The salicylic acid signal in plants

Genome-wide identification of the ICS family genes and its role in resistance to Plasmodiophora brassicae in Brassica napus L

The isochorismate synthase (ICS) proteins are essential regulators of salicylic acid (SA) synthesis, which has been reported to regulate resistance to biotic and abiotic stresses in plants. Clubroot caused by Plasmodiophora brassicae is a common disease that threatens the yield and quality of Oilseed rape (Brassica napus L.). Exogenous application of salicylic acid reduced the incidence of clubroot in oilseed rape. However, the potential importance of the ICS genes family in B. napus and its diploid progenitors has been unclear. Here, we identified 16, 9, and 10 ICS genes in the allotetraploid B. napus, diploid ancestor Brassica rapa and Brassica oleracea, respectively. These ICS genes were classified into three subfamilies (I-III), and member of the same subfamilies showed relatively conserved gene structures, motifs, and protein domains. Furthermore, many hormone-response and stress-related promoter cis-acting elements were observed in the BnaICS genes. Exogenous application of SA delayed the growth of clubroot galls, and the expression of BnaICS genes was significantly different compared to the control groups. Protein-protein interaction analysis identified 58 proteins involved in the regulation of ICS in response to P. brassicae in B. napus. These results provide new clues for understanding the resistance mechanism to P. brassicae.

The Invasive Mechanisms of the Noxious Alien Plant Species Bidens pilosa

Bidens pilosa L. is native to tropical America and has widely naturized from tropical to warm temperate regions in Europe, Africa, Asia, Australia, and North and South America. The species has infested a wide range of habitats such as grasslands, forests, wetlands, streamlines, coastal areas, pasture, plantations, agricultural fields, roadsides, and railway sides and has become a noxious invasive weed species. B. pilosa forms thick monospecific stands, quickly expands, and threatens the indigenous plant species and crop production. It is also involved in pathogen transmission as a vector. The species was reported to have (1) a high growth ability, producing several generations in a year; (2) a high achene production rate; (3) different biotypes of cypselae, differently germinating given the time and condition; (4) a high adaptative ability to various environmental conditions; (5) an ability to alter the microbial community, including mutualism with arbuscular mycorrhizal fungi; and (6) defense functions against natural enemies and allelopathy. The species produces several potential allelochemicals such as palmitic acid, p-coumaric acid, caffeic acid, ferulic acid, p-hydroxybenzoic acid, vanillic acid, salycilic acid, quercetin, α-pinene, and limonene and compounds involved in the defense functions such as 1-phenylhepta-1,3,5-trine, 5-phenyl-2-(1-propynyl)-thiophene, 5-actoxy-2-phenylethinyl-thiophene, and icthyothereol acetate. These characteristics of B. pilosa may contribute to the naturalization and invasiveness of the species in the introduced ranges. This is the first review article focusing on the invasive mechanisms of the species.

Strategies to Increase the Production of Triterpene Acids in Ligzhi or Reishi Medicinal Mushroom (Ganoderma lucidum, Agaricomycetes): A Review

Ganoderic acids (GAs) are the main active ingredient of Ganoderma lucidum, which has been widely accepted as a medicinal mushroom. Due to the low yield of GAs produced by liquid cultured Ganoderma mycelium and solid cultured fruiting bodies, the commercial production and clinical application of GAs are limited. Therefore, it is important to increase the yield of GA in G. lucidum. A comprehensive literature search was performed with no set data range using the following keywords such as "triterpene," "ganoderic acids," "Ganoderma lucidum," and "Lingzhi" within the main databases including Web of Science, PubMed, and China National Knowledge Infrastructure (CNKI). The data were screened using titles and abstracts and those relevant to the topic were included in the paper and was not limited to studies published in English. Present review focuses on the four aspects: fermentation conditions and substrate, extrinsic elicitor, genetic engineering, and mutagenesis, which play significant roles in increasing triterpene acids production, thus providing an available reference for further research on G. lucidum fermentation.

Flagellin, a plant-defense-activating protein identified from Xanthomonas axonopodis pv. Dieffenbachiae invokes defense response in tobacco

BACKGROUND

Secretome analysis is a valuable tool to study host-pathogen protein interactions and to identify new proteins that are important for plant health. Microbial signatures elicit defense responses in plants, and by that, the plant immune system gets triggered prior to pathogen infection. Functional properties of secretory proteins from Xanthomonas axonopodis pv. dieffenbachiae (Xad1) involved in priming plant immunity was evaluated.

RESULTS

In this study, the secretome of Xad1 was analyzed under host plant extract-induced conditions, and mass spectroscopic analysis of differentially expressed protein was identified as plant-defense-activating protein viz., flagellin C (FliC). The flagellin and Flg22 peptides both elicited hypersensitive reaction (HR) in non-host tobacco, activated reactive oxygen species (ROS) scavenging enzymes, and increased pathogenesis-related (PR) gene expression viz., NPR1, PR1, and down-regulation of PR2 (β-1,3-glucanase). Protein docking studies revealed the Flg22 epitope of Xad1, a 22 amino acid peptide region in FliC that recognizes plant receptor FLS2 to initiate downstream defense signaling.

CONCLUSION

The flagellin or the Flg22 peptide from Xad1 was efficient in eliciting an HR in tobacco via salicylic acid (SA)-mediated defense signaling that subsequently triggers systemic immune response epigenetically. The insights from this study can be used for the development of bio-based products (small PAMPs) for plant immunity and health.

Molecular Mechanisms of Pseudomonas-Assisted Plant Nitrogen Uptake: Opportunities for Modern Agriculture

Pseudomonas spp. make up 1.6% of the bacteria in the soil and are found throughout the world. More than 140 species of this genus have been identified, some beneficial to the plant. Several species in the family Pseudomonadaceae, including Azotobacter vinelandii AvOP, Pseudomonas stutzeri A1501, Pseudomonas stutzeri DSM4166, Pseudomonas szotifigens 6HT33bT, and Pseudomonas sp. strain K1 can fix nitrogen from the air. The genes required for these reactions are organized in a nitrogen fixation island, obtained via horizontal gene transfer from Klebsiella pneumoniae, Pseudomonas stutzeri, and Azotobacter vinelandii. Today, this island is conserved in Pseudomonas spp. from different geographical locations, which, in turn, have evolved to deal with different geo-climatic conditions. Here, we summarize the molecular mechanisms behind Pseudomonas-driven plant growth promotion, with particular focus on improving plant performance at limiting nitrogen (N) and improving plant N content. We describe Pseudomonas-plant interaction strategies in the soil, noting that the mechanisms of denitrification, ammonification, and secondary metabolite signaling are only marginally explored. Plant growth promotion is dependent on the abiotic conditions and differs at sufficient and deficient N. The molecular controls behind different plant responses are not fully elucidated. We suggest that superposition of transcriptome, proteome, and metabolome data and their integration with plant phenotype development through time will help fill these gaps. The aim of this review is to summarize the knowledge behind Pseudomonas-driven nitrogen fixation and to point to possible agricultural solutions. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Uncovering the mechanisms of salicylic acid-mediated abiotic stress tolerance in horticultural crops

Salicylic acid (SA) has been recognized as a promising molecule for improving abiotic stress tolerance in plants due to its ability to enhance antioxidant defense system, and promote root architecture system. Recent research has focused on uncovering the mechanisms by which SA confers abiotic stress tolerance in horticultural crops. SA has been shown to act as a signaling molecule that triggers various physiological and morphological responses in plants. SA regulates the production of reactive oxygen species (ROS). Moreover, it can also act as signaling molecule that regulate the expression of stress-responsive genes. SA can directly interact with various hormones, proteins and enzymes involved in abiotic stress tolerance. SA regulates the antioxidant enzymes activities that scavenge toxic ROS, thereby reducing oxidative damage in plants. SA can also activate protein kinases that phosphorylate and activate transcription factors involved in stress responses. Understanding these mechanisms is essential for developing effective strategies to improve crop resilience in the face of changing environmental conditions. Current information provides valuable insights for farmers and plant researchers, offering new strategies to enhance crop resilience and productivity in the face of environmental challenges. By harnessing the power of SA and its signaling pathways, farmers can develop more effective stress management techniques and optimize crop performance. Plant researchers can also explore innovative approaches to breed or engineer crops with enhanced stress tolerance, thereby contributing to sustainable agriculture and food security.

