Chitinase and peroxidase activities are induced in eucalyptus roots according to aggressiveness of Australian ectomycorrhizal strains of Pisolithus sp

Improvement of Sphaeropsis Shoot Blight Disease Resistance by Applying the Ectomycorrhizal Fungus Hymenochaete sp. Rl and Mycorrhizal Helper Bacterium Bacillus pumilus HR10 to Pinus thunbergii

Ectomycorrhizal fungi (EMFs) form symbioses with plant roots to promote nutrient uptake by plants but it is controversial as to whether they induce disease resistance in plants. Here, we inoculated pine seedlings with Sphaeropsis sapinea, which was presymbiotic with the EMF Hymenochaete sp. Rl, and the mycorrhizal helper bacterium (MHB) Bacillus pumilus HR10, which promotes the formation of Pinus thunbergia-Hymenochaete sp. Rl mycorrhizae. The results showed that inoculation with Hymenochaete sp. Rl, B. pumilus HR10, and the consortium significantly reduced pine shoot blight disease caused by S. sapinea. After inoculation with pathogenic fungi, callose deposition was significantly increased in needles of pine seedlings inoculated with Hymenochaete sp. Rl, B. pumilus HR10, and the consortium, together with an increase in enzymatic and nonenzymatic systemic antioxidant activity as well as early priming for upregulated expression of PR3 and PR5 genes. Our findings suggest that ectomycorrhizal colonization enhances the resistance of pine seedlings to Sphaeropsis shoot blight by triggering a systemic defense response and that interactions between EMFs and MHBs are essential for mycorrhizal-induced disease resistance.

Local Responses and Systemic Induced Resistance Mediated by Ectomycorrhizal Fungi

Ectomycorrhizal fungi (EMF) grow as saprotrophs in soil and interact with plants, forming mutualistic associations with roots of many economically and ecologically important forest tree genera. EMF ensheath the root tips and produce an extensive extramatrical mycelium for nutrient uptake from the soil. In contrast to other mycorrhizal fungal symbioses, EMF do not invade plant cells but form an interface for nutrient exchange adjacent to the cortex cells. The interaction of roots and EMF affects host stress resistance but uncovering the underlying molecular mechanisms is an emerging topic. Here, we focused on local and systemic effects of EMF modulating defenses against insects or pathogens in aboveground tissues in comparison with arbuscular mycorrhizal induced systemic resistance. Molecular studies indicate a role of chitin in defense activation by EMF in local tissues and an immune response that is induced by yet unknown signals in aboveground tissues. Volatile organic compounds may be involved in long-distance communication between below- and aboveground tissues, in addition to metabolite signals in the xylem or phloem. In leaves of EMF-colonized plants, jasmonate signaling is involved in transcriptional re-wiring, leading to metabolic shifts in the secondary and nitrogen-based defense metabolism but cross talk with salicylate-related signaling is likely. Ectomycorrhizal-induced plant immunity shares commonalities with systemic acquired resistance and induced systemic resistance. We highlight novel developments and provide a guide to future research directions in EMF-induced resistance.

Trees, fungi and bacteria: tripartite metatranscriptomics of a root microbiome responding to soil contamination

BACKGROUND

One method for rejuvenating land polluted with anthropogenic contaminants is through phytoremediation, the reclamation of land through the cultivation of specific crops. The capacity for phytoremediation crops, such as Salix spp., to tolerate and even flourish in contaminated soils relies on a highly complex and predominantly cryptic interacting community of microbial life.

METHODS

Here, Illumina HiSeq 2500 sequencing and de novo transcriptome assembly were used to observe gene expression in washed Salix purpurea cv. 'Fish Creek' roots from trees pot grown in petroleum hydrocarbon-contaminated or non-contaminated soil. All 189,849 assembled contigs were annotated without a priori assumption as to sequence origin and differential expression was assessed.

RESULTS

The 839 contigs differentially expressed (DE) and annotated from S. purpurea revealed substantial increases in transcripts encoding abiotic stress response equipment, such as glutathione S-transferases, in roots of contaminated trees as well as the hallmarks of fungal interaction, such as SWEET2 (Sugars Will Eventually Be Exported Transporter). A total of 8252 DE transcripts were fungal in origin, with contamination conditions resulting in a community shift from Ascomycota to Basidiomycota genera. In response to contamination, 1745 Basidiomycota transcripts increased in abundance (the majority uniquely expressed in contaminated soil) including major monosaccharide transporter MST1, primary cell wall and lamella CAZy enzymes, and an ectomycorrhiza-upregulated exo-β-1,3-glucanase (GH5). Additionally, 639 DE polycistronic transcripts from an uncharacterised Enterobacteriaceae species were uniformly in higher abundance in contamination conditions and comprised a wide spectrum of genes cryptic under laboratory conditions but considered putatively involved in eukaryotic interaction, biofilm formation and dioxygenase hydrocarbon degradation.

CONCLUSIONS

Fungal gene expression, representing the majority of contigs assembled, suggests out-competition of white rot Ascomycota genera (dominated by Pyronema), a sometimes ectomycorrhizal (ECM) Ascomycota (Tuber) and ECM Basidiomycota (Hebeloma) by a poorly characterised putative ECM Basidiomycota due to contamination. Root and fungal expression involved transcripts encoding carbohydrate/amino acid (C/N) dialogue whereas bacterial gene expression included the apparatus necessary for biofilm interaction and direct reduction of contamination stress, a potential bacterial currency for a role in tripartite mutualism. Unmistakable within the metatranscriptome is the degree to which the landscape of rhizospheric biology, particularly the important but predominantly uncharacterised fungal genetics, is yet to be discovered.

Peroxidases have more functions