'Jeju17', was identified a decade ago on the Korean island of Jeju. Using short-reads, long-reads, and Hi-C, a chromosome-scale genome spanning 676 Mbp, with high contiguity, was assembled. Aligning the 'Jeju17' genome to the 'PC002' Chinese species revealed significant collinearity with respect to the total length. A total of 31,769 coding sequences were predicted, among which 3331 were 'Jeju17'-specific. Gene enrichment of the species-specific gene repertoire highlighted environment adaptation, fatty acid metabolism, and plant-specialized metabolite biosynthesis. Using a homology-based approach, genes involved in fatty acid and lipid triacylglycerol biosynthesis were identified. A total of 22 fatty acid desaturases were found and comprehensively characterized. Expression of the FAD genes in 'Jeju17' was examined at the seed level, and hormone signaling factors were identified. The results showed that the expression of FAD genes in 'Jeju17' at the seed level was high 25 days after flowering, and their responses of hormones and stress were mainly associated with hormone signal transduction and abiotic stress via cis-elements patterns. This study presents a chromosome-level genome assembly of P. citriodora 'Jeju17', the first wild Perilla to be sequenced from the Korean island of Jeju. The analyses provided can be useful in designing ALA-enhanced Perilla genotypes in the future.

Plant Disease Resistance-Related Signaling Pathways: Recent Progress and Future Prospects

Plant-pathogen interactions induce a signal transmission series that stimulates the plant's host defense system against pathogens and this, in turn, leads to disease resistance responses. Plant innate immunity mainly includes two lines of the defense system, called pathogen-associated molecular pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). There is extensive signal exchange and recognition in the process of triggering the plant immune signaling network. Plant messenger signaling molecules, such as calcium ions, reactive oxygen species, and nitric oxide, and plant hormone signaling molecules, such as salicylic acid, jasmonic acid, and ethylene, play key roles in inducing plant defense responses. In addition, heterotrimeric G proteins, the mitogen-activated protein kinase cascade, and non-coding RNAs (ncRNAs) play important roles in regulating disease resistance and the defense signal transduction network. This paper summarizes the status and progress in plant disease resistance and disease resistance signal transduction pathway research in recent years; discusses the complexities of, and interactions among, defense signal pathways; and forecasts future research prospects to provide new ideas for the prevention and control of plant diseases.

Synchronised regulation of disease resistance in primed finger millet plants against the blast disease

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Varied phytohormone levels was recorded in the primed plants infected with Magnoporthe grisea.

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The role of MAPK was studied for the first time in finger millet-blast disease response.

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Callose deposition at the site of pathogen entry in the primed plants indicating its role during plant defense.

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Temporal accumulation of lipoxygenase (LOX) activity confirms its role during jasmonate synthesis in the plant cells.

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The study has portrayed the SA mediated defense mechanism involved in finger millet plants against blast pathogen M. grisea.

Plants, being sessile, are exposed to an array of abiotic and biotic stresses. To adapt towards the changing environments, plants have evolved mechanisms that help in perceiving stress signals wherein phytohormones play a critical role. They have the ability to network enabling them to mediate defense responses. These endogenous signals, functioning at low doses are a part of all the developmental stages of the plant. Phytohormones possess specific functions as they interact with each other positively or negatively through cross-talks. In the present study, variations in the amount of phytohormones produced during biotic stress caused due to Magnoporthe grisea infection was studied through targeted metabolomics in both primed and control finger millet plants. Histochemical studies revealed callose deposition at the site of pathogen entry in the primed plants indicating its role during plant defense. The knowledge on the genetic makeup during infection was obtained by quantification of MAP kinase kinases 1 and 2 (MKK1/2) and lipoxygenase (LOX) genes, wherein the expression levels were high in the primed plants at 6 hours post-inoculation (hpi) compared to mock-control. Studies indicate the pivotal role of mitogen-activated protein kinase (MAPK or MAP kinases) during defense signalling. It is the first report to be studied on MAPK role in finger millet-blast disease response. Temporal accumulation of LOX enzyme along with its activity was also investigated due to its significant role during jasmonate synthesis in the plant cells. Results indicated its highest activity at 12 hpi. This is the first report on the variation in phytohormone levels in fingermillet - M. grisea pathosystem upon priming which were substantiated through salicylic acid (SA) pathway.