The role of plant growth promoting rhizobacteria in plant drought stress responses

Climate change has exacerbated the effects of abiotic stresses on plant growth and productivity. Drought is one of the most important abiotic stress factors that interfere with plant growth and development. Plant selection and breeding as well as genetic engineering methods used to improve crop drought tolerance are expensive and time consuming. Plants use a myriad of adaptative mechanisms to cope with the adverse effects of drought stress including the association with beneficial microorganisms such as plant growth promoting rhizobacteria (PGPR). Inoculation of plant roots with different PGPR species has been shown to promote drought tolerance through a variety of interconnected physiological, biochemical, molecular, nutritional, metabolic, and cellular processes, which include enhanced plant growth, root elongation, phytohormone production or inhibition, and production of volatile organic compounds. Therefore, plant colonization by PGPR is an eco-friendly agricultural method to improve plant growth and productivity. Notably, the processes regulated and enhanced by PGPR can promote plant growth as well as enhance drought tolerance. This review addresses the current knowledge on how drought stress affects plant growth and development and describes how PGPR can trigger plant drought stress responses at the physiological, morphological, and molecular levels.

Nucleoredoxin gene SINRX1 negatively regulates tomato immunity by activating SA signaling pathway

The tomato (Solanum lycopersicum) is widely consumed globally and renowned for its health benefits, including the reduction of cardiovascular disease and prostate cancer risk. However, tomato production faces significant challenges, particularly due to various biotic stresses such as fungi, bacteria, and viruses. To address this challenges, we employed the CRISPR/Cas9 system to modify the tomato NUCLEOREDOXIN (SlNRX) genes (SlNRX1 and SlNRX2) belonging to the nucleocytoplasmic THIOREDOXIN subfamily. CRISPR/Cas9-mediated mutations in SlNRX1 (slnrx1) plants exhibited resistance against bacterial leaf pathogen Pseudomonas syringae pv. maculicola (Psm) ES4326, as well as the fungal pathogen Alternaria brassicicola. However, the slnrx2 plants did not display resistance. Notably, the slnrx1 demonstrated elevated levels of endogenous salicylic acid (SA) and reduced levels of jasmonic acid after Psm infection, in comparison to both wild-type (WT) and slnrx2 plants. Furthermore, transcriptional analysis revealed that genes involved in SA biosynthesis, such as ISOCHORISMATE SYNTHASE 1 (SlICS1) and ENHANCED DISEASE SUSCEPTIBILITY 5 (SlEDS5), were upregulated in slnrx1 compared to WT plants. In addition, a key regulator of systemic acquired resistance, PATHOGENESIS-RELATED 1 (PR1), exhibited increased expression in slnrx1 compared to WT. These findings suggest that SlNRX1 acts as a negative regulator of plant immunity, facilitating infection by the Psm pathogen through interference with the phytohormone SA signaling pathway. Thus, targeted mutagenesis of SlNRX1 is a promising genetic means to enhance biotic stress resistance in crop breeding.

Preventive and Curative Effects of Salicylic and Methyl Salicylic Acid Having Antifungal Potential against Monilinia laxa and the Development of Phenolic Response in Apple Peel

The effects of salicylic acid (SA) and one of its better-known derivatives-methyl salicylic acid (MeSA)-on the infection of apple fruits with the fungus Monilinia laxa, which causes brown rot, were investigated. Since research to date has focused on preventive effects, we also focused on the curative use of SA and MeSA. Curative use of SA and MeSA slowed the progression of the infection. In contrast, preventive use was generally unsuccessful. HPLC-MS was used to analyze the content of phenolic compounds in apple peels in healthy and boundary peel tissues around lesions. The boundary tissue around the lesions of untreated infected apple peel had up to 2.2-times higher content of total analyzed phenolics (TAPs) than that in the control. Flavanols, hydroxycinnamic acids and dihydrochalcones were also higher in the boundary tissue. During the curative treatment with salicylates, the ratio of TAP content between healthy and boundary tissue was lower (SA up to 1.2-times higher and MeSA up to 1.3-times higher content of TAPs in boundary compared to those in healthy tissue) at the expense of also increasing the content in healthy tissues. The results confirm that salicylates and infection with the fungus M. laxa cause an increased content of phenolic compounds. Curative use of salicylates has a greater potential than preventive use in infection control.

Antioxidant Potential of Glutathione and Crosstalk with Phytohormones in Enhancing Abiotic Stress Tolerance in Crop Plants

Glutathione (GSH) is an abundant tripeptide that can enhance plant tolerance to biotic and abiotic stress. Its main role is to counter free radicals and detoxify reactive oxygen species (ROS) generated in cells under unfavorable conditions. Moreover, along with other second messengers (such as ROS, calcium, nitric oxide, cyclic nucleotides, etc.), GSH also acts as a cellular signal involved in stress signal pathways in plants, directly or along with the glutaredoxin and thioredoxin systems. While associated biochemical activities and roles in cellular stress response have been widely presented, the relationship between phytohormones and GSH has received comparatively less attention. This review, after presenting glutathione as part of plants' feedback to main abiotic stress factors, focuses on the interaction between GSH and phytohormones, and their roles in the modulation of the acclimatation and tolerance to abiotic stress in crops plants.

Regulation of Reactive Oxygen Species during Salt Stress in Plants and Their Crosstalk with Other Signaling Molecules-Current Perspectives and Future Directions

Salt stress is a severe type of environmental stress. It adversely affects agricultural production worldwide. The overproduction of reactive oxygen species (ROS) is the most frequent phenomenon during salt stress. ROS are extremely reactive and, in high amounts, noxious, leading to destructive processes and causing cellular damage. However, at lower concentrations, ROS function as secondary messengers, playing a critical role as signaling molecules, ensuring regulation of growth and adjustment to multifactorial stresses. Plants contain several enzymatic and non-enzymatic antioxidants that can detoxify ROS. The production of ROS and their scavenging are important aspects of the plant's normal response to adverse conditions. Recently, this field has attracted immense attention from plant scientists; however, ROS-induced signaling pathways during salt stress remain largely unknown. In this review, we will discuss the critical role of different antioxidants in salt stress tolerance. We also summarize the recent advances on the detrimental effects of ROS, on the antioxidant machinery scavenging ROS under salt stress, and on the crosstalk between ROS and other various signaling molecules, including nitric oxide, hydrogen sulfide, calcium, and phytohormones. Moreover, the utilization of "-omic" approaches to improve the ROS-regulating antioxidant system during the adaptation process to salt stress is also described.

