Cytological Analysis of Defense-Related Mechanisms Induced in Pea Root Tissues in Response to Colonization by Nonpathogenic Fusarium oxysporum Fo47

Resistance strategies for defense against Albugo candida causing white rust disease

Albugo candida, the causal organism of white rust, is an oomycete obligate pathogen infecting crops of Brassicaceae family occurred on aerial part, including vegetable and oilseed crops at all growth stages. The disease expression is characterized by local infection appearing on the abaxial region developing white or creamy yellow blister (sori) on leaves and systemic infections cause hypertrophy and hyperplasia leading to stag-head of reproductive organ. To overcome this problem, several disease management strategies like fungicide treatments were used in the field and disease-resistant varieties have also been developed using conventional and molecular breeding. Due to high variability among A. candida isolates, there is no single approach available to understand the diverse spectrum of disease symptoms. In absence of resistance sources against pathogen, repetitive cultivation of genetically-similar varieties locally tends to attract oomycete pathogen causing heavy yield losses. In the present review, a deep insight into the underlying role of the non-host resistance (NHR) defence mechanism available in plants, and the strategies to exploit available gene pools from plant species that are non-host to A. candida could serve as novel sources of resistance. This work summaries the current knowledge pertaining to the resistance sources available in non-host germ plasm, the understanding of defence mechanisms and the advance strategies covers molecular, biochemical and nature-based solutions in protecting Brassica crops from white rust disease.

Differential Colonization of the Plant Vasculature Between Endophytic Versus Pathogenic Fusarium oxysporum Strains

Plant xylem colonization is the hallmark of vascular wilt diseases caused by phytopathogens within the Fusarium oxysporum species complex. Recently, xylem colonization has also been reported among endophytic F. oxysporum strains, resulting in some uncertainty. This study compares xylem colonization processes by pathogenic versus endophytic strains in Arabidopsis thaliana and Solanum lycopersicum, using Arabidopsis pathogen Fo5176, tomato pathogen Fol4287, and the endophyte Fo47, which can colonize both plant hosts. We observed that all strains were able to advance from epidermis to endodermis within 3 days postinoculation (dpi) and reached the root xylem at 4 dpi. However, this shared progression was restricted to lateral roots and the elongation zone of the primary root. Only pathogens reached the xylem above the primary-root maturation zone (PMZ). Related to the distinct colonization patterns, we also observed stronger induction of callose at the PMZ and lignin deposition at primary-lateral root junctions by the endophyte in both plants. This observation was further supported by stronger induction of Arabidopsis genes involved in callose and lignin biosynthesis during the endophytic colonization (Fo47) compared with the pathogenic interaction (Fo5176). Moreover, both pathogens encode more plant cell wall-degrading enzymes than the endophyte Fo47. Therefore, observed differences in callose and lignin deposition could be the combination of host production and the subsequent fungal degradation. In summary, this study demonstrates spatial differences between endophytic and pathogenic colonization, strongly suggesting that further investigations of molecular arm-races are needed to understand how plants differentiate friend from foe. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Pea Breeding for Resistance to Rhizospheric Pathogens

Pea (Pisum sativum L.) is a grain legume widely cultivated in temperate climates. It is important in the race for food security owing to its multipurpose low-input requirement and environmental promoting traits. Pea is key in nitrogen fixation, biodiversity preservation, and nutritional functions as food and feed. Unfortunately, like most crops, pea production is constrained by several pests and diseases, of which rhizosphere disease dwellers are the most critical due to their long-term persistence in the soil and difficulty to manage. Understanding the rhizosphere environment can improve host plant root microbial association to increase yield stability and facilitate improved crop performance through breeding. Thus, the use of various germplasm and genomic resources combined with scientific collaborative efforts has contributed to improving pea resistance/cultivation against rhizospheric diseases. This improvement has been achieved through robust phenotyping, genotyping, agronomic practices, and resistance breeding. Nonetheless, resistance to rhizospheric diseases is still limited, while biological and chemical-based control strategies are unrealistic and unfavourable to the environment, respectively. Hence, there is a need to consistently scout for host plant resistance to resolve these bottlenecks. Herein, in view of these challenges, we reflect on pea breeding for resistance to diseases caused by rhizospheric pathogens, including fusarium wilt, root rots, nematode complex, and parasitic broomrape. Here, we will attempt to appraise and harmonise historical and contemporary knowledge that contributes to pea resistance breeding for soilborne disease management and discuss the way forward.

Pathogen Adaptation to the Xylem Environment

A group of aggressive pathogens have evolved to colonize the plant xylem. In this vascular tissue, where water and nutrients are transported from the roots to the rest of the plant, pathogens must be able to thrive under acropetal xylem sap flow and scarcity of nutrients while having direct contact only with predominantly dead cells. Nevertheless, a few bacteria have adapted to exclusively live in the xylem, and various pathogens may colonize other plant niches without causing symptoms unless they reach the xylem. Once established, the pathogens modulate its physicochemical conditions to enhance their growth and virulence. Adaptation to the restrictive lifestyle of the xylem leads to genome reduction in xylem-restricted bacteria, as they have a higher proportion of pseudogenes in their genome. The basis of xylem adaptation is not completely understood; therefore, a need still exists for model systems to advance the knowledge on this topic.

