Potassium phosphite primes defense responses in potato against Phytophthora infestans

Phosphite Compounds Suppress Anthracnose in Soybean Seeds Infected by Colletotrichum truncatum and Stimulate Growth and Defense Mechanisms

Soybean is one of the main agricultural commodities, and its productivity is limited by several diseases, such as anthracnose, which is caused by a complex of fungal species, with Colletotrichum truncatum being the most prevalent. Management is mainly carried out through chemical seed treatment. However, a reduction in the sensitivity of C. truncatum to fungicides was observed. Therefore, it is extremely important to search for products that are effective in controlling the disease. The objectives of this study were to evaluate the efficacy of commercial formulations of copper, potassium, manganese, and zinc phosphites in the treatment of soybean seeds infected by C. truncatum, as well as their direct fungitoxicity and ability to induce soybean defense mechanisms. For this purpose, seeds inoculated with C. truncatum were subjected to phosphites and a fungicide (carbendazim + thiram). The seeds were exposed to germination, health, and vigor tests. Fungal toxicity and the ability of phosphites to induce defense through the activities of catalase, peroxidase, and superoxide dismutase enzymes, as well as the levels of lignin and total soluble phenols, were also evaluated. Mn and Zn phosphites showed direct toxicity to C. truncatum and were as effective as the fungicide (carbendazim + thiram) in treating soybean seeds infected by the fungus. Mn phosphite induced the production of catalase (CAT), peroxidase (POX) and lignin, while Zn phosphite increased the production of CAT and POX. These results demonstrate the efficacy of Mn and Zn phosphites in controlling C. truncatum in infected soybean seeds, their direct toxic action, and their ability to induce resistance.

Modulation of key sterol-related genes of Nicotiana benthamiana by phosphite treatment during infection with Phytophthora cinnamomi

Phytophthora cinnamomi is a globally destructive pathogen causing disease in over 5000 plant species. As sterol auxotrophs, Phytophthora species rely on host-derived phytosterols for reproduction, yet the effects of pathogen infection on plant sterol biosynthesis remains unclear. We utilised a soil-free plant growth system to analyze the impacts of P. cinnamomi on Nicotiana benthamiana roots, a new model for studying P. cinnamomi -plant root interactions. Our results show that P. cinnamomi successfully infected all ecotypes tested, but infection was inhibited by the systemic chemical, phosphite. While phosphite is traditionally associated with the activation of plant defence mechanisms, we show that phosphite also modulates plant immune receptors and phytosterol biosynthesis. qPCR analyses revealed a two-fold upregulation of the N. benthamiana elicitin receptor, Responsive to Elicitins (REL ), and its co-receptor, suppressor of BIR1-1 (SOBIR ) during P. cinnamomi infection when compared with infected, phosphite-treated plants. Furthermore, key genes related to plant sterol biosynthesis were upregulated in their expression during pathogen infection but were suppressed in phosphite-treated and infected plants. Notably, the cytochrome P450 family 710 (CYP710A ) gene encoding a C22-sterol desaturase, involved in stigmasterol production, a phytosterol known to be linked to plant susceptibility to pathogens, was downregulated in phosphite-treated plants, independent of infection status. These findings reveal novel insights into the role of phosphite in modulating plant immune responses and sterol metabolism, with potential in managing diseases caused by P. cinnamomi .

The functional mechanisms of phosphite and its applications in crop plants

Phosphite (Phi), the reduced form of phosphate (Pi), is characterized by its stability, high solubility, efficient transport, resistance to fixation in soil, and widespread occurrence in natural environments. Although Phi exhibits greater suitability than Pi as a soil fertilizer, it cannot be metabolized by plants. In agricultural applications, Phi serves as a bio-stimulant, fungicide, herbicide, and has other purposes. As a bio-stimulant, Phi has been shown to promote plant growth, enhance stress resistance, and improve fruit quality. Additionally, when used as a fungicide or pesticide, it effectively inhibits the growth of phytopathogens in various crop species. The discovery of the phosphite dehydrogenase (ptxD) gene in microorganisms has significantly expanded the potential applications of Phi, including its use as a herbicide, phosphatic fertilizer, and a selectable chemical for generating marker-free transgenic plants. Therefore, the dual fertilization and weed control system of ptxD/Phi facilitates the utilization of Phi as the sole phosphorus source while concurrently suppressing the evolution of herbicide-resistant weeds in the future. Notably, ptxD also acts as an ideal selectable marker because its resistant is specific to Phi, thereby eliminating the risk of false positive clones. The application of Phi provides a promising strategy for addressing phosphorus resource shortages and improving the efficiency of phosphatic fertilizers in agriculture. Furthermore, Phi is considered an environmentally friendly fertilizer, as it contributes to the mitigation of eutrophication. In prospect, Phi is anticipated to play a significant role as a chemical fertilizer that promotes the sustainable development of agriculture. In this review, we provide a comprehensive analysis of the functional mechanisms of Phi and its current applications in agriculture, with the aim of offering deeper insights into its potential benefits and practical utility.

Identification of plant resistance inducers and evaluation of genotype receptivity for carrot protection against Alternaria leaf blight

The use of biopesticides represents an alternative strategy to synthetic chemical products for crop protection. To promote their adoption and effective use by growers, it is crucial to understand their modes of action and the optimal conditions for their application in crops, including their compatibility with specific varieties. Through a series of greenhouse experiments, this study describes the development and validation of a robust molecular diagnostic tool for enabling the evaluation of defence gene activation. The results identified plant resistance inducers (PRIs) among biopesticide products capable of protecting carrots against Alternaria leaf blight. By applying a PRI to a range of carrot varieties exhibiting varying levels of resistance to Alternaria dauci, preliminary findings on plant receptivity suggest that the efficacy of PRIs in conferring protection is highly dependent on the treated variety. Two distinct genotype-dependent effects were observed: sensitivity to the PRI and an enhancement of resistance. This study offers new insights into optimising biopesticide use in carrot cultivation.

New insights into azelaic acid-induced resistance against Alternaria Solani in tomato plants

BACKGROUND

The effect of azelaic acid (Aza) on the response of tomato plants to Alternaria solani was investigated in this study. After being treated with Aza, tomato plants were infected with A. solani, and their antioxidant, biochemical, and molecular responses were analyzed.

RESULTS

The results demonstrated that H2O2 and MDA accumulation increased in control plants after pathogen infection. Aza-treated plants exhibited a remarkable rise in peroxidase (POD) and catalase (CAT) activities during the initial stages of A. solani infection. Gene expression analysis revealed that both Aza treatment and pathogen infection altered the expression patterns of the SlNPR1, SlERF2, SlPR1, and SlPDF1.2 genes. The expression of SlPDF1.2, a marker gene for the jasmonic acid/ethylene (JA/ET) signaling pathway, showed a remarkable increase of 4.2-fold upon pathogen infection. In contrast, for the SlNPR1, a key gene in salicylic acid (SA) pathway, this increased expression was recorded with a delay at 96 hpi. Also, the phytohormone analysis showed significantly increased SA accumulation in plant tissues with disease development. It was also revealed that tissue accumulation of JA in Aza-treated plants was increased following pathogen infection, while it was not increased in plants without pathogen inoculation.

CONCLUSION

The results suggest that the resistance induced by Aza is mainly a result of modulations in both SA and JA pathways following complex antioxidant and molecular defense responses in tomato plants during A. solani infection. These findings provide novel information regarding inducing mechanisms of azelaic acid which would add to the current body of knowledge of SAR induction in plants as result of Aza application.