Abscisic Acid Mediates Salicylic Acid Induced Chilling Tolerance of Grafted Cucumber by Activating H2O2 Biosynthesis and Accumulation

Grafting is widely applied to enhance the tolerance of some vegetables to biotic and abiotic stress. Salicylic acid (SA) is known to be involved in grafting-induced chilling tolerance in cucumber. Here, we revealed that grafting with pumpkin (Cucurbita moschata, Cm) as a rootstock improved chilling tolerance and increased the accumulation of SA, abscisic acid (ABA) and hydrogen peroxide (H₂O₂) in grafted cucumber (Cucumis sativus/Cucurbita moschata, Cs/Cm) leaves. Exogenous SA improved the chilling tolerance and increased the accumulation of ABA and H₂O₂ and the mRNA abundances of CBF1, COR47, NCED, and RBOH1. However, 2-aminoindan-2-phosphonic acid (AIP) and L-a-aminooxy-b-phenylpropionic acid (AOPP) (biosynthesis inhibitors of SA) reduced grafting-induced chilling tolerance, as well as the synthesis of ABA and H₂O₂, in cucumber leaves. ABA significantly increased endogenous H₂O₂ production and the resistance to chilling stress, as proven by the lower electrolyte leakage (EL) and chilling injury index (CI). However, application of the ABA biosynthesis inhibitors sodium tungstate (Na₂WO₄) and fluridone (Flu) abolished grafting or SA-induced H₂O₂ accumulation and chilling tolerance. SA-induced plant response to chilling stress was also eliminated by N,N'-dimethylthiourea (DMTU, an H₂O₂ scavenger). In addition, ABA-induced chilling tolerance was attenuated by DMTU and diphenyleneiodonium (DPI, an H₂O₂ inhibitor) chloride, but AIP and AOPP had little effect on the ABA-induced mitigation of chilling stress. Na₂WO₄ and Flu diminished grafting- or SA-induced H₂O₂ biosynthesis, but DMTU and DPI did not affect ABA production induced by SA under chilling stress. These results suggest that SA participated in grafting-induced chilling tolerance by stimulating the biosynthesis of ABA and H₂O₂. H₂O₂, as a downstream signaler of ABA, mediates SA-induced chilling tolerance in grafted cucumber plants.

Effects of Exogenous Salicylic Acid Application to Aboveground Part on the Defense Responses in Bt (Bacillus thuringiensis) and Non-Bt Corn (Zea mays L.) Seedlings

Bt (Bacillus thuringiensis) corn is one of the top three large-scale commercialized anti-insect transgenic crops around the world. In the present study, we tested the Bt protein content, defense chemicals contents, and defense enzyme activities in both the leaves and roots of Bt corn varieties 5422Bt1 and 5422CBCL, as well as their conventional corn 5422 seedlings, with two fully expanded leaves which had been treated with 2.5 mM exogenous salicylic acid (SA) to the aboveground part for 24 h. The result showed that the SA treatment to the aboveground part could significantly increase the polyphenol oxidase activity of conventional corn 5422, the Bt protein content, and peroxidase activities of Bt corn 5422Bt1, as well as the polyphenol oxidase and peroxidase activity of Bt corn 5422CBCL in the leaves. In the roots, the polyphenol oxidase and peroxidase activity of conventional corn 5422, the polyphenol oxidase and superoxide dismutase activities of Bt corn 5422Bt1, the DIMBOA (2,4-dihydroxy-7-methoxy-2H, 1, 4-benzoxazin-3 (4H)-one) content, and four defense enzymes activities of Bt corn 5422CBCL were systematically increased. These findings suggest that the direct effect of SA application to aboveground part on the leaf defense responses in Bt corn 5422CBCL is stronger than that in non-Bt corn. Meanwhile, the systemic effect of SA on the root defense responses in Bt corn 5422CBCL is stronger than that in conventional corn 5422 and Bt corn 5422Bt1. It can be concluded that the Bt gene introduction and endogenous chemical defense responses of corns act synergistically during the SA-induced defense processes to the aboveground part. Different transformation events affected the root defense response when the SA treatment was applied to the aboveground part.

Defense response changes in roots of oil palm (Elaeis guineensis Jacq.) seedlings after internal symptoms of Ganoderma boninense Pat. infection

BACKGROUND

The development of basal stem rot (BSR) disease in oil palm is associated with lignin during vegetative growth and salicylic acid (SA) biosynthesis. The increase in the lignin content, SA accumulation, growth, and root biomass could indicate the resistance of oil palm seedlings to BSR disease. Therefore, although there are many studies on the interactions between the Ganoderma boninense and oil palm, research on evaluation of physiological processes, biochemistry, and molecules occurring during early internal symptoms of BSR in roots of oil palm (Elaeis guineensis Jacq.) are essential.

RESULTS

Ganoderma boninense inoculation indicated that C01, C02, and C05 seedlings were susceptible, while the other three seedlings, C03, C07, and C08, were resistant based on Ganoderma Disease Index (GDI). Infection by G. boninense in the most susceptible seedlings C05 reduced fresh weight of roots (FW) by 9.0%, and lignin content by 10.9%. The most resistant seedlings C08 were reduced by only 8.4%, and 0.2% regarding their fresh weight and lignin content, respectively. BSR disease induced SA accumulation in the most susceptible C08 and decreased peroxidase (PRX) enzyme (EC 1.11.1.7) activities in root tissues of oil palm seedlings except C07 and C08 where PRX activities remained high in the 4 months after planting. Infection with G. boninense also increased glutathione S-transferase U19-like (EgGSTU19) gene expression in the root tissues of susceptible seedlings, while laccase-24 (EgLCC24) gene expression was associated with resistance against BSR disease. Based on the relative expression of twelve genes, two genes are categorized as receptors (EgWAKL5, EgMIK1), two genes as biosynthesis signal transduction compound (EgOPR5, EgACO1), five genes as defense responses (EgROMT, EgSOT12, EgLCC24, EgGLT3, EgGSTU19), and one gene as trans-resveratrol di-O-methyltransferase-like (EgRNaseIII) predicted related to BSR infection. While two other genes remain unknown (EgUnk1, EgUnk2).

CONCLUSIONS

Ganoderma infection-induced SA accumulation and lignification in resistant accessions promote the seedlings root biomass. Oil palm seedlings have a synergistic physical, biochemical, and molecular defense mechanism to the BSR disease. The utilization of nucleotide-based molecular markers using EgLCC24 gene is able to detect resistant oil palm seedlings to G. boninense.

Immunity elicitors for induced resistance against the downy mildew pathogen in pearl millet

Pearl millet (Pennisetum glaucum (L.) R. Br.) is a globally important cereal whose production is severely constrained by downy mildew caused by Sclerospora graminicola (Sacc.). In this study, immunity eliciting properties of 3,5-dichloroanthranilic acid (DCA), Cell Wall Glucan (CWG), Lipopolysaccharide (LPS), and Glycinebetaine (GB) was deciphered through enzymatic and protein studies based on elicitor treatment activated defense mechanisms. Glycinebetaine, LPS, CWS and DCA elicited enzyme activities and gene expression of the defense enzymes, such as β-1,3-glucanase, phenylalanine ammonia lyase (PAL), peroxidase (POX), polyphenol oxidase (PPO), lipoxygenase (LOX) and defense protein hydroxyproline-rich glycoproteins (HRGPs). However, the speed and the extent of elicitation differed. High levels of enzyme activities and gene expression in elicitor-treated P. glaucum positively correlated with the increased downy mildew resistance. A very rapid and large changes in elicitor-treated seedlings, in contrast to the delayed, smaller changes in the untreated susceptible control seedlings suggests that the rate and magnitude of defense gene expression are important for effective manifestation of defense against pathogen. As compared to other elicitors and control, GB promoted increase in enzyme activities and gene expression, implicating that GB is a promising elicitor of downy mildew resistance in P. glaucum.

