Trichoderma/pathogen/plant interaction in pre-harvest food security

Large losses before crop harvesting are caused by plant pathogens, such as viruses, bacteria, oomycetes, fungi, and nematodes. Among these, fungi are the major cause of losses in agriculture worldwide. Plant pathogens are still controlled through application of agrochemicals, causing human disease and impacting environmental and food security. Biological control provides a safe alternative for the control of fungal plant pathogens, because of the ability of biocontrol agents to establish in the ecosystem. Some Trichoderma spp. are considered potential agents in the control of fungal plant diseases. They can interact directly with roots, increasing plant growth, resistance to diseases, and tolerance to abiotic stress. Furthermore, Trichoderma can directly kill fungal plant pathogens by antibiosis, as well as via mycoparasitism strategies. In this review, we will discuss the interactions between Trichoderma/fungal pathogens/plants during the pre-harvest of crops. In addition, we will highlight how these interactions can influence crop production and food security. Finally, we will describe the future of crop production using antimicrobial peptides, plants carrying pathogen-derived resistance, and plantibodies.     

Induction and accumulation of PR proteins activityduring early stages of root colonizationby the mycoparasite Trichoderma harzianum strain T-203

The biochemical nature of the interaction between the antagonistic fungus Trichoderma harzianum strain T-203 and cucumber roots was studied during the early stages of root colonization by the fungus. Pathogenesis related (PR) proteins of the plant and enzyme activity of the fungus following the penetration and colonization of the roots by T. harzianum were explored up to 72 h post-inoculation. Scanning electron microscopy (SEM) revealed typical fungal structures previously associated with mycoparasitic interactions of T. harzianum strains during biological control. These included hyphal coiling and appressoria formation. Compared to untreated control, cucumber roots treated with T. harzianum T-203 exhibited higher activities of chitinase (EC 3.2.1.14), β-1,3-glucanase (EC 3.2.1.6), cellulase (EC 3.2.1.4) and peroxidase (EC 1.11.1.7), up to 72 h post-inoculation. Plants treated with a chemical inducer of the plant defence response, 2,6-dichloroisonicotinic acid (INA) displayed responses that were similar but not identical to those of plants inoculated with T. harzianum. In vivo staining of chitinase activity in fresh root sections allowed the localization of the activity in roots treated with either T. harzianum T-203 or INA. The formation of fluorescent products mainly in intercellular spaces of the induced roots provided evidence for the involvement of the plant defence system. In addition to its well-recognized mycoparasitic nature, it is suggested that Trichoderma’s association with roots reduce root disease through activation of the plant’s defence response.     

Secretome of Trichoderma Interacting With Maize Roots: Role in Induced Systemic Resistance

