Salicylic acid-mediated plant defense: Recent developments, missing links, and future outlook

Effects of Altering Three Steps of Monolignol Biosynthesis on Sorghum Responses to Stalk Pathogens and Water Deficit

The drought-resilient crop sorghum (Sorghum bicolor [L.] Moench) is grown worldwide for multiple uses, including forage or potential lignocellulosic bioenergy feedstock. A major impediment to biomass yield and quality are the pathogens Fusarium thapsinum and Macrophomina phaseolina, which cause Fusarium stalk rot and charcoal rot, respectively. These fungi are more virulent with abiotic stresses such as drought. Monolignol biosynthesis plays a critical role in plant defense. The genes Brown midrib (Bmr)6, Bmr12, and Bmr2 encode the monolignol biosynthesis enzymes cinnamyl alcohol dehydrogenase, caffeic acid O-methyltransferase, and 4-coumarate:CoA ligase, respectively. Plant stalks from lines overexpressing these genes and containing bmr mutations were screened for pathogen responses with controlled adequate or deficit watering. Additionally, near-isogenic bmr12 and wild-type lines in five backgrounds were screened for response to F. thapsinum with adequate and deficit watering. All mutant and overexpression lines were no more susceptible than corresponding wild-type under both watering conditions. The bmr2 and bmr12 lines, near-isogenic to wild-type, had significantly shorter mean lesion lengths (were more resistant) than RTx430 wild-type when inoculated with F. thapsinum under water deficit. Additionally, bmr2 plants grown under water deficit had significantly smaller mean lesions when inoculated with M. phaseolina than under adequate-water conditions. When well-watered, bmr12 in cultivar Wheatland and one of two Bmr2 overexpression lines in RTx430 had shorter mean lesion lengths than corresponding wild-type lines. This research demonstrates that modifying monolignol biosynthesis for increased usability may not impair plant defenses but can even enhance resistance to stalk pathogens under drought conditions.

Identification and Expression Analysis of the Solanum tuberosum StATG8 Family Associated with the WRKY Transcription Factor

Autophagy is an evolutionarily well-conserved cellular catabolic pathway in eukaryotic cells and plays an important role in cellular processes. Autophagy is regulated by autophagy-associated (ATG) proteins. Among these ATG proteins, the ubiquitin-like protein ATG8/LC3 is essential for autophagosome formation and function. In this study, the potato StATG8 family showed clade I and clade II with significantly different sequences. Expression of the StATG8 family was also increased in senescence. Interestingly, the expression of the StATG8 and other core StATG genes decreased in potato tubers as the tubers matured. The StATG8 family also responded to a variety of stresses such as heat, wounding, salicylic acid, and salt stress. We also found that some Arabidopsis WRKY transcription factors interacted with the StATG8 protein in planta. Based on group II-a WRKY, StATG8-WRKY interaction is independent of the ATG8 interacting motif (AIM) or LC3 interacting region (LIR) motif. This study showed that the StATG8 family had diverse functions in tuber maturation and multiple stress responses in potatoes. Additionally, StATG8 may have an unrelated autophagy function in the nucleus with the WRKY transcription factor.

Transcriptome analysis reveals pathogenesis-related gene 1 pathway against salicylic acid treatment in grapevine (Vitis vinifera L)

