Sublethal heavy metal stress stimulates innate immunity in tomato

Responses of plant immune system and rhizosphere soil microbiome to the elicitor BAR11 in Arabidopsis thaliana

Microbial elicitors have been shown to boost plant immunity by inducing defense responses to reduce plant disease. However, little is known about the changes in plant microbiome and metabolism in the process of enhancing plant immunity with elicitors. The protein elicitor BAR11, from Saccharothrix yanglingensis Hhs.015, induces defense responses in Arabidopsis thaliana that enhances resistance to pathogens. In this study, bar11 was inserted into Col-0 A. thaliana to obtain BAR11-Trans plant by Agrobacterium-mediated immersion transformation. BAR11-Trans exhibited an elevated defense level against Pseudomonas syringae pv. tomato DC3000 while experiencing a decline in biomass production of above-ground parts. In the process, BAR11-Trans increased the activity of phenylalanine ammonia lyase (PAL) and catalase (CAT), and up-regulated genes related to plant defense pathways. Furthermore, BAR11-Trans decreased root tip reactive oxygen species (ROS) levels while increasing ROS burst in the leaves. Soil transplantation experiments showed that soil planted with BAR11-Trans could enhance the resistance of Col-0 A. thaliana to DC3000. Analysis of A. thaliana rhizosphere soil through 16S rRNA amplified sequencing revealed that BAR11-Trans increased the relative abundance and diversity of the rhizosphere microbial community, leading to the recruitment of more plant probiotics. Additionally, the accumulation of kaempferitrin and robinin in BAR11-Trans influenced the physicochemical properties of rhizosphere soil and the composition of the bacterial community. In summary, BAR11-Trans exhibited heightened defense levels compared to Col-0, leading to increased secretion of secondary metabolites and the recruitment of a greater number of microorganisms to adapt to the environment. These findings offer novel insights for the potential application of elicitors in agricultural disease control.

Spermine induced endogenous signal ameliorates osmotic stress in Lens culinaris

The present study evaluates the alleviation of polyethylene glycol (PEG) mediated osmotic stress by exogenous spermine (SP) application on seeds of Lens culinaris. Seeds were soaked overnight with SP (0.05 M) and PEG (10%) separately. Third set was prepared by combined treatment with SP (0.05 M) and PEG (10%). Further, various physiological parameters, defense enzymes, phenol, flavonoid, nitric oxide (NO), proline content, chromosomal abnormality studies etc. was conducted after 5 days of incubation. PEG application significantly hampered the seed germination, root length, shoot length, relative water content and vigour index. Application of PEG induced electrolytic leakage in root cells. PEG application also significantly reduced the activities of different defense parameters viz. peroxidase, polyphenol oxidase, phenylalanine ammonia lyase and β-1,3 glucanase, total phenol and flavonoid accumulation over control. Reactive oxygen species, lipid peroxidation, cell death was found significantly high in PEG treated sets. However, NO level has been significantly decreased by PEG treatment. Cytological studies showed reduced mitotic index and highest abnormalities under osmotic stress. Contrary to that, application of SP either alone or in combination with PEG showed improved responses of aforesaid defense parameters compared to control and solely PEG treated sets. Interestingly, cytological abnormalities were also reduced in SP treated roots. Overall, these results suggests that SP application on seeds reduced the PEG induced damage and intricately improved the defense mechanisms which might help to fight against oxidative stress.

The Role of NO in the Amelioration of Heavy Metal Stress in Plants by Individual Application or in Combination with Phytohormones, Especially Auxin

Since the time of the Industrial Revolution, the accumulation of various heavy metals (HMs), such as cadmium (Cd), arsenic (As), lead (Pb), chromium (Cr), mercury (Hg), copper (Cu), zinc (Zn), nickel (Ni), etc., has increased substantially in the soil, causing a real risk to all kinds of consumers in the food chain. Moreover, excess HM accumulation is considered a major factor in decreasing plant growth and productivity. A number of recent studies have exhibited the astonishing impact of nitric oxide (NO), a multifunctional, gaseous signal molecule, on alleviating the destructive effects of HMs. Many reports revealed the noteworthy contribution of NO in reducing HM uptake and toxicity levels. In the present review, focus is given to the contribution of NO to decrease the toxicity levels of different HMs in a variety of plant species and their accumulation in those species. Simultaneously, this review also demonstrates the effects of NO on HM-stressed species, by its use both individually and along with auxin, a plant-growth-promoting phytohormone. Different perspectives about the reaction to the co-application of NO and auxin, as well as the differential role of NO to overcome HM stress, have been expanded.