Trichoderma virens is a biocontrol agent used in agriculture to antagonize pathogens of crop plants. In addition to direct mycoparasitism of soil-borne fungal pathogens, T. virens interacts with roots. This interaction induces systemic resistance (ISR), which reduces disease in above-ground parts of the plant. In the molecular dialog between fungus and plant leading to ISR, proteins secreted by T. virens provide signals. Only a few such proteins have been characterized previously. To study the secretome, proteins were characterized from hydroponic culture systems with T. virens alone or with maize seedlings, and combined with a bioassay for ISR in maize leaves infected by the pathogen Cochliobolus heterostrophus. The secreted protein fraction from coculture of maize roots and T. virens (Tv+M) was found to have a higher ISR activity than from T. virens grown alone (Tv). A total of 280 fungal proteins were identified, 66 showing significant differences in abundance between the two conditions: 32 were higher in Tv+M and 34 were higher in Tv. Among the 34 found in higher abundance in Tv and negatively regulated by roots were 13 SSCPs (small, secreted, cysteine rich proteins), known to be important in the molecular dialog between plants and fungi. The role of four SSCPs in ISR was studied by gene knockout. All four knockout lines showed better ISR activity than WT without affecting colonization of maize roots. Furthermore, the secreted protein fraction from each of the mutant lines showed improved ISR activity compared with WT. These SSCPs, apparently, act as negative effectors reducing the defense levels in the plant and may be important for the fine tuning of ISR by Trichoderma. The down-regulation of SSCPs in interaction with plant roots implies a revision of the current model for the Trichoderma-plant symbiosis and its induction of resistance to pathogens.Fungi belonging to the genus Trichoderma are used as biocontrol agents in agriculture. In addition to direct antagonism of soil-borne pathogens, these fungi intimately interact with plant roots, and are thus considered rhizosphere-competent (1). The interaction is, in general, a beneficial one, promoting plant growth as well as inducing systemic resistance (ISR)¹ to pathogens (2–6). The elicitation of defense response in the leaves of plants whose roots are colonized with Trichoderma enhances the plant''s resistance to foliar pathogens. This clear potential for application in agriculture is already beginning to be realized (7–9).Secreted proteins are central to the molecular dialog between fungi and their plant hosts. Recent studies addressed, for example, the molecular basis for mutualistic interactions between soil fungi and plants in mycorrhizae, a fungus-root symbiosis of widespread importance for nutrient acquisition. Specific secreted proteins were found to have targets in the plant (10, 11). The Trichoderma-root mutualism is distinct from these well-studied mycorrhizal symbioses, but some of the principles may be shared. Proteomic studies on several Trichoderma species have been reviewed (12). These studies employed total protein extracts from Trichoderma interacting with plants, or the three-way Trichoderma-plant-pathogen interaction (13, 14), and led to the identification of some secreted proteins expressed during the interaction with plant and fungal hosts. Indeed, the first studies of secreted proteins demonstrated an abundant Trichoderma secreted protein, belonging to the ceratoplatanins, which are a fungal family of secreted elicitors and toxins. This protein, Sm1 (in T. virens)/Epl1 (in T. atroviride (15–20) was shown to elicit ISR. The ceratoplatanin Sm1/Epl1 also belongs to a larger class of fungal proteins defined as SSCPs or SSPs: small, secreted (cysteine rich) proteins (21–23). There are no sequence motifs or domains common to the members of the entire SSCP class. Within the wide definition, though, there are subfamilies of proteins that do share sequence homology, for example the ceratoplatanin family to which Sm1 belongs.A bioinformatic survey of the SSCPs encoded in the genomes of three Trichoderma species, T. virens (Tv), T. atrovirde (Ta), and T. reesei (Tr) revealed several hundred candidate SSCPs in each species (24). Approximately half of the SSCPs from each species have homologs in the same and/or in the other two species, whereas the other half are unique and do not share homology in or between the species. This diversity between the three species suggests that SSCPs are evolving rapidly.Given the known importance of Sm1, and the wide host range of Trichoderma species, it seemed likely that many SSCPs might be involved in the Trichoderma-root interaction. Secreted proteins (with emphasis on SSCPs), whose abundance changes in response to association with plant roots, may function in the fungal-plant molecular dialog. To test the hypothesis that the abundance of specific SSCPs and other secreted proteins is regulated by the interaction with plant roots, we compared the secretome of Trichoderma alone to the secretome of Trichoderma cocultured with the roots of maize seedlings. Functional experiments using knock out mutants in the genes encoding some of the regulated SSCPs were carried out in order to shed light on their role in the molecular interaction between the plant and the fungus.     

Interactions between functionally diverse fungal mutualists inconsistently affect plant performance and competition

Plants form mutualistic relationship with a variety of belowground fungal species. Such a mutualistic relationship can enhance plant growth and resistance to pathogens. Yet, we know little about how interactions between functionally diverse groups of fungal mutualists affect plant performance and competition. We experimentally determined the effects of interaction between two functional groups of belowground fungi that form mutualistic relationship with plants, arbuscular mycorrhizal (AM) fungi and Trichoderma, on interspecific competition between pairs of closely related plant species from four different genera. We hypothesized that the combination of two functionally diverse belowground fungal species would allow plants and fungi to partition their symbiotic relationships and relax plant–plant competition. Our results show that: 1) the AM fungal species consistently outcompeted the Trichoderma species independent of plant combinations; 2) the fungal species generally had limited effects on competitive interactions between plants; 3) however, the combination of fungal species relaxed interspecific competition in one of the four instances of plant–plant competition, despite the general competitive superiority of AM fungi over Trichoderma. We highlight that the competitive outcome between functionally diverse fungal species may show high consistency across a broad range of host plants and their combinations. However, despite this consistent competitive hierarchy, the consequences of their interaction for plant performance and competition can strongly vary among plant communities.     