Salicylic acid (SA) is a well-studied phenolic plant hormone that plays an important role in plant defense against the hemi-biothrophic and biothrophic pathogens and depends on the living cells of host for the successful infection. In this study, a pathogenesis test was performed between Vitis davidii and V. vinifera cultivars against grape white rot disease (Coniella diplodiella). V. davidii was found to be resistant against this disease. SA contents were found to be higher in the resistant grape cultivar after different time points. RNA-seq analysis was conducted on susceptible grapevine cultivars after 12, 24, and 48 h of SA application with the hypothesis that SA may induce defense genes in susceptible cultivars. A total of 511 differentially expressed genes (DEGs) were identified from the RNA-seq data, including some important genes, VvWRKY1/2, VvNPR1, VvTGA2, and VvPR1, for the SA defense pathway. DEGs related to phytohormone signal transduction and flavonoid biosynthetic pathways were also upregulated. The quantitative real-time PCR (qRT-PCR) results of the significantly expressed transcripts were found to be consistent with the transcriptome data, with a high correlation between the two analyses. The pathogenesis-related gene 1 (VvPR1), which is an important marker gene for plant defense, was selected for further promoter analysis. The promoter sequence showed that it contains some important cis-elements (W-box, LS7, as-1, and TCA-element) to recruit the transcription factors VvWRKY, VvNPR1, and VvTGA2 to express the VvPR1 gene in response to SA treatment. Furthermore, the VvPR1 promoter was serially deleted into different fragments (-1,837, -1,443, -1,119, -864, -558, -436, and -192 ) bp and constructed vectors with the GUS reporter gene. Deletion analysis revealed that the VvPR1 promoter between -1837 bp to -558 bp induced significant GUS expression with respect to the control. On the basis of these results, the -558 bp region was assumed to be an important part of the VvPR1 promoter, and this region contained the important cis-elements related to SA, such as TCA-element (-1,472 bp), LS7 (-1,428 bp), and as-1 (-520 bp), that recruit the TFs and induce the expression of the VvPR1 gene. This study expanded the available information regarding SA-induced defense in susceptible grapes and recognized the molecular mechanisms through which this defense might be mediated.

Reprogramming of sorghum proteome in response to sugarcane aphid infestation

Sugarcane aphid (SCA; Melanaphis sacchari Zehntner) is a key piercing-sucking pest of sorghum (Sorghum bicolor) that cause significant yield losses. While feeding on host plants, complex signaling networks are invoked from recognition of insect attack to induction of plant defenses. Consequently, these signaling networks lead to the production of insecticidal compounds or limited access of nutrients to insects. Previously, several studies were published on the transcriptomics analysis of sorghum in response to SCA infestation, but no information is available on the physiological changes of sorghum at the proteome level. We used the SCA resistant sorghum genotype SC265 for the global proteomics analysis after 1 and 7 days of SCA infestation using the TMT-plex technique. Peptides matching a total of 4211 proteins were identified and 158 proteins were differentially expressed at day 1 and 7. Overall, proteome profiling of SC265 after SCA infestation at days 1 and 7 revealed the suppression of plant defense-related proteins and upregulation of plant defense and signaling-related proteins, respectively. The plant defense responses based on proteome data were validated using electrical penetration graph (EPG) technique to observe changes in aphid feeding. Feeding behavior analyses revealed that SCA spent significantly longer time in phloem phase on SCA infested plants for day 1 and lesser time in day 7 SCA infested sorghum plants, compared to their respective control plants. Overall, our study provides insights into underlying mechanisms that contribute to sorghum resistance to SCA.

Perception and First Defense Responses Against Pseudomonas syringae pv. phaseolicola in Phaseolus vulgaris: Identification of Wall-Associated Kinase Receptors

Common bean (Phaseolus vulgaris) is attacked by several pathogens such as the biotrophic gamma-proteobacterium Pseudomonas syringae pv. phaseolicola. To study the P. syringae pv. phaseolicola-bean interaction during the first stages of infection, leaf discs of a susceptible bean cultivar Riñón were infected with pathogenic P. syringae pv. phaseolicola. Using this experimental system, we tested six new putative wall-associated kinase (WAK) receptors, previously identified in silico. These six P. vulgaris WAKs (PvWAKs) showed high protein sequence homology to the well-described Arabidopsis thaliana WAK1 (AtWAK1) receptor and, by phylogenetic analysis, clustered together with AtWAKs. The expression of PvWAK1 increased at very early stages after the P. syringae pv. phaseolicola infection. Time course experiments were performed to evaluate the accumulation of apoplastic H₂O₂, Ca²⁺ influx, total H₂O₂, antioxidant enzymatic activities, lipid peroxidation, and the concentrations of abscisic acid and salicylic acid (SA), as well as the expression of six defense-related genes: MEKK-1, MAPKK, WRKY33, RIN4, PR1, and NPR1. The results showed that overexpression of PR1 occurred 2 h after P. syringae pv. phaseolicola infection without a concomitant increase in SA levels. Although apoplastic H₂O₂ increased after infection, the oxidative burst was neither intense nor rapid, and an efficient antioxidant response did not occur, suggesting that the observed cellular damage was caused by the initial increase in total H₂O₂ early after infection. In conclusion, Riñón can perceive the presence of P. syringae pv. phaseolicola, but this recognition results in only a modest and slow activation of host defenses, leading to high susceptibility to P. syringae pv. phaseolicola.