Identification and Validation of Candidate Genes Conferring Resistance to Downy Mildew in Maize (Zea mays L.)

Downy mildew (DM) is a major disease of maize that causes significant yield loss in subtropical and tropical regions around the world. A variety of DM strains have been reported, and the resistance to them is polygenically controlled. In this study, we analyzed the quantitative trait loci (QTLs) involved in resistance to Peronosclerospora sorghi (sorghum DM), P. maydis (Java DM), and Sclerophthora macrospora (crazy top DM) using a recombinant inbred line (RIL) from a cross between B73 (susceptible) and Ki11 (resistant), and the candidate genes for P. sorghi, P. maydis, and S. macrospora resistance were discovered. The linkage map was constructed with 234 simple sequence repeat (SSR) and restriction fragment length polymorphism (RFLP) markers, which was identified seven QTLs (chromosomes 2, 3, 6, and 9) for three DM strains. The major QTL, located on chromosome 2, consists of 12.95% of phenotypic variation explained (PVE) and a logarithm of odds (LOD) score of 14.12. Sixty-two candidate genes for P. sorghi, P. maydis, and S. macrospora resistance were obtained between the flanked markers in the QTL regions. The relative expression level of candidate genes was evaluated by quantitative real-time polymerase chain reaction (qRT-PCR) using resistant (CML228, Ki3, and Ki11) and susceptible (B73 and CML270) genotypes. For the 62 candidate genes, 15 genes were upregulated in resistant genotypes. Among these, three (GRMZM2G028643, GRMZM2G128315, and GRMZM2G330907) and AC210003.2_FG004 were annotated as leucine-rich repeat (LRR) and peroxidase (POX) genes, respectively. These candidate genes in the QTL regions provide valuable information for further studies related to P. sorghi, P. maydis, and S. macrospora resistance.

Proteomic analysis of the compatible interaction of wheat and powdery mildew (Blumeria graminis f. sp. tritici)

Proteome characteristics of wheat leaves with the powdery mildew pathogen Blumeria graminis f. sp. tritici (Bgt) infection were investigated by two-dimensional electrophoresis and tandem MALDI-TOF/TOF-MS. We identified 46 unique proteins which were differentially expressed at 24, 48, and 72 h post-inoculation. The functional classification of these proteins showed that most of them were involved in photosynthesis, carbohydrate and nitrogen metabolism, defense responses, and signal transduction. Upregulated proteins included primary metabolism pathways and defense responses, while proteins related to photosynthesis and signal transduction were mostly downregulated. As expected, more antioxidative proteins were activated at the later infection stage than the earlier stage, suggesting that the antioxidative system of host plays a role in maintaining the compatible interaction between wheat and powdery mildew. A high accumulation of 6-phosphogluconate dehydrogenase and isocitrate dehydrogenase in infected leaves indicated the regulation of the TCA cycle and pentose phosphate pathway in parallel to the activation of host defenses. The downregulation of MAPK5 could be facilitated for the compatible interaction of wheat plants and Bgt. qRT-PCR analysis supported the data of protein expression profiles. Our results reveal the relevance of primary plant metabolism and defense responses during compatible interaction, and provide new insights into the biology of susceptible wheat in response to Bgt infection.

Exogenous Trehalose Treatment Enhances the Activities of Defense-Related Enzymes and Triggers Resistance against Downy Mildew Disease of Pearl Millet