Role and genetic basis of specialised secondary metabolites in Trichoderma ecophysiology

Species of fungal genus Trichoderma are characterized by a versatile lifestyle, high adaptability to the changing environmental conditions and the ability to establish sophisticated interactions with other organisms. Due to their ability to antagonize plant pathogens and to elicit the plant defence responses against biotic/abiotic stresses, Trichoderma spp. are commonly used as commercially biopesticides and biofertilizers. The Trichoderma success in the rhizosphere is supported by a wide arsenal of specialised metabolites (SMs) providing morphological and physiological autoregulation, self-protection and facilitating fungal communication. This review aims to explore the roles of SMs in the biology of fungi, with special emphasis on the genus Trichoderma and on how divergence in the SMs genetic structure determine Trichoderma lifestyles. Trichoderma genomes are endowed with a high number of SMs biosynthetic genes, and understanding the genetic basis of their biosynthesis is crucial for determining the role of these metabolites in Trichoderma ecophysiology and for expanding their application in crop protection. Recent advances on the characterization of the Trichoderma SMs genetic inventory driven by computational biology are discussed.     

Caffeine fostering of mycoparasitic fungi against phytopathogens

Caffeine (1,3,7-trimethixanthine) is a typical purine alkaloid produced in more than 80 plant species. Its biological role is considered to strengthen plant''s defense capabilities, directly as a toxicant to biotic attackers (allelopathy) and indirectly as an activator of defense system (priming). Caffeine is actively secreted into rhizosphere through primary root, and possibly affects the structure of microbe community nearby. The fungal community in coffee plant rhizosphere is enriched with particular species, including Trichoderma family, a mycoparasite that attacks and kills phytopathogens by coiling and destroying their hyphae. In the present study, the caffeine response of 8 filamentous fungi, 4 mycoparasitic Trichoderma, and 4 prey phytopathogens, was examined. Results showed that allelopathic effect of caffeine on fungal growth and development was differential, being stronger on pathogens than on Trichoderma species. Upon confronting, the prey immediately ceased the growth, whereas the predator continued to grow, indicating active mycoparasitism to have occurred. Caffeine enhanced mycoparasitism up to 1.7-fold. Caffeine thus functions in a double-track manner against fungal pathogens: first by direct suppression of growth and development, and second by assisting their natural enemy. These observations suggest that caffeine is a powerful weapon in the arms race between plants and pathogens by fostering enemy''s enemy, and we propose the idea of "caffeine fostering" as the third role of caffeine.     

Salicylic acid prevents Trichoderma harzianum from entering the vascular system of roots

Trichoderma is a soil‐borne fungal genus that includes species with a significant impact on agriculture and industrial processes. Some Trichoderma strains exert beneficial effects in plants through root colonization, although little is known about how this interaction takes place. To better understand this process, the root colonization of wild‐type Arabidopsis and the salicylic acid (SA)‐impaired mutant sid2 by a green fluorescent protein (GFP)‐marked Trichoderma harzianum strain was followed under confocal microscopy. Trichoderma harzianum GFP22 was able to penetrate the vascular tissue of the sid2 mutant because of the absence of callose deposition in the cell wall of root cells. In addition, a higher colonization of sid2 roots by GFP22 compared with that in Arabidopsis wild‐type roots was detected by real‐time polymerase chain reaction. These results, together with differences in the expression levels of plant defence genes in the roots of both interactions, support a key role for SA in Trichoderma early root colonization stages. We observed that, without the support of SA, plants were unable to prevent the arrival of the fungus in the vascular system and its spread into aerial parts, leading to later collapse.     

A soil-free root observation system for the study of root-microorganism interactions in maize

Background and aims

The root surface of a plant usually exceeds the leaf area and is constantly exposed to a variety of soil-borne microorganisms. Root pathogens and pests, as well as belowground interactions with beneficial microbes, can significantly influence a plants' performance. Unfortunately, the analysis of these interactions is often limited because of the arduous task of accessing roots growing in soil. Here, we present a soil-free root observation system (SF-ROBS) designed to grow maize (Zea mays) plants and to study root interactions with either beneficial or pathogenic microbes.

Methods

The SF-ROBS consists of pouches lined with wet filter paper supplying nutrient solution.

Results

The aspect of maize grown in the SF-ROBS was similar to soil-grown maize; the plant growth was similar for the shoot but different for the roots (biomass and length increased in the SF-ROBS). SF-ROBS-grown roots were successfully inoculated with the hemi-biotrophic maize fungal pathogen Colletotrichum graminicola and the beneficial rhizobacteria Pseudomonas putida KT2440. Thus, the SF-ROBS is a system suitable to study two major belowground phenomena, namely root fungal defense reactions and interactions of roots with beneficial soil-borne bacteria.