Beneficial Effects of Phosphite in Arabidopsis thaliana Mediated by Activation of ABA, SA, and JA Biosynthesis and Signaling Pathways

Phosphite (Phi) has gained attention in agriculture due to its biostimulant effect on crops. This molecule has been found to benefit plant performance by providing protection against pathogens, improving yield and fruit quality as well as nutrient and water use efficiency. It is still unclear how Phi enhances plant growth and protects against multiple stresses. It has been hypothesized that Phi acts by directly affecting the pathogens and interacting with the plant cellular components and molecular machinery to elicit defense responses. This study elucidates the mechanisms underlying Phi's beneficial effects on plants, revealing their complex interplay with fundamental signaling pathways. An RNA-seq study of Arabidopsis seedlings under optimal and limiting phosphate conditions helped us unveil Phi's role in promoting plant growth by activating the expression of the genes involved in the biosynthesis and signaling pathways associated with abscisic acid (ABA), salicylic acid (SA), and jasmonic acid (JA). The expression of ABA-related genes, known for their involvement in stress response and development regulation, is triggered by Phi treatment, contributing to enhanced resilience and growth. Simultaneously, the activation of the SA pathway, associated with defense responses, suggests Phi's potential in bolstering plant immunity. Moreover, Phi influences JA biosynthesis and signaling, which are crucial for defense against herbivores and pathogens, thereby strengthening plants' defenses. Our findings reveal a multifaceted mechanism through which Phi benefits Arabidopsis development. Understanding its intricate interplay with key signaling pathways opens avenues for leveraging Phi as a strategic tool to enhance plant resilience, immunity, and growth in agricultural and ecological contexts.

Proteomic analysis revealed that the oomyceticide phosphite exhibits multi-modal action in an oomycete pathosystem

Phytopathogenic oomycetes constitute some of the most devastating plant pathogens and cause significant crop and horticultural yield and economic losses. The phytopathogen Phytophthora cinnamomi causes dieback disease in native vegetation and several crops. The most commonly used chemical to control P. cinnamomi is the oomyceticide phosphite. Despite its widespread use, the mode of action of phosphite is not well understood and it is unclear whether it targets the pathogen, the host, or both. Resistance to phosphite is emerging in P. cinnamomi isolates and other oomycete phytopathogens. The mode of action of phosphite on phosphite-sensitive and resistant isolates of the pathogen and through a model host was investigated using label-free quantitative proteomics. In vitro treatment of sensitive P. cinnamomi isolates with phosphite hinders growth by interfering with metabolism, signalling and gene expression; traits that are not observed in the resistant isolate. When the model host Lupinus angustifolius was treated with phosphite, proteins associated with photosynthesis, carbon fixation and lipid metabolism in the host were enriched. Increased production of defence-related proteins was also observed in the plant. We hypothesise the multi-modal action of phosphite and present two models constructed using comparative proteomics that demonstrate mechanisms of pathogen and host responses to phosphite. SIGNIFICANCE: Phytophthora cinnamomi is a significant phytopathogenic oomycete that causes root rot (dieback) in a number of horticultural crops and a vast range of native vegetation. Historically, areas infected with phosphite have been treated with the oomyceticide phosphite despite its unknown mode of action. Additionally, overuse of phosphite has driven the emergence of phosphite-resistant isolates of the pathogen. We conducted a comparative proteomic study of a sensitive and resistant isolate of P. cinnamomi in response to treatment with phosphite, and the response of a model host, Lupinus angustifolius, to phosphite and its implications on infection. The present study has allowed for a deeper understanding of the bimodal action of phosphite, suggested potential biochemical factors contributing to chemical resistance in P. cinnamomi, and unveiled possible drivers of phosphite-induced host plant immunity to the pathogen.

Potassium Phosphite Activates Components Associated with Constitutive Defense Responses in Coffea arabica Cultivars

Phosphites have been used as inducers of resistance, activating the defense of plants and increasing its ability to respond to the invasion of the pathogen. However, the mode of action of phosphites in defense responses has not yet been fully elucidated. The objective of this study was to evaluate the effect of potassium phosphite (KPhi) in coffee cultivars with different levels of resistance to rust to clarify the mechanism by which KPhi activates the constitutive defense of plants. To this end, we studied the expression of genes and the activity of enzymes involved in the defense pathway of salicylic acid (SA) and reactive oxygen species (ROS), in addition to the levels of total soluble phenolic compounds and soluble lignin. Treatment with KPhi induced constitutive defense responses in cultivars resistant and susceptible to rust. The results suggest that KPhi acts in two parallel defense pathways, SA and ROS, which are essential for the induction of systemic acquired resistance (SAR) when activated simultaneously. The activation of the mechanisms associated with defense routes demonstrates that KPhi is a potential inducer of resistance in coffee plants.

Functional inhibition of the StERF3 gene by dual targeting through CRISPR/Cas9 enhances resistance to the late blight disease in Solanum tuberosum L

BACKGROUND

Disease-resistant cultivars are the best solution to get their maximum yield potential and avoid fungicide application. There is no doubt about the contribution, and use of R genes (resistance genes) in resistance development in plants, while S genes (susceptibility genes) also hold a strong position in pathogenesis by resistance repression, and their loss of function contributes to enhanced resistance. Hence, we attempted to knock out the function of the StERF3 gene in potatoes through CRISPR/Cas9-based genome editing and investigated the CRISPR/Cas9 approach as strategic control against late blight disease in potato plants.

METHODS AND RESULTS

The StERF3 gene was edited in late blight susceptible cv. Lady Rosetta. Full allelic edited plants were identified through DnpI, and N1aIV mediated restriction digestion and then further analyzed through Indel Detection by Amplicon Analysis. Sequence analysis of targeted plants for indel identification showed full allelic editing. The detached leaf assay of full allelic edited plants demonstrated the role of the StERF3 gene in susceptibility to late blight in potatoes. In planta disease assay also showed reduced, slowed, and delayed disease progression in StERF3-loss-of-function mutants compared to wild-type (control) plants. Less fungal biomass was quantified in knockouts through Real-time qPCR that supported less susceptibility of edited plants to late blight. Besides, relatively high expression of pathogens-related genes, StPR1, and StNPR1, were also observed in StERF3-loss-of-function mutants compared to the corresponding control.

CONCLUSION

The results showed the functional inhibition of StERF3 genes using the CRISPR/Cas9 approach. The functional knockouts (StERF3 gene-edited potato plants) revealed enhanced resistance against Phytophthora infestans, thereby demonstrating the best strategic control for late blight disease in potato plants.

Borate and phosphite treatments of potato plants (Solanum tuberosum L.) as a proof of concept to reinforce the cell wall structure and reduce starch digestibility

Potatoes are one of the main sources of carbohydrates in human diet, however they have a high glycaemic index (GI). Hence, developing new agricultural and industrial strategies to produce low GI potatoes represents a health priority to prevent obesity and related diseases. In this work, we investigated whether treatments of potato plants with elicitors of plant defence responses can lead to a reduction of tuber starch availability and digestibility, through the induction of cell wall remodelling and stiffening. Treatments with phosphites (KPhi) and borate were performed, as they are known to activate plant defence responses that cause modifications in the architecture and composition of the plant cell wall. Data of suberin autofluorescence demonstrated that potato plants grown in a nutrition medium supplemented with KPhi and borate produced tubers with a thicker periderm, while pectin staining demonstrated that KPhi treatment induced a reinforcement of the wall of storage parenchyma cells. Both compounds elicited the production of H₂O₂, which is usually involved in cell-wall remodelling and stiffening reactions while only KPhi caused an increase of the total content of phenolic compounds. A two-phase digestion in vitro assay showed that treatment with KPhi determined a significant decrease of the starch hydrolysis rate in potato tubers. This work highlights the ability of cell wall architecture in modulating starch accessibility to digestive enzymes, paving the way for new agronomic practices to produce low GI index potatoes.

