Green Leaf Volatile Confers Management of Late Blight Disease: A Green Vaccination in Potato

A review of biocontrol agents in controlling late blight of potatoes and tomatoes caused by Phytophthora infestans and the underlying mechanisms

Phytophthora infestans causes late blight on potatoes and tomatoes, which has a significant economic impact on agriculture. The management of late blight has been largely dependent on the application of synthetic fungicides, which is not an ultimate solution for sustainable agriculture and environmental safety. Biocontrol strategies are expected to be alternative methods to the conventional chemicals in controlling plant diseases in the integrated pest management (IPM) programs. Well-studied biocontrol agents against Phytophthora infestans include fungi, oomycetes, bacteria, and compounds produced by these antagonists, in addition to certain bioactive metabolites produced by plants. Laboratory and glasshouse experiments suggest a potential for using biocontrol in practical late blight disease management. However, the transition of biocontrol to field applications is problematic for the moment, due to low and variable efficacies. In this review, we provide a comprehensive summary on these biocontrol strategies and the underlying corresponding mechanisms. To give a more intuitive understanding of the promising biocontrol agents against Phytophthora infestans in agricultural systems, we discuss the utilizations, modes of action and future potentials of these antagonists based on their taxonomic classifications. To achieve a goal of best possible results produced by biocontrol agents, it is suggested to work on field trials, strain modifications, formulations, regulations, and optimizations of application. Combined biocontrol agents having different modes of action or biological adaptation traits may be used to strengthen the biocontrol efficacy. More importantly, biological control agents should be applied in the coordination of other existing and forthcoming methods in the IPM programs. © 2023 Society of Chemical Industry.

Molecular analysis of broad-spectrum induced resistance in rice by the green leaf volatile Z-3-hexenyl acetate

Green leaf volatiles (GLVs), volatile organic compounds released by plants upon tissue damage, are key signaling molecules in plant immunity. The ability of exogenous GLV application to trigger an induced resistance (IR) phenotype against arthropod pests has been widely reported, but its effectiveness against plant pathogens is less well understood. In this study, we combined mRNA sequencing-based transcriptomics and phytohormone measurements with multispectral imaging-based precision phenotyping to gain insights into the molecular basis of Z-3-hexenyl acetate-induced resistance (Z-3-HAC-IR) in rice. Furthermore, we evaluated the efficacy of Z-3-HAC-IR against a panel of economically significant rice pathogens: Pyricularia oryzae, Rhizoctonia solani, Xanthomonas oryzae pv. oryzae, Cochliobolus miyabeanus, and Meloidogyne graminicola. Our data revealed rapid induction of jasmonate metabolism and systemic induction of plant immune responses upon Z-3-HAC exposure, as well as a transient allocation cost due to accelerated chlorophyll degradation and nutrient remobilization. Z-3-HAC-IR proved effective against all tested pathogens except for C. miyabeanus, including against the (hemi)biotrophs M. graminicola, X. oryzae pv. oryzae, and P. oryzae. The Z-3-HAC-IR phenotype was lost in the jasmonate (JA)-deficient hebiba mutant, which confirms the causal role of JA in Z-3-HAC-IR. Together, our results show that GLV exposure in rice induces broad-spectrum, JA-mediated disease resistance with limited allocation costs, and may thus be a promising alternative crop protection approach.

Characterization of bifunctional alginate lyase Aly644 and antimicrobial activity of enzymatic hydrolysates

An alginate lyase gene aly644 encoding a member of polysaccharide lyase family 6 was obtained from a metagenome of Antarctic macroalgae-associated microbes. The gene was expressed heterologously in Escherichia coli, and the recombinant protein was purified using a Ni-NTA His Tag Kit. With sodium alginate as the substrate, recombinant Aly644 exhibited an optimum reaction temperature of 50°C and an optimum reaction pH of 7.0. The Vmax and Km values of Aly644 toward sodium alginate were 112.36 mg/mL·min and 16.75 mg/mL, respectively. Substrate specificity analysis showed that Aly644 was a bifunctional alginate lyase that hydrolyzed both polyguluronic acid and polymannuronic acid. The hydrolysis products of Aly644 with sodium alginate as the substrate were detected by thin-layer chromatography, and were mainly di- and trisaccharides. The oligosaccharides produced by degradation of sodium alginate by Aly644 inhibited the mycelial growth of the plant pathogens Phytophthora capsici and Fulvia fulva; the 50% maximal effective concentration (EC50) values were 297.45 and 452.89 mg/L, and the 90% maximal effective concentration (EC90) values were 1341.45 and 2693.83 mg/L, respectively. This highlights that Aly644 is a potential candidate enzyme for the industrial production of alginate oligosaccharides with low degree of polymerization. Enzyme-hydrolyzed alginate oligosaccharides could support the development of green agriculture as natural antimicrobial agents. KEY POINTS: • An alginate lyase was obtained from a metagenome of Antarctic macroalgae-associated microbes. • Aly644 is a bifunctional alginate lyase with excellent thermostability and pH stability. • The enzymatic hydrolysates of Aly644 directly inhibited Phytophthora capsici and Fulvia fulva.