Conclusions

This system contributes to a better understanding of belowground plant microbe interactions in maize and most likely also in other crops.     

Root endophyte symbiosis in vitro between the ectomycorrhizal basidiomycete Tricholoma matsutake and the arbuscular mycorrhizal plant Prunus speciosa

We previously reported that Tricholoma matsutake and Tricholoma fulvocastaneum, ectomycorrhizal basidiomycetes that associate with Pinaceae and Fagaceae, respectively, in the Northern Hemisphere, could interact in vitro as a root endophyte of somatic plants of Cedrela odorata (Meliaceae), which naturally harbors arbuscular mycorrhizal fungi in South America, to form a characteristic rhizospheric colony or “shiro”. We questioned whether this phenomenon could have occurred because of plant–microbe interactions between geographically separated species that never encounter one another in nature. In the present study, we document that these fungi formed root endophyte interactions and shiro within 140 days of inoculation with somatic plants of Prunus speciosa (=Cerasus speciosa, Rosaceae), a wild cherry tree that naturally harbors arbuscular mycorrhizal fungi in Japan. Compared with C. odorata, infected P. speciosa plants had less mycelial sheath surrounding the exodermis, and the older the roots, especially main roots, the more hyphae penetrated. In addition, a large number of juvenile roots were not associated with hyphae. We concluded that such root endophyte interactions were not events isolated to the interactions between exotic plants and microbes but could occur generally in vitro. Our pure culture system with a somatic plant allowed these fungi to express symbiosis-related phenotypes that varied with the plant host; these traits are innately programmed but suppressed in nature and could be useful in genetic analyses of plant–fungal symbiosis.     

Rhizosphere disturbance influences fungal colonization and community development on dead fine roots

Little is known about the community dynamics of fungi on decomposing fine roots, despite the importance of fine roots as a source of carbon to detrital systems in forests. We examined fungal communities on dead roots in a sugar-maple dominated northern hardwood forest to test the hypothesis that community development is sensitive to rhizosphere disruption. We generated cohorts of dead fine roots in root windows and disturbed the rhizosphere microbial community in half of the windows by moving roots into sieved bulk soil. We sampled root fragments repeatedly over time and cultured fungi from these fragments to explore temporal patterns of fungal species composition. Disturbing the root rhizosphere prior to initiating decomposition changed the dominant fungal taxa, the distribution of dominant species within the community, and the temporal development in the culturable fungal community. Dominance in control roots shifted from Neonectria in early decay to Umbelopsis in later decay. Disturbance roots were more evenly dominated over time by Trichoderma, Neonectria, another species of Umbelopsis, and Pochonia. Our results suggest that species interactions are important in the ecology of fine root decay fungi, with the rhizosphere community of the living root influencing development of the decay community.     

Root endophytic fungi show low levels of interspecific competition in planta

Roots are associated with fungal communities that affect plant growth and health. Individual root-associated fungi have different effects on plant performance, from detrimental to beneficial, but it is barely known how their inter-species interactions determine plant fitness. Here, we evaluate in planta interactions among dominant root-colonizing fungi with different degrees of phylogenetic and trait similarity, and the impact of their co-occurrence on their respective ability to colonize roots and their effects on plant growth. An in vitro bioassay with Arabidopsis thaliana as host plant was used for the co-cultivation with individual or paired combinations of fungal strains. Root colonization by strains was monitored using real-time quantitative PCR, and the effects on their host's growth were estimated by measuring plant biomass. Strains had variable effects on plant growth, although these effects were mostly modest and little affected by the presence of other fungi. Abundance of each fungus in roots responded differently to co-inoculation, but competition between strains was not associated with their similarity in functional traits. Our findings show little competition between dominant fungal root endophytes, which suggests that they occupy complementary root niches and could explain the high fungal diversity colonizing healthy hosts in natural conditions.     