Phosphite Reduces the Predation Impact of Poterioochromonasmalhamensis on Cyanobacterial Culture

Contamination by the predatory zooplankton Poterioochromonas malhamensis is one of the major threats that causes catastrophic damage to commercial-scale microalgal cultivation. However, knowledge of how to manage predator contamination is limited. Previously, we established a phosphite (Pt)-based culture system by engineering Synechococcus elongatus, which exerted a competitive growth advantage against microbial contaminants that compete with phosphate source. Here, we examined whether Pt is effective in suppressing predator-type contamination. Co-culture experiment of Synechococcus with isolated P. malhamensis revealed that, although an addition of Pt at low concentrations up to 2.0 mM was not effective, increased dosage of Pt (~20 mM) resulted in the reduced grazing impact of P. malhamensis. By using unsterilized raw environmental water collected from rivers or ponds, we found that the suppression effect of Pt was dependent on the type of environmental water used. Eukaryotic microbial community analysis of the cultures using environmental water samples revealed that Paraphysomonas, a colorless Chrysophyceae, emerged and dominated under high-Pt conditions, suggesting that Paraphysomonas is insensitive to Pt compared to P. malhamensis. These findings may provide a clue for developing a strategy to reduce the impact of grazer contamination in commercial-scale microalgal cultivation.

ROS and Oxidative Response Systems in Plants Under Biotic and Abiotic Stresses: Revisiting the Crucial Role of Phosphite Triggered Plants Defense Response

Phosphite (Phi) is a chemical analog of orthophosphate [HPO4 3-]. It is a systemic pesticide generally known to control the prevalence of oomycetes and soil-borne diseases such as Phytophthora, Pythium, and Plasmopora species. Phi can also control disease symptoms and the spread of pathogenic bacteria, fungi, and nematodes. Phi plays critical roles as a fungicide, pesticide, fertilizer, or biostimulator. Overall, Phi can alleviate the severity of the disease caused by oomycete, fungi, pathogenic bacteria, and nematodes (leave, stem, fruit, tuber, and root) in various plants (vegetables, fruits, crops, root/tuber crops, ornamental plants, and forests). Advance research in molecular, physiological, and biochemical approaches has approved the key role of Phi in enhancing crop growth, quantity, and quality of several plant species. Phi is chemically similar to orthophosphate, and inside the cells, it is likely to get involved in different features of phosphate metabolism in both plants and pathogens. In plants, a range of physiobiochemical alterations are induced by plant pathogen stress, which causes lowered photosynthesis activities, enzymatic activities, increased accumulation of reactive oxygen species (ROS), and modification in a large group of genes. To date, several attempts have been made to study plant-pathogen interactions with the intent to minimize the loss of crop productivity. Phi's emerging function as a biostimulant in plants has boost plant yield and tolerance against various stress factors. This review discusses Phi-mediated biostimulant effects against biotic and abiotic stresses.

Phosphonic acid: a long-standing and versatile crop protectant

Phosphonic acid-based fungicides, also referred to as phosphonates, have been used extensively as crop protectants in horticulture since the late 1970s, and more recently in native ecosystems and forestry. Discovering that phosphonates are effective against foliar and soilborne oomycete diseases, such as those caused by species of Phytophthora, Pythium and Plasmopara, was a significant breakthrough, especially for soilborne pathogens that are notoriously difficult to manage. Phosphonates have played an important role in protection of forests and sensitive natural ecosystems, under threat from these pathogens. Since introduction, their increased application in management of non-oomycete diseases, along with other functionalities, demonstrates their versatility in agriculture and more broadly. Continued use of phosphonic acid crop protectants will be underpinned by demonstrated efficacy and safety, and a better understanding of specific interactions within the plant, pathogen and environment. © 2020 Society of Chemical Industry.

Effects of phosphite as a plant biostimulant on metabolism and stress response for better plant performance in Solanum tuberosum

Food availability represents a major worldwide concern due to population growth, increased demand, and climate change. Therefore, it is imperative to identify compounds that can improve crop performance. Plant biostimulants have gained prominence because of their potentials to increase germination, productivity and quality of a wide range of horticultural and agronomic crops. Phosphite (Phi), an analog of orthophosphate, is an emerging biostimulant used in horticulture and agronomy. The aim of this study was to uncover the molecular mechanisms through which Phi acts as a biostimulant with potential effects of overall plant growth. Field and greenhouse experiments, using 4 potato cultivars, showed that following Phi applications, plant performance, including several physio-biochemical traits, crop productivity, and quality traits, were significantly improved. RNA sequencing of control and Phi-treated plants of cultivar Xingjia No. 2, at 0 h, 6 h, 24 h, 48 h, 72 h and 96 h after the Phi application for 24 h revealed extensive changes in the gene expression profiles. A total of 2856 differentially expressed genes were identified, suggesting that multiple pathways of primary and secondary metabolism, such as flavonoids biosynthesis, starch and sucrose metabolism, and phenylpropanoid biosynthesis, were strongly influenced by foliar applications of Phi. GO (Gene Ontology) and KEGG (Kyoto Encyclopedia of Genes and Genomes) enrichment analyses associated with defense responses revealed significant effects of Phi on a plethora of defense mechanisms. These results suggest that Phi acted as a biostimulant by priming the plants, that was, by triggering dynamic changes in gene expression and modulating metabolic fluxes in a way that allowed plants to perform better. Therefore, Phi usage has the potential to improve crop yield and health, alleviating the challenges posed by the need of feeding a growing world population, while minimizing the agricultural impact on human health and environment.

Phosphite Integrated in Late Blight Treatment Strategies in Starch Potato Does Not Cause Residues in the Starch Product

Currently available fungicides against potato late blight are effective but there are concerns about the sustainability of frequent applications and the risks of fungicide resistance. Therefore, we investigated how potassium phosphite can be integrated into late blight control programs with reduced fungicides in field trials. Phosphite was somewhat less effective than the conventional fungicides at suppressing late blight in the foliage, and the tubers contained less starch. However, when we reduced the amount of phosphite and combined it with reduced amounts of conventional fungicides, we observed no differences in disease suppression, total yields, and tuber starch contents compared with the full treatments with conventional fungicides. The amount of phosphite detected in the harvested tubers was linearly associated with the amount of phosphite applied to the foliage. Our analyses indicate that phosphite could replace some fungicides without exceeding the current European Union standards for the maximum residue levels in potato tubers. No phosphite was detected in the starch from the tubers. In 1 of 2 years, early blight (caused by Alternaria solani) was less severe in the phosphite treatments than in the treatments without phosphite. The integration of phosphite into current treatment strategies would reduce the dependence on conventional fungicides.