Harnessing Plant's Arsenal: Essential Oils as Promising Tools for Sustainable Management of Potato Late Blight Disease Caused by Phytophthora infestans-A Comprehensive Review

Potato late blight disease is caused by the oomycete Phytophthora infestans and is listed as one of the most severe phytopathologies on Earth. The current environmental issues require new methods of pest management. For that reason, plant secondary metabolites and, in particular, essential oils (EOs) have demonstrated promising potential as pesticide alternatives. This review presents the up-to-date work accomplished using EOs against P. infestans at various experimental scales, from in vitro to in vivo. Additionally, some cellular mechanisms of action on Phytophthora spp., especially towards cell membranes, are also presented for a better understanding of anti-oomycete activities. Finally, some challenges and constraints encountered for the development of EOs-based biopesticides are highlighted.

Comparative transcriptome profiling of potato cultivars infected by late blight pathogen Phytophthora infestans: Diversity of quantitative and qualitative responses

The Estonia potato cultivar Ando has shown elevated field resistance to Phytophthora infestans, even after being widely grown for over 40 years. A comprehensive transcriptional analysis was performed using RNA-seq from plant leaf tissues to gain insight into the mechanisms activated for the defense after infection. Pathogen infection in Ando resulted in about 5927 differentially expressed genes (DEGs) compared to 1161 DEGs in the susceptible cultivar Arielle. The expression levels of genes related to plant disease resistance such as serine/threonine kinase activity, signal transduction, plant-pathogen interaction, endocytosis, autophagy, mitogen-activated protein kinase (MAPK), and others were significantly enriched in the upregulated DEGs in Ando, whereas in the susceptible cultivar, only the pathway related to phenylpropanoid biosynthesis was enriched in the upregulated DEGs. However, in response to infection, photosynthesis was deregulated in Ando. Multi-signaling pathways of the salicylic-jasmonic-ethylene biosynthesis pathway were also activated in response to Phytophthora infestans infection.

The effectors of Phytophthora infestans impact host immunity upon regulation of antagonistic hormonal activities

MAIN CONCLUSION

Phytophthora infestans effectors manipulate the antagonism of host hormones to interfere with the immune response of plants at different infection stages. Phytophthora infestans (P. infestans) poses a serious threat to global crop production, and its effectors play an indispensable role in its pathogenicity. However, the function of these effectors during the switch from biotrophy to necrotrophy of P. infestans remains unclear. Further research on the effectors that manipulate the antagonistic response of host hormones is also lacking. In this study, a coexpression analysis and infection assays were performed to identify distinct gene expression changes in both P. infestans and tomato. During the switch from biotrophy to necrotrophy, P. infestans secretes three types of effectors to interfere with host salicylic acid (SA), jasmonic acid (JA), ethylene (ET), and abscisic acid (ABA) levels. The three aforementioned effectors also regulate the host gene expression including NPR1, TGA2.1, PDF1.2, NDR1, ERF3, NCED6, GAI4, which are involved in hormone crosstalk. The changes in plant hormones are mediated by the three types of effectors, which may accelerate infection and drive completion of the P. infestans lifecycle. Our findings provide new insight into plant‒pathogen interactions that may contribute to the prevention growth of hemibiotrophic pathogens.

Impacts of Dutch elm disease-causing fungi on foliage photosynthetic characteristics and volatiles in Ulmus species with different pathogen resistance

Global warming affects the abiotic and biotic growth environment of plants, including the spread of fungal diseases such as Dutch elm disease (DED). Dutch elm disease-resistance of different Ulmus species varies, but how this is reflected in leaf-level physiological pathogen responses has not been investigated. We studied the impacts of mechanical injury alone and mechanical injury plus inoculation with the DED-causing pathogens Ophiostoma novo-ulmi subsp. novo-ulmi and O. novo-ulmi subsp. americana on Ulmus glabra, a more vulnerable species, and U. laevis, a more resistant species. Plant stress responses were evaluated for 12 days after stress application by monitoring leaf net CO2 assimilation rate (A), stomatal conductance (gs), ratio of ambient to intercellular CO2 concentration (Ca/Ci) and intrinsic water-use efficiency (A/gs), and by measuring biogenic volatile (VOC) release by plant leaves. In U. glabra and U. laevis, A was not affected by time, stressors or their interaction. Only in U. glabra, gs and Ca/Ci decreased in time, yet recovered by the end of the experiment. Although the emission compositions were affected in both species, the stress treatments enhanced VOC emission rates only in U. laevis. In this species, mechanical injury especially when combined with the pathogens increased the emission of lipoxygenase pathway volatiles and dimethylallyl diphosphate and geranyl diphosphate pathway volatiles. In conclusion, the more resistant species U. laevis had a more stable photosynthesis, but stronger pathogen-elicited volatile response, especially after inoculation by O. novo-ulmi subsp. novo-ulmi. Thus, stronger activation of defenses might underlay higher DED-resistance in this species.

