Crosstalk of Zn in Combination with Other Fertilizers Underpins Interactive Effects and Induces Resistance in Tomato Plant against Early Blight Disease

Transcriptome analysis and functional verification reveal the roles of copper in resistance to potato virus Y infection in tobacco

Potato virus Y (PVY) is one of the most important pathogens in the genus Potyvirus that seriously harms agricultural production. Copper (Cu), as a micronutrient, is closely related to plant immune response. In this study, we found that foliar application of Cu could inhibit PVY infection to some extent, especially at 7 days post inoculation (dpi). To explore the effect of Cu on PVY infection, transcriptome sequencing analysis was performed on PVY-infected tobacco with or without Cu application. Several key pathways regulated by Cu were identified, including plant-pathogen interaction, inorganic ion transport and metabolism, and photosynthesis. Moreover, the results of virus-induced gene silencing (VIGS) assays revealed that NbMLP423, NbPIP2, NbFd and NbEXPA played positive roles in resistance to PVY infection in Nicotiana benthamiana. In addition, transgenic tobacco plants overexpressing NtEXPA11 showed increased resistance to PVY infection. These results contribute to clarify the role and regulatory mechanism of Cu against PVY infection, and provide candidate genes for disease resistance breeding.

Advances in the Involvement of Metals and Metalloids in Plant Defense Response to External Stress

Plants, as sessile organisms, uptake nutrients from the soil. Throughout their whole life cycle, they confront various external biotic and abiotic threats, encompassing harmful element toxicity, pathogen infection, and herbivore attack, posing risks to plant growth and production. Plants have evolved multifaceted mechanisms to cope with exogenous stress. The element defense hypothesis (EDH) theory elucidates that plants employ elements within their tissues to withstand various natural enemies. Notably, essential and non-essential trace metals and metalloids have been identified as active participants in plant defense mechanisms, especially in nanoparticle form. In this review, we compiled and synthetized recent advancements and robust evidence regarding the involvement of trace metals and metalloids in plant element defense against external stresses that include biotic stressors (such as drought, salinity, and heavy metal toxicity) and abiotic environmental stressors (such as pathogen invasion and herbivore attack). We discuss the mechanisms underlying the metals and metalloids involved in plant defense enhancement from physiological, biochemical, and molecular perspectives. By consolidating this information, this review enhances our understanding of how metals and metalloids contribute to plant element defense. Drawing on the current advances in plant elemental defense, we propose an application prospect of metals and metalloids in agricultural products to solve current issues, including soil pollution and production, for the sustainable development of agriculture. Although the studies focused on plant elemental defense have advanced, the precise mechanism under the plant defense response still needs further investigation.

Seed biopriming with Ochrobactrum ciceri mediated defense responses in Zea mays (L.) against Fusarium rot

Seed bio-priming is a simple and friendly technique to improve stress resilience against fungal diseases in plants. An integrated approach of maize seeds biopriming with Ochrobactrum ciceri was applied in Zn-amended soil to observe the response against Fusarium rot disease of Zea mays (L.) caused by Fusarium verticillioides. Initially, the pathogen isolated from the infected corn was identified as F. verticillioides based on morphology and sequences of the internally transcribed spacer region of the ribosomal RNA gene. Re-inoculation of maize seed with the isolated pathogen confirmed the pathogenicity of the fungus on the maize seeds. In vitro, the inhibitory potential of O. ciceri assessed on Zn-amended/un-amended growth medium revealed that antifungal potential of O. ciceri significantly improved in the Zn-amended medium, leading to 88% inhibition in fungal growth. Further assays with different concentrations (25, 50, and 75%) of cell pellet and the cultural filtrate of O. ciceri (with/without the Zn-amendment) showed a dose-dependent inhibitory effect on mycelial growth of the pathogen that also led to discoloration, fragmentation, and complete disintegration of the fungus hyphae and spores at 75% dose. In planta, biopriming of maize seeds with O. ciceri significantly managed disease, improved the growth and biochemical attributes (up to two-fold), and accelerated accumulation of lignin, polyphenols, and starch, especially in the presence of basal Zn. The results indicated that bioprimed seeds along with Zn as the most promising treatment for managing disease and improving plant growth traits through the enhanced accumulation of lignin, polyphenols, and starch, respectively.