Induced tolerance to abiotic and biotic stresses of broccoli and Arabidopsis after treatment with elicitor molecules

The plant hormones salicylic acid (SA) and jasmonic acid (JA) regulate defense mechanisms capable of overcoming different plant stress conditions and constitute distinct but interconnected signaling pathways. Interestingly, several other molecules are reported to trigger stress-specific defense responses to biotic and abiotic stresses. In this study, we investigated the effect of 14 elicitors against diverse but pivotal types of abiotic (drought) and biotic (the chewing insect Ascia monuste, the hemibiotrophic bacterium Pseudomonas syringae DC 3000 and the necrotrophic fungus Alternaria alternata) stresses on broccoli and Arabidopsis. Among the main findings, broccoli pre-treated with SA and chitosan showed the highest drought stress recovery in a dose-dependent manner. Several molecules led to increased drought tolerance over a period of three weeks. The enhanced drought tolerance after triggering the SA pathway was associated with stomata control. Moreover, methyl jasmonate (MeJA) reduced A. monuste insect development and plant damage, but unexpectedly, other elicitors increased both parameters. GUS reporter assays indicated expression of the SA-dependent PR1 gene in plants treated with nine elicitors, whereas the JA-dependent LOX2 gene was only expressed upon MeJA treatment. Overall, elicitors capable of tackling drought and biotrophic pathogens mainly triggered the SA pathway, but adversely also induced systemic susceptibility to chewing insects. These findings provide directions for potential future in-depth characterization and utilization of elicitors and induced resistance in plant protection.

Some Plant Defense Stimulators can induce IL-1β production in human immune cells in vitro

Among Plant Protection Products (PPP), a new emerging category of pesticides act by stimulating plant defense in order to improve plant resistance against microbial pathogens. Given that these compounds, the so-called Plant Defense Stimulators (PDS) act on innate immunity, we tested, using an in vitro approach on human mononuclear leucocytes (PBMC), the potential toxicity (XTT assay) and inflammatory effects (production of IL-1β) of 4 PPP belonging to different chemical families. We found that two products (LBG-01F34® and Regalis®) did not induce any cytotoxicity or IL-1 β production. The product BION-50 WG®, that contains Acibenzolar-S-methyl (ASM) and silica particles did not present any cytotoxicity but induced a significant increase in the production of the inflammatory cytokine IL-1 β. Finally, Vacciplant® that contains laminarin, was highly cytotoxic and pro-inflammatory. It induced a strong production of IL-1 β when used at a concentration in the culture medium, as low as 0.02 mg/mL. We also tested the potential toxic effect of these 4 PPP on 4 days old zebra fish larvae. After 24 h of exposure, our results indicate that Vacciplant® induced zebra fish larvae mortality at concentration of 20 μg/mL. LBG did not induced significant mortality at concentrations up to 1 mg/mL whereas Regalis was lethal for 0,3 mg/mL concentrations and BION-50 WG began to induce mortality at 2,5 mg/mL.

Our results indicate possible effects of PDS on IL-1β production in human cells and fish survival, calling for more studies on the potential noxious side effects of these compounds.

Sulfur Amendment of Soil Improves Establishment and Growth of Firs in a Field Naturally Infested with Phytophthora1

Acidification of soil from pH 6 to 4 by incorporating elemental sulfur reduced mortality and improved color and initial growth of Fraser fir, Abies fraseri (Pursh) Poir., and Canaan fir, Abies balsamea (L.) Mill. var. phanerolepis Fernald, planted into a field that had previously experienced significant losses consistent with phytophthora root rot. Acidifying the soil improved tree color starting the year of planting and persisting through five years. During their second year after planting, extension of terminal growth was 12.5 vs. 5.6 cm (4.9 vs. 2.2 in) for plots with soil pH of 4 and 6, respectively, averaged across tree species. In subsequent years, the growth rate of trees was unaffected by having acidified the soil. Over the course of five years, the average annual mortality rate for the trees was 1.4, 4.0, 9.7, and 12.2% for Canaan fir (pH 4), Canaan fir (pH 6), Fraser fir (pH 4), and Fraser fir (pH 6), respectively. However, all tree mortality for Canaan fir planted into acidified soil occurred during the first two years. A root dip with potassium phosphite at the time of planting only benefitted Fraser fir. Minimizing losses of trees in this field would require planting species less susceptible or resistant to phytophthora root rot infection and soil acidification.

Index words: soil acidification, phytophthora root rot, potassium phosphite, root dip

Species used in this study: Fraser fir; Abies fraseri (Pursh) Poir.; Canaan fir; Abies balsamea (L.) Mill. var. phanerolepis Fernald

Chemicals used in this study: acibenzolar-S-methyl (Actigard 50WDG); clothianidin (Arena 50WDG); imidacloprid (Xytect 2F); potassium phosphite (Helena Prophyt); sulfur

Phosphite Application Alleviates Pythophthora infestans by Modulation of Photosynthetic and Physio-Biochemical Metabolites in Potato Leaves

Potato late blight (Phytophtora infestans) is among the most severely damaging diseases of potato (Solanum tuberusom L.) worldwide, causing serious damages in potato leaves and tubers. In the present study, the effects of potassium phosphite (KPhi) applications on photosynthetic parameters, enzymatic and non-enzymatic antioxidant properties, hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), total protein and total carbohydrate of potato leaves challenged with P. infestans pathogen were investigated. Potato leaves were sprayed five times with KPhi (0.5%) during the growing season prior to inoculation with P. infestans. The potato leaves were artificially infected by the LC06-44 pathogen isolate. The leaves were sampled at 0, 24, 48, 72 and 96 h after the infection for evaluations. P. infestans infection reduced chlorophyll (Chl) pigments contents, chlorophyll fluorescence, carotenoid (Car) and anthocyanin contents and increased the accumulation of H₂O₂ and MDA. Meanwhile, our result showed that KPhi treatment alleviated adverse effect of late blight in potato leaves. KPhi application also increased plant tolerance to the pathogen with improved photosynthetic parameters Chl a, b, total Chl, Car, and anthocyanin compare to controls. Moreover, the increased oxidative enzymes activity of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APx), and non-enzymatic substances such as phenolics, flavonoids and proline were found in KPhi treated plants, compared to untreated plants after inoculation. In addition, KPhi application followed by P. infestans infection also decreased the content of H₂O₂ and MDA, but increased the total protein and total carbohydrate contents in potato leaves. The consequence of current research indicated that KPhi played a vital role in pathogen tolerance, protecting the functions of photosynthetic apparatus by improved oxidative levels and physio-biochemical compounds in potato leaves.

Identification of potassium phosphite responsive miRNAs and their targets in potato

Micro RNAs (miRNAs) are small single strand non-coding RNAs that regulate gene expression at the post-transcriptional level, either by translational inhibition or mRNA degradation based on the extent of complementarity between the miRNA and its target mRNAs. Potato (Solanum tuberosum L.) is the most important horticultural crop in Argentina. Achieving an integrated control of diseases is crucial for this crop, where frequent agrochemical applications, particularly fungicides, are carried out. A promising strategy is based on promoting induced resistance through the application of environmentally friendly compounds such as phosphites, inorganic salts of phosphorous acid. The use of phosphites in disease control management has proven to be effective. Although the mechanisms underlying their effect remain unclear, we postulated that miRNAs could be involved. Therefore we performed next generation sequencing (NGS) in potato leaves treated and non treated with potassium phosphite (KPhi). We identified 25 miRNAs that were expressed differentially, 14 already annotated in miRBase and 11 mapped to the potato genome as potential new miRNAs. A prediction of miRNA targets showed genes related to pathogen resistance, transcription factors, and oxidative stress. We also analyzed in silico stress and phytohormone responsive cis-acting elements on differentially expressed pre miRNAs. Despite the fact that some of the differentially expressed miRNAs have been already identified, this is to our knowledge the first report identifying miRNAs responsive to a biocompatible stress resistance inducer such as potassium phosphite, in plants. Further characterization of these miRNAs and their target genes might help to elucidate the molecular mechanisms underlying KPhi-induced resistance.