Diversity and host preference of fungi co-inhabiting Cenococcum mycorrhizae

Diverse fungal assemblages colonize the fine feeder roots of woody plants, including mycorrhizal fungi, fungal root endophytes and soil saprotrophs. The fungi co-inhabiting Cenococcum geophilum ectomycorrhizae (ECM) of Abies balsamea, Betula papyrifera and Picea glauca were studied at two boreal forest sites in Eastern Canada by direct PCR of ITS rDNA. 50 non-Cenococcum fungal sequence types were detected, including several potentially mycorrhizal species as well as fungal root endophytes. Non-melanized ascomycetes dominated, in contrast to the dark septate endophytes (DSE) reported in most culture dependent studies. The results demonstrate significant differences in root associated fungal assemblages among the host species studied. Fungal diversity was also host dependent, with P. glauca roots supporting a more diverse community than A. balsamea. Differences in root associated fungal communities may well influence ecological interactions among host plant species.     

MycoRed: Betalain pigments enable in vivo real-time visualisation of arbuscular mycorrhizal colonisation

Arbuscular mycorrhiza (AM) are mutualistic interactions formed between soil fungi and plant roots. AM symbiosis is a fundamental and widespread trait in plants with the potential to sustainably enhance future crop yields. However, improving AM fungal association in crop species requires a fundamental understanding of host colonisation dynamics across varying agronomic and ecological contexts. To this end, we demonstrate the use of betalain pigments as in vivo visual markers for the occurrence and distribution of AM fungal colonisation by Rhizophagus irregularis in Medicago truncatula and Nicotiana benthamiana roots. Using established and novel AM-responsive promoters, we assembled multigene reporter constructs that enable the AM-controlled expression of the core betalain synthesis genes. We show that betalain colouration is specifically induced in root tissues and cells where fungal colonisation has occurred. In a rhizotron setup, we also demonstrate that betalain staining allows for the noninvasive tracing of fungal colonisation along the root system over time. We present MycoRed, a useful innovative method that will expand and complement currently used fungal visualisation techniques to advance knowledge in the field of AM symbiosis.

Arbuscular mycorrhiza are mutualistic interactions formed between soil fungi and plant roots. This study presents the MycoRed system, which uses red plant pigments derived from beetroot to reveal how fungi establish symbiosis with living legume and wild tobacco roots.     

Trichoderma–Plant–Pathogen Interactions: Advances in Genetics of Biological Control

Trichoderma spp. are widely used in agriculture as biofungicides. Induction of plant defense and mycoparasitism (killing of one fungus by another) are considered to be the most important mechanisms of Trichoderma-mediated biological control. Understanding these mechanisms at the molecular level would help in developing strains with superior biocontrol properties. In this article, we review our current understanding of the genetics of interactions of Trichoderma with plants and plant pathogens.     

Contrasting impacts of defoliation on root colonization by arbuscular mycorrhizal and dark septate endophytic fungi of Medicago sativa

Individual plants typically interact with multiple mutualists and enemies simultaneously. Plant roots encounter both arbuscular mycorrhizal (AM) and dark septate endophytic (DSE) fungi, while the leaves are exposed to herbivores. AMF are usually beneficial symbionts, while the functional role of DSE is largely unknown. Leaf herbivory may have a negative effect on root symbiotic fungi due to decreased carbon availability. However, evidence for this is ambiguous and no inoculation-based experiment on joint effects of herbivory on AM and DSE has been done to date. We investigated how artificial defoliation impacts root colonization by AM (Glomus intraradices) and DSE (Phialocephala fortinii) fungi and growth of Medicago sativa host in a factorial laboratory experiment. Defoliation affected fungi differentially, causing a decrease in arbuscular colonization and a slight increase in DSE-type colonization. However, the presence of one fungal species had no effect on colonization by the other or on plant growth. Defoliation reduced plant biomass, with this effect independent of the fungal treatments. Inoculation by either fungal species reduced root/shoot ratios, with this effect independent of the defoliation treatments. These results suggest AM colonization is limited by host carbon availability, while DSE may benefit from root dieback or exudation associated with defoliation. Reductions in root allocation associated with fungal inoculation combined with a lack of effect of fungi on plant biomass suggest DSE and AMF may be functional equivalent to the plant within this study. Combined, our results indicate different controls of colonization, but no apparent functional consequences between AM and DSE association in plant roots in this experimental setup.     

When do arbuscular mycorrhizal fungi protect plant roots from pathogens?