Improvement in salt tolerance of Iris pseudacorus L. in constructed wetland by exogenous application of salicylic acid and calcium chloride

Wetland plants play a major role in the process of wastewater treatment in constructed wetlands (CWs). The inhibitory effect of salt stress on plants may reduce the performance of CWs. In this study, salicylic acid (SA) and/or calcium ion (Ca²⁺) were used for root pretreatment to alleviate the salt stress in Iris pseudacorus L. The results showed that root pretreatment with SA and/or Ca²⁺ improved the response of Iris pseudacorus L. to salinity by increasing growth, photosynthetic pigments, Pro content, enzymes activities and K⁺ content. In addition, SA and/or Ca²⁺ application in saline conditions decreased the relative conductivity and content of malondialdehyde. RNA-seq analysis showed the expression of hormone signaling genes, potassium ion transporter genes, oxidative stress genes and photosynthesis genes were up-regulated after pretreating with SA and CaCl₂. In conclusion, the addition of SA and Ca²⁺ could improve the saline wastewater treatment efficiency of CWs by enhancing the salt tolerance of Iris pseudacorus L.

Salicylic acid: A key regulator of redox signalling and plant immunity

In plants, the reactive oxygen species (ROS) formed during normal conditions are essential in regulating several processes, like stomatal physiology, pathogen immunity and developmental signaling. However, biotic and abiotic stresses can cause ROS over-accumulation leading to oxidative stress. Therefore, a suitable equilibrium is vital for redox homeostasis in plants, and there have been major advances in this research arena. Salicylic acid (SA) is known as a chief regulator of ROS; however, the underlying mechanisms remain largely unexplored. SA plays an important role in establishing the hypersensitive response (HR) and systemic acquired resistance (SAR). This is underpinned by a robust and complex network of SA with Non-Expressor of Pathogenesis Related protein-1 (NPR1), ROS, calcium ions (Ca²⁺), nitric oxide (NO) and mitogen-activated protein kinase (MAPK) cascades. In this review, we summarize the recent advances in the regulation of ROS and antioxidant defense system signalling by SA at the physiological and molecular levels. Understanding the molecular mechanisms of how SA controls redox homeostasis would provide a fundamental framework to develop approaches that will improve plant growth and fitness, in order to meet the increasing global demand for food and bioenergy.

Aromatic compounds that attract Meloidogyne species second-stage juveniles in soil

BACKGROUND

Nematode attractants could serve in nematode control strategies by combining with chemical or biological nematicides or by interrupting the nematodes' host-finding process. The attractiveness of some benzenoid aromatic compounds, mainly benzoic acids, alcohols, aldehydes and phenols, to second-stage juveniles (J2) of four Meloidogyne species (M. hapla, M. incognita, M. javanica and M. marylandi) was evaluated by using trap tubes and balls filled with washed dune sand buried in nematode-inoculated sand in Petri dishes.

RESULTS

Two-methoxybenzaldehyde, 2-methoxycinnamaldehyde, 2-hydroxybenzoic acid (salicylic acid), 2-hydroxy-3-methoxybenzaldehyde (o-vanillin), 3-methoxybenzoic acid, 4-methoxybenzoic acid and trans-cinnamic acid effectively attracted J2 of all or most of the four Meloidogyne species to trap tubes in a one-compound assay. When nematodes were exposed to three different compounds simultaneously in the three-compound assay, J2 of all Meloidogyne species were attracted mainly to 2-methoxycinnamaldehyde, salicylic acid and 4-methoxybenzoic acid. Exceptions were M. hapla J2, which were not attracted to salicylic acid. In the soil column assay, M. javanica and M. incognita J2 were attracted upward to 4-methoxybenzoic acid in a trap ball located 4 or 8 cm above the inoculation point, whereas salicylic acid and 3-methoxybenzoic acid demonstrated slight, if any attraction.

CONCLUSION

Although some of the tested compounds exist in root exudates, it is not clear whether they are involved in the nematode host-searching process in nature. The attractants found in the study have potential for use in Meloidogyne species control, probably as a nematode trap constituent or as compounds that disrupt the nematodes' host-finding process. © 2021 Society of Chemical Industry.

Comparative studies of eco-friendly compounds like humic acid, salicylic, and glycyrrhizic acids and their nanocomposites on French basil (Ocimum basilicum L. cv. Grand verde)

As the green tactics for enhancing plant growth and production using naturally occurring materials are highly needed, it is important to use the nanoformulation of these materials as an attractive novel technique. Therefore, this research has been performed to evaluate the plants' morphological traits, the qualitative parameters, and molecular genetic characteristics using random amplified polymorphic DNA (RAPD) of French basil independence on growth biostimulators and their nanocomposite. The treatments included normal formulations and nanocomposite formulation of humic acid (5 mM HA), salicylic acid (1.4 mM SA), and glycyrrhizic acid ammonium salt (0.4 mM GA) and control treatment (water application). The results show that foliar spray with HA, SA, GA, and their nanocomposites significantly increased (p ≤ 0.05) on all vegetative growth characters, photosynthetic pigments, oil yield/plant, mineral content, and antibacterial activity as compared with control plants. Also, 1,1-diphenyl-2-picrylhydrazyl (DPPH) values of different samples used varied from 70.63 to 74.93%, with a significant increase compared to untreated plants. The most marked increases have been observed in treated plants with biostimulants in the nanocomposites form than in the natural form. On the other hand, GA and its nanocomposite showed variable effects on basil plants and gave the lowest increase values in all parameters than the other biostimulant but have high antimicrobial activity. For the molecular study, ten selected primers displayed a total of 288 amplified fragments scored per primer ranging from 7 to 46 fragments; 157 bands were polymorphic with 69% polymorphism. It could be concluded that humic acid and its nanocomposite are the most effective biostimulants that increased plant productivity and oil content.

Electrical Stimulation Enhances Plant Defense Response in Grapevine through Salicylic Acid-Dependent Defense Pathway

Concern over environmental pollution generated by chemical fungicides has led to the introduction of alternative pest management strategies to chemical fungicide application. One of those strategies is the induction of plant defense response by an abiotic elicitor. In the present study, field-grown grapevines were subjected to electrical stimulation using a solar panel from two weeks before flowering to harvest in the 2016 and 2020 growing seasons. In both years, electrical stimulation decreased the incidence of gray mold and/or ripe rot on bunches and downy mildew on leaves of the field-grown grapevine. Transcription of a gene encoding β-1,3-glucanase but not class IV chitinase in leaves of potted grapevine seedlings was upregulated 20 days after electrical stimulation, suggesting that electrical stimulation acts as an abiotic elicitor of plant defense response to fungal diseases. The gene expression of PR1 but not PDF1.2 was upregulated in Arabidopsis plants subjected to electrical stimulation. On the other hand, PR1 gene expression was not induced in salicylic acid (SA)-insensitive Arabidopsis mutant npr1-5 subjected to electrical stimulation. Taken together, electrical stimulation is responsible for plant defense response through the SA-dependent defense pathway. These findings would help us develop a novel and innovative practical technique that uses electrical stimulation in integrated pest management.