Molecular insights into the jasmonate signaling and associated defense responses against wilt caused by Fusarium oxysporum

Biotic and abiotic stress factors drastically limit plant growth and development as well as alter the physiological, biochemical and cellular processes. This negatively impacts plant productivity, ultimately leading to agricultural and economical loss. Plant defense mechanisms elicited in response to these stressors are crucially regulated by the intricate crosstalk between defense hormones such as jasmonic acid (JA), salicylic acid and ethylene. These hormones orchestrate adaptive responses by modulating the gene regulatory networks leading to sequential changes in the root architecture, cell wall composition, secondary metabolite production and expression of defense-related genes. Fusarium wilt is a widespread vascular disease in plants caused by the soil-borne ascomycete Fusarium oxysporum and is known to attack several economically important plant cultivars. JA along with its conjugated forms methyl jasmonate and jasmonic acid isoleucine critically tunes plant defense mechanisms by regulating the expression of JA-associated genes imparting resistance phenotype. However, it should be noted that some members of F. oxysporum utilize the JA signaling pathway for disease development leading to susceptibility in plants. Therefore, JA signaling pathway becomes one of the important targets amenable for modulation to develop resistance response against Fusarium wilt in plants. In this review, we have emphasized on the physiological and molecular aspects of JA and its significant role in mounting an early defense response against Fusarium wilt disease. Further, utilization of the inherent JA signaling pathway and/or exogenous application of JA in generating Fusarium wilt resistant plants is discussed.

Comparative transcriptomic analysis of races 1, 2, 5 and 6 of Fusarium oxysporum f.sp. pisi in a susceptible pea host identifies differential pathogenicity profiles

BACKGROUND

The fungal pathogen Fusarium oxysporum f.sp. pisi (Fop) causes Fusarium wilt in peas. There are four races globally: 1, 2, 5 and 6 and all of these races are present in Australia. Molecular infection mechanisms have been studied in a few other F. oxysporum formae speciales; however, there has been no transcriptomic Fop-pea pathosystem study.

RESULTS

A transcriptomic study was carried out to understand the molecular pathogenicity differences between the races. Transcriptome analysis at 20 days post-inoculation revealed differences in the differentially expressed genes (DEGs) in the Fop races potentially involved in fungal pathogenicity variations. Most of the DEGs in all the races were engaged in transportation, metabolism, oxidation-reduction, translation, biosynthetic processes, signal transduction, proteolysis, among others. Race 5 expressed the most virulence-associated genes. Most genes encoding for plant cell wall degrading enzymes, CAZymes and effector-like proteins were expressed in race 2. Race 6 expressed the least number of genes at this time point.

CONCLUSION

Fop races deploy various factors and complex strategies to mitigate host defences to facilitate colonisation. This investigation provides an overview of the putative pathogenicity genes in different Fop races during the necrotrophic stage of infection. These genes need to be functionally characterised to confirm their pathogenicity/virulence roles and the race-specific genes can be further explored for molecular characterisation.

Metatranscriptomic Comparison of Endophytic and Pathogenic Fusarium-Arabidopsis Interactions Reveals Plant Transcriptional Plasticity

Plants are continuously exposed to beneficial and pathogenic microbes, but how plants recognize and respond to friends versus foes remains poorly understood. Here, we compared the molecular response of Arabidopsis thaliana independently challenged with a Fusarium oxysporum endophyte Fo47 versus a pathogen Fo5176. These two F. oxysporum strains share a core genome of about 46 Mb, in addition to 1,229 and 5,415 unique accessory genes. Metatranscriptomic data reveal a shared pattern of expression for most plant genes (about 80%) in responding to both fungal inoculums at all timepoints from 12 to 96 h postinoculation (HPI). However, the distinct responding genes depict transcriptional plasticity, as the pathogenic interaction activates plant stress responses and suppresses functions related to plant growth and development, while the endophytic interaction attenuates host immunity but activates plant nitrogen assimilation. The differences in reprogramming of the plant transcriptome are most obvious in 12 HPI, the earliest timepoint sampled, and are linked to accessory genes in both fungal genomes. Collectively, our results indicate that the A. thaliana and F. oxysporum interaction displays both transcriptome conservation and plasticity in the early stages of infection, providing insights into the fine-tuning of gene regulation underlying plant differential responses to fungal endophytes and pathogens.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Number of Candidate Effector Genes in Accessory Genomes Differentiates Pathogenic From Endophytic Fusarium oxysporum Strains

The fungus Fusarium oxysporum (Fo) is widely known for causing wilt disease in over 100 different plant species. Endophytic interactions of Fo with plants are much more common, and strains pathogenic on one plant species can even be beneficial endophytes on another species. However, endophytic and beneficial interactions have been much less investigated at the molecular level, and the genetic basis that underlies endophytic versus pathogenic behavior is unknown. To investigate this, 44 Fo strains from non-cultivated Australian soils, grass roots from Spain, and tomato stems from United States were characterized genotypically by whole genome sequencing, and phenotypically by examining their ability to symptomlessly colonize tomato plants and to confer resistance against Fusarium Wilt. Comparison of the genomes of the validated endophytic Fo strains with those of 102 pathogenic strains revealed that both groups have similar genomes sizes, with similar amount of accessory DNA. However, although endophytic strains can harbor homologs of known effector genes, they have typically fewer effector gene candidates and associated non-autonomous transposons (mimps) than pathogenic strains. A pathogenic 'lifestyle' is associated with extended effector gene catalogs and a set of "host specific" effectors. No candidate effector genes unique to endophytic strains isolated from the same plant species were found, implying little or no host-specific adaptation. As plant-beneficial interactions were observed to be common for the tested Fo isolates, the propensity for endophytism and the ability to confer biocontrol appears to be a predominant feature of this organism. These findings allow prediction of the lifestyle of a Fo strain based on its genome sequence as a potential pathogen or as a harmless or even beneficial endophyte by determining its effectorome and mimp number.