Biochemical responses of common bean to white mold potentiated by phosphites

Considering that the mechanisms for phosphite-afforded disease control remain elusive, this study investigated whether zinc (Zn) and copper (Cu) phosphites could possible potentiate common bean resistance to white mold, caused by Sclerotinia sclerotiorum, through the stimulation of biochemical defence responses. Lesion area and disease severity were decreased by phosphites spray, but Zn phosphite outcompeted Cu phosphite. Histopathological observations revealed fewer fungal hyphae and less collapse of the mesophyll cells in the Zn and Cu phosphite-sprayed plants compared to water-sprayed ones. The S. sclerotiorum-triggered accumulation of reactive oxygen species, oxalic acid (a fungal secreted toxin) and malondialdehyde (an indicator of cellular damage) were constrained as a result of Zn and Cu phosphites spray. Activities of antioxidant enzymes (superoxide dismutase, peroxidase, ascorbate peroxidase and glutathione-S-transferase at 12 h after inoculation (hai) and catalase at 60 and 84 hai) were higher for Zn and Cu phosphites-sprayed plants than for water-sprayed ones. Activities of defence-related enzymes chitinase (CHI) at 12 hai, β-1,3-glucanase (GLU) and polyphenoloxidase (PPO) were higher at 12-84 hai for Zn, and Cu phosphites sprayed plants, phenylalanine ammonia-lyase at 36-84 hai for the Zn phosphite sprayed ones, CHI at 12-36 hai, GLU at 12-60 hai, PPO at 36 hai and PAL and lipoxygenase at 12 hai for the Cu phosphite sprayed ones upon inoculation with S. sclerotiorum relative to their water-sprayed counterparts. Concentrations of total soluble phenols and lignin-thioglycolic acid derivatives were not affected by Cu phosphite spray on infected plants but were higher and lower, respectively, for Zn phosphite sprayed plants at 60 hai compared to water-sprayed ones. Taken together, the findings from the present study shed light on the biochemical defence mechanisms involved in the Zn and Cu phosphites-mediated suppression of white mold in common bean.

Translocation of phosphite encourages the protection against Phytophthora infestans in potato: The efficiency and efficacy

Phosphite (Phi)-based fungicides, such as the commercial product Phostrol™, are widely used in potato late blight control. However, the Phi translocation efficiency and the efficacy against pathogen are less discussed. In this study, the Phi concentration were quantified by high performance ion chromatography (HPIC) and the Phi translocation efficiency in potato tissues was evaluated using potato cultivar Russet Burbank with foliar application of the Phostrol solution both under greenhouse and field conditions. In the greenhouse trials, it was found that Phi was translocated from leaves to roots within 3 h and its concentration was significantly increased in the roots 24 h after the Phostrol application. In the field trials, the application rate of Phostrol affected the Phi translocation in potato tubers. To assess the efficacy of Phi against P. infestans, both the inhibition and infection tests were carried out. In the inhibition tests, three most common strains of P. infestans in Canada (US-8, US-23 and US-24) were inoculated on pea agar containing different levels of Phi. In the infection tests, both of detached leaves and whole tubers that received Phi were infected by the three strains of P. infestans. The in vitro tests indicated that the US-8 strain is the most tolerant whereas the US-23 strain is the most sensitive to Phi. Also, the in vivo tests demonstrated the dose-dependent translocation of Phi in potato leaves and tubers decreased the severity of infection by P. infestans. Moreover, potential defense mechanisms related to salicylic acid (SA) and jasmonic acid (JA) pathways that might be activated by Phi were also explored. Overall, the results of the study provided evidences that high Phi translocation efficiency encouraged late blight suppression in potato production.

Fertigation: Nutrition, Stimulation and Bioprotection of the Root in High Performance

Temperature changes, drought, frost, and the presence of pest and diseases place enormous stress on crops, which implies that the potential performance of these crops may be affected. One of the main goals for agronomists, horticulturists, growers, physiologists, soil scientists, geneticists, plant breeders, phytopathologists, and microbiologists is to increase the food production on the same cultivable area and to ensure that they are safe and of high quality. Understanding the biophysical changes in soil will help to manage the crop's ability to cope with biotic and abiotic stress. Optimization is needed in the nutrition of crops, which involves the use of biostimulants to counter oxidative stress and the management of strain bioformulations (bacteria and fungi) that protect and stimulate roots for the acquisition of nutrients. The implementation of these strategies in fertigation programs improves crop yields. This article addresses the importance of the stimulation and the bioprotection of the root as a fundamental pillar in ensuring the high performance of a crop.

Gene Profiling in Late Blight Resistance in Potato Genotype SD20

Late blight caused by the oomycete fungus Phytophthora infestans (Pi) is the most serious obstacle to potato (Solanum tuberosum) production in the world. A super race isolate, CN152, which was identified from Sichuan Province, China, could overcome nearly all known late blight resistance genes and caused serious damage in China. The potato genotype SD20 was verified to be highly resistant to CN152; however, the molecular regulation network underlying late blight resistance pathway remains unclear in SD20. Here, we performed a time-course experiment to systematically profile the late blight resistance response genes using RNA-sequencing in SD20. We identified 3354 differentially expressed genes (DEGs), which mainly encoded transcription factors and protein kinases, and also included four NBS-LRR genes. The late blight responsive genes showed time-point-specific induction/repression. Multi-signaling pathways of salicylic acid, jasmonic acid, and ethylene signaling pathways involved in resistance and defense against Pi in SD20. Gene Ontology and KEGG analyses indicated that the DEGs were significantly enriched in metabolic process, protein serine/threonine kinase activity, and biosynthesis of secondary metabolites. Forty-three DEGs were involved in immune response, of which 19 were enriched in hypersensitive response reaction, which could play an important role in broad-spectrum resistance to Pi infection. Experimental verification confirmed the induced expression of the responsive genes in the late blight resistance signaling pathway, such as WRKY, ERF, MAPK, and NBS-LRR family genes. Our results provided valuable information for understanding late blight resistance mechanism of potato.

Effect of potassium and manganese phosphites in the control of Pythium damping-off in soybean: a feasible alternative to fungicide seed treatments

BACKGROUND

Use of fungicide seed treatments for control of soybean soilborne diseases such as Pythium damping-off has increased worldwide. However, emergence of Pythium strains resistant to metalaxyl-M has prompted the need for alternative technologies to fungicides for damping-off control. The use of phosphites (Phis) has been proposed as a method to control oomycetes, but their use as seed treatments in soybean is limited by the lack of information on their efficacy. The effect of potassium (K) and manganese (Mn) Phis (as seed treatments) in the control of Pythium damping-off in soybean was evaluated in vitro and in vivo. In vitro, treated seeds and a control were placed on potato dextrose agar and the damping-off severity caused by Pythium aphanidermatum (Edson) Fitzpatrick, Pythium irregulare Buisman, and Pythium ultimum Trow was assessed 5 days after incubation using an ordinal scale. In vivo, treated seeds and a control were planted in polystyrene pots and emergence was evaluated 21 days after planting.