Non-Targeted Metabolite Profiling Reveals Host Metabolomic Reprogramming during the Interaction of Black Pepper with Phytophthora capsici

Phytophthora capsici is one of the most destructive pathogens causing quick wilt (foot rot) disease in black pepper (Piper nigrum L.) to which no effective resistance has been defined. To better understand the P. nigrum-P. capsici pathosystem, we employed metabolomic approaches based on flow-infusion electrospray-high-resolution mass spectrometry. Changes in the leaf metabolome were assessed in infected and systemic tissues at 24 and 48 hpi. Principal Component Analysis of the derived data indicated that the infected leaves showed a rapid metabolic response by 24 hpi whereas the systemic leaves took 48 hpi to respond to the infection. The major sources of variations between infected leaf and systemic leaf were identified, and enrichment pathway analysis indicated, major shifts in amino acid, tricarboxylic acid cycle, nucleotide and vitamin B6 metabolism upon infection. Moreover, the individual metabolites involved in defensive phytohormone signalling were identified. RT-qPCR analysis of key salicylate and jasmonate biosynthetic genes indicated a transient reduction of expression at 24 hpi but this increased subsequently. Exogenous application of jasmonate and salicylate reduced P. capsici disease symptoms, but this effect was suppressed with the co-application of abscisic acid. The results are consistent with abscisic acid reprogramming, salicylate and jasmonate defences in infected leaves to facilitate the formation of disease. The augmentation of salicylate and jasmonate defences could represent an approach through which quick wilt disease could be controlled in black pepper.

Full-Length Transcriptome Analysis of Four Different Tissues of Cephalotaxus oliveri

Cephalotaxus oliveri is a tertiary relict conifer endemic to China, regarded as a national second-level protected plant in China. This species has experienced severe changes in temperature and precipitation in the past millions of years, adapting well to harsh environments. In view of global climate change and its endangered conditions, it is crucial to study how it responds to changes in temperature and precipitation for its conservation work. In this study, single-molecule real-time (SMRT) sequencing and Illumina RNA sequencing were combined to generate the complete transcriptome of C. oliveri. Using the RNA-seq data to correct the SMRT sequencing data, the four tissues obtained 63,831 (root), 58,108 (stem), 33,013 (leaf) and 62,436 (male cone) full-length unigenes, with a N50 length of 2523, 3480, 3181, and 3267 bp, respectively. Additionally, 35,887, 11,306, 36,422, and 25,439 SSRs were detected for the male cone, leaf, root, and stem, respectively. The number of long non-coding RNAs predicted from the root was the largest (11,113), and the other tissues were 3408 (stem), 3193 (leaf), and 3107 (male cone), respectively. Functional annotation and enrichment analysis of tissue-specific expressed genes revealed the special roles in response to environmental stress and adaptability in the different four tissues. We also characterized the gene families and pathways related to abiotic factors. This work provides a comprehensive transcriptome resource for C. oliveri, and this resource will facilitate further studies on the functional genomics and adaptive evolution of C. oliveri.

Elicitor Application in Strawberry Results in Long-Term Increase of Plant Resilience Without Yield Loss

For a first step integrating elicitor applications into the current IPM strategy increasing plant resilience against pests, we investigated repeated elicitor treatments in a strawberry everbearer nursery and cropping cycle under glass. During nursery methyl-jasmonate (MeJA), testing induction of defenses with plant bioassays was applied every 3 weeks. Thrips damage and reproduction by spider mites, whitefly and aphids were strongly reduced upon elicitor treatment. Subsequently, we applied MeJA every 3 weeks or based on scouting pests during a whole cropping cycle. Thrips leaf bioassays and LC-MS leaf metabolomics were applied to investigate the induction of defenses. Leaf damage by thrips was lower for both MeJA application schemes compared to the control except for the last weeks. While elicitor treatments after scouting also reduced damage, its effect did not last. Thrips damage decreased from vegetative to mature plants during the cropping cycle. At the end of the nursery phase, plants in the elicitor treatment were smaller. Surprisingly, growth during production was not affected by MeJA application, as were fruit yield and quality. LC-MS leaf metabolomics showed strong induction of vegetative plants decreasing during the maturation of plants toward the end of cultivation. Concurrently, no increase in the JA-inducible marker PPO was observed when measured toward the end of cultivation. Mostly flavonoid and phenolic glycosides known as plant defense compounds were induced upon MeJA application. While induced defense decreased with the maturation of plants, constitutive defense increased as measured in the leaf metabolome of control plants. Our data propose that young, relatively small plant stages lack constitutive defense necessitating an active JA defense response. As plants, mature constitutive defense metabolites seem to accumulate, providing a higher level of basal resistance. Our results have important implications for but are not limited to strawberry cultivation. We demonstrated that repeated elicitor application could be deployed as part of an integrated approach for sustainable crop protection by vertical integration with other management tactics and horizontal integration to control multiple pests concurrently. This approach forms a promising potential for long-term crop protection in greenhouses.