Integrating Transcriptome and Coexpression Network Analyses to Characterize Salicylic Acid- and Jasmonic Acid-Related Genes in Tolerant Poplars Infected with Rust

Melampsora larici-populina causes serious poplar foliar diseases called rust worldwide. Salicylic acid (SA) and jasmonic acid (JA) are important phytohormones that are related to plant defence responses. To investigate the transcriptome profiles of SA- and JA-related genes involved in poplar rust interaction, two tolerant poplars and one intolerant poplar were selected for this study. Weighted gene coexpression network analysis (WGCNA) was applied to characterize the changes in the transcriptome profiles and contents of SA and JA after infection with the virulent E4 race of M. larici-populina. In response to infection with the E4 race of M. larici-populina, tolerant symptoms were correlated with the expression of genes related to SA and JA biosynthesis, the levels of SA and JA, and the expression of defence-related genes downstream of SA and JA. Tolerant poplars could promptly regulate the occurrence of defence responses by activating or inhibiting SA or JA pathways in a timely manner, including regulating the expression of genes related to programmed cell death, such as Kunitz-type trypsin inhibitor (KTI), to limit the growth of E4 and protect themselves. WGCNA suggested that KTI might be regulated by a Cytochrome P450 family (CYP) gene. Some CYPs should play an important role in both JA- and SA-related pathways. In contrast, in intolerant poplar, the inhibition of SA-related defence signalling through increasing JA levels in the early stage led to continued inhibition of a large number of plant–pathogen interaction-related and signalling-related genes, including NBS-LRRs, EDS1, NDR1, WRKYs, and PRs. Therefore, timely activation or inhibition of the SA or JA pathways is the key difference between tolerant and intolerant poplars.

Improving ATPase and PPase activities, nutrient uptake and growth of salt stressed ajowan plants by salicylic acid and iron-oxide nanoparticles

Salicylic acid and iron-oxide nanoparticles alleviated salt toxicity and improved plant growth by stimulating the activities of H⁺-ATPase and H⁺-PPase and preventing nutrient imbalance. Two factorial experiments were undertaken in a greenhouse during 2018 and 2019, to evaluate the impacts of SA (1 mM) and nano-Fe₂O₃ (3 mM) sprays at 7 leaves and flowering stages on vacuolar H⁺-pumps, growth and essential oil of salt-subjected (0, 4, 8 and 12 dS m^-1 NaCl) ajowan plants. Measurements of plant traits were started at about 12 days after the last foliar spray and continued up to maturity. The H⁺-ATPase and H⁺-PPase activities and root ATP content were enhanced under low salinity, but higher salinities reduced these parameters. Rising salinity enhanced Na uptake and translocation, endogenous SA and DPPH activity, while reduced K⁺/Na⁺ ratio and nutrients uptake, leading to a reduction in plant biomass. Treatment with SA, nano-Fe₂O₃ and their combination improved H⁺-pumps activities and ATP content in roots and leaves. The SA-related treatments caused the highest activities of H⁺-pumps in roots, but Fe-related treatments resulted in the highest activities of these pumps in leaves. Increasing H⁺-pumps activities reduced sodium uptake and translocation and enhanced nutrients uptake. Foliar treatments, especially SA + nano-Fe₂O₃ augmented endogenous SA, DPPH activity, and plant growth in salt-stressed plants. Essential oil contents of vegetative and inflorescence organs under severe salinity and seeds under moderate and severe salinities were enhanced. Maximum essential oil was obtained from seeds of SA + nano-Fe₂O₃-treated plants, which was strongly correlated with endogenous SA and DPPH. Nevertheless, the SA + nano-Fe₂O₃ was the best treatment for diminishing salt toxicity and improving ajowan plant growth and essential oil production.

Effects of potential synbiotic interaction between Lactobacillus rhamnosus GG and salicylic acid on human colon and prostate cancer cells

Salicylic acid, widely distributed in the whole plant kingdom, is a benzoic acid derivative acting as a signal substance in plants, but could be related to differences in cancer incidence, as many herbs and spices contain high amounts. Lactobacillus rhamnosus GG (LGG) is one of the best-known lactic acid bacteria that has been studied for over 30 years. Probiotic and/or commensal bacteria of the human microbiota are known to respond to diet constituents. Therefore, the present study aims at investigating the possible effects of salicylic acid on the probiotic properties of LGG, and in vitro cytotoxic effects of combination of salicylic acid and LGG on human colon and prostate cancer cells. Salicylic acid significantly (p < 0.05) increased co-aggregation of LGG with E. coli (~ twofold) and anti-oxidant properties. Furthermore, it also induced the cytotoxic effects of LGG against human colon cancer cells. These results suggest that interaction of LGG with salicylic acid can exert more probiotic properties.

APICAL SPIKELET ABORTION (ASA) Controls Apical Panicle Development in Rice by Regulating Salicylic Acid Biosynthesis

Panicle degradation causes severe yield reduction in rice. There are two main types of panicle degradation: apical spikelet abortion and basal degeneration. In this study, we isolated and characterized the apical panicle abortion mutant apical spikelet abortion (asa), which exhibits degeneration and defects in the apical spikelets. This mutant had a pleiotropic phenotype, characterized by reduced plant height, increased tiller number, and decreased pollen fertility. Map-based cloning revealed that OsASA encodes a boric acid channel protein that showed the highest expression in the inflorescence, peduncle, and anther. RNA-seq analysis of the asa mutant vs wild-type (WT) plants revealed that biological processes related to reactive oxygen species (ROS) homeostasis and salicylic acid (SA) metabolism were significantly affected. Furthermore, the asa mutants had an increased SA level and H₂O₂ accumulation in the young panicles compared to the WT plants. Moreover, the SA level and the expression of OsPAL3, OsPAL4, and OsPAL6 genes (related to SA biosynthesis) were significantly increased under boron-deficient conditions in the asa mutant and in OsASA-knockout plants. Collectively, these results suggest that the boron distribution maintained by OsASA is required for normal panicle development in a process that involves modulating ROS homeostasis and SA biosynthesis.

Hydrogen sulfide is required for salicylic acid-induced chilling tolerance of cucumber seedlings

Salicylic acid (SA) and hydrogen sulfide (H₂S) have been proved to be multifunctional signal molecules to participate in the response of plants to abiotic stresses. However, it is still unclear whether there is interaction between SA and H₂S in response to chilling intensity of cucumber seedlings. Here, we found SA was sensitive to chilling intensity. Under normal condition, NaHS (H₂S donor) or removing endogenous H₂S with hypotaurine (HT, a specific scavenger of H₂S) and DL-propargylglycine (PAG, a specific inhibitor of H₂S) has no effect on endogenous SA level; however, SA induced endogenous H₂S content and activated the activities and mRNA level of L-/D-cysteine desulfhydrase (L-/D-CD), and inhibiting endogenous SA with paclobutrazol (PAC) or 2-aminoindan-2-phosphonic acid (AIP) blocked this effect, implying H₂S may play a role after SA signal. Further studies showed that both SA and NaHS notably alleviated chilling injury, which was evidenced by lower electrolyte leakage (EL), MDA content, and ROS accumulation, compared with H₂O treatment. Of note, SA and H₂S improved the activities and mRNA level of antioxidant enzymes (SOD, POD, CAT, APX, and GR) as well as the contents of AsA and GSH. Additionally, the chilling-response genes (ICE, CBF1, and COR) were obviously upregulated by exogenous SA and NaHS. However, the positive effect of SA on chilling tolerance was inhibited by HT, whereas PAC or AIP did not affect NaHS-induced chilling tolerance. Taken together, the data reveals that H₂S acts as a downstream signal of SA-induced chilling tolerance of cucumber via modulating antioxidant system and chilling-response genes.