In recent years, diverse physiological functions of various sugars are the subject of investigations. Their roles in signal transduction in plant responses to adverse biotic and abiotic stress conditions have become apparent, and growing scientific evidence has indicated that disaccharides like sucrose and trehalose mediate plant defense responses in similar way as those induced by elicitors against the pathogens. Trehalose is a well-known metabolic osmoregulator, stress-protectant and non-reducing disaccharide existing in a variety of organisms, including fungi, bacteria, and plants. Commercially procured trehalose was applied to seeds of susceptible pearl millet (Pennisetum glaucum) cultivar "HB3," and tested for its ability to reduce downy mildew disease incidence by induction of resistance. Seed treatment with trehalose at 200 mM for 9 h recorded 70.25% downy mildew disease protection, followed by those with 100 and 50 mM trehalose which offered 64.35 and 52.55% defense, respectively, under greenhouse conditions. Furthermore, under field conditions treatment with 200 mM trehalose for 9 h recorded 67.25% downy mildew disease protection, and reduced the disease severity to 32.75% when compared with untreated control which displayed 90% of disease severity. Trehalose did not affect either sporangial formation or zoospore release from sporangia, indicating that the reduction in disease incidence was not due to direct inhibition but rather through induction of resistance responses in the host. Additionally, trehalose was shown to enhance the levels of polyphenol oxidase, phenylalanine ammonia lyase, and peroxidase, which are known as markers of both biotic and abiotic stress responses. Our study shows that osmoregulators like trehalose could be used to protect plants against pathogen attacks by seed treatment, thus offering dual benefits of biotic and abiotic stress tolerance.

De novo Transcriptome Sequencing to Dissect Candidate Genes Associated with Pearl Millet-Downy Mildew (Sclerospora graminicola Sacc.) Interaction

Understanding the plant-pathogen interactions is of utmost importance to design strategies for minimizing the economic deficits caused by pathogens in crops. With an aim to identify genes underlying resistance to downy mildew, a major disease responsible for productivity loss in pearl millet, transcriptome analysis was performed in downy mildew resistant and susceptible genotypes upon infection and control on 454 Roche NGS platform. A total of ~685 Mb data was obtained with 1 575 290 raw reads. The raw reads were pre-processed into high-quality (HQ) reads making to ~82% with an average of 427 bases. The assembly was optimized using four assemblers viz. Newbler, MIRA, CLC and Trinity, out of which MIRA with a total of 14.10 Mb and 90118 transcripts proved to be the best for assembling reads. Differential expression analysis depicted 1396 and 936 and 1000 and 1591 transcripts up and down regulated in resistant inoculated/resistant control and susceptible inoculated/susceptible control respectively with a common of 3644 transcripts. The pathways for secondary metabolism, specifically the phenylpropanoid pathway was up-regulated in resistant genotype. Transcripts up-regulated as a part of defense response included classes of R genes, PR proteins, HR induced proteins and plant hormonal signaling transduction proteins. The transcripts for skp1 protein, purothionin, V type proton ATPase were found to have the highest expression in resistant genotype. Ten transcripts, selected on the basis of their involvement in defense mechanism were validated with qRT-PCR and showed positive co-relation with transcriptome data. Transcriptome analysis evoked potentials of hypersensitive response and systemic acquired resistance as possible mechanism operating in defense mechanism in pearl millet against downy mildew infection.

Molecular cloning of a coiled-coil-nucleotide-binding-site-leucine-rich repeat gene from pearl millet and its expression pattern in response to the downy mildew pathogen

Downy mildew caused by Sclerospora graminicola is a devastating disease of pearl millet. Based on candidate gene approach, a set of 22 resistance gene analogues were identified. The clone RGPM 301 (AY117410) containing a partial sequence shared 83% similarity to rice R-proteins. A full-length R-gene RGA RGPM 301 of 3552 bp with 2979 bp open reading frame encoding 992 amino acids was isolated by the degenerate primers and rapid amplification of cDNA ends polymerase chain reaction (RACE-PCR) approach. It had a molecular mass of 113.96 kDa and isoelectric point (pI) of 8.71. The sequence alignment and phylogenetic analysis grouped it to a non-TIR NBS LRR group. The quantitative real-time PCR (qRT-PCR) analysis revealed higher accumulation of the transcripts following inoculation with S. graminicola in the resistant cultivar (IP18296) compared to susceptible cultivar (7042S). Further, significant induction in the transcript levels were observed when treated with abiotic elicitor β-aminobutyric acid (BABA) and biotic elicitor Pseudomonas fluorescens. Exogenous application of phytohormones jasmonic acid or salicylic acid also up-regulated the expression levels of RGA RGPM 301. The treatment of cultivar IP18296 with mitogen-activated protein kinase (MPK) inhibitors (PD98059 and U0126) suppressed the levels of RGA RGPM 301. A 3.5 kb RGA RGPM 301 which is a non-TIR NBS-LRR protein was isolated from pearl millet and its up-regulation during downy mildew interaction was demonstrated by qRT-PCR. These studies indicate a role for this RGA in pearl millet downy mildew interaction.

