植物与病原真菌互作的形态变化及其生理、生化机制和基因调控

本文从生理、生化、酶学及基因调控等方面对植物与病原真菌互作过程中的形态变化进行了综述,以期为植物抗病方面的研究提供参考     

植物与病原真菌互作的形态变化及其生理 生化机制和基因调控

本文从生理、生化、酶学及基因调控等方面对植物与病原真菌互作过程中的形态变化进行了综述,以期为植物抗病方面的研究提供参考。     

Flowering‐mediated root‐fungus symbiosis loss is related to jasmonate‐dependent root soluble sugar deprivation

The role of flowering in root‐fungal symbiosis is not well understood. Because flowering and fungal symbionts are supported by carbohydrates, we hypothesized that flowering modulates root‐beneficial fungal associations through alterations in carbohydrate metabolism and transport. We monitored fungal colonization and soluble sugars in the roots of Arabidopsis thaliana following inoculation with a mutualistic fungus Phomopsis liquidambari across different plant developmental stages. Jasmonate signalling of wild‐type plants, sugar transport, and root invertase of wild‐type and jasmonate‐insensitive plants were exploited to assess whether and how jasmonate‐dependent sugar dynamics are involved in flowering‐mediated fungal colonization alterations. We found that flowering restricts root‐fungal colonization and activates root jasmonate signalling upon fungal inoculation. Jasmonates reduce the constitutive and fungus‐induced accumulation of root glucose and fructose at the flowering stage. Further experiments with sugar transport and metabolism mutant lines revealed that root glucose and fructose positively influence fungal colonization. Diurnal, jasmonate‐dependent inhibitions of sugar transport and soluble invertase activity were identified as likely mechanisms for flowering‐mediated root sugar depletion upon fungal inoculation. Collectively, our results reveal that flowering drives root‐fungus cooperation loss, which is related to jasmonate‐dependent root soluble sugar depletion. Limiting the spread of root‐fungal colonization may direct more resources to flower development.     

The fungal NADPH oxidase is an essential element for the molecular dialog between Trichoderma and Arabidopsis

Members of the fungal genus Trichoderma stimulate growth and reinforce plant immunity. Nevertheless, how fungal signaling elements mediate the establishment of a successful Trichoderma?plant interaction is largely unknown. In this work, we analyzed growth, root architecture and defense in an Arabidopsis?Trichoderma co‐cultivation system, including the wild‐type (WT) strain of the fungus and mutants affected in NADPH oxidase. Global gene expression profiles were assessed in both the plant and the fungus during the establishment of the interaction. Trichoderma atroviride WT improved root branching and growth of seedling as previously reported. This effect diminished in co‐cultivation with the ?nox1, ?nox2 and ?noxR null mutants. The data gathered of the Arabidopsis interaction with the ?noxR strain showed that the seedlings had a heightened immune response linked to jasmonic acid in roots and shoots. In the fungus, we observed repression of genes involved in complex carbohydrate degradation in the presence of the plant before contact. However, in the absence of NoxR, such repression was lost, apparently due to a poor ability to adequately utilize simple carbon sources such as sucrose, a typical plant exudate. Our results unveiled the critical role played by the Trichoderma NoxR in the establishment of a fine‐tuned communication between the plant and the fungus even before physical contact. In this dialog, the fungus appears to respond to the plant by adjusting its metabolism, while in the plant, fungal perception determines a delicate growth?defense balance.     