Silicon and nitric oxide synergistically modulate the production of essential oil and rosmarinic acid in Salvia officinalis under Cu stress

The individual impact of silicon (Si) and nitric oxide (NO) on secondary metabolism in several plant species has been reported, but their combined effect has never been evaluated yet. Therefore, in this study, single and combined impacts of NO and Si on the biosynthesis of rosmarinic acid (RA) and essential oil (EO) content in leaves of Salvia officinalis were investigated under both non-stress and Cu stress conditions. The results indicated that high Cu concentration decreased biomass and the content of polyphenols, but elevated electrolyte leakage, while lower Cu concentrations, especially 200 μM Cu, increased the content of polyphenols, EO, and antioxidant capacity in leaves of S. officinalis. The foliar application of sodium silicate (1 mM Si) and sodium nitroprusside (200 μM SNP as a NO donor) alone and particularly in combination improved shoot dry biomass, restored chlorophyll and carotenoids, increased EO content, the amounts of flavonoids, and phenolic compounds especially RA, and enhanced antioxidant capacity in the leaves of S. officinalis under both non-stress and Cu stress conditions. Copper treatment increased NO content, upregulated expression of PAL, TAT, and RAS genes, and enhanced phenylalanine ammonia-lyase activity in the leaves, which were responsible for improving the production of phenolic compounds, particularly rosmarinic acid. Foliar spraying with Si and SNP intensified these attributes. All responses were more pronounced when NO and Si were simultaneously applied under Cu stress. These findings suggest that NO and Si synergistically modulate secondary metabolism through upregulation of related gene expression and enzyme activities under both non-stress and Cu stress conditions.

Salicylic acid and nitric oxide cross-talks to improve innate immunity and plant vigor in tomato against Fusarium oxysporum stress

Foliar application of SA cross-talks and induce endogenous nitric oxide and reactive oxygen species to improve innate immunity and vigor of tomato plant against Fusarium oxysporum stress. The present investigation was aimed to demonstrate the efficacy of salicylic acid (SA), as a powerful elicitor or plant growth regulator (PGR) and its cross-talk with nitric oxide (NO) in tomato against the biotic stress caused by wilt pathogen, Fusarium oxysporum f. sp. lycopersici. Different defense-related enzymes and gene expression, phenol, flavonoid, and phenolic acid content along with NO generation and other physiological characters have been estimated after foliar application of SA. Total chlorophyll content was steadily maintained and the amount of death of cells was negligible after 72 h of SA treatment. Significant reduction of disease incidence was also recorded in SA treated sets. Simultaneously, NO generation was drastically improved at this stage, which has been justified by both spectrophotometrically and microscopically. A direct correlation between reactive oxygen species (ROS) generation and NO has been established. Production of defense enzymes, gene expressions, different phenolic acids was positively influenced by SA treatment. However, tomato plants treated with SA along with NO synthase (NOS) inhibitor or NO scavenger significantly reduce all those parameters tested. On the other hand, NO donor-treated plants showed the same inductive effect like SA. Furthermore, SA treated seeds of tomato also showed improved physiological parameters like higher seedling vigor index, shoot and root length, mean trichome density, etc. It is speculated that the cross-talk between SA and endogenous NO have tremendous ability to improve defense responses and growth of the tomato plant. It can be utilized in future sustainable agriculture for bimodal action.

Class III peroxidase: an indispensable enzyme for biotic/abiotic stress tolerance and a potent candidate for crop improvement

Class III peroxidases are secretory enzymes which belong to a ubiquitous multigene family in higher plants and have been identified to play role in a broad range of physiological and developmental processes. Potentially, it is involved in generation and detoxification of hydrogen peroxide (H₂O₂), and their subcellular localization reflects through three different cycles, namely peroxidative cycle, oxidative and hydroxylic cycles to maintain the ROS level inside the cell. Being an antioxidant, class III peroxidases are an important initial defence adapted by plants to cope with biotic and abiotic stresses. Both these stresses have become a major concern in the field of agriculture due to their devastating effect on plant growth and development. Despite numerous studies on plant defence against both the stresses, only a handful role of class III peroxidases have been uncovered by its functional characterization. This review will cover our current understanding on class III peroxidases and the signalling involved in their regulation under both types of stresses. The review will give a view of class III peroxidases and highlights their indispensable role under stress conditions. Its future application will be discussed to showcase their importance in crop improvement by genetic manipulation and by transcriptome analysis.