Biocontrol by Fusarium oxysporum Using Endophyte-Mediated Resistance

Interactions between plants and the root-colonizing fungus Fusarium oxysporum (Fo) can be neutral, beneficial, or detrimental for the host. Fo is infamous for its ability to cause wilt, root-, and foot-rot in many plant species, including many agronomically important crops. However, Fo also has another face; as a root endophyte, it can reduce disease caused by vascular pathogens such as Verticillium dahliae and pathogenic Fo strains. Fo also confers protection to root pathogens like Pythium ultimum, but typically not to pathogens attacking above-ground tissues such as Botrytis cinerea or Phytophthora capsici. Endophytes confer biocontrol either directly by interacting with pathogens via mycoparasitism, antibiosis, or by competition for nutrients or root niches, or indirectly by inducing resistance mechanisms in the host. Fo endophytes such as Fo47 and CS-20 differ from Fo pathogens in their effector gene content, host colonization mechanism, location in the plant, and induced host-responses. Whereas endophytic strains trigger localized cell death in the root cortex, and transiently induce immune signaling and papilla formation, these responses are largely suppressed by pathogenic Fo strains. The ability of pathogenic strains to compromise immune signaling and cell death is likely attributable to their host-specific effector repertoire. The lower number of effector genes in endophytes as compared to pathogens provides a means to distinguish them from each other. Co-inoculation of a biocontrol-conferring Fo and a pathogenic Fo strain on tomato reduces disease, and although the pathogen still colonizes the xylem vessels this has surprisingly little effect on the xylem sap proteome composition. In this tripartite interaction the accumulation of just two PR proteins, NP24 (a PR-5) and a β-glucanase, was affected. The Fo-induced resistance response in tomato appears to be distinct from induced systemic resistance (ISR) or systemic acquired resistance (SAR), as the phytohormones jasmonate, ethylene, and salicylic acid are not required. In this review, we summarize our molecular understanding of Fo-induced resistance in a model and identify caveats in our knowledge.

Differences in Production, Composition, and Antioxidant Activities of Exopolymeric Substances (EPS) Obtained from Cultures of Endophytic Fusarium culmorum Strains with Different Effects on Cereals

Exopolymeric substances (EPS) can determine plant-microorganism interactions and have great potential as bioactive compounds. The different amounts of EPS obtained from cultures of three endophytic Fusarium culmorum strains with different aggressiveness-growth promoting (PGPF), deleterious (DRMO), and pathogenic towards cereal plants-depended on growth conditions. The EPS concentrations (under optimized culture conditions) were the lowest (0.2 g/L) in the PGPF, about three times higher in the DRMO, and five times higher in the pathogen culture. The EPS of these strains differed in the content of proteins, phenolic components, total sugars, glycosidic linkages, and sugar composition (glucose, mannose, galactose, and smaller quantities of arabinose, galactosamine, and glucosamine). The pathogen EPS exhibited the highest total sugar and mannose concentration. FTIR analysis confirmed the β configuration of the sugars. The EPS differed in the number and weight of polysaccharidic subfractions. The EPS of PGPF and DRMO had two subfractions and the pathogen EPS exhibited a subfraction with the lowest weight (5 kDa). The three EPS preparations (ethanol-precipitated EP, crude C, and proteolysed P) had antioxidant activity (particularly high for the EP-EPS soluble in high concentrations). The EP-EPS of the PGPF strain had the highest antioxidant activity, most likely associated with the highest content of phenolic compounds in this EPS.

Evaluation of the Biocontrol Potential of Endophytic Fungus Fusarium solani CEF559 against Verticillium dahliae in Cotton Plant

Verticillium wilt of cotton, caused by the soilborne pathogen Verticillium dahliae, is one of the most serious diseases of cotton worldwide. Increased concerns about the side effects of synthetic pesticides have resulted in greater interest in developing biocontrol strategies against Verticillium wilt. We evaluated a Fusarium solani CEF559 isolate, obtained from the endosphere of healthy cotton plants, for its biocontrol potential against V. dahliae in vitro and in vivo. In addition to disease assessment, three key genes in the lignin metabolism pathway and four pathogenesis-related (PR) genes were monitored using qRT-PCR. In the laboratory tests, F. solani CEF559 inhibited V. dahliae colony growth by 75% and sporulation by nearly 80% and completely suppressed conidial production. However, volatile metabolites from CEF559 did not affect V. dahliae colony growth. In the greenhouse study, CEF559 significantly reduced wilt development, with a control efficacy greater than 60% when assessed 25 days postinoculation. In a field experiment, CEF559 reduced wilt development, with the efficacy ranting from 30.1% to 56.3%. PR genes and those key genes in the lignin metabolism pathway were transiently upregulated in the cotton roots pretreated with CEF559 when subsequently inoculated with V. dahliae, compared with those plants inoculated with V. dahliae only. Moreover, CEF559 inhibited V. dahliae colonisation of both the roots and hypocotyls. The present results suggest that this cotton endophytic fungal strain, F. solani CEF559, confers protection against V. dahliae.

Endophyte-Mediated Resistance in Tomato to Fusarium oxysporum Is Independent of ET, JA, and SA

Root endophytes can confer resistance against plant pathogens by direct antagonism or via the host by triggering induced resistance. The latter response typically relies on jasmonic acid (JA)/ethylene (ET)-depended signaling pathways, but can also be triggered via salicylic acid (SA)-dependent signaling pathways. Here, we set out to determine if endophyte-mediated resistance (EMR), conferred by the Fusarium endophyte Fo47, against Fusarium wilt disease in tomato is mediated via SA, ET or JA. To test the contribution of SA, ET, and JA in EMR we performed bioassays with Fo47 and Fusarium oxysporum f. sp. lycopersici in tomato plants impaired in SA accumulation (NahG), JA biosynthesis (def1) or ET-production (ACD) and -sensing (Nr). We observed that the colonization pattern of Fo47 in stems of wildtype plants was indistinguishable from that of the hormone mutants. Surprisingly, EMR was not compromised in the lines affected in JA, ET, or SA signaling. The independence of EMR on SA, JA, and ET implies that this induced resistance-response against Fusarium wilt disease is distinct from the classical Induced Systemic Resistance (ISR) response, providing exciting possibilities for control of wilt diseases independent of conventional defense pathways.