Agrobacterium strains and strain improvement: Present and outlook

Almost 40 years ago the first transgenic plant was generated through Agrobacterium tumefaciens-mediated transformation, which, until now, remains the method of choice for gene delivery into plants. Ever since, optimized Agrobacterium strains have been developed with additional (genetic) modifications that were mostly aimed at enhancing the transformation efficiency, although an optimized strain also exists that reduces unwanted plasmid recombination. As a result, a collection of very useful strains has been created to transform a wide variety of plant species, but has also led to a confusing Agrobacterium strain nomenclature. The latter is often misleading for choosing the best-suited strain for one's transformation purposes. To overcome this issue, we provide a complete overview of the strain classification. We also indicate different strain modifications and their purposes, as well as the obtained results with regard to the transformation process sensu largo. Furthermore, we propose additional improvements of the Agrobacterium-mediated transformation process and consider several worthwhile modifications, for instance, by circumventing a defense response in planta. In this regard, we will discuss pattern-triggered immunity, pathogen-associated molecular pattern detection, hormone homeostasis and signaling, and reactive oxygen species in relationship to Agrobacterium transformation. We will also explore alterations that increase agrobacterial transformation efficiency, reduce plasmid recombination, and improve biocontainment. Finally, we recommend the use of a modular system to best utilize the available knowledge for successful plant transformation.

Reactive oxygen species metabolism and photosynthetic performance in leaves of Hordeum vulgare plants co-infested with Heterodera filipjevi and Aceria tosichella

KEY MESSAGE

Defence responses of cyst nematode and/or wheat curl mite infested barley engage the altered reactive oxygen species production, antioxidant machinery, carbon dioxide assimilation and photosynthesis efficiency. The primary aim of this study was to determine how barley responds to two pests infesting separately or at once; thus barley was inoculated with Heterodera filipjevi (Madzhidov) Stelter (cereal cyst nematode; CCN) and Aceria tosichella Keifer (wheat curl mite; WCM). To verify hypothesis about the involvement of redox metabolism and photosynthesis in barley defence responses, biochemical, photosynthesis efficiency and chlorophyll a fluorescence measurements as well as transmission electron microscopy were implemented. Inoculation with WCM (apart from or with CCN) brought about a significant suppression in the efficiency of electron transport outside photosystem II reaction centres. This limitation was an effect of diminished pool of rapidly reducing plastoquinone and decreased total electron carriers. Infestation with WCM (apart from or with CCN) also significantly restricted the electron transport on the photosystem I acceptor side, therefore produced reactive oxygen species oxidized lipids in cells of WCM and double infested plants and proteins in cells of WCM-infested plants. The level of hydrogen peroxide was significantly decreased in double infested plants because of glutathione-ascorbate cycle involvement. The inhibition of nitrosoglutathione reductase promoted the accumulation of S-nitrosoglutathione increasing antioxidant capacity in cells of double infested plants. Moreover, enhanced arginase activity in WCM-infested plants could stimulate synthesis of polyamines participating in plant antioxidant response. Infestation with WCM (apart from or with CCN) significantly reduced the efficiency of carbon dioxide assimilation by barley leaves, whereas infection only with CCN expanded photosynthesis efficiency. These were accompanied with the ultrastructural changes in chloroplasts during CCN and WCM infestation.