Arbuscular mycorrhizal (AM) fungi are mainly thought to facilitate phosphorus uptake in plants, but they can also perform several other functions that are equally beneficial. Our recent study sheds light on the factors determining one such function, enhanced plant protection from root pathogens. Root infection by the fungal pathogen Fusarium oxysporum was determined by both plant susceptibility and the ability of an AM fungal partner to suppress the pathogen. The non-susceptible plant species (Allium cepa) had limited F. oxysporum infection even without AM fungi. In contrast, the susceptible plant species (Setaria glauca) was heavily infected and only AM fungi in the family Glomeraceae limited pathogen abundance. Plant susceptibility to pathogens was likely determined by contrasting root architectures between plants, with the simple rooted plant (A. cepa) presenting fewer sites for infection. AM fungal colonization, however, was not limited in the same way in part because plants with fewer, simple roots are more mycorrhizal dependent. Protection only by Glomus species also indicates that whatever the mechanism(s) of this function, it responds to AM fungal families differently. While poor at pathogen protection, AM fungal species in the family Gigasporaceae most benefited the growth of the simple rooted plant species. Our research indicates that plant trait differences, such as root architecture can determine how important each mycorrhizal function is to plant growth but the ability to provide these functions differs among AM fungi.Key words: arbuscular mycorrhizal fungi, Fusarium oxysporum, root architecture, pathogen protection, multi-functionalityArbuscular mycorrhizas (AM) represent the oldest and most widespread symbiosis with land plants.¹ Most mycorrhizal research has focused on the ability of AM fungi to facilitate nutrient uptake, particularly phosphorus.² Although researchers recognize that AM fungi are multi-functional,³ it is not clear what factors determine which function an AM fungus performs or its relative importance to the plant.⁴ Newsham et al. (1995)³ hypothesized that AM function is based on root architecture: plants with simple rooting systems are dependent on mycorrhizas for nutrient uptake, while those with complex root systems are less dependent on mycorrhizas for nutrient uptake, but are more susceptible to root pathogens because of increased numbers of infections sites.³ These two functions, phosphorus uptake and enhanced pathogen protection from mycorrhizas also depend on the identity of the fungus. Arbuscular mycorrhizal fungi in the family Gigasporaceae are more effective at enhancing plant phosphorus, while AM fungi in the Glomeraceae better protect plants from root pathogens.⁵Our results support both plant and fungal control of a common pathogen, Fusarium oxysporum, and the interaction between these two factors ultimately determined the level of pathogen infection and plant mycorrhizal benefit. We inoculated two plant species that have contrasting root architectures with one of six AM fungal species from two families (or no AM fungi). After five months of growth, plants were inoculated with F. oxysporum, grown for another month and then harvested. All plant seeds and fungi were collected in a local old field community.⁶ Allium cepa (garden onion) was not susceptible to F. oxysporum likely because it has only a few adventitious roots below the main bulb that do not present many sites for infection. In contrast, Setaria glauca (yellow foxtail) was heavily infected by F. oxysporum and has fine roots with increased numbers of branching points and lateral meristems where fungi can colonize.⁷ For the susceptible plant (S. glauca), AM fungal species from the family Glomeraceae were effective at reducing pathogen abundance while species from the Gigasporaceae were not. Forming a symbiosis with a Glomus species resulted in S. glauca plants that were as large as control plants. AM fungal species from the family Gigaspoaceae were more beneficial to growth of the simple rooted A. cepa, which had fewer roots to take up soil nutrients.Reduced rooting structures may limit pathogen infection sites, but AM fungal colonization was not limited in the same way and may actually alter plant root architecture. While the simple rooted A. cepa had limited pathogen susceptibility, it had twice the AM fungal colonization of the complex rooted S. glauca. Because the simple rooted plant has a greater dependence on mycorrhizas,⁸ it likely transmits chemical signals to rapidly initiate mycorrhizal formation,⁹ but then may have less control on the spread of AM fungi within the root. In contrast, S. glauca is more susceptible to fungal pathogens and may be less mycorrhizal dependent in nature.¹⁰ As a result, S. glauca may treat all colonizing root fungi as potential parasites. Colonization by AM fungi from the Glomeraceae was also much greater than those in the Gigasporaceae due to differences in fungal life history strategy between these families.^11,12 AM fungal colonization can reduce root branching in plants and alter plant allocation to roots, thereby increasing mycorrhizal dependence for nutrients^10,13 and potentially reducing pathogen infection sites. Mycorrhizal induced changes to plant root architecture may therefore reinforce current mycorrhizal associations and alter future fungal colonization attempts.¹⁴ An important next step is to test if AM fungal families (or species) alter plant root architecture in different ways and the degree to which these effects depend on colonization timing and the plant host.Our study did not isolate the particular mechanism by which AM fungi control pathogens, but this mechanism clearly differentiates between AM fungal families. AM fungi can control pathogens through several mechanisms including direct competition for colonization sites, indirect initiation of plant defensive responses or altering other rhizosphere biota.¹⁵ Although these AM fungal families differ in the intensity of root colonization,¹¹ percentage of root length colonized by an AM fungus is a poor predictor of pathogen limitation compared to family identity,^12,16 suggesting that direct competition for space is unlikely. AM fungi share many cell surface molecules with pathogenic fungi like Fusarium.¹⁷ These molecules can act as signals that initiate plant production of defensive compounds such as phytoalexins, phenolics and other compounds.¹⁸ While AM fungi appear to evade these defenses, only AM fungal species in the family Glomeraceae would have elicited plant responses which altered future infection by F. oxysporum. AM fungi in the Gigasporaceae may differ more from F. oxysporum in their chemical signals or not colonize roots sufficiently to induce a sustained, system-wide plant response. In addition, many rhizosphere related microbes are antagonistic to pathogenic fungi¹⁵ and may differ in their response to the different AM fungal families.¹⁹ Because rhizosphere microbes also differ among plant species, plant pathogen protection may be influenced by multiple ecological interactions that determine the specific cases when mycorrhizal pathogen protection occurs. To distinguish between these mechanisms, future experiments could test whether biochemical similarity or ecological similarity (especially with other soil biota) between an AM fungus and fungal pathogen can predict mycorrhizal induced pathogen protection.Plant and fungal identity clearly affect AM fungal function and benefit, but to accurately use AM fungi in agriculture and restoration^20,21 we must clearly understand how functional mechanisms differ. Different mycorrhizal functions may be based on common plant traits like root architecture, but ecology, colonization timing and environment may alter the specific function AM fungi provide and its importance to plants. While it may be useful to establish greenhouse rules about which fungal species perform specific mycorrhizal functions, predicting their role in more complex systems relies on understanding if other factors will enhance or negate these effects. Most AM fungal species vary in their ability to perform each function and these can be locally adapted to limiting soil nutrients.²² In plants, there is also a range to which specific mycorrhizal functions may benefit plant fitness, and these responses are based on both plant traits (which change throughout a plant''s life cycle) and the local environment.^23,24 Given this variation, it is critical to understand if AM fungi can respond to cues from the plant or the environment to identify what factors limit plant growth and whether a the most effective AM fungus shows a greater response.     