Physical and Chemical Barriers in Root Tissues Contribute to Quantitative Resistance to Fusarium oxysporum f. sp. pisi in Pea

Fusarium wilt caused by Fusarium oxysporum f. sp. pisi (Fop) is one of the most destructive diseases of pea worldwide. Control of this disease is difficult and it is mainly based on the use of resistant cultivars. While monogenic resistance has been successfully used in the field, it is at risk of breakdown by the constant evolution of the pathogen. New sources of quantitative resistance have been recently identified from a wild relative Pisum spp. collection. Here, we characterize histologically the resistance mechanisms occurring in these sources of quantitative resistance. Detailed comparison, of the reaction at cellular level, of eight pea accessions with differential responses to Fop race 2, showed that resistant accessions established several barriers at the epidermis, exodermis, cortex, endodermis and vascular stele efficiently impeding fungal progression. The main components of these different barriers were carbohydrates and phenolic compounds including lignin. We found that these barriers were mainly based on three defense mechanisms including cell wall strengthening, formation of papilla-like structures at penetration sites and accumulation of different substances within and between cells. These defense reactions varied in intensity and localization between resistant accessions. Our results also clarify some steps of the infection process of F. oxysporum in plant and support the important role of cell wall-degrading enzymes in F. oxysporum pathogenicity.

Combined action of the major secreted exo- and endopolygalacturonases is required for full virulence of Fusarium oxysporum

The genome of the tomato pathogen Fusarium oxysporum f. sp. lycopersici encodes eight different polygalacturonases (PGs): four endoPGs and four exoPGs. Quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR) revealed that endoPGs pg1 and pg5 and exoPGs pgx4 and pgx6 are expressed at significant levels during growth on citrus pectin, polygalacturonic acid or the monomer galacturonic acid, as well as during the infection of tomato plants. The remaining PG genes exhibit low expression levels under all the conditions tested. Secreted PG activity was decreased significantly during growth on pectin in the single deletion mutants lacking either pg1 or pgx6, as well as in the double mutant. Although the single deletion mutants did not display a significant virulence reduction on tomato plants, the Δpg1Δpgx6 double mutant was significantly attenuated in virulence. The combined action of exoPGs and endoPGs is thus essential for plant infection by the vascular wilt fungus F. oxysporum.

Highly diverse endophytic and soil Fusarium oxysporum populations associated with field-grown tomato plants

The diversity and genetic differentiation of populations of Fusarium oxysporum associated with tomato fields, both endophytes obtained from tomato plants and isolates obtained from soil surrounding the sampled plants, were investigated. A total of 609 isolates of F. oxysporum were obtained, 295 isolates from a total of 32 asymptomatic tomato plants in two fields and 314 isolates from eight soil cores sampled from the area surrounding the plants. Included in this total were 112 isolates from the stems of all 32 plants, a niche that has not been previously included in F. oxysporum population genetics studies. Isolates were characterized using the DNA sequence of the translation elongation factor 1α gene. A diverse population of 26 sequence types was found, although two sequence types represented nearly two-thirds of the isolates studied. The sequence types were placed in different phylogenetic clades within F. oxysporum, and endophytic isolates were not monophyletic. Multiple sequence types were found in all plants, with an average of 4.2 per plant. The population compositions differed between the two fields but not between soil samples within each field. A certain degree of differentiation was observed between populations associated with different tomato cultivars, suggesting that the host genotype may affect the composition of plant-associated F. oxysporum populations. No clear patterns of genetic differentiation were observed between endophyte populations and soil populations, suggesting a lack of specialization of endophytic isolates.

Analysis of the defence-related mechanism in cucumber seedlings in relation to root colonization by nonpathogenic Fusarium oxysporum CS-20

A defence response can be induced by nonpathogenic Fusarium oxysporum CS-20 in several crops, but the molecular mechanism has not been clearly demonstrated. In the present study, we analysed the defence mechanism of a susceptible cucumber cultivar (Cucumis sativus L. 9930) against a pathogen (F. oxysporum f. sp. cucumerinum) through the root precolonization of CS-20. A challenge inoculation assay indicated that the disease severity index (DSI) was reduced, ranging from 18.83 to 61.67 in comparison with the pathogen control. Root colonization analysis indicated that CS-20 clearly did not appear to influence the growth of cucumber seedlings. Quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) revealed that CS-20-mediated defence response was activated by PR3, LOX1 and PAL1 and the pathogen-mediated resistance response was regulated by PR1 and PR3. Moreover, both nonpathogenic and pathogenic F. oxysporum were able to upregulate NPR1 expression. In contrast to a pathogen, CS-20 can activate the Ca(2+) /CaM signal transduction pathway, and the gene expression of both CsCam7 and CsCam12 increased significantly. The gene expression analysis indicated that CS-20 strongly enhanced the expression of PR3, LOX1, PAL1, NPR1, CsCam7 and CsCam12 after inoculation. Overall, the defence response induced by CS-20 can be controlled by multiple genes in the cucumber plant.

Biological control of plant pathogens: advantages and limitations seen through the case study of Pythium oligandrum

The management of certain plant beneficial microorganisms [biological control agents (BCAs)] seems to be a promising and environmental friendly method to control plant pathogens. However, applications are still limited because of the lack of consistency of BCAs when they are applied in the field. In the present paper, the advantages and limitations of BCAs are seen through the example of Pythium oligandrum, an oomycete that has received much attention in the last decade. The biological control exerted by P. oligandrum is the result of a complex process, which includes direct effects through the control of pathogens and/or indirect effects mediated by P. oligandrum, i.e. induction of resistance and growth promotion. P. oligandrum antagonism is a multifaceted and target fungus-dependent process. Interestingly, it does not seem to disrupt microflora biodiversity on the roots. P. oligandrum has an atypical relationship with the plant because it rapidly penetrates into the root tissues but it cannot stay alive in planta. After root colonisation, because of the elicitation by P. oligandrum of the plant-defence system, plants are protected from a range of pathogens. The management of BCAs, here P. oligandrum, is discussed with regard to its interactions with the incredibly complex agrosystems.