Zinc and Boron Soil Applications Affect Athelia rolfsii Stress Response in Sugar Beet (Beta vulgaris L.) Plants

Generation of reactive oxygen species (ROS) constitutes an initial defense approach in plants during pathogen infection. Here, the effects of the two micronutrients, namely, zinc (Zn) and boron (B), on enzymatic and non-enzymatic antioxidant properties, as well as malondialdehyde (MDA) contents in leaves and roots challenged with Athelia rolfsii, which cause root rot disease, were investigated. The findings revealed that Zn and B application to the potting soil alleviated the adverse effect of A. rolfsii on sugar beet plants and increased the chlorophyll content in leaves. The increased enzymatic antioxidant activities such as catalase (CAT), peroxidase (POX), and ascorbate peroxidase (APX), and non-enzymatic antioxidants such as ascorbic acid (AsA) were observed in Zn applied plants compared to both uninoculated and inoculated control plants. A significant rise in CAT activity was noted in both leaves (335.1%) and roots (264.82%) due to the Zn2B1.5 + Ar treatment, in comparison to the inoculated control plants. On the other hand, B did not enhance the activity of any one of them except AsA. Meanwhile, A. rolfsii infection led to the increased accumulation of MDA content both in the leaves and roots of sugar beet plants. Interestingly, reduced MDA content was recorded in leaves and roots treated with both Zn and B. The results of this study demonstrate that both Zn and B played a vital role in A. rofsii tolerance in sugar beet, while Zn enhances antioxidant enzyme activities, B appeared to have a less pronounced effect on modulating the antioxidant system to alleviate the adverse effect of A. rolfsii.

Fungicide-free management of Alternaria leaf blotch and fruit spot on apple indicates Alternaria spp. as secondary colonizer

The fungal genus Alternaria is a pan-global pathogen of > 100 crops, and is associated with the globally expanding Alternaria leaf blotch in apple (Malus x domestica Borkh.) which leads to severe leaf necrosis, premature defoliation, and large economic losses. Up to date, the epidemiology of many Alternaria species is still not resolved as they can be saprophytic, parasitic or shift between both lifestyles and are also classified as primary pathogen able to infect healthy tissue. We argue that Alternaria spp. does not act as primary pathogen, but only as a necrosis-dependent opportunist. We studied the infection biology of Alternaria spp. under controlled conditions and monitored disease prevalence in real orchards and validated our ideas by applying fungicide-free treatments in 3-years field experiments. Alternaria spp. isolates were not able to induce necroses in healthy tissue, but only when prior induced damages existed. Next, leaf-applied fertilizers, without fungicidal effect, reduced Alternaria-associated symptoms (- 72.7%, SE: ± 2.5%) with the same efficacy as fungicides. Finally, low leaf magnesium, sulphur, and manganese concentrations were consistently linked with Alternaria-associated leaf blotch. Fruit spot incidence correlated positively with leaf blotch, was also reduced by fertilizer treatments, and did not expand during storage unlike other fungus-mediated diseases. Our findings suggest that Alternaria spp. may be a consequence of leaf blotch rather than its primary cause, as it appears to colonize the physiologically induced leaf blotch. Taking into account existing observations that Alternaria infection is connected to weakened hosts, the distinction may appear slight, but is of great significance, as we can now (a) explain the mechanism of how different stresses result in colonization with Alternaria spp. and (b) substitute fungicides for a basic leaf fertilizer. Therefore, our findings can result in significant decreases in environmental costs due to reduced fungicide use, especially if the same mechanism applies to other crops.

Reactive oxygen species-related genes participate in resistance to cucumber green mottle mosaic virus infection regulated by boron in Nicotiana benthamiana and watermelon

Cucumber green mottle mosaic virus (CGMMV) infection causes acidification and rot of watermelon flesh, resulting in serious economic losses. It is widely reported the interaction relationship between boron and reactive oxygen species (ROS) in regulating normal growth and disease resistance in plants. Our previous results demonstrated that exogenous boron could improve watermelon resistance to CGMMV infection. However, the roles of ROS-related genes regulated by boron in resistance to CGMMV infection are unclear. Here, we demonstrated that CGMMV symptoms were alleviated, and viral accumulations were decreased by boron application in Nicotiana benthamiana, indicating that boron contributed to inhibiting CGMMV infection. Meanwhile, we found that a number of differentially expressed genes (DEGs) associated with inositol biosynthesis, ethylene synthesis, Ca²⁺ signaling transduction and ROS scavenging system were up-regulated, while many DEGs involved in ABA catabolism, GA signal transduction and ascorbic acid metabolism were down-regulated by boron application under CGMMV infection. Additionally, we individually silenced nine ROS-related genes to explore their anti-CGMMV roles using a tobacco rattle virus (TRV) vector. The results showed that NbCat1, NbGME1, NbGGP and NbPrx Q were required for CGMMV infection, while NbGST and NbIPS played roles in resistance to CGMMV infection. The similar results were obtained in watermelon by silencing of ClCat, ClPrx or ClGST expression using a pV190 vector. This study proposed a new strategy for improving plant resistance to CGMMV infection by boron-regulated ROS pathway and provided several target genes for watermelon disease resistance breeding.