The arbuscular mycorrhizal mycelium from barley differentially influences various defense parameters in the non-host sugar beet under co-cultivation

The interactions between arbuscular mycorrhizal fungi (AMF) and non-host species are poorly studied. Particularly scarce is information on members of the Amaranthaceae/Chenopodiaceae family. Sugar beet (Beta vulgaris) plants were co-cultivated with a host species (Hordeum vulgare) in the presence (+AMF) or absence of Rhizophagus intraradices to explore the hypothesis that the presence of an active, pre-established AMF mycelium induces defense responses in the non-host species. Biomass of sugar beet did not respond to the +AMF treatment, while its root exudation of organic acids and phenolic acids was drastically decreased upon co-cultivation with +AMF barley. The most conspicuous effect was observed on a wide range of potential defense parameters being differentially influenced by the +AMF treatment in this non-host species. Antioxidant defense enzymes were activated and the level of endogenous jasmonic acid was elevated accompanied by nitric oxide accumulation and lignin deposition in the roots after long-term +AMF treatment. In contrast, significant reductions in the levels of endogenous salicylic acid and tissue concentration and exudation of phenolic acids indicated that AM fungus hyphae in the substrate did not induce a hypersensitive-type response in the sugar beet roots and downregulated certain chemical defenses. Our results imply that the fitness of this non-host species is not reduced when grown in the presence of an AMF mycelium because of balanced defense costs. Further studies should address the question of whether or not such modulation of defense pattern influences the pest resistance of sugar beet plants under field conditions.

The atypical E2F transcription factor DEL1 modulates growth-defense tradeoffs of host plants during root-knot nematode infection

In plants, growth-defense tradeoffs are essential for optimizing plant performance and adaptation under stress conditions, such as pathogen attack. Root-knot nematodes (RKNs) cause severe economic losses in many crops worldwide, although little is known about the mechanisms that control plant growth and defense responses during nematode attack. Upon investigation of Arabidopsis thaliana infected with RKN (Meloidogyne incognita), we observed that the atypical transcription factor DP-E2F-like 1 (DEL1) repressed salicylic acid (SA) accumulation in RKN-induced galls. The DEL1-deficient Arabidopsis mutant (del1-1) exhibited excessive SA accumulation in galls and is more resistant to RKN infection. In addition, excessive lignification was observed in galls of del1-1. On the other hand, the root growth of del1-1 is reduced after RKN infection. Taken together, these findings suggest that DEL1 plays an important role in the balance between plant growth and defense responses to RKN infection by controlling SA accumulation and lignification.

MiMIF-2 Effector of Meloidogyne incognita Exhibited Enzyme Activities and Potential Roles in Plant Salicylic Acid Synthesis

Plant-parasitic nematodes secrete a series of effectors to promote parasitism by modulating host immunity, but the detailed molecular mechanism is ambiguous. Animal parasites secrete macrophage migration inhibitory factor (MIF)-like proteins for evasion of host immune systems, in which their biochemical activities play essential roles. Previous research demonstrated that MiMIF-2 effector was secreted by Meloidogyne incognita and modulated host immunity by interacting with annexins. In this study, we show that MiMIF-2 had tautomerase activity and protected nematodes against H₂O₂ damage. MiMIF-2 expression not only decreased the amount of H₂O₂ generation during nematode infection in Arabidopsis, but also suppressed Bax-induced cell death by inhibiting reactive oxygen species burst in Nicotiana benthamiana. Further, RNA-seq transcriptome analysis and RT-qPCR showed that the expression of some heat-shock proteins was down regulated in MiMIF-2 transgenic Arabidopsis. After treatment with flg22, RNA-seq transcriptome analysis indicated that the differentially expressed genes in MiMIF-2 expressing Arabidopsis were pointed to plant hormone signal transduction, compound metabolism and plant defense. RT-qPCR and metabolomic results confirmed that salicylic acid (SA) related marker genes and SA content were significantly decreased. Our results provide a comprehensive understanding of how MiMIF-2 modulates plant immunity and broaden knowledge of the intricate relationship between M. incognita and host plants.

Nanoapplication of a Resistance Inducer to Reduce Phytophthora Disease in Pineapple (Ananas comosus L.)