Chitin synthase-deficient mutant of Fusarium oxysporum elicits tomato plant defence response and protects against wild-type infection

A mutant of the root pathogen Fusarium oxysporum f. sp. lycopersici, deficient in class V chitin synthase, has been shown previously to be nonvirulent. In this study, we tested the hypothesis that the cause of its avirulence could be the elicitation of the induced plant defence response, leading to the restriction of fungal infection. Co-inoculation of tomato plants with the wild-type strain and the DeltachsV mutant resulted in a significant reduction in symptom development, supporting a protective mechanism exerted by the mutant. The ability of the mutant to penetrate and colonize plant tissues was determined by scanning and transmission electron microscopy, as well as fluorescence microscopy using green fluorescent protein- or cherry fluorescent protein-labelled fungal strains. The extent of wild-type strain colonization in co-inoculated plants decreased steadily throughout the infection process, as shown by the quantification of fungal biomass using real-time polymerase chain reaction. The hypothesis that defence responses are activated by the DeltachsV mutant was confirmed by the analysis of plant pathogenesis-related genes using real-time reverse transcriptase-polymerase chain reaction. Tomato plants inoculated with the DeltachsV mutant showed a three fold increase in endochitinase activity in comparison with wild-type inoculated plants. Taken together, these results suggest that the perturbation of fungal cell wall biosynthesis results in elicitation of the plant defence response during the infection process.

Microbiological control of soil-borne phytopathogenic fungi with special emphasis on wilt-inducing Fusarium oxysporum

Plant diseases induced by soil-borne plant pathogens are among the most difficult to control. In the absence of effective chemical control methods, there is renewed interest in biological control based on application of populations of antagonistic micro-organisms. In addition to Pseudomonas spp. and Trichoderma spp., which are the two most widely studied groups of biological control agents, the protective strains of Fusarium oxysporum represent an original model. These protective strains of F. oxysporum can be used to control wilt induced by pathogenic strains of the same species. Exploring the mechanisms involved in the protective capability of these strains is not only necessary for their development as commercial biocontrol agents but raises many basic questions related to the determinism of pathogenicity versus biocontrol capacity in the F. oxysporum species complex. In this paper, current knowledge regarding the interaction between the plant and the protective strains is reviewed in comparison with interactions between the plant and pathogenic strains. The success of biological control depends not only on plant-microbial interactions but also on the ecological fitness of the biological control agents.

Endomembrane system of aspen root cells plays a key role in defense against a common fungal root endophyte, Cryptosporiopsis radicicola

The host-endophyte interaction between roots of aspen (Populus tremuloides) and Cryptosporiopsis radicicola was examined primarily by transmission electron microscopy. Hyphae growing on the exterior of the inoculated roots had a thick, electron-dense, adhesive sheath. At hyphal contact and penetration, host epidermal cells exhibited a series of defense responses (viz. formation of papillae and partition walls, general wall thickening and walling-off of internal hyphae). In papilla formation, loop-shaped, rough endoplasmic reticula (rER) gave rise to globose secretory vesicles that accumulated around and then fused to the developing papilla. Unlike papillae, general wall thickening was associated with the Golgi apparatus (GA) that produced cell wall materials; 1-3 layers of Golgi cisternae were in contact with or in the immediate proximity (mostly within 0-0.5 microm) of and lying parallel to the host cell wall, where they budded out numerous subglobose vesicles that fused directly to the host cell wall and made it thicker. Partition wall formation and walling-off of internal hyphae also were common; the former was associated with an extended single cisterna, which was indistinguishable from rER or individual cisternae of GA, and in the latter phenomenon internal hyphae were encased by electron-dense material containing numerous ribosomes and membranous elements that were derived apparently from proliferated rER. These pronounced defense responses protected the stele and contributed to making C. radicicola endophytic rather than pathogenic.

Colonization of barley roots by endophytic fungi and their reduction of take-all caused by Gaeumannomyces graminis var. tritici

Fungal root endophytes obtained from natural vegetation were tested for antifungal activity in dual culture tests against the root pathogen Gaeumannomyces graminis var. tritici. Fifteen isolates, including Acremonium blochii, Acremonium furcatum, Aspergillus fumigatus, Cylindrocarpon sp., Cylindrocarpon destructans, Dactylaria sp., Fusarium equiseti, Phoma herbarum, Phoma leveillei, and a sterile mycelium, selected based on the dual culture test, were inoculated on barley roots in growth tubes under axenic conditions, both in the absence and presence of G. graminis var. tritici. All isolates colonized the rhizosphere and very often the root cortex without causing disease symptoms and without affecting plant growth. Eight isolates significantly reduced the symptoms caused by G. graminis var. tritici, and 6 of them reduced its presence in the roots.

Screenhouse and field persistence of nonpathogenic endophytic Fusarium oxysporum in Musa tissue culture plants

Two major biotic constraints to highland cooking banana (Musa spp., genome group AAA-EA) production in Uganda are the banana weevil Cosmopolites sordidus and the burrowing nematode Radopholus similis. Endophytic Fusarium oxysporum strains inoculated into tissue culture banana plantlets have shown control of the banana weevil and the nematode. We conducted screenhouse and field experiments to investigate persistence in the roots and rhizome of two endophytic Fusarium oxysporum strains, V2w2 and III4w1, inoculated into tissue-culture banana plantlets of highland cooking banana cultivars Kibuzi and Nabusa. Re-isolation of F. oxysporum showed that endophyte colonization decreased faster from the rhizomes than from the roots of inoculated plants, both in the screenhouse and in the field. Whereas rhizome colonization by F. oxysporum decreased in the screenhouse (4-16 weeks after inoculation), root colonization did not. However, in the field (17-33 weeks after inoculation), a decrease was observed in both rhizome and root colonization. The results show a better persistence in the roots than rhizomes of endophytic F. oxysporum strains V2w2 and III4w1.