RESULTS

Analysis of the in vitro data using a multinomial generalized linear model showed that the probabilities of non-germinated, dead seeds ranged from 0.64 to 1.00 in the control and from 0 to 0.13 in the Phi treatments in each of the Pythium species. Probabilities of seed germination without or with damping-off symptoms were significantly higher for seeds treated with the Phi products than for the control. In the in vivo experiment, the Phi-based products increased seedling emergence by up to 29% on average compared with the untreated control.

CONCLUSION

Mn and K Phis are feasible alternatives as seed treatments to control Pythium damping-off in soybean. This study is the first, worldwide, to document the efficacy of K and Mn Phis in the control of soybean Pythium damping-off. © 2017 Society of Chemical Industry.

Botanicals and Phosphonate Show Potential to Replace Copper for Control of Potato Late Blight

Potato late blight (PLB) caused by Phytophthora infestans (Pi) is the most harmful disease in potato production worldwide. In organic farming, copper is used despite its persistence in soil and toxicity to soil organisms. To replace copper, suspensions of powders from three promising botanicals, including bark of buckthorn (Frangula alnus, FA), roots of medicinal rhubarb (Rheum palmatum) and galls of the nutgall tree (Galla chinensis), were tested in multi-year field experiments. The current study shows for the first time that botanicals could replace copper under field conditions and best PLB reduction on leaves was achieved with FA, reaching a level close to that of 2 to 3 kg copper per hectare and year. Better results than with copper were achieved with Phosfik® (Ph), a phosphonate-based product. For both FA and Ph, the mode of action is based on induced resistance, for Ph also on direct fungicidal effects. A disadvantage of Ph is the accumulation of residues in potato tubers. Nevertheless, two to three applications with 2 to 3 L/ha of Ph would be feasible to not exceed a minimal risk level (MLR) of 20 mg/kg of phosphorous acid as proposed by the European Food Safety Authority. Due to an excellent environmental profile and a complex mode of action counteracting Pi resistance, phosphonate-based products would be most suitable for sustainable PLB management in integrated pest management (IPM) programmes.

Reactive oxygen species accumulation and homeostasis are involved in plant immunity to an opportunistic fungal pathogen

Alternaria blight is a major and destructive disease of potato worldwide. In recent years, A. tenuissima is recognized as the most prevalent species of this phytopathogenic fungus in potato fields of Asian countries, which causes high yield losses every year. Any potato cultivar with complete resistance to this disease is not recognized, so far. Therefore, screening resistance levels of potatoes and identification of plant defense mechanisms against this fungus might be important for designing novel and effective disease management strategies for controlling the disease. In this research, the role of reactive oxygen species, antioxidants, lignin and phenolics in potato basal resistance to A. tenuissima was compared in the partially resistant Ramus and susceptible Bamba cultivars. Priming O₂^- and H₂O₂ production and enhanced activity of peroxidase (POX) and catalase (CAT) during interaction with A. tenuissima were observed in Ramus cultivar. Application of ROS generating systems and scavengers revealed critical role of O₂^- and H₂O₂ in potato defense, which was associated with lignification and phenolics production. More OH^- and lipid peroxidation in the susceptible Bamba compared to Ramus cultivar showed their negative effects on resistance. Priming the POX and CAT activity, in correlation with upregulation of the corresponding genes was observed in Ramus. The POX and CAT inhibitors increased disease progress, which was related with decreased lignification. This assay demonstrated not only POX-dependency of lignification, but also its dependence on CAT. However, POX had more importance than CAT in potato defense and in lignification. These findings highlight the function of ROS accumulation and homeostasis in potato resistance against A. tenuissima.

Phosphite shifts physiological and hormonal profile of Monterey pine and delays Fusarium circinatum progression

Fusarium circinatum is the causal agent of pitch canker disease affecting Pinus spp. and Pseudotsuga menziesii worldwide. Under strict quarantine measures, alternative approaches for disease control are necessary. Phosphite (Phi) salts are known for their fungicidal activity and as plant resistance elicitors; however, its potential is yet to be acknowledged in the Pinus-F. circinatum model. The main aim of this study was to assess whether the application of a Phi-based commercial formulation would delay the progression of the pitch canker on Pinus radiata plants, and on the in vitro fungal growth. In vitro assays were performed using different Phi concentrations (1% and 4%) and a non-treated control (0%), and repeated in vivo using inoculated and non-inoculated plants. Plant physiological parameters and hormonal content were evaluated. Phi was effective at inhibiting in vitro mycelial growth in a dose dependent manner. Regardless of fungal inoculation, Phi application induced positive effects on plant performance, despite phytotoxic effects found at 4%. Fusarium circinatum infection led to a reduction in gas exchange and chlorophyll fluorescence (Fv/Fm and φPSII), while proline and hormone (JA, ABA and SA) levels increased. Phi was effective in delaying disease symptom development in a dose dependent manner, concurrent with in vitro observations: gas exchange and chlorophyll fluorescence (Fv/Fm) were unaffected; proline, MDA and ABA decreased; electrolyte leakage and total soluble sugars increased. This suggests a direct (pathogen growth inhibition) and indirect (host defense priming) action of Phi, showing that Phi represents a potential strategy to control F. circinatum infection.

Plant Resistance Inducers against Pathogens in Solanaceae Species-From Molecular Mechanisms to Field Application

This review provides a current summary of plant resistance inducers (PRIs) that have been successfully used in the Solanaceae plant family to protect against pathogens by activating the plant's own defence. Solanaceous species include many important crops such as potato and tomato. We also present findings regarding the molecular processes after application of PRIs, even if the number of such studies still remains limited in this plant family. In general, there is a lack of patterns regarding the efficiency of induced resistance (IR) both between and within solanaceous species. In many cases, a hypersensitivity-like reaction needs to form in order for the PRI to be efficient. "-Omics" studies have already given insight in the complexity of responses, and can explain some of the differences seen in efficacy of PRIs between and within species as well as towards different pathogens. Finally, examples of field applications of PRIs for solanaceous crops are presented and discussed. We predict that PRIs will play a role in future plant protection strategies in Solanaceae crops if they are combined with other means of disease control in different spatial and temporal combinations.

Phosphite-induced reactive oxygen species production and ethylene and ABA biosynthesis, mediate the control of Phytophthora capsici in pepper (Capsicum annuum)

Phytophthora capsici is an oomycete pathogen with a broad host range that inflicts significant damage in vegetables. Phosphite (Phi) is used to control oomycete diseases, but the molecular mechanisms underlying Phi-induced resistance to P. capsici are unknown. Thus, Phi-inhibited mycelial growth on strain LT1534 and primed host defence were analysed. We demonstrated that Phi (>5µgmL-1) had a direct antibiotic effect on mycelial growth and zoospore production, and that mortality and DNA content were significantly reduced by pre-treatment with Phi. In addition, elevated hydrogen peroxide (H2O2) promoted callose deposition and increased the levels of soluble proteins and Capsicum annuum L. pathogenesis-related 1 (CaPR1) expression. Furthermore, Phi (1gL-1) significantly increased the transcription of the antioxidant enzyme genes, and the genes involved in ethylene (ET) and abscisic acid (ABA) biosynthesis, as well as mitogen-activated protein kinase (MAPK) cascades. However, pre-treatment with reactive oxygen species (ROS), ABA and ET biosynthesis inhibitors decreased Phi-induced resistance and reduced the expression of ABA-responsive 1 (CaABR1) and lipoxygenase 1 (CaLOX1). In addition, the decreased ROS and ABA inhibited Phi-induced expression of CaMPK17-1. We propose that Phi-induced ROS production, ET and ABA biosynthesis mediate the control of P. capsici, and that ABA functions through CaMPK17-1-mediated MAPK signalling.