Sugar for my honey: carbohydrate partitioning in ectomycorrhizal symbiosis

Simple, readily utilizable carbohydrates, necessary for growth and maintenance of large numbers of microbes are rare in forest soils. Among other types of mutualistic interactions, the formation of ectomycorrhizas, a symbiosis between tree roots and certain soil fungi, is a way to overcome nutrient and carbohydrate limitations typical for many forest ecosystems. Ectomycorrhiza formation is typical for trees in boreal and temperate forests of the northern hemisphere and alpine regions world-wide. The main function of this symbiosis is the exchange of fungus-derived nutrients for plant-derived carbohydrates, enabling the colonization of mineral nutrient-poor environments. In ectomycorrhizal symbiosis up to 1/3 of plant photoassimilates could be transferred toward the fungal partner. The creation of such a strong sink is directly related to the efficiency of fungal hexose uptake at the plant/fungus interface, a modulated fungal carbohydrate metabolism in the ectomycorrhiza, and the export of carbohydrates towards soil growing hyphae. However, not only the fungus but also the plant partner increase its expression of hexose importer genes at the plant/fungus interface. This increase in hexose uptake capacity of plant roots in combination with an increase in photosynthesis may explain how the plant deals with the growing fungal carbohydrate demand in symbiosis and how it can restrict this loss of carbohydrates under certain conditions to avoid fungal parasitism.     

Secondary metabolites produced by a root‐inhabiting sterile fungus antagonistic towards pathogenic fungi

Aims: A sterile red fungus (SRF) isolated from cortices of roots of both wheat (Triticum aestivum cv. Gamenya) and ryegrass (Lolium rigidum cv. Wimmera) was found to protect the hosts from phytopathogens and promote plant growth. In this work, the major secondary metabolites produced by this SRF were analysed, and their antibiotic and plant‐growth‐promoting activities investigated. Methods and Results: Two main compounds, veratryl alcohol (VA) and 4‐(hydroxymethyl)‐quinoline, were isolated from the culture filtrate of the fungus. In antifungal assays, VA inhibited the growth of Sclerotinia sclerotiorum and Pythium irregulare even at low amounts, while high concentrations (>100 μg per plug) of 4‐(hydroxymethyl)‐quinoline were needed. Both metabolites revealed weak inhibition of Rhizoctonia solani. Furthermore, both compounds showed a growth promotion activity on canola (Brassica napus) seedlings used as bioassays. Conclusions: Isolation and characterization of the main secondary metabolites from culture filtrates of a root‐inhabiting sterile fungus are reported. The biological assays indicate that these secondary metabolites may have a role in both plant growth regulation and antifungal activity. Significance and Impact of the Study: This study provides a better understanding of the metabolism of a cortical fungus that may have a useful role in the biological suppression of root‐infecting soil‐borne plant pathogens.     

Arbuscular mycorrhizal symbiosis induces strigolactone biosynthesis under drought and improves drought tolerance in lettuce and tomato

Arbuscular mycorrhizal (AM) symbiosis alleviates drought stress in plants. However, the intimate mechanisms involved, as well as its effect on the production of signalling molecules associated with the host plant–AM fungus interaction remains largely unknown. In the present work, the effects of drought on lettuce and tomato plant performance and hormone levels were investigated in non‐AM and AM plants. Three different water regimes were applied, and their effects were analysed over time. AM plants showed an improved growth rate and efficiency of photosystem II than non‐AM plants under drought from very early stages of plant colonization. The levels of the phytohormone abscisic acid, as well as the expression of the corresponding marker genes, were influenced by drought stress in non‐AM and AM plants. The levels of strigolactones and the expression of corresponding marker genes were affected by both AM symbiosis and drought. The results suggest that AM symbiosis alleviates drought stress by altering the hormonal profiles and affecting plant physiology in the host plant. In addition, a correlation between AM root colonization, strigolactone levels and drought severity is shown, suggesting that under these unfavourable conditions, plants might increase strigolactone production in order to promote symbiosis establishment to cope with the stress.     