Cryptosporiopsis species isolated from the roots of aspen in central Alberta: identification, morphology, and interactions with the host, in vitroThis article is one of a selection of papers published in the Special Issue on Poplar Research in Canada

Cryptosporiopsis Bubák & Kabát isolates were obtained for the first time from roots of apparently healthy aspen seedlings in Alberta. These isolates were similar in all the major morphological features previously used to separate Cryptosporiopsis species, but sequencing data of the ITS1-5.8S-ITS2 region indicated that they were separated into two groups, one belonging to Cryptosporiopsis ericae Sigler and the other to Cryptosporiopsis radicicola Kowalski & Bartnik. Scanning electron microscopy of ex-type cultures and selected isolates from aspen roots revealed that C. ericae and C. radicicola differed in morphogenesis and structure of conidiomata: those of C. ericae were either synnematous or sporodochial, whereas those of C. radicicola possessed a peridium-like mycelial envelope bearing amorphous adhesive material. Phialides in the hymenium of C. radicicola were also embedded in amorphous matrix material but such material was absent in C. ericae. Microscopic examination of artificially inoculated aspen roots indicated that both species are endophytes of the host. Hyphal penetration by C. ericae was only occasional and confined to the host epidermis, whereas C. radicicola was more aggressive and its hyphal ingress extended to the cortical region.

Some fungal endophytes from vegetable crops and their anti-oomycete activities against tomato late blight

AIMS

To isolate endophytic fungi from vegetable plants and examine their in vivo anti-oomycete activity against Phytophthora infestans in tomato plants.

METHODS AND RESULTS

Endophytic fungi were isolated from surface-sterilized plant tissues and anti-oomycete activity was measured by in vivo assay using tomato seedlings. Endophytic fungi showing potent anti-oomycete activity were identified by morphological characteristics and nuclear ribosomal ITS1-5.8S-ITS2 sequence analysis. A total of 152 isolates were obtained from 66 healthy tissue samples of cucumber, red pepper, tomato, pumpkin and Chinese cabbage and the fermentation broths of 23 isolates showed potent in vivo anti-oomycete activity against tomato late blight with control values over 90%. The Fusarium oxysporum strain EF119, which was isolated from roots of red pepper, showed the most potent disease control efficacy against tomato late blight. In dual-culture tests, it inhibited the growth of Pythium ultimum, P. infestans and Phytophthora capsici.

CONCLUSIONS

Among endophytic fungi isolated from healthy tissues of vegetable plants, F. oxysporum EF119 showed the most potent in vivo anti-oomycete activity against tomato late blight and in vitro anti-oomycete activity against several oomycete pathogens.

SIGNIFICANCE AND IMPACT OF THE STUDY

Endophytic fungi showing anti-oomycete activity in vitro and in vivo may be used as biocontrol agents particularly of tomato late blight.

Modeling competition for infection sites on roots by nonpathogenic strains of Fusarium oxysporum

By use of plane and solid geometry and probability models, efficiencies of infection and competition for nutrients and infection sites by a nonpathogenic strain of Fusarium oxysporum (C14) with F. oxysporum f. sp. cucumerinum on the rhizoplane of cucumber were calculated. The model is derived from previously published data. Efficiencies for successful infection were 0.04 chlamydospores per infection site for both pathogen and nonpathogen. Observed successful infections by the pathogen in competition with the nonpathogen were close in values to the competition ratio (CR) calculated as the number of chlamydospores on the infection court of the pathogen divided by the total number of both pathogen and nonpathogen at relatively low densities. When total chlamydospores were, on average, closer than 175 microm apart, however, competition for nutrients/mutual inhibition occurred. At such densities there was an overestimation of the effect of competition for infection sites. These relationships were modeled at inoculum densities of pathogen and/or nonpathogen of 5000 chlamydospores per g soil and above, however, in the field, maximum densities of 1000 colony forming units/g (cfu) were observed. Most likely models of competition for infection sites at this density of the pathogen revealed that infection efficiency was only approximately halved, even when 0.98 of the possible 30 infection sites were occupied by the nonpathogen. It is conclude that competition for nutrients and/or infection sites is an insignificant factor in biocontrol of Fusarium wilt diseases by nonpathogenic fusaria.

Seed-transmitted beneficial endophytic Stagonospora sp. can penetrate the walls of the root epidermis, but does not proliferate in the cortex, of Phragmites australis

Stagonospora sp. (4/99-1) is a beneficial endophytic fungus frequently transmitted by seeds of Phragmites australis [Cav.] Trin. ex Steudel. Here we show that this fungus also penetrates the root epidermis. At first, hyphae were attracted by the root and proliferated on the root surface, preferably over the anticlinal walls. Penetration occurred directly by undifferentiated hyphae or was facilitated by hyphopodia. Hyphal growth within the root was restricted to the walls of epidermal cells and the walls of the cells of the outermost cortical layer. Deeper growth by the fungus elicited wall appositions and ingress into the cytoplasm of cortical cells was blocked by papillae. In the rare cases, the fungus managed to penetrate into cortical cells, these reacted with necrosis. Immunological studies suggested that fungal material reached the host plasmalemma and may have been taken up by endocytotic events. Our observations explain the endophytic lifestyle of hyphae close to the epidermis and the restricted development within the cortex.

Early physiological responses of Arabidopsis thaliana cells to fusaric acid: toxic and signalling effects

Fusaric acid (FA) is a toxin produced by Fusarium species. Most studies on FA have reported toxic effects (for example, alteration of cell growth, mitochondrial activity and membrane permeability) at concentrations greater than 10(-5) m. FA participates in fungal pathogenicity by decreasing plant cell viability. However, FA is also produced by nonpathogenic Fusarii, potential biocontrol agents of vascular wilt fusaria. The aim of this study was to determine whether FA, at nontoxic concentrations, could induce plant defence responses. Nontoxic concentrations of FA were determined from cell-growth and O2-uptake measurements on suspensions of Arabidopsis thaliana cells. Ion flux variations were analysed from electrophysiological and pH measurements. H2O2 and cytosolic calcium were quantified by luminescence techniques. FA at nontoxic concentrations (i.e. below 10(-6) m) was able to induce the synthesis of phytoalexin, a classic delayed plant response to pathogen. FA could also induce rapid responses putatively involved in signal transduction, such as the production of reactive oxygen species, and an increase in cytosolic calcium and ion channel current modulations. FA can thus act as an elicitor at nanomolar concentrations.