Green Synthesized Copper Oxide Nanoparticles Ameliorate Defence and Antioxidant Enzymes in Lens culinaris

Biosynthesis of copper oxide nanoparticles (CuONPs) in a cost-effective and eco-friendly way has gained its importance. CuONPs has been prepared from copper sulfate by using Adiantum lunulatum whole plant extract. CuONPs have been characterized by X-ray diffraction, Fourier transform infrared spectroscopic, transmission electron microscope, etc. Mono-disperse, spherical, pure, and highly stable CuONPs have formed with an average diameter of 6.5 ± 1.5 nm. Biosynthesized CuONPs at different concentrations were applied to seeds of Lens culinaris. Physiological characteristics were investigated in the germinated seeds. Roots obtained from the seeds treated with 0.025 mgmL-1 concentration of CuONPs showed highest activity of different defence enzymes and total phenolics. However, at higher concentration it becomes close to control. It showed gradual increase of antioxidative enzymes, in accordance with the increasing dose of CuONPs. Likewise, lipid peroxidation and proline content gradually increased with the increasing concentration. Reactive oxygen species and nitric oxide generation was also altered due to CuONPs treatment indicating stress signal transduction. Finally, this study provides a new approach of the production of valuable CuONPs, is a unique, economical, and handy tool for large scale saleable production which can also be used as a potent plant defence booster instead of other commercial uses.

Interaction between Bean and Colletotrichum gloeosporioides: Understanding Through a Biochemical Approach

In addition to its role in animals, nowadays nitric oxide (NO) is considered as an emerging signaling molecule in plant systems. It is now believed that NO exerts its pivotal role in various plant physiological processes, such as in seed germination, plant developmental stages, and plant defense mechanisms. In this study, we have taken an initiative to show the biochemical basis of defense response activation in bean leaves during the progression of Colletotrichum gloeosporioides (Penz.) Penz. and Sacc. in detached bean leaves. Stages of pathogen penetration and colonization were successfully established in the detached bean leaves. Results showed up-regulation of different defense-related enzymes and other defense molecules, such as phenols, flavonoids, callose, and lignin molecules, along with NO at early stages of pathogen invasion. Although in the later stages of the disease, development of NO and other defense components (excluding lignin) were down-regulated, the production of reactive oxygen species in the form of H2O2 became elevated. Consequently, other stress markers, such as lipid peroxidation, proline content, and chlorophyll content, were changed accordingly. Correlation between the disease index and other defense molecules, along with NO, indicate that production of NO and reactive oxygen species (ROS) might influence the development of anthracnose in common bean.

Protective role of selenium against chromium stress involving metabolites and essential elements in Brassica juncea L. seedlings

The present study aimed at the potential role of selenium in providing protection to plants subjected to chromium toxicity. The study was carried out on 15-day-old seedlings of Brassica juncea raised in the solutions of Cr (300 µM) and Se (2, 4 and 6 µM), both alone and in combinations under controlled laboratory environment. The effects were studied on growth, plant metabolites (involved in osmotic homeostasis and stress protection), and essential elements. The results showed that the exposure of B. juncea seedlings to 300 µM Cr led to an increase in the contents of total sugars, reducing sugars, non-reducing sugars, total phenols and flavonoids. However, a significant decline in growth characteristics, the contents of proteins and free amino acids was observed. The essential elements (Na, K, Ca, Mg, C, H, N) also decreased in response to Cr. Se application in binary combinations, on the other hand, aided in improving seed germination (19%), root (88.3%) and shoot (18.2%) lengths. It also helped to increase the contents of sugars [total (16.3%), reducing (21.6%) and non-reducing (15.2%)], phenols (36.7%) and flavonoids (27.4%), thereby aiding in alleviating the phytotoxicity of Cr. The profiling of polyphenols and amino acids, and histological study of phenols supported the above results. The contents of essential elements also showed a significant increase, while Cr uptake was observed to decline by Se supplementation. The observations from the present study indicate that Se has the ability to influence primary and secondary metabolism, improve mineral nutrition and reduce Cr uptake in B. juncea seedlings to combat the Cr phytotoxicity and enhance the tolerance against stress.