Salicylate-Induced Chitinases in Pea Roots

Three proteins induced by salicylic acid were revealed in pea roots. These proteins were identified as chitinase isozymes belonging to the glycoside hydrolases family 18. The PsCam050724 transcript encoding at least one of these isoforms was found, allowing us to determine its primary structure, which lacks the signal peptide.

Improving Regulation of Enzymatic and Non-Enzymatic Antioxidants and Stress-Related Gene Stimulation in Cucumber mosaic cucumovirus-Infected Cucumber Plants Treated with Glycine Betaine, Chitosan and Combination

Cucumber mosaic cucumovirus (CMV) is a deadly plant virus that results in crop-yield losses with serious economic consequences. In recent years, environmentally friendly components have been developed to manage crop diseases as alternatives to chemical pesticides, including the use of natural compounds such as glycine betaine (GB) and chitosan (CHT), either alone or in combination. In the present study, the leaves of the cucumber plants were foliar-sprayed with GB and CHT—either alone or in combination—to evaluate their ability to induce resistance against CMV. The results showed a significant reduction in disease severity and CMV accumulation in plants treated with GB and CHT, either alone or in combination, compared to untreated plants (challenge control). In every treatment, growth indices, leaf chlorophylls content, phytohormones (i.e., indole acetic acid, gibberellic acid, salicylic acid and jasmonic acid), endogenous osmoprotectants (i.e., proline, soluble sugars and glycine betaine), non-enzymatic antioxidants (i.e., ascorbic acid, glutathione and phenols) and enzymatic antioxidants (i.e., superoxide dismutase, peroxidase, polyphenol oxidase, catalase, lipoxygenase, ascorbate peroxidase, glutathione reductase, chitinase and β-1,3 glucanase) of virus-infected plants were significantly increased. On the other hand, malondialdehyde and abscisic acid contents have been significantly reduced. Based on a gene expression study, all treated plants exhibited increased expression levels of some regulatory defense genes such as PR1 and PAL1. In conclusion, the combination of GB and CHT is the most effective treatment in alleviated virus infection. To our knowledge, this is the first report to demonstrate the induction of systemic resistance against CMV by using GB.

Improving Storability of “Nanfeng” Mandarins by Treating with Postharvest Hot Water Dipping

The current research aimed at studying the possibility of improving the postharvest storability of “Nanfeng” mandarins by hot water dipping (HWD) treatment. The research was conducted in two phases. Firstly, two different temperatures (50 and 55°C) were tested for three different dipping durations (2, 3, and 4 min) on the mandarin fruits, and the best combination was defined for the prevention of weight loss and fruit decay. Next, the optimal treatment (HWD at 50°C for 3 min) was used in further studies to test the effects on the postharvest fruit quality attributes. Regular measurements were performed to determine total soluble solid (TSS) content, titratable acid (TA) content, vitamin C (VC) content, total sugar content, respiration rate, malondialdehyde (MDA) content, and activities of superoxide dismutase (SOD) enzyme, polyphenoloxidase (PPO) enzyme, and peroxidase (POD) enzyme. According to the results obtained, HWD treatment was found to prevent the loss of TSS, TA, and VC contents during the storage period. The HWD-treated fruits were also found to have a lower respiration rate and MDA content as compared with control treatment. Furthermore, HWD treatment significantly enhanced the activities of SOD, POD, and PPO which are known to enhance tolerance to lipid peroxidation and are associated with the fruit protection from injuries and pathogens. Present results also suggest that the activation of the SOD and POD enzymes is highly related to the respiratory activities of the fresh produce. This suggests that the HWD can be used to improve the storability of “Nanfeng” mandarins by maintaining the postharvest physical and biochemical quality.

Effects of Hot Air Treatments on Postharvest Storage of Newhall Navel Orange

The effects of hot air flow (HAF) treatment on the postharvest storage of 'Newhall' navel oranges were investigated in this study. Studies were conducted with two separate sections. First of all, the effects of HAF at 37 °C for 36 h, for 48 h, and for 60 h were tested on fruit decay and weight loss. Thus, the optimal treatment was found as HAF at 37 °C for 48 h based on the fruit decay percentage and weight loss, and further studies were carried out with this treatment. The HAF-treated and control fruits were flowed at 37 °C and 20 °C with relative humidity (RH) of 85-95% for 48 h, respectively. After flowing, fruits of both treatments were individually film-packed, precooled (10-12 °C, 12 h), and stored (6 ± 0.5 °C and 85-90% relative humidity) for 120 days. Regular (0, 15, 30, 45, 60, 90, and 120 days) measurements were carried out for analyzing total soluble solid (TSS) content, titratable acid (TA) content, vitamin C (VC) content, total sugar content, respiration rate, malondialdehyde (MDA) content, and protective enzyme activities. The results indicated that HAF treatment significantly inhibited the MDA content and respiration rate of navel orange fruits after 45 d storage. The superoxide dismutase (SOD) and peroxidase (POD) enzyme activities were enhanced after 60 d storage, while polyphenol oxidase (PPO) enzyme activities were enhanced throughout the storage period. Results suggested that the SOD and POD activities are highly related with respiratory activities and could be enhanced with hot air flow. Meanwhile, HAF treatment maintained high content of TSS, total sugar, TA, and VC.

Silicon and salicylic acid confer high-pH stress tolerance in tomato seedlings

Alkalinity is a known threat to crop plant growth and production, yet the role of exogenous silicon (Si) and salicylic acid (SA) application has been largely unexplored. Here, we sought to understand the beneficial impacts of Si and SA on tomato seedlings during high-pH (9.0) stress. Results showed that Si- and SA-treated plants displayed higher biomass, chlorophyll contents, relative leaf water and better root system than none-treated plants under alkaline conditions. Both Si and SA counteracted the alkaline stress-induced oxidative damage by lowering the accumulation of reactive oxygen species and lipid peroxidation. The major antioxidant defence enzyme activities were largely stimulated by Si and SA, and these treatments caused significantly increased K+ and lowered Na+ concentrations in shoot and root under stress. Moreover, Si and SA treatments modulated endogenous SA levels and dramatically decreased abscisic acid levels in both shoot and root. Additionally, key genes involved in Si uptake, SA biosynthesis, the antioxidant defence system and rhizosphere acidification were up-regulated in Si and SA treatments under alkaline conditions. These results demonstrate that Si and SA play critical roles in improving alkaline stress tolerance in tomato seedlings, by modifying the endogenous Na+ and K+ contents, regulating oxidative damage and key genes and modulating endogenous hormone levels. These findings will help to broaden our understanding regarding the physiological and molecular mechanisms associated with the alkaline soil tolerance in plants.

Establishment of Actinorhizal Symbiosis in Response to Ethylene, Salicylic Acid, and Jasmonate

Phytohormones play a crucial role in regulating plant developmental processes. Among them, ethylene and jasmonate are known to be involved in plant defense responses to a wide range of biotic stresses as their levels increase with pathogen infection. In addition, these two phytohormones have been shown to inhibit plant nodulation in legumes. Here, exogenous salicylic acid (SA), jasmonate acid (JA), and ethephon (ET) were applied to the root system of Casuarina glauca plants before Frankia inoculation, in order to analyze their effects on the establishment of actinorhizal symbiosis. This protocol further describes how to identify putative ortholog genes involved in ethylene and jasmonate biosynthesis and/or signaling pathways in plant, using the Arabidopsis Information Resource (TAIR), Legume Information System (LIS), and Genevestigator databases. The expression of these genes in response to the bacterium Frankia was analyzed using the gene atlas for Casuarina-Frankia symbiosis (SESAM web site).