Potential of non-pathogenic Fusarium oxysporum isolates for control of Fusarium wilt of tomato

This study was done to evaluate the efficiency of non-pathogenic Fusarium oxysporum isolates (141/3, 233, 233/1, 245, 245/1, 251, 251/2, 251/5, and 257) in controlling vascular wilt caused by F. oxysporum f. sp. lycopersici, race 2 (isolates C-21A, TO11, and TO245) in tomato (Lycopersicon esculentum) cv. Viradoro seedlings. In order to determine the effect of non-pathogenic F. oxysporum isolates in tomato plants, the root system of 30-day-old seedlings was immersed in conidial suspensions (10(6) ml-1) of each isolate and the seedlings were transplanted to a cultivation substrate. Thirty-five days after transplanting it was observed that the non-pathogenic F. oxysporum isolates were not pathogenic to the cv. Viradoro nor did they affect seedling development. The efficiency of the non-pathogenic F. oxysporum isolates in controlling Fusarium wilt was determined by immersing the tomato seedling roots in the conidial suspension (10(6) ml-1) of each isolate and then transplanting them into substrates previously infested with isolates of F. oxysporum f.sp. lycopersici, race 2 (10(5) conidia ml-1 of substrate). Evaluations were performed 35 days after transplanting, for severity in scale with 1=healthy plant to 6=dead plant or plant showing vessel browning and wilted leaves up to the leader shoot and seedling height. The non-pathogenic F. oxysporum isolates were efficient in reducing the severity of the disease and maintaining normal plant development. These results provide evidence of the antagonistic activity of non-pathogenic F. oxysporum isolates in controlling vascular wilt caused by F. oxysporum f. sp. lycopersici race 2 in tomato.

Introduction of plant and fungal genes into pea (Pisum sativum L.) hairy roots reduces their ability to produce pisatin and affects their response to a fungal pathogen

Pisatin is an isoflavonoid phytoalexin synthesized by pea (Pisum sativum L.). Previous studies have identified two enzymes apparently involved in the synthesis of this phytoalexin, isoflavone reductase (IFR), which catalyzes an intermediate step in pisatin biosynthesis, and (+)6a-hydroxymaackiain 3-O-methyltransferase (HMM), an enzyme catalyzing the terminal step. To further evaluate the involvement of these enzymes in pisatin biosynthesis, sense- and antisense-oriented cDNAs of Ifr and Hmm fused to the 35s CaMV promoter, and Agrobacterium rhizogenes, were used to produce transgenic pea hairy root cultures. PDA, a gene encoding pisatin demethylating activity (pda) in the pea-pathogenic fungus Nectria haematococca, also was used in an attempt to reduce pisatin levels. Although hairy root tissue with either sense or antisense Ifr cDNA produced less pisatin, the greatest reduction occurred with sense or antisense Hmm cDNA. The reduced pisatin production in these lines was associated with reduced amounts of Hmm transcripts, HMM protein, and HMM enzyme activity. Hairy roots containing the PDA gene also produced less pisatin. To evaluate the role of pisatin in disease resistance, the virulence of N. haematococca on the transgenic roots that produced the lowest levels of pisatin was tested. Hairy roots expressing antisense Hmm were more susceptible than the control hairy roots to isolates of N. haematococca that are either virulent or nonvirulent on wild-type pea plants. This appears to be the first case of producing transgenic plant tissue with a reduced ability to produce a phytoalexin and demonstrating that such tissue is less resistant to fungal infection: these results support the hypothesis that phytoalexin production is a disease resistance mechanism.

Control of Verticillium Yellows in Chinese Cabbage by the Dark Septate Endophytic Fungus LtVB3

ABSTRACT Three hundred forty-nine fungal endophytes were obtained from a total of 1,214 root segments of eggplant, melon, barley, and Chinese cabbage grown as bait plants in a mixed soil made up of samples from different forest soils in Alberta and British Columbia, Canada. Three of the 349 isolates, when inoculated in axenically reared Chinese cabbage seedlings grown in petri dishes, almost completely suppressed the effects of a postinoculated and virulent strain of Verticillium longisporum. Two isolates effective against the pathogen were Phialocephala fortinii, which had been obtained from the roots of eggplant and Chinese cabbage. The third isolate was a dark septate endophytic (DSE) fungus obtained from barley roots. Hyphae of P. fortinii grew along the surface of the root and formed microsclerotia on or in the epidermal layer. Hyphae of the DSE fungus heavily colonized root cells of the cortex. Seedlings grown for 1 week in the presence of the endophytes were then challenged with the Verticillium pathogen. In DSE-treated roots, some of cell walls in the epidermal and cortical layers showed cell wall appositions and thickenings, which appeared to limit the ingress of the pathogen into adjacent cells. Such marked host reactions were not observed in the root cells colonized by P. fortinii. Chinese cabbage preinoculated with the above endophytes and, for comparison, a previously reported disease-suppressive fungal endophyte, Heteroconium chaetospira, were transplanted into the field and disease symptoms were assessed. The DSE could most effectively inhibit the development of Verticillium yellows, with reductions in the percentages of external and internal disease symptoms of 84 and 88%, respectively. The protective values against the disease are extremely high compared with those of other isolates. Most of the DSE-treated plants in the plots achieved marketable quality.

Overexpression of a cell wall glycoprotein in Fusarium oxysporum increases virulence and resistance to a plant PR-5 protein

Fusarium oxysporum f. sp. nicotianae is a causal agent for vascular wilt disease in tobacco. It is sensitive to osmotin, a tobacco pathogenesis-related protein (PR-5) that is implicated in plant defense against phytopathogenic fungi. We show that osmotin susceptibility of F. oxysporum f. sp. nicotianae was reduced by overexpression of the heterologous cell wall glycoprotein Saccharomyces cerevisiae protein containing inverted repeats (PIR2), a member of the PIR family of fungal cell wall glycoproteins that protect S. cerevisiae from the toxic action of osmotin. S. cerevisiae PIR2 was targeted to the cell wall of F. oxysporum. Disease severity and fungal growth were increased in tobacco seedlings inoculated with F. oxysporum transformed with PIR2 compared to seedlings infected with untransformed F. oxysporum or that transformed with vector, although accumulation of transcript and protein of defense genes was similar. The results show that fungal cell wall components can increase resistance to plant defense proteins and affect virulence.