Volatile-mediated signalling in barley induces metabolic reprogramming and resistance against the biotrophic fungus Blumeria hordei

Plants have evolved diverse secondary metabolites to counteract biotic stress. Volatile organic compounds (VOCs) are released upon herbivore attack or pathogen infection. Recent studies suggest that VOCs can act as signalling molecules in plant defence and induce resistance in distant organs and neighbouring plants. However, knowledge is lacking on the function of VOCs in biotrophic fungal infection on cereal plants. We analysed VOCs emitted by 13 ± 1-day-old barley plants (Hordeum vulgare L.) after mechanical wounding using passive absorbers and TD-GC/MS. We investigated the effect of pure VOC and complex VOC mixtures released from wounded plants on the barley-powdery mildew interaction by pre-exposure in a dynamic headspace connected to a powdery mildew susceptibility assay. Untargeted metabolomics and lipidomics were applied to investigate metabolic changes in sender and receiver barley plants. Green leaf volatiles (GLVs) dominated the volatile profile of wounded barley plants, with (Z)-3-hexenyl acetate (Z3HAC) as the most abundant compound. Barley volatiles emitted after mechanical wounding enhanced resistance in receiver plants towards fungal infection. We found volatile-mediated modifications of the plant-pathogen interaction in a concentration-dependent manner. Pre-exposure with physiologically relevant concentrations of Z3HAC resulted in induced resistance, suggesting that this GLV is a key player in barley anti-pathogen defence. The complex VOC mixture released from wounded barley and Z3HAC induced e.g. accumulation of chlorophyll, linolenic acid and linolenate-conjugated lipids, as well as defence-related secondary metabolites, such as hordatines in receiving plants. Barley VOCs hence induce a complex physiological response and disease resistance in receiver plants.

The Green Leaf Volatile (Z)-3-Hexenyl Acetate Is Differently Emitted by Two Varieties of Tulbaghia violacea Plants Routinely and after Wounding

While studying aromas produced by the edible flowers of Tulbaghia violacea, we noticed a different production of (Z)-3-Hexenyl acetate (a green-leaf volatile, GLV) by purple (var. ‘Violacea’) and white (var. ‘Alba’) flowers. The white Tulbaghia flowers constantly emits (Z)-3-Hexenyl acetate, which is instead produced in a lower amount by the purple-flowered variety. Thus, we moved to analyze the production of (Z)-3-Hexenyl acetate by whole plants of the two varieties by keeping them confined under a glass bell for 5 h together with a SPME (Solid Phase Micro Extraction) fiber. Results show that six main volatile compounds are emitted by T. violacea plants: (Z)-3-Hexenyl acetate, benzyl alcohol, nonanal, decanal, (Z)-3-Hexenyl-α-methylbutyrate, and one unknown compound. By cutting at half-height of the leaves, the (Z)-3-Hexenyl acetate is emitted in high quantities from both varieties, while the production of (Z)-3-Hexenyl-α-methylbutyrate increases. (Z)-3-Hexenyl acetate is a GLV capable of stimulating plant defenses, attracting herbivores and their natural enemies, and it is also involved in plant-to-plant communication and defense priming. Thus, T. violacea could represent a useful model for the study of GLVs production and a ‘signal’ plant capable of stimulating natural defenses in the neighboring plants.

Green Leaf Volatiles-The Forefront of Plant Responses Against Biotic Attack

Green leaf volatiles (GLVs) are six-carbon volatile oxylipins ubiquitous in vascular plants. GLVs are produced from acyl groups in the biological membranes via oxygenation by a pathway-specific lipoxygenase (LOX) and a subsequent cleavage reaction by hydroperoxide lyase. Because of the universal distribution and ability to form GLVs, they have been anticipated to play a common role in vascular plants. While resting levels in intact plant tissues are low, GLVs are immediately synthesized de novo in response to stresses, such as insect herbivory, that disrupt the cell structure. This rapid GLV burst is one of the fastest responses of plants to cell-damaging stresses; therefore, GLVs are the first plant-derived compounds encountered by organisms that interact with plants irrespective of whether the interaction is competitive or friendly. GLVs should therefore be considered important mediators between plants and organisms that interact with them. GLVs can have direct effects by deterring herbivores and pathogens as well as indirect effects by attracting predators of herbivores, while other plants can recruit them to prepare their defenses in a process called priming. While the beneficial effects provided to plants by GLVs are often less dramatic and even complementary, the buildup of these tiny effects due to the multiple functions of GLVs can amass to levels that become substantially beneficial to plants. This review summarizes the current understanding of the spatiotemporal resolution of GLV biosynthesis and GLV functions and outlines how GLVs support the basic health of plants.