Salicylic Acid and Melatonin Alleviate the Effects of Heat Stress on Essential Oil Composition and Antioxidant Enzyme Activity in Mentha × piperita and Mentha arvensis L

The aim of this study was to evaluate changes in the chemical profile of essential oils and antioxidant enzymes activity (catalase CAT, superoxide dismutase SOD, Glutathione S-transferases GST, and Peroxidase POX) in Mentha × piperita L. (Mitcham variety) and Mentha arvensis L. (var. piperascens), in response to heat stress. In addition, we used salicylic acid (SA) and melatonin (M), two brassinosteroids that play an important role in regulating physiological processes, to assess their potential to mitigate heat stress. In both species, the heat stress caused a variation in the composition of the essential oils and in the antioxidant enzymatic activity. Furthermore both Salicylic acid (SA) and melatonin (M) alleviated the effect of heat stress.

Small-Scale Bioreactor for Sterile Hydroponics and Hairy Roots: Metabolic Diversity and Salicylic Acid Exudation by Hairy Roots of Hyoscyamus niger

The volume and complexity of commercial bioreactors for sterile hydroponics and hairy roots are too large for comparative analysis of many cultures. Here a small-scale bioreactor fabricated from standard glass materials and suitable for both airlift and bubble aeration mode is described. The performance of the bioreactor was tested by growing oilseed rape (Brassica napus L.) and rose plants (Rosa canina L.) in sterile hydroponics and by cultivating hairy roots of henbane (Hyoscyamus niger L.) and sesame (Hyoscyamus niger L.). Plants grown in hydroponics for up to six weeks did not show chloroses or necroses. Hairy roots grew faster or comparably fast in bioreactors as compared to shaking flasks. Root exudates of roses and exudates of hairy roots of henbane were subjected to targeted and nontargeted analysis by HPLC coupled with optical and mass spectrometric detectors. The diversity and concentration of hairy root exudates were higher in bioreactors than in shaking flasks. The composition of hairy root exudates of three accessions of H. niger did not match the genetic relatedness among the accessions. Hairy roots of Hyoscyamus niger exuded salicylic acid in amounts varying among plant accessions and between bioreactors and shaking flask cultures.

Salicylic Acid Alleviated Salt Damage of Populus euphratica: A Physiological and Transcriptomic Analysis

Populus euphratica Oliv. is a model tree for studying abiotic stress, especially salt stress response. Salt stress is one of the most extensive abiotic stresses, which has an adverse effect on plant growth and development. Salicylic acid (SA) is an important signaling molecule that plays an important role in modulating the plant responses to abiotic stresses. To answer whether the endogenous SA can be induced by salt stress, and whether SA effectively alleviates the negative effects of salt on poplar growth is the main purpose of the study. To elucidate the effects of SA and salt stress on the growth of P. euphratica, we examined the morphological and physiological changes of P. euphratica under 300 mM NaCl after treatment with different concentrations of SA. A pretreatment of P. euphratica with 0.4 mM SA for 3 days effectively improved the growth status of plants under subsequent salt stress. These results indicate that appropriate concentrations of exogenous SA can effectively counteract the negative effect of salt stress on growth and development. Subsequently, transcripts involved in salt stress response via SA signaling were captured by RNA sequencing. The results indicated that numerous specific genes encoding mitogen-activated protein kinase, calcium-dependent protein kinase, and antioxidant enzymes were upregulated. Potassium transporters and Na+/H+ antiporters, which maintain K+/Na+ balance, were also upregulated after SA pretreatment. The transcriptome changes show that the ion transport and antioxidant enzymes were the early enhanced systems in response of P. euphratica to salt via SA, expanding our knowledge about SA function in salt stress defense in P. euphratica. This provides a solid foundation for future study of functional genes controlling effective components in metabolic pathways of trees.

A Long Non-Coding RNA of Citrus tristeza virus: Role in the Virus Interplay with the Host Immunity

During infection, Citrus tristeza virus (CTV) produces a non-coding subgenomic RNA referred to as low-molecular-weight tristeza 1 (LMT1), which for a long time has been considered as a by-product of the complex CTV replication machinery. In this study, we investigated the role of LMT1 in the virus infection cycle using a CTV variant that does not produce LMT1 (CTV-LMT1d). We showed that lack of LMT1 did not halt virus ability to replicate or form proper virions. However, the mutant virus demonstrated significantly reduced invasiveness and systemic spread in Nicotiana benthamiana as well as an inability to establish infection in citrus. Introduction of CTV-LMT1d into the herbaceous host resulted in elevation of the levels of salicylic acid (SA) and SA-responsive pathogenesis-related genes beyond those upon inoculation with wild-type (WT) virus (CTV-WT). Further analysis showed that the LMT1 RNA produced by CTV-WT or via ectopic expression in the N. benthamiana leaves suppressed SA accumulation and up-regulated an alternative oxidase gene, which appeared to mitigate the accumulation of reactive oxygen species. To the best of our knowledge, this is the first report of a plant viral long non-coding RNA being involved in counter-acting host response by subverting the SA-mediated plant defense.

Abiotic and biotic stresses induce a core transcriptome response in rice

Environmental stresses greatly limit crop yield. With the increase in extreme weather events due to climate change and the constant pressure of diseases and pests, there is an urgent need to develop crop varieties that can tolerate multiple stresses. However, our knowledge of how plants broadly respond to stress is limited. Here, we explore the rice core stress response via meta-analysis of publicly available rice transcriptome data. Our results confirm that rice universally down-regulates photosynthesis in response to both abiotic and biotic stress. Rice also generally up-regulates hormone-responsive genes during stress response, most notably genes in the abscisic acid, jasmonic acid and salicylic acid pathways. We identified several promoter motifs that are likely involved in stress-responsive regulatory mechanisms in rice. With this work, we provide a list of candidate genes to study for improving rice stress tolerance in light of environmental stresses. This work also serves as a proof of concept to show that meta-analysis of diverse transcriptome data is a valid approach to develop robust hypotheses for how plants respond to stress.

LSD1-, EDS1- and PAD4-dependent conditional correlation among salicylic acid, hydrogen peroxide, water use efficiency and seed yield in Arabidopsis thaliana

In Arabidopsis thaliana, LESION SIMULATING DISEASE 1 (LSD1), ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1) and PHYTOALEXIN DEFICIENT 4 (PAD4) proteins are regulators of cell death (CD) in response to abiotic and biotic stresses. Hormones, such as salicylic acid (SA), and reactive oxygen species, such as hydrogen peroxide (H₂ O₂ ), are key signaling molecules involved in plant CD. The proposed mathematical models presented in this study suggest that LSD1, EDS1 and PAD4 together with SA and H₂ O₂ are involved in the control of plant water use efficiency (WUE), vegetative growth and generative development. The analysis of Arabidopsis wild-type and single mutants lsd1, eds1, and pad4, as well as double mutants eds1/lsd1 and pad4/lsd1, demonstrated the strong conditional correlation between SA/H₂ O₂ and WUE that is dependent on LSD1, EDS1 and PAD4 proteins. Moreover, we found a strong correlation between the SA/H₂ O₂ homeostasis of 4-week-old Arabidopsis leaves and a total seed yield of 9-week-old plants. Altogether, our results prove that SA and H₂ O₂ are conditionally regulated by LSD1/EDS/PAD4 to govern WUE, biomass accumulation and seed yield. Conditional correlation and the proposed models presented in this study can be used as the starting points in the creation of a plant breeding algorithm that would allow to estimate the seed yield at the initial stage of plant growth, based on WUE, SA and H₂ O₂ content.