Mastering ectomycorrhizal symbiosis: the impact of carbohydrates

Mycorrhiza formation is the consequence of a mutualistic interaction between certain soil fungi and plant roots that helps to overcome nutritional limitations faced by the respective partners. In symbiosis, fungi contribute to tree nutrition by means of mineral weathering and mobilization of nutrients from organic matter, and obtain plant-derived carbohydrates as a response. Support with easily degradable carbohydrates seems to be the driving force for fungi to undergo this type of interaction. As a consequence, the fungal hexose uptake capacity is strongly increased in Hartig net hyphae of the model fungi Amanita muscaria and Laccaria bicolor. Next to fast carbohydrate uptake and metabolism, storage carbohydrates are of special interest. In functional A. muscaria ectomycorrhizas, expression and activity of proteins involved in trehalose biosynthesis is mainly localized in hyphae of the Hartig net, indicating an important function of trehalose in generation of a strong carbon sink by fungal hyphae. In symbiosis, fungal partners receive up to approximately 19 times more carbohydrates from their hosts than normal leakage of the root system would cause, resulting in a strong carbohydrate demand of infected roots and, as a consequence, a more efficient plant photosynthesis. To avoid fungal parasitism, the plant seems to have developed mechanisms to control carbohydrate drain towards the fungal partner and link it to the fungus-derived mineral nutrition. In this contribution, current knowledge on fungal strategies to obtain carbohydrates from its host and plant strategies to enable, but also to control and restrict (under certain conditions), carbon transfer are summarized.     

Endophyte or parasite--what decides?

Symbiosis between a fungus and a plant is a widespread phenomenon in nature. The outcome of such an interaction can vary in a seamless manner from mutualism to parasitism. In most cases, the host plant does not suffer, in fact it often gains an advantage from colonization by a fungus. This benefit is based on a fine-tuned balance between the demands of the invader and the plant response. If the interaction becomes unbalanced, disease symptoms appear or the fungus is excluded by induced host defence reactions. Symbioses of plants with beneficial or neutral endophytes share many common attributes with plant interactions with pathogens. Recent findings emerging from studies of compatible host-fungus interactions have enhanced our understanding of what determines whether the fungus behaves as an endophyte or a parasite and of how plants avoid exploitation by detrimental parasites but benefit from mutualistic endophytes.     

Mycorrhiza modulates aboveground tri‐trophic interactions to the fitness benefit of its host plant

1. Arbuscular mycorrhiza (AM), the association of AM fungi and plant roots, may alter morphological and physiological attributes of aboveground plant parts and thereby influence plant‐associated organisms such as herbivores and their natural enemies, predators and parasitoids. 2. The interactions between AM and the players of aboveground tri‐trophic systems have mainly been considered in isolation from each other. The effects of AM on aboveground herbivore–carnivore population dynamics and the consequences to plant fitness are unknown. 3. We explored AM‐induced compensatory mechanisms for AM‐promoted proliferation of the herbivorous spider mite, Tetranychus urticae Koch, on whole bean plants, Phaseolus vulgaris L. Vegetative and reproductive plant growth, AM fungal colonisation levels, and mite densities were assessed on spider mite‐infested plants colonised or not by the AM fungus Glomus mosseae Nicol. & Gerd, and harbouring the natural enemy of the spider mites, the predatory mite Phytoseiulus persimilis Anthias‐Henriot or not. 4. AM symbiosis modulated the aboveground tri‐trophic system to the fitness benefit of the plant. AM‐increased plant productivity outweighed the fitness decrease due to AM‐promoted herbivory: at similar vegetative growth, mycorrhizal plants produced more seeds than non‐mycorrhizal plants. 5. AM‐increased spider mite population levels were compensated for by enhanced population growth of the predators and increased plant tolerance to herbivory. 6. AM‐enhanced predator performance looped back to the AM fungus and stabilised its root colonisation levels, providing the first experimental evidence of a mutually beneficial interaction between AM and an aboveground third trophic level natural enemy.     