Plant resistance signalling hijacked by a necrotrophic fungal pathogen

The strategies used by necrotrophic fungal pathogens to infect plants are often perceived as lacking the sophistication of their haustorium producing, host defence suppressing, biotrophic counterparts. There is also a relative paucity of knowledge regarding how effective gene-for-gene based resistance reactions might function against necrotrophic plant pathogens. However, recent data has emerged from a number of systems which has highlighted that particular species of necrotrophic (and/or hemibiotrophic) fungi, have evolved very sophisticated strategies for plant infection which appear, in fact, to hijack the host resistance responses that are commonly deployed against biotrophs. Both disease resistance (R) protein homologues and mitogen-activated protein kinase (MAPK) cascades commonly associated with incompatible disease resistance responses; appear to be targeted by necrotrophic fungi during compatible disease interactions. These findings highlight an emerging sophistication in the strategies deployed by necrotrophic fungi to infect plants.Key words: Mycosphaerella graminicola, Septoria tritici, Triticum aestivum, mitogen-activated protein kinase, programmed cell death, fungal pathogen, disease resistance, disease susceptibility, toxin     

Mycoparasitism studies of Trichoderma species against three phytopathogenic fungi: evaluation of antagonism and hydrolytic enzyme production

Trichoderma spp. are used for biocontrol of several plant pathogens. However, their efficient interaction with the host needs to be accompanied by production of secondary metabolites and cell wall-degrading enzymes. Three parameters were evaluated after interaction between four Trichoderma species and plant-pathogenic fungi: Fusarium solani, Rhizoctonia solani and Sclerotinia sclerotiorum. Trichoderma harzianum and T. asperellum were the most effective antagonists against the pathogens. Most of the Trichoderma species produced toxic volatile metabolites, having significant effects on growth and development of the plant pathogens. When these species were grown in liquid cultures with cell walls from these plant pathogens, they produced and secreted β-1,3-glucanase, NAGAse, chitinase, acid phosphatase, acid proteases and alginate lyase.