Biochemical basis of improvement of defense in tomato plant against Fusarium wilt by CaCl2

The objective of this study was to investigate the effectiveness of calcium chloride (CaCl2), as potential elicitor, on tomato plants against Fusarium oxysporum f. sp. lycopersici. Foliar application of CaCl2 showed significant reduction of wilt incidence after challenge inoculation. Increased production of defense and antioxidant enzymes was observed in elicitor treated sets over control. Simultaneously, altered amount of phenolic acids were analyzed spectrophotometrically and by using high performance liquid chromatography. Significant induction of defense-related genes expressions was measured by semi-quantitative RT-PCR. Greater lignifications by microscopic analysis were also recorded in elicitor treated plants. Simultaneously, generation of nitric oxide (NO) in elicitor treated plants was confirmed by spectrophotometrically and microscopically by using membrane permeable fluorescent dye. Furthermore, plants treated with potential NO donor and NO modulators showed significant alteration of all those aforesaid defense molecules. Transcript analysis of nitrate reductase and calmodulin gene showed positive correlation with elicitor treatment. Furthermore, CaCl2 treatment showed greater seedling vigor index, mean trichome density etc. The result suggests that CaCl2 have tremendous potential to elicit defense responses as well as plant growth in co-relation with NO, which ultimately leads to resistance against the wilt pathogen.

Abiotic elicitors mediated elicitation of innate immunity in tomato: an ex vivo comparison

Improvement of the host resistance by using hazard free chemical elicitors is emerging as an alternative approach in the field of plant disease management. In our present work, we have screened the efficacy and possible mechanism of abiogenic elicitors like Dipotassium hydrogen orthophosphate (K2HPO4), Oxalic acid (OA), Isonicotinic acid (INA), Salicylic acid (SA), Acetylsalicylate (AS), Arachidonic acid (AA) and Calcium chloride (CaCl2) to stimulate innate immune responses in Lycopersicum esculentum Mill. Excised tomato leaves, treated with elicitors at three different concentrations, were found to stimulate defense and antioxidative enzymes, total phenol and flavonoid content after 24 h of incubation. CaCl2 (0.5 %) followed by INA (2.5 mM) were found most effective in activation of all such defense molecules in tomato leaves. Furthermore, nitric oxide (NO), a key gaseous mediator in plant defense signaling, was also measured after subsequent elicitor application. Higher doses of elicitors showed an elevated level of reactive oxygen species (ROS) generation, enhanced lipid peroxidation rate and proline content, which indicates the extent of abiotic stress generation on the leaves. However, ROS production, lipid peroxidation rate and proline concentration remain significantly reduced as a result of CaCl2 (0.5 %) and INA (2.5 mM) application. A sharp increase of total chlorophyll content was also recorded due to treatment of CaCl2 (0.5 %). These results demonstrate the effects of different abiogenic elicitors to regulate the production of defense molecules. Results also suggest that among all such chemicals, CaCl2 (0.5 %) and INA (2.5 mM) can be used as a potential elicitor in organic farming of tomato.

Chitosan nanoparticles: A positive modulator of innate immune responses in plants

The immunomodulatory role of the natural biopolymer, chitosan, has already been demonstrated in plants, whilst its nanoparticles have only been examined for biomedical applications. In our present study, we have investigated the possible ability and mechanism of chitosan nanoparticles (CNP) to induce and augment immune responses in plants. CNP-treatment of leaves produced significant improvement in the plant's innate immune response through induction of defense enzyme activity, upregulation of defense related genes including that of several antioxidant enzymes as well as elevation of the levels of total phenolics. It is also possible that the extracellular localization of CNP may also play a role in the observed upregulation of defense response in plants. Nitric oxide (NO), an important signaling molecule in plant defense, was also observed to increase following CNP treatment. However, such CNP-mediated immuno-stimulation was significantly mitigated when NO production was inhibited, indicating a possible role of NO in such immune induction. Taken together, our results suggest that CNP may be used as a more effective phytosanitary or disease control agent compared to natural chitosan for sustainable organic cultivation.