Insights to plant–microbe interactions provide opportunities to improve resistance breeding against root diseases in grain legumes

Root and foot diseases severely impede grain legume cultivation worldwide. Breeding lines with resistance against individual pathogens exist, but these resistances are often overcome by the interaction of multiple pathogens in field situations. Novel tools allow to decipher plant–microbiome interactions in unprecedented detail and provide insights into resistance mechanisms that consider both simultaneous attacks of various pathogens and the interplay with beneficial microbes. Although it has become clear that plant‐associated microbes play a key role in plant health, a systematic picture of how and to what extent plants can shape their own detrimental or beneficial microbiome remains to be drawn. There is increasing evidence for the existence of genetic variation in the regulation of plant–microbe interactions that can be exploited by plant breeders. We propose to consider the entire plant holobiont in resistance breeding strategies in order to unravel hidden parts of complex defence mechanisms. This review summarizes (a) the current knowledge of resistance against soil‐borne pathogens in grain legumes, (b) evidence for genetic variation for rhizosphere‐related traits, (c) the role of root exudation in microbe‐mediated disease resistance and elaborates (d) how these traits can be incorporated in resistance breeding programmes.     

低毒病毒及板栗疫病菌低毒力机制

低毒病毒是一类存在于板栗疫病菌细胞质中自主复制的无衣壳正链RNA病毒.感染低毒病毒后,板栗疫病菌的致病力显著降低,色素分泌减少,菌丝体由感染病毒前的桔黄色变为白色,产孢量降低或不产孢,漆酶表达水平明显下降.低毒病毒侵染性克隆的获得以及高效转化和转染体系的建立,使得低毒病毒成为目前唯一可以进行全面遗传操作的真菌病毒.利用低毒病毒作为探针来探测板栗疫病菌的致病力组成和毒力调节机制,已获得了一些很有意义的发现.本文介绍近几年低毒病毒及其与真菌相互作用的研究进展,包括低毒病毒的基因组和功能基因研究、低毒病毒和线粒体损害引起的板栗疫病菌低毒力机制、板栗疫病菌的RNA沉默系统以及低毒病毒抗RNA沉默的机制.低毒病毒/板栗疫病菌系统已经成为研究病毒与宿主相互作用以及病原真菌致病机理的很好的模式系统.     

Ergosterol,an orphan fungal microbe‐associated molecular pattern (MAMP)

Fungal pathogens continue to pose a significant threat to crop production and food supply. The early stages of plant–fungus interactions are mostly mediated by microbe‐associated molecular pattern (MAMP) molecules, perceived by plant pattern recognition receptors (PRRs). Currently, the identified fungal MAMP molecules include chitin, chitosan, β‐glucans, elicitins and ergosterol. Although the molecular battles between host plants and infecting fungal phytopathogens have been studied extensively, many aspects still need to be investigated to obtain a holistic understanding of the intrinsic mechanisms, which is paramount in combating fungal plant diseases. Here, an overview is given of the most recent findings concerning an ‘orphan’ fungal MAMP molecule, ergosterol, and we present what is currently known from a synopsis of different genes, proteins and metabolites found to play key roles in induced immune responses in plant–fungus interactions. Clearly, integrative investigations are still needed to provide a comprehensive systems‐based understanding of the dynamics associated with molecular mechanisms in plant–ergosterol interactions and associated host responses.     

Informationsaustausch in der Ektomykorrhiza

To see the wood for the trees: Communication in ectomycorrhizal symbiosis The mutual symbiosis of ectomycorrhiza has been established in a co‐evolution that depends on a specific communication between the woody plant and the fungus. The exchange of inorganic nutrients and water (delivered by the fungus) for sugar (supplied by the host tree) provides the basis for the symbiosis. The interaction is initiated with signals that can be associated with root exudates and volatiles in the soil matrix. After recognition, the fungus is able to modulate plant response functions that usually suppress pathogens by excretion of effector molecules, which allows entry into the root. Within the root, specific cell wall proteins of the fungus like hydrophobins are important for host specificity. Signals in the mycorrhizal root like the auxin indole‐acetic acid modify the morphology of both partners resulting in the intimate interactions of fully established mycorrhiza. The soil hyphae of the fungus, at the same time, respond to other bacteria and fungi in the mycorrhizosphere.     