Treatment of plants with a variety of abiotic and biotic inducers causes induced resistance to pathogen attack. In this study, the effect of four resistance inducers on plant diseases caused by Phytophthora cinnamomi was screened in vivo initially by using lupin, a susceptible model plant. Lupin pretreated with 0.5 mM salicylic acid (SA) showed effective resistance against P. cinnamomi with restricted lesions. Then, mesoporous silica nanoparticles (MSNs) with particle size around 20 nm and approximate pore size of 3.0 nm were synthesized and functionalized for loading and importing SA to pineapple plantlets. Decanethiol gatekeepers were introduced to the surface of MSNs via glutathione (GSH)-cleavable disulfide linkages to cover the pore entrance, which was confirmed through using Raman spectroscopy. Through free diffusion, the loading efficiency of SA in MSNs gated with gatekeepers was 11.7%, but was lower in MSNs without gatekeepers (8.0%). In addition, in vitro release profile of SA from gatekeeper-capped MSNs indicated that higher concentrations of GSH resulted in more cargo release. Moreover, the experiments in planta showed that the application of MSNs as a resistance inducer delivery system significantly improved pineapple resistance to P. cinnamomi in terms of inhibiting lesion development and improving root growth of infected plants, compared to the use of free SA and MSNs without gatekeepers. The analysis of SA, GSH, and defense-related genes, of PR1 and PR5, further confirmed that the slow and prolonged release of SA from MSNs inside the roots of pineapple plants was achieved through a redox-stimuli release mechanism. Therefore, the application of MSNs with redox-responsive gatekeepers has shown great potential as an efficient tool for delivering chemicals into plants in a controllable way.

Novel Salicylic Acid Analogs Induce a Potent Defense Response in Arabidopsis

The master regulator of salicylic acid (SA)-mediated plant defense, NPR1 (NONEXPRESSER OF PR GENES 1) and its paralogs NPR3 and NPR4, act as SA receptors. After the perception of a pathogen, plant cells produce SA in the chloroplast. In the presence of SA, NPR1 protein is reduced from oligomers to monomers, and translocated into the nucleus. There, NPR1 binds to TGA, TCP, and WRKY transcription factors to induce expression of plant defense genes. A list of compounds structurally similar to SA was generated using ChemMine Tools and its Clustering Toolbox. Several of these analogs can induce SA-mediated defense and inhibit growth of Pseudomonas syringae in Arabidopsis. These analogs, when sprayed on Arabidopsis, can induce the accumulation of the master regulator of plant defense NPR1. In a yeast two-hybrid system, these analogs can strengthen the interactions among NPR proteins. We demonstrated that these analogs can induce the expression of the defense marker gene PR1. Furthermore, we hypothesized that these SA analogs could be potent tools against the citrus greening pathogen Candidatus liberibacter spp. In fact, our results suggest that the SA analogs we tested using Arabidopsis may also be effective for inducing a defense response in citrus. Several SA analogs consistently strengthened the interactions between citrus NPR1 and NPR3 proteins in a yeast two-hybrid system. In future assays, we plan to test whether these analogs avoid degradation by SA hydroxylases from plant pathogens. In future assays, we plan to test whether these analogs avoid degradation by SA hydroxylases from plant pathogens.

Bioprospecting from plant waste composting: Actinobacteria against phytopathogens producing damping-off

Strains isolated during composting processes of plant waste, and identified as Actinobacteria, proved to be significant producers of compounds that actively participate in the control of phytopathogens, such as those that cause Damping-off disease. Although most of the actinomycetes analyzed showed to be antagonistic strains against common phytopathogens, only some 30% proved to be capable of producing bioactive substances, such as siderophores, salicylic acid, chitinase enzymes or cyanide, so that antibiosis could be considered the most probable antagonistic mechanism for a high proportion of the strains investigated. 6% of the microorganisms identified in this work, were selected as potential strains to be investigated in depth, since they further stimulated plant growth (germination index tests greater than 100%). Microbacteriaceae was one of the most prominent families.