Integrated management of charcoal rot disease in susceptible genotypes of mungbean with soil application of micronutrient zinc and green manure (prickly sesban)

Charcoal rot disease is incited by the soil-borne fungus Macrophomina phaseolina (Tassi). Goid is a challenging disease due to long-term persistence of fungus sclerotia in the soil. This study assessed the potential of zinc (Zn: 1.25, 2.44, and 5 mg/kg) and green manure (GM: 1 and 2%) in solitary and bilateral combinations to alleviate infection stress incited by M. phaseolina on disease, growth, physiology, and yield attributes in mungbean. A completely randomized design experiment was conducted in potted soil, artificially inoculated with the pathogen, and sown with surface-sterilized seeds of mungbean genotypes (susceptible: MNUYT-107 and highly susceptible: MNUYT-105). Concealment of plant resistance by M. phaseolina in both genotypes resulted in 53-55% disease incidence and 40-50% plant mortality, which contributed in causing a significant reduction of 30-90% in attributes of growth, biomass, yield, photosynthetic pigment, and total protein content with an imbalance of production of antioxidant enzymes (polyphenol oxidase, superoxide dismutase, catalase, and peroxidase). Soil application with Zn-based fertilizer (ZnSO4: 33%) in combination with GM significantly managed up to 80% of the charcoal rot disease, hence improving growth (50-100%) and physiochemical (30-100%) attributes and sustainably enhancing grain average yield (300-600%), biological yield (100-200%), and harvest index (100-200%) in mungbean plants. The heat map and principal component analyses based on 19 measured attributes with 16 treatments separated Zn (2.44 or 5 mg/kg) combined with 2% GM as the best treatments for alleviating charcoal rot disease stress by improving growth, yield, and biological attributes to an extent to profitable farming in terms of harvest index (HI) and benefit-cost ratio (BCR).

Transcriptome analysis reveals the mechanism of zinc ion-mediated plant resistance to TMV in Nicotiana benthamiana

Zinc ions (Zn2+) are used to promote plant growth and treat multiple diseases. However, it is still unclear which pathways in plants respond to Zn2+. In this study, we found that supplying (CH3COO)2Zn can effectively delay tobacco mosaic virus (TMV) replication and movement in Nicotiana benthamiana. To further understand the regulatory mechanism of antiviral activity mediated by Zn2+, we examined the transcriptomic changes of leaves treated with Zn2+. Three days after treatment, 7575 differential expression genes (DEGs) were enriched in the Zn2+ treatment group compared with the control group. Through GO and KEGG analysis, the pathway of phosphatidylinositol signaling system and inositol phosphate metabolism were significantly enriched after treated with Zn2+, and a large number of ethylene-responsive transcription factors (ERFs) involved in inositol phosphate metabolism were found to be enriched. We identified ERF5 performed a positive effect on plant immunity. Our findings demonstrated that Zn2+-mediated resistance in N. benthamiana activated signal transduction and regulated the expression of resistance-related genes. The results of the study uncover a global view of mRNA changes in Zn2+-mediated cellular processes involved in the competition between plants and viruses.