Production of the Green Leaf Volatile (Z)-3-Hexenal by a Zea mays Hydroperoxide Lyase

Plant-produced volatile compounds play important roles in plant signaling and in the communication of plants with other organisms. Many plants emit green leaf volatiles (GLVs) in response to damage or attack, which serve to warn neighboring plants or attract predatory or parasitic insects to help defend against insect pests. GLVs include aldehydes, esters, and alcohols of 6-carbon compounds that are released rapidly following wounding. One GLV produced by maize (Zea mays) is the volatile (Z)-3-hexenal; this volatile is produced from the cleavage of (9Z,11E,15Z)-octadecatrienoic acid by hydroperoxide lyases (HPLs) of the cytochrome P450 CYP74B family. The specific HPL in maize involved in (Z)-3-hexenal production had not been determined. In this study, we used phylogenetics with known HPLs from other species to identify a candidate HPL from maize (ZmHPL). To test the ability of the putative HPL to produce (Z)-3-hexenal, we constitutively expressed the gene in Arabidopsis thaliana ecotype Columbia-0 that contains a natural loss-of-function mutant in AtHPL and examined the transgenic plants for restored (Z)-3-hexenal production. Volatile analysis of leaves from these transgenic plants showed that they did produce (Z)-3-hexenal, confirming that ZmHPL can produce (Z)-3-hexenal in vivo. Furthermore, we used gene expression analysis to show that expression of ZmHPL is induced in maize in response to both wounding and the insect pests Spodoptera frugiperda and Spodoptera exigua. Our study demonstrates that ZmHPL can produce GLVs and highlights its likely role in (Z)-3-hexenal production in response to mechanical damage and herbivory in maize.

Late Blight Resistance Conferred by Rpi-Smira2/R8 in Potato Genotypes In Vitro Depends on the Genetic Background

Potato production worldwide is threatened by late blight, caused by the oomycete Phytophthora infestans (Mont.) de Bary. Highly resistant potato cultivars were developed in breeding programs, using resistance gene pyramiding methods. In Sárpo Mira potatoes, five resistance genes (R3a, R3b, R4, Rpi-Smira1, and Rpi-Smira2/R8) are reported, with the latter gene assumed to be the major contributor. To study the level of late blight resistance conferred by the Rpi-Smira2/R8 gene, potato genotypes with only the Rpi-Smira2/R8 gene were selected from progeny population in which susceptible cultivars were crossed with Sárpo Mira. Ten R8 potato genotypes were obtained using stepwise marker-assisted selection, and agroinfiltration of the avirulence effector gene Avr4. Nine of these R8 genotypes were infected with both Slovenian P. infestans isolates and aggressive foreign isolates. All the progeny R8 genotypes are resistant to the Slovenian P. infestans isolate 02_07, and several show milder late blight symptoms than the corresponding susceptible parent after inoculation with other isolates. When inoculated with foreign P. infestans isolates, the genotype C571 shows intermediate resistance, similar to that of Sárpo Mira. These results suggest that Rpi-Smira2/R8 contributes to late blight resistance, although this resistance is not guaranteed solely by the presence of the R8 in the genome.

The Omics Hunt for Novel Molecular Markers of Resistance to Phytophthora infestans

Wild Solanum accessions are a treasured source of resistance against pathogens, including oomycete Phytophthora infestans, causing late blight disease. Here, Solanum pinnatisectum, Solanum tuberosum, and the somatic hybrid between these two lines were analyzed, representing resistant, susceptible, and moderately resistant genotypes, respectively. Proteome and metabolome analyses showed that the infection had the highest impact on leaves of the resistant plant and indicated, among others, an extensive remodeling of the leaf lipidome. The lipidome profiling confirmed an accumulation of glycerolipids, a depletion in the total pool of glycerophospholipids, and showed considerable differences between the lipidome composition of resistant and susceptible genotypes. The analysis of putative resistance markers pinpointed more than 100 molecules that positively correlated with resistance including phenolics and cysteamine, a compound with known antimicrobial activity. Putative resistance protein markers were targeted in an additional 12 genotypes with contrasting resistance to P. infestans. At least 27 proteins showed a negative correlation with the susceptibility including HSP70-2, endochitinase B, WPP domain-containing protein, and cyclase 3. In summary, these findings provide insights into molecular mechanisms of resistance against P. infestans and present novel targets for selective breeding.