Biochemical and physiological mechanisms underlying effects of Cucumber mosaic virus on host‐plant traits that mediate transmission by aphid vectors

The transmission of insect‐vectored diseases entails complex interactions among pathogens, hosts and vectors. Chemistry plays a key role in these interactions; yet, little work has addressed the chemical ecology of insect‐vectored diseases, especially in plant pathosystems. Recently, we documented effects of Cucumber mosaic virus (CMV) on the phenotype of its host (Cucurbita pepo) that influence plant‐aphid interactions and appear conducive to the non‐persistent transmission of this virus. CMV reduces host‐plant quality for aphids, causing rapid vector dispersal. Nevertheless, aphids are attracted to the elevated volatile emissions of CMV‐infected plants. Here, we show that CMV infection (1) disrupts levels of carbohydrates and amino acids in leaf tissue (where aphids initially probe plants and acquire virions) and in the phloem (where long‐term feeding occurs) in ways that reduce plant quality for aphids; (2) causes constitutive up‐regulation of salicylic acid; (3) alters herbivore‐induced jasmonic acid biosynthesis as well as the sensitivity of downstream defences to jasmonic acid; and (4) elevates ethylene emissions and free fatty acid precursors of volatiles. These findings are consistent with previously documented patterns of aphid performance and behaviour and provide a foundation for further exploration of the genetic mechanisms responsible for these effects and the evolutionary processes that shape them.     

Reflections on the role of environmentally-governed reproductive versatility in the adaptation of plant populations

Summary

Mycorrhizal associations vary widely in structure and function, but the commonest interaction is the Arbuscular Mycorrhizal (AM) symbiosis which forms between the roots of over 80% of all terrestrial plant species and Zygomycete fungi of the Order Glomales. These are obligate symbionts which colonise plant root cells. This symbiosis confers benefits directly to the host plants through the acquisition of phosphate and other mineral nutrients from the soil by the fungus while the fungus receives a carbon source from the host. In addition, the symbiosis may also enhance the plants resistance to biotic and abiotic stresses. The beneficial effects of AM symbioses occur as a result of a complex molecular dialogue between the two symbiotic partners. Identifying the molecules and genes involved in the dialogue is necessary for a greater understanding of the symbiosis. This paper reviews the process of AM fungal colonisation of plant roots and the underlying molecular mechanisms associated with the formation and functioning of an AM symbiosis.     

The sucrose transporter SlSUT2 from tomato interacts with brassinosteroid functioning and affects arbuscular mycorrhiza formation

Mycorrhizal plants benefit from the fungal partners by getting better access to soil nutrients. In exchange, the plant supplies carbohydrates to the fungus. The additional carbohydrate demand in mycorrhizal plants was shown to be balanced partially by higher CO₂ assimilation and increased C metabolism in shoots and roots. In order to test the role of sucrose transport for fungal development in arbuscular mycorrhizal (AM) tomato, transgenic plants with down‐regulated expression of three sucrose transporter genes were analysed. Plants that carried an antisense construct of SlSUT2 (SlSUT2as) repeatedly exhibited increased mycorrhizal colonization and the positive effect of plants to mycorrhiza was abolished. Grafting experiments between transgenic and wild‐type rootstocks and scions indicated that mainly the root‐specific function of SlSUT2 has an impact on colonization of tomato roots with the AM fungus. Localization of SISUT2 to the periarbuscular membrane indicates a role in back transport of sucrose from the periarbuscular matrix into the plant cell thereby affecting hyphal development. Screening of an expression library for SlSUT2‐interacting proteins revealed interactions with candidates involved in brassinosteroid (BR) signaling or biosynthesis. Interaction of these candidates with SlSUT2 was confirmed by bimolecular fluorescence complementation. Tomato mutants defective in BR biosynthesis were analysed with respect to mycorrhizal symbiosis and showed indeed decreased mycorrhization. This finding suggests that BRs affect mycorrhizal infection and colonization. If the inhibitory effect of SlSUT2 on mycorrhizal growth involves components of BR synthesis and of the BR signaling pathway is discussed.