Modulation of host cell biology by plant pathogenic microbes

Plant-pathogen interactions can result in dramatic visual changes in the host, such as galls, phyllody, pseudoflowers, and altered root-system architecture, indicating that the invading microbe has perturbed normal plant growth and development. These effects occur on a cellular level but range up to the organ scale, and they commonly involve attenuation of hormone homeostasis and deployment of effector proteins with varying activities to modify host cell processes. This review focuses on the cellular-reprogramming mechanisms of filamentous and bacterial plant pathogens that exhibit a biotrophic lifestyle for part, if not all, of their lifecycle in association with the host. We also highlight strategies for exploiting our growing knowledge of microbial host reprogramming to study plant processes other than immunity and to explore alternative strategies for durable plant resistance.

Inhibition of SlMPK1, SlMPK2, and SlMPK3 Disrupts Defense Signaling Pathways and Enhances Tomato Fruit Susceptibility to Botrytis cinerea

Mitogen-activated protein kinases (MAPKs) are major components of defense signaling pathways that transduce extracellular stimuli into intracellular responses in plants. Our previous study indicated that SlMPK1/2/3 were associated with nitric oxide-induced defense response in tomato fruit. In this study, we determine whether SlMPK1/2/3 influence the tomato fruit's innate immunity and whether plant hormones and reactive oxygen species (ROS) are involved in SlMPK1/2/3 defense signaling pathways. Treatment with 10 μM U0126 significantly inhibited the relative expression of SlMPK1, SlMPK2, and SlMPK3 (P < 0.05). U0126-treated fruit showed higher concentrations of auxin indole acetic acid (IAA), abscisic acid (ABA), and gibberellic acid (GA), but a lower concentration of methyl jasmonate (MeJA). The activities of defense enzymes, including β-1,3-glucanases (GLU), chitinase (CHI), phenylalanine ammonia lyase (PAL), and polyphenol oxidase (PPO), decreased after U0126 treatment. Meanwhile, H2O2 content increased, and catalase (CAT), ascorbate peroxidase (APX), and peroxidase (POD) activities decreased after U0126 treatment. U0126 treatment enhanced the susceptibility of tomato fruit to Botrytis cinerea and resulted in more severe gray mold rot. These results demonstrate that inhibition of SlMPK1/2/3 disrupts tomato fruit defense signaling pathways and enhances the susceptibility to B. cinerea and also that plant hormones and ROS are associated with SlMPK1/2/3 defense signaling pathways.

The pearl millet mitogen-activated protein kinase PgMPK4 is involved in responses to downy mildew infection and in jasmonic- and salicylic acid-mediated defense

Plant mitogen-activated protein kinases (MPKs) transduce signals required for the induction of immunity triggered by host recognition of pathogen-associated molecular patterns. We isolated a full-length cDNA of a group B MPK (PgMPK4) from pearl millet. Autophosphorylation assay of recombinant PgMPK4 produced in Escherichia coli confirmed it as a kinase. Differential accumulation of PgMPK4 mRNA and kinase activity was observed between pearl millet cultivars 852B and IP18292 in response to inoculation with the downy mildew oomycete pathogen Sclerospora graminicola. This increased accumulation of PgMPK4 mRNA, kinase activity as well as nuclear-localization of PgMPK protein(s) was only detected in the S. graminicola resistant cultivar IP18292 with a ~tenfold peak at 9 h post inoculation. In the susceptible cultivar 852B, PgMPK4 mRNA and immuno-detectable nuclear PgMPK could be induced by application of the chemical elicitor β-amino butyric acid, the non-pathogenic bacteria Pseudomonas fluorescens, or by the phytohormones jasmonic acid (JA) or salicylic acid (SA). Furthermore, kinase inhibitor treatments indicated that PgMPK4 is involved in the JA- and SA-mediated expression of three defense genes, lipoxygenase, catalase 3 and polygalacturonase-inhibitor protein. These findings indicate that PgMPK/s contribute to pearl millet defense against the downy mildew pathogen by activating the expression of defense proteins.