Mixtures of Macro and Micronutrients Control Grape Powdery Mildew and Alter Berry Metabolites

Powdery mildew caused by the fungus Erysiphe necator is a major grape disease worldwide. It attacks foliage and berries and reduces yield and wine quality. Fungicides are mainly used for combating the disease. Fungicide resistance and the global requisite to reduce pesticide deployment encourage the use of environment-friendly alternatives for disease management. Our field experiments showed that the foliar application of the potassium phosphate fertilizer Top-KP+ (1-50-33 NPK) reduced disease incidence on leaves and clusters by 15-65% and severity by 75-90%, compared to untreated vines. Top-KP+ mixed with Nanovatz (containing the micronutrients boron (B) and zinc (Zn)) or with TruPhos Platinum (a mixture containing N, P₂O₅, K₂O, Zn, B, Mg, Fe, Mn, Cu, Mo, and CO) further reduced disease incidence by 30-90% and disease severity by 85-95%. These fertilizers were as effective as the fungicide tebuconazole. Tank mixtures of fertilizers and tebuconazole further enhanced control efficacy in the vineyards. The modes of action of fertilizers in disease control were elucidated via tests with grape seedlings, microscopy, and berry metabolomics. Fertilizers applied preventively to the foliage of grape seedlings inhibited powdery mildew development. Application onto existing mildew colonies plasmolyzed mycelia and conidia and arrested the development of the disease. Berries treated with fertilizers or with a fungicide showed a significant increase in anti-fungal and antioxidant metabolites. Twenty-two metabolites, including non-protein amino acids and carbohydrates, known for their anti-fungal and bioactive effects, were significantly upregulated in grapes treated with fertilizers as compared to grapes treated with a fungicide, suggesting possible indirect activity against the pathogen. Esters and organic acids that contribute to wine quality were also upregulated. We conclude that integrating macro and micronutrients in spray programs in commercial vineyards shall control powdery mildew, reduce fungicide deployment, delay the buildup of fungicide resistance, and may improve wine quality.

Combining Biocontrol Agent With Plant Nutrients for Integrated Control of Tomato Early Blight Through the Modulation of Physio-Chemical Attributes and Key Antioxidants

Early blight (EB) is one of the major fungal diseases caused by Alternaria solani that is responsible for destructive tomato production around the globe. Biocontrol agent/s can be adequately implemented in an integrated management framework by using it in combination with vital plant nutrients, e.g., nitrogen, phosphorus, and potassium (NPK) and zinc (Zn). The current study was aimed to assess the integrated effect of a biocontrol agent Bacillus subtilis (BS-01) and the selective plant nutrients (NPK and Zn) on EB disease management and tomato crop performance. A field experiment was conducted for the off-season tomato production (under walk-in tunnels) in Punjab, Pakistan. The trial was set in a randomized complete block design (RCBD) and comprised nine treatments of a biocontrol agent (BS-01) either alone or in combination with the plant nutrients, viz., NPK (64:46:50 kg acre^–1) and Zn (10 kg acre^–1) as sustainable disease managing approach against EB. In addition, the biocontrol efficacy of B. subtilis (BS-01) on a fungal load of A. solani was estimated by quantitative PCR assays, where the foliar application of BS-01 on tomato plants either alone or in combination with the plant nutrients was done as a preventive measure. Our results revealed that the interactive effect of BS-01 with plant nutrients conferred significantly a varying degree of resilience in the infected tomato plants against EB by effectively modifying the content of total chlorophyll, carotenoids, and total phenolics along with the activities of antioxidant enzymes (SOD, CAT, POX, PPO, and PAL). In addition, the integrative effect of BS-01 and plant nutrients proved significantly effective in reducing pathogen load on inoculated tomato foliage, displaying the desired level of protection against A. solani infection. Besides, the complementary interaction of BS-01 + Zn + NPK worked synergistically to improve crop productivity by providing the highest marketable yield (21.61 tons acre^–1) and net profit (361,363 Pakistani rupees acre^–1). This integrated approach is put forward as a way to reduce the fungicide doses to control EB that would act as a sustainable plant protection strategy to generate profitable tomato production.

Effects of Microelements on Downy Mildew (Peronospora belbahrii) of Sweet Basil

We recently demonstrated that spraying or irrigating with Ca, Mg and K reduces the severity of sweet basil downy mildew (SBDM). Here, the effects of Mn, Zn, Cu and Fe on SBDM were tested in potted plants. The effects of Mn and Zn were also tested under semi-commercial and commercial-like field conditions. Spray applications of a mixture of EDTA-chelated microelements (i.e., Fe-EDTA, Mn-EDTA, Zn-EDTA, Cu-EDTA and Mo) reduces SBDM severity. The application of EDTA chelates of individual microelements (i.e., Fe-EDTA, Mn-EDTA and Zn-EDTA) significantly reduces SBDM in potted plants. Foliar applications of Mn-EDTA and Zn-EDTA are found to be effective under semi-commercial conditions and were, thus, further tested under commercial-like conditions. Under commercial-like conditions, foliar-applied Mn-EDTA and Zn-EDTA decreased SBDM severity by 46-71%. When applied through the irrigation solution, those two microelements reduce SBDM by more than 50%. Combining Mg with Mn-EDTA and Zn-EDTA in the irrigation solution does not provide any additional disease reduction. In the commercial-like field experiment, the microelement-mixture treatment, applied as a spray or via the irrigation solution, was combined with fungicides spray treatments. This combination provides synergistic disease control. The mode of action in this plant-pathogen system may involve features of altered host resistance.