Transcription factor StWRKY1 regulates phenylpropanoid metabolites conferring late blight resistance in potato

Quantitative resistance is polygenically controlled and durable, but the underlying molecular and biochemical mechanisms are poorly understood. Secondary cell wall thickening is a critical process in quantitative resistance, regulated by transcriptional networks. This paper provides compelling evidence on the functionality of StWRKY1 transcription factor, in a compatible interaction of potato-Phytophthora infestans, to extend our knowledge on the regulation of the metabolic pathway genes leading to strengthening the secondary cell wall. A metabolomics approach was used to identify resistance-related metabolites belonging to the phenylpropanoid pathway and their biosynthetic genes regulated by StWRKY1. The StWRKY1 gene in resistant potato was silenced to decipher its role in the regulation of phenylpropanoid pathway genes to strengthen the secondary cell wall. Sequencing of the promoter region of StWRKY1 in susceptible genotypes revealed the absence of heat shock elements (HSEs). Simultaneous induction of both the heat shock protein (sHSP17.8) and StWRKY1 following pathogen invasion enables functioning of the latter to interact with the HSE present in the resistant StWRKY1 promoter region. EMSA and luciferase transient expression assays further revealed direct binding of StWRKY1 to promoters of hydroxycinnamic acid amide (HCAA) biosynthetic genes encoding 4-coumarate:CoA ligase and tyramine hydroxycinnamoyl transferase. Silencing of the StWRKY1 gene was associated with signs of reduced late blight resistance by significantly increasing the pathogen biomass and decreasing the abundance of HCAAs. This study provides convincing evidence on the role of StWRKY1 in the regulation of downstream genes to biosynthesize HCAAs, which are deposited to reinforce secondary cell walls.

Differentiating phosphate-dependent and phosphate-independent systemic phosphate-starvation response networks in Arabidopsis thaliana through the application of phosphite

Phosphite is a less oxidized form of phosphorus than phosphate. Phosphite is considered to be taken up by the plant through phosphate transporters. It can mimic phosphate to some extent, but it is not metabolized into organophosphates. Phosphite could therefore interfere with phosphorus signalling networks. Typical physiological and transcriptional responses to low phosphate availability were investigated and the short-term kinetics of their reversion by phosphite, compared with phosphate, were determined in both roots and shoots of Arabidopsis thaliana. Phosphite treatment resulted in a strong growth arrest. It mimicked phosphate in causing a reduction in leaf anthocyanins and in the expression of a subset of the phosphate-starvation-responsive genes. However, the kinetics of the response were slower than for phosphate, which may be due to discrimination against phosphite by phosphate transporters PHT1;8 and PHT1;9 causing delayed shoot accumulation of phosphite. Transcripts encoding PHT1;7, lipid-remodelling enzymes such as SQD2, and phosphocholine-producing NMT3 were highly responsive to phosphite, suggesting their regulation by a direct phosphate-sensing network. Genes encoding components associated with the 'PHO regulon' in plants, such as At4, IPS1, and PHO1;H1, generally responded more slowly to phosphite than to phosphate, except for SPX1 in roots and MIR399d in shoots. Two uncharacterized phosphate-responsive E3 ligase genes, PUB35 and C3HC4, were also highly phosphite responsive. These results show that phosphite is a valuable tool to identify network components directly responsive to phosphate.

Potassium phosphite increases tolerance to UV-B in potato

The use of biocompatible chemical compounds that enhance plant disease resistance through Induced Resistance (IR) is an innovative strategy to improve the yield and quality of crops. Phosphites (Phi), inorganic salts of phosphorous acid, are environment friendly, and have been described to induce disease control. Phi, similar to other plant inductors, are thought to be effective against different types of biotic and abiotic stress, and it is assumed that the underlying signaling pathways probably overlap and interact. The signaling pathways triggered by UV-B radiation, for instance, are known to crosstalk with other signaling routes that respond that biotic stress. In the present work, the effect of potassium phosphite (KPhi) pre-treatment on UV-B stress tolerance was evaluated in potato leaves. Plants were treated with KPhi and, after 3 days, exposed to 2 h/day of UV-B (1.5 Watt m(-2)) for 0, 3 and 6 days. KPhi pre-treatment had a beneficial effect on two photosynthetic parameters, specifically chlorophyll content and expression of the psbA gene. Oxidative stress caused by UV-B was also prevented by KPhi. A decrease in the accumulation of hydrogen peroxide (H2O2) in leaves and an increase in guaiacol peroxidase (POD) and superoxide dismutase (SOD) activities were also observed. In addition, the expression levels of a gene involved in flavonoid synthesis increased in UV-B-stressed plants only when pre-treated with KPhi. Finally, accumulation of glucanases and chitinases was induced by UV-B stress and markedly potentiated by KPhi pre-treatment. Altogether, this is the first report that shows a contribution of KPhi in UV-B stress tolerance in potato plants.

Phenolic characterization and antimicrobial activity of folk medicinal plant extracts for their applications in olive production

Phytophthora spp is important in plant pathology due to the importance of the diseases it causes. In olive trees, severe damages are caused by the disease known as "dry branch" occasioned by Phytophthora nicotianae, P. citrophthora and P. palmivora. Much effort has been made to find efficient methods of control, with a low negative impact on environment. In this regard, treatment with plant extracts is a valid strategy. The aims of the present study are (i) to determine the polyphenol composition of extracts of Thymus vulgaris, Origanum vulgare, Matricaria recutita, and Larrea divaricata by CZE, (ii) correlate the analytical composition of these extracts with the inhibition on the mycelial growth, and (iii) determine the individual antimicrobial activity of the most active ingredients. A simple methodology was developed for the determination of catechin, naringenin, cinnamic acid, syringic acid, chlorogenic acid, apigenin, vanillic acid, luteolin, quercetin, and caffeic acid in plant extracts by CZE. The extraction of phenolic compounds in extract was performed by a miniaturized solid phase extraction using a home-made minicolumn packed with suitable filtering material (C18 , 50 mg). The optimized analyses conditions were: 30 mM boric acid buffer, pH 9.50; capillary, 57 cm full length, 50 cm effective length, 75 μm id, hydrodynamic injection 30 mbar, 2 s; 25 kV; 25°C, detection by UV absorbance at 290 nm. Sample results suggest that phenolic composition seems to have a great influence on inhibition of pathogens. The highest inhibitions of mycelial growth were observed for cinnamic acid and naringenin.

Phosphite-induced changes of the transcriptome and secretome in Solanum tuberosum leading to resistance against Phytophthora infestans

BACKGROUND

Potato late blight caused by the oomycete pathogen Phytophthora infestans can lead to immense yield loss. We investigated the transcriptome of Solanum tubersoum (cv. Desiree) and characterized the secretome by quantitative proteomics after foliar application of the protective agent phosphite. We also studied the distribution of phosphite in planta after application and tested transgenic potato lines with impaired in salicylic and jasmonic acid signaling.