Colonization of flax roots and early physiological responses of flax cells inoculated with pathogenic and nonpathogenic strains of Fusarium oxysporum

Fusarium oxysporum includes nonpathogenic strains and pathogenic strains that can induce necrosis or tracheomycosis in plants. The objective of this study was to compare the abilities of a pathogenic strain (Foln3) and a nonpathogenic strain (Fo47) to colonize flax roots and to induce early physiological responses in flax cell culture suspensions. Both strains colonized the outer cortex of the root; however, plant defense reactions, i.e., the presence of wall appositions, osmiophilic material, and collapsed cells, were less frequent and less intense in a root colonized by Foln3 than by Fo47. Early physiological responses were measured in flax cell suspensions confronted with germinated microconidia of both strains. Both pathogenic (Foln3) and nonpathogenic strains (Fo47) triggered transient H(2)O(2) production in the first few minutes of the interaction, but the nonpathogenic strain also induced a second burst 3 h postinoculation. Ca(2+) influx was more intense in cells inoculated with Fo47 than in cells inoculated with Foln3. Similarly, alkalinization of the extracellular medium was higher with Fo47 than with Foln3. Inoculation of the fungi into flax cell suspensions induced cell death 10 to 20 h postinoculation, with a higher percentage of dead cells observed with Fo47 than with Foln3 beginning at 14 h. This is the first report showing that early physiological responses of flax cells can be used to distinguish pathogenic and nonpathogenic strains of the soil-borne fungus F. oxysporum.

Fusarium oxysporum and its biocontrol

Fusarium oxysporum is well represented among the rhizosphere microflora. While all strains exist saprophytically, some are well-known for inducing wilt or root rots on plants whereas others are considered as nonpathogenic. Several methods based on phenotypic and genetic traits have been developed to characterize F. oxysporum strains. Results showed the great diversity affecting the soil-borne populations of F. oxysporum. In suppressive soils, interactions between pathogenic and nonpathogenic strains result in the control of the disease. Therefore nonpathogenic strains are developed as biocontrol agents. The nonpathogenic F. oxysporum strains show several modes of action contributing to their biocontrol capacity. They are able to compete for nutrients in the soil, affecting the rate of chlamydospore germination of the pathogen. They can also compete for infection sites on the root, and can trigger plant defence reactions, inducing systemic resistance. These mechanisms are more or less important depending on the strain. The nonpathogenic F. oxysporum are easy to mass produce and formulate, but application conditions for biocontrol efficacy under field conditions have still to be determined.

Recovery of Fusarium oxysporum Fo47 Mutants Affected in Their Biocontrol Activity After Transposition of the Fot1 Element

ABSTRACT To investigate the biocontrol mechanisms by which the antagonistic Fusarium oxysporum strain Fo47 is active against Fusarium wilt, a Fot1 transposon-mediated insertional mutagenesis approach was adopted to generate mutants affected in their antagonistic activity. Ninety strains in which an active Fot1 copy had transposed were identified with a phenotypic assay for excision and tested for their biocontrol activity against F. oxysporum f. sp. lini on flax in greenhouse experiments. Sixteen strains were affected in their capacity to protect flax plants, either positively (more antagonistic than Fo47) or negatively (less antagonistic). The molecular characterization of these mutants confirms the excision of Fot1 and its reinsertion in most of the cases. Moreover, we demonstrate that other transposable elements such as Fot2, impala, and Hop have no transposition activity in the mutant genomes. The phenotypic characterization of these mutants shows that they are affected neither in their in vitro growth habit nor in their competitiveness in soil compared with wild-type strain Fo47. These results show that mutants are not impaired in their saprophytic phase and suggest that the altered biocontrol phenotype should likely be expressed during the interaction with the host plant.

Ability of nonpathogenic Fusarium oxysporum strain Fo47 to induce resistance against Pythium ultimum infection in cucumber

The influence exerted by nonpathogenic Fusarium oxysporum strain Fo47 in triggering cucumber protection against infection by Pythium ultimum was investigated ultrastructurally. Macroscopic and microscopic observations of the pathogen colony in dual cultures revealed that reduction of Pythium growth was associated with marked disorders, including generalized disorganization of the host cytoplasm, retraction of the plasmalemma, and complete loss of the protoplasm. Cytochemical labeling of cellulose with an exoglucanase-gold complex showed that the cellulose component of the host cell walls was structurally preserved at a time when the host cytoplasm had undergone complete disorganization. A similar antagonistic process was observed at the root cell surface. Most striking and interesting was the finding that mycoparasitism, as evidenced by the frequent occurrence of Fo47 hyphae within nearly empty cells of the pathogen, occurred not only at the root surface but also within the invaded root tissues. The specific labeling pattern obtained with the exoglucanase-gold complex confirmed that Fo47 successfully penetrated cells of the pathogen, both in the rhizosphere and inside the root tissues. Pythium cells that could evade the first defensive line in the rhizosphere could penetrate the root epidermis, but their growth was restricted to the outermost tissues. Positive correlations between Fo47 treatment and induced resistance to infection by P. ultimum in cucumber were confirmed by (i) the reduction of pathogen viability; (ii) the elaboration of newly formed barriers, a phenomenon which was not seen in Fo47-free plants, where the pathogen proliferated in all root tissues within a few days; and (iii) the occlusion of intercellular spaces with a dense material likely enriched in phenolics. Taken together, our observations provide the first convincing evidence that Fo47 exerts a direct inhibitory effect on P. ultimum through a combination of antibiosis and mycoparasitism, in addition to being a strong inducer of plant defense reactions.