Bio-genetic analysis of resistance in tomato to early blight disease, Alternaria alternata

Early blight disease (EB), Alternaria alternata, is destructive on Solanum lycopersicum Mill. The responses of 35 domestic and exotic commercial tomato genotypes to early blight were examined at transplanting and maturing stages using genetic diversity analysis, with 15 Inter Simple Sequence Repeat (ISSR) primers, total phenolic content (TPC), and enzymatic activity of catalase (CAT), phenylalanine ammonia lyase (PAL), peroxidase (POD) and superoxide dismutase (SOD) assays. The disease severity ranged from 18 to 87.5%. Eleven of 15 ISSR primers generated 68 loci of which 63 (90%) were polymorphic. Polymorphism information content value varied from 0.3 to 0.5 with an average of 0.4. Nei's measure of the average gene diversity ranged from 0.06 to 0.5. The Tomato genotypes were divided into five clusters in Un-weighted Pair Group Method with Arithmetic Mean (UPGMA) analysis, showing a considerable similarity between resistance level and molecular classification pattern. Antioxidant analysis indicated a significant increase in TPC and CAT, POD, PAL and SOD activities in most inoculated tomato genotypes at both growth stages. The highest increase in activity was seen in PAL (5-fold) and TPC (4-fold) at transplanting stage, whereas the highest TPC (2 to 3-fold) and POD activity (3-fold) were found at maturing stage in all the inoculated resistant genotypes in comparison with controls. Esfahan Local and H. a.s 2274 showed the highest level of activity in POD (2.5- and 3- fold, respectively) and TPC (2.5- and 4-fold, respectively). Our results suggest that using both genetic diversity and enzymatic diversity as markers, it is possible to discriminate resistant from susceptible tomato genotypes to early blight disease.

Contending charcoal rot disease of mungbean by employing biocontrol Ochrobactrum ciceri and zinc

Role of rhizobacteria and zinc (Zn) was investigated in the management of charcoal rot disease in mungbean [Vigna radiata (L.) Wilczek] caused by Macrophomina phaseolina (Tassi) Goid. In vitro, screening tests with eight rhizobacteria [Bacillus subtilis (FCBP-0324), B. subtilis (FCBP-0189), Rhizobacter daucus (FCBP-0450), Azospirillum brasilense (FCBP-0025), Azospirillum lipoferum (FCBP-0022), Pseudomonas malophilia (FCBP-0099), Pseudomonas florescense (FCBP-0083) and Ochrobactrum ciceri (FCBP-0727)] were conducted against M. phaseolina and FCBP-0727 were found as the most effective biocontrol agent. Molecular analyses of 16S rDNA combined with cultural and biochemical analyses confirmed FCBP-0727 identification (GeneBank Accession No. LC415039). Cell-free culture filtrate (CFCF) and cell culture of O. ciceri were separated and antifungal trials of both substrates indicated inhibition in mycelial growth and suppression in sclerotia formation, although the CFCF appeared to be more destructive against the pathogen. Ethyl-acetate and chloroform extracts of bacterial secondary metabolites completely halted the growth of M. phaseolina. The GC-MS analysis of CFCF of chloroform extract proved to be rich sources of bioactive fungicide like phthalates, adipic acid, propanoic acid, and linoleic acid. Likewise, CFCF of ethyl acetate also exhibited important organic compounds like phthalates, diisopropylglycol and octasiloxan. Pot experiment revealed that soil inoculation with O. ciceri in combination with Zn (2.5 mg/kg) protected mungbean plants against M. phaseolina through improving photosynthetic pigment, total protein content and activities of antioxidant enzymes (catalase, peroxidase and polyphenol oxidase). The present study will open new vistas for biological management of charcoal rot disease of mungbean using a combination of rhizobacteria and Zn.