RESULTS

Phosphite had a rapid and transient effect on the transcriptome, with a clear response 3 h after treatment. Strikingly this effect lasted less than 24 h, whereas protection was observed throughout all time points tested. In contrast, 67 secretome proteins predominantly associated with cell-wall processes and defense changed in abundance at 48 h after treatment. Transcripts associated with defense, wounding, and oxidative stress constituted the core of the phosphite response. We also observed changes in primary metabolism and cell wall-related processes. These changes were shown not to be due to phosphate depletion or acidification caused by phosphite treatment. Of the phosphite-regulated transcripts 40% also changed with β-aminobutyric acid (BABA) as an elicitor, while the defence gene PR1 was only up-regulated by BABA. Although phosphite was shown to be distributed in planta to parts not directly exposed to phosphite, no protection in leaves without direct foliar application was observed. Furthermore, the analysis of transgenic potato lines indicated that the phosphite-mediated resistance was independent of the plant hormones salicylic and jasmonic acid.

CONCLUSIONS

Our study suggests that a rapid phosphite-triggered response is important to confer long-lasting resistance against P. infestans and gives molecular understanding of its successful field applications.

Suppression of the auxin response pathway enhances susceptibility to Phytophthora cinnamomi while phosphite-mediated resistance stimulates the auxin signalling pathway

BACKGROUND

Phytophthora cinnamomi is a devastating pathogen worldwide and phosphite (Phi), an analogue of phosphate (Pi) is highly effective in the control of this pathogen. Phi also interferes with Pi starvation responses (PSR), of which auxin signalling is an integral component. In the current study, the involvement of Pi and the auxin signalling pathways in host and Phi-mediated resistance to P. cinnamomi was investigated by screening the Arabidopsis thaliana ecotype Col-0 and several mutants defective in PSR and the auxin response pathway for their susceptibility to this pathogen. The response to Phi treatment was also studied by monitoring its effect on Pi- and the auxin response pathways.

RESULTS

Here we demonstrate that phr1-1 (phosphate starvation response 1), a mutant defective in response to Pi starvation was highly susceptible to P. cinnamomi compared to the parental background Col-0. Furthermore, the analysis of the Arabidopsis tir1-1 (transport inhibitor response 1) mutant, deficient in the auxin-stimulated SCF (Skp1 - Cullin - F-Box) ubiquitination pathway was also highly susceptible to P. cinnamomi and the susceptibility of the mutants rpn10 and pbe1 further supported a role for the 26S proteasome in resistance to P. cinnamomi. The role of auxin was also supported by a significant (P < 0.001) increase in susceptibility of blue lupin (Lupinus angustifolius) to P. cinnamomi following treatment with the inhibitor of auxin transport, TIBA (2,3,5-triiodobenzoic acid). Given the apparent involvement of auxin and PSR signalling in the resistance to P. cinnamomi, the possible involvement of these pathways in Phi mediated resistance was also investigated. Phi (especially at high concentrations) attenuates the response of some Pi starvation inducible genes such as AT4, AtACP5 and AtPT2 in Pi starved plants. However, Phi enhanced the transcript levels of PHR1 and the auxin responsive genes (AUX1, AXR1and AXR2), suppressed the primary root elongation, and increased root hair formation in plants with sufficient Pi.

CONCLUSIONS

The auxin response pathway, particularly auxin sensitivity and transport, plays an important role in resistance to P. cinnamomi in Arabidopsis, and phosphite-mediated resistance may in some part be through its effect on the stimulation of the PSR and auxin response pathways.

Differential accumulation of Phytophthora cambivora cox II gene transcripts in infected chestnut tissue

This study provides a novel qRT-PCR protocol for specific detection and proof of viability of Phytophthora in environmental samples based on differential accumulation of cox II transcripts. Chemical and physical treatments were tested for their ability to induce in vitro the accumulation of cytochrome oxidase genes encoding subunits II (cox II) transcripts in Phytophthora cambivora. Glucose 170 mM, KNO3 0.25 mM and K3 PO3 0.5 and 0.8 mM induced the transcription of cox II in P. cambivora living mycelium while no transcription was observed in mycelium previously killed with 0.5% (p/v) RidomilGold(®) R WG. Living chestnut tissue was artificially infected with P. cambivora and treated with inducers. In vivo experiments confirmed the ability of glucose to induce the accumulation of P. cambivora cox II transcripts. Based on these results, pretreatment of environmental samples with glucose prior to nucleic acid extraction increased the accumulation of specific cox II transcripts, and therefore the sensitivity of qRT-PCR assay for detection of P. cambivora in living tissues. Furthermore, differential accumulation of transcripts between treated and untreated samples represents an unequivocal proof of inoculum viability.

Phosphite protects Fagus sylvatica seedlings towards Phytophthora plurivora via local toxicity, priming and facilitation of pathogen recognition

Phytophthora plurivora causes severe damage on Fagus sylvatica and is responsible for the extensive decline of European Beech throughout Europe. Unfortunately, no effective treatment against this disease is available. Phosphite (Phi) is known to protect plants against Phytophthora species; however, its mode of action towards P. plurivora is still unknown. To discover the effect of Phi on root infection, leaves were sprayed with Phi and roots were subsequently inoculated with P. plurivora zoospores. Seedling physiology, defense responses, colonization of root tissue by the pathogen and mortality were monitored. Additionally the Phi concentration in roots was quantified. Finally, the effect of Phi on mycelial growth and zoospore formation was recorded. Phi treatment was remarkably efficient in protecting beech against P. plurivora; all Phi treated plants survived infection. Phi treated and infected seedlings showed a strong up-regulation of several defense genes in jasmonate, salicylic acid and ethylene pathways. Moreover, all physiological parameters measured were comparable to control plants. The local Phi concentration detected in roots was high enough to inhibit pathogen growth. Phi treatment alone did not harm seedling physiology or induce defense responses. The up-regulation of defense genes could be explained either by priming or by facilitation of pathogen recognition of the host.

Proteomics analysis suggests broad functional changes in potato leaves triggered by phosphites and a complex indirect mode of action against Phytophthora infestans

Phosphite (salts of phosphorous acid; Phi)-based fungicides are increasingly used in controlling oomycete pathogens, such as the late blight agent Phytophthora infestans. In plants, low amounts of Phi induce pathogen resistance through an indirect mode of action. We used iTRAQ-based quantitative proteomics to investigate the effects of phosphite on potato plants before and after infection with P. infestans. Ninety-three (62 up-regulated and 31 down-regulated) differentially regulated proteins, from a total of 1172 reproducibly identified proteins, were identified in the leaf proteome of Phi-treated potato plants. Four days post-inoculation with P. infestans, 16 of the 31 down-regulated proteins remained down-regulated and 42 of the 62 up-regulated proteins remained up-regulated, including 90% of the defense proteins. This group includes pathogenesis-related, stress-responsive, and detoxification-related proteins. Callose deposition and ultrastructural analyses of leaf tissues after infection were used to complement the proteomics approach. This study represents the first comprehensive proteomics analysis of the indirect mode of action of Phi, demonstrating broad effects on plant defense and plant metabolism. The proteomics data and the microscopy study suggest that Phi triggers a hypersensitive response that is responsible for induced resistance of potato leaves against P. infestans.

BIOLOGICAL SIGNIFICANCE

Phosphie triggers complex functional changes in potato leaves that are responsible for the induced resistance against Phytophthora infestans. This article is part of a Special Issue entitled: Translational Plant Proteomics.