Synthetic Salicylic acid inducible recombinant promoter for translational research

In the present study, we have developed an inter-molecularly shuffled caulimoviral promoter for protein over-expression by placing the Upstream Activation Sequence (UAS) of Figwort Mosaic Virus (FMV; -249 to -54) at the 5'-end of the Cassava Vein Mosaic Virus (CsVMV) promoter fragment 8 (CsVMV8; -215 to +166) to design a hybrid promoter; FUASCsV8CP. The FUASCsV8CP promoter exhibited approximately 2.1 and 2.0 times higher GUS-activities than that obtained from the CaMV35S promoter, in tobacco (Xanthi Brad) protoplasts and in Agroinfiltration assays respectively. Hereto, when FUASCsV8CP was assayed using transgenic tobacco plants (T2- generation), it showed 2.0 times stronger activity than CaMV35S promoter and almost equivalent activity to that of CaMV35S2 promoter. The promoter displayed Salicylic acid (SA) inducibility and hence can also be used for ensuring effective gene expression in plants under constitutive as well as specific inducible conditions. Furthermore, FUASCsV8CP was used to drive the expression of victoviral Vin gene (encoding Victoriocin) transiently in tobacco. The recombinant Victoriocin could be successfully detected by western blotting three days post infiltration. Also, the in vitro Agar-based killing zone assays employing plant-derived Victoriocin-His (obtained from transient expression of Vin) revealed enhanced antifungal activity of Victoriocin against hemi-biotrophic pathogen Phoma exigua Desm. var. exigua.

Pandemonium Breaks Out: Disruption of Salicylic Acid-Mediated Defense by Plant Pathogens

Salicylic acid (SA) or 2-hydroxybenoic acid is a phenolic plant hormone that plays an essential role in plant defense against biotrophic and semi-biotrophic pathogens. In Arabidopsis, SA is synthesized from chorismate in the chloroplast through the ICS1 (isochorismate synthase I) pathway during pathogen infection. The transcription co-activator NPR1 (Non-Expresser of Pathogenesis-Related Gene 1), as the master regulator of SA signaling, interacts with transcription factors to induce the expression of anti-microbial PR (Pathogenesis-Related) genes. To establish successful infections, plant bacterial, oomycete, fungal, and viral pathogens have evolved at least three major strategies to disrupt SA-mediated defense. The first strategy is to reduce SA accumulation directly by converting SA into its inactive derivatives. The second strategy is to interrupt SA biosynthesis by targeting the ICS1 pathway. In the third major strategy, plant pathogens deploy different mechanisms to interfere with SA downstream signaling. The wide array of strategies deployed by plant pathogens highlights the crucial role of disruption of SA-mediated plant defense in plant pathogenesis. A deeper understanding of this topic will greatly expand our knowledge of how plant pathogens cause diseases and consequently pave the way for the development of more effective ways to control these diseases.

Salicylic acid improves arbuscular mycorrhizal symbiosis, and chickpea growth and yield by modulating carbohydrate metabolism under salt stress

Salt stress is a major abiotic stress restricting plant growth and reproductive yield. Salicylic acid (SA) and arbuscular mycorrhizal (AM) symbioses play key roles in eliminating adverse effects of salt stress by modulating ion homeostasis and carbohydrate metabolism in crop plants. Sugars synthesized via carbohydrate metabolism act as osmotic adjustors and signaling molecules in activation of various defense responses against salt stress. The present study investigated the role of SA (0.5 mM) seed priming in establishment of AM symbiosis with Rhizoglomus intraradices and the impact on growth, ion-homeostasis, nutrient uptake, and sugar metabolism in Cicer arietinum L. (chickpea) genotypes under salt stress. Salinity had a negative correlation with plant growth and AM symbiosis in both genotypes with more negative effects in relatively salt-sensitive genotype than tolerant. SA enhanced the percent root colonization by significantly increasing the number of arbuscules and vesicles under salt stress. AM symbiosis was more effective in improving root biomass, root to shoot ratio, and nutrient acquisition than SA, while SA was more effective in maintaining ion equilibrium and modulating carbohydrate metabolism and reproductive yield when compared with AM inoculation. SA priming directed the utilization of total soluble sugars (TSS) towards reproductive attributes more efficiently than did AM inoculation by activating TSS metabolic consumption. In AM plants, TSS concentrations were more directed towards sink demand by the fungus itself rather than developing reproductive structures. SA priming further increased sugar export to roots of AM plants, thus favored AM symbiosis. Hence, SA seed priming-induced improvement in AM symbiosis can be a promising strategy in achieving sustainable production of chickpea genotypes under salt stress.