Manipulation of Phytohormone Pathways by Effectors of Filamentous Plant Pathogens

Phytohormones regulate a large variety of physiological processes in plants. In addition, salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) are responsible for primary defense responses against abiotic and biotic stresses, while plant growth regulators, such as auxins, brassinosteroids (BRs), cytokinins (CKs), abscisic acid (ABA), and gibberellins (GAs), also contribute to plant immunity. To successfully colonize plants, filamentous pathogens like fungi and oomycetes have evolved diverse strategies to interfere with phytohormone pathways with the help of secreted effectors. These include proteins, toxins, polysaccharides as well as phytohormones or phytohormone mimics. Such pathogen effectors manipulate phytohormone pathways by directly altering hormone levels, by interfering with phytohormone biosynthesis, or by altering or blocking important components of phytohormone signaling pathways. In this review, we outline the various strategies used by filamentous phytopathogens to manipulate phytohormone pathways to cause disease.

Systemic Acquired Resistance and Salicylic Acid: Past, Present, and Future

This article is part of the Distinguished Review Article Series in Conceptual and Methodological Breakthroughs in Molecular Plant-Microbe Interactions. Salicylic acid (SA) is a critical plant hormone that regulates numerous aspects of plant growth and development as well as the activation of defenses against biotic and abiotic stress. Here, we present a historical overview of the progress that has been made to date in elucidating the role of SA in signaling plant immune responses. The ability of plants to develop acquired immunity after pathogen infection was first proposed in 1933. However, most of our knowledge about plant immune signaling was generated over the last three decades, following the discovery that SA is an endogenous defense signal. During this timeframe, researchers have identified i) two pathways through which SA can be synthesized, ii) numerous proteins that regulate SA synthesis and metabolism, and iii) some of the signaling components that function downstream of SA, including a large number of SA targets or receptors. In addition, it has become increasingly evident that SA does not signal immune responses by itself but, rather, as part of an intricate network that involves many other plant hormones. Future efforts to develop a comprehensive understanding of SA-mediated immune signaling will therefore need to close knowledge gaps that exist within the SA pathway itself as well as clarify how crosstalk among the different hormone signaling pathways leads to an immune response that is both robust and optimized for maximal efficacy, depending on the identity of the attacking pathogen.

Involvement of CAT in the detoxification of HT-induced ROS burst in rice anther and its relation to pollen fertility

HT-induced ROS burst in developing anther is closely related to the lowered CAT activity as the result of the markedly suppressed OsCATB transcript, thereby causing severe fertility injury for rice plants exposed to HT at meiosis stage. The reproductive stage of rice plants is highly sensitive to heat stress. In this paper, different rice cultivars were used to investigate the relationship of HT-induced floret sterility with reactive oxygen species (ROS) detoxification in rice anthers under well-controlled climatic conditions. Results showed that high temperature (HT) exposure significantly enhanced the ROS level and malondialdehyde (MDA) content in developing anther, and the increase in ROS amount in rice anther under HT exposure was closely associated with HT-induced decline in the activities of several antioxidant enzymes. For various antioxidant enzymes, SOD and CAT were more susceptible to the ROS burst in rice anther induced by HT exposure than APX and POD, in which SOD and CAT activity in developing anther decreased significantly by HT exposure, whereas APX activity was relatively stable among different temperature regimes. HT-induced decrease in CAT activity was attributable to the suppressed transcript of OsCATB. This occurrence was strongly responsible for HT-induced increase in ROS level and oxidative-damage in rice anther, thereby it finally caused significant reduction in pollen viability and floret fertility for the rice plants exposed to HT during meiosis. Exogenous application of 1000 µM salicylic acid (SA) may alleviate HT-induced reduction in pollen viability and floret fertility, concomitantly with the increased CAT activity and reduced ROS level in rice anther.

Valsa mali Pathogenic Effector VmPxE1 Contributes to Full Virulence and Interacts With the Host Peroxidase MdAPX1 as a Potential Target

The Valsa canker, caused by Valsa mali (V. mali), is a destructive disease of apple in Eastern Asia. Effector proteins are important for fungal pathogenicity. We studied a candidate effector VmPxE1 isolated based on the genome information of V. mali. By using the yeast invertase secretion assay system, VmPxE1 was shown to contain a signal peptide with secretory functions. VmPxE1 can suppress BCL-2-associated X protein (BAX)-induced cell death with a high efficacy of 92% in Nicotiana benthamiana. The expression of VmPxE1 was upregulated during the early infection stage and deletion of VmPxE1 led to significant reductions in virulence on both apple twigs and leaves. VmPxE1 was also shown to target an apple ascorbate peroxidase (MdAPX1) by the yeast two-hybrid screening, bimolecular fluorescence complementation and in vivo co-immunoprecipitation. Sequence phylogenetic analysis suggested that MdAPX1 was an ascorbate peroxidase belonging to a subgroup of heme-dependent peroxidases of the plant superfamily. The ectopic expression of MdAPX1 in the mutant of VmPxE1 significantly enhanced resistance to H₂O₂, while the presence of VmPxE1 seems to disturb MdAPX1 function. The present results provide insights into the functions of VmPxE1 as a candidate effector of V. mali in causing apple canker.

SA inhibits complex III activity to generate reactive oxygen species and thereby induces GA overproduction in Ganoderma lucidum

Ganoderma lucidum has high commercial value because it produces many active compounds, such as ganoderic acids (GAs). Salicylic acid (SA) was previously reported to induce the biosynthesis of GA in G. lucidum. In this study, we found that SA induces GA biosynthesis by increasing ROS production, and further research found that NADPH oxidase-silenced strains exhibited a partial reduction in the response to SA, resulting in the induction of increased ROS production. Furthermore, the localization of ROS shows that mitochondria are sources of ROS production in response to SA treatment. An additional analysis focused on the relationship between SA-induced ROS production and mitochondrial functions, and the results showed that inhibitors of mitochondrial complexes I and II exert approximately 40–50% superimposed inhibitory effects on the respiration rate and H₂O₂ content when co-administered with SA. However, no obvious superimposed inhibition effects were observed in the sample co-treated with mitochondrial complex III inhibitor and SA, implying that the inhibitor of mitochondrial complex III and SA might act on the same site in mitochondria. Additional experiments revealed that complex III activity was decreased 51%, 62% and 75% after treatment with 100, 200, and 400 µM SA, respectively. Our results highlight the finding that SA inhibits mitochondrial complex III activity to increase ROS generation. In addition, inhibition of mitochondrial complex III caused ROS accumulation, which plays an essential role in SA-mediated GA biosynthesis in G. lucidum. This conclusion was also demonstrated in complex III-silenced strains. To the best of our knowledge, this study provides the first demonstration that SA inhibits complex III activity to increase the ROS levels and thereby regulate secondary metabolite biosynthesis.

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Mitochondria as a source of salicylic acid (SA) induced reactive oxygen species (ROS) production in Ganoderma lucidum.

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SA induces the accumulation of ganoderic acids in Ganoderma lucidum by mitochondria ROS overproduction.

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SA inhibits mitochondrial complex III activity to increase ROS and thereby induces ganoderic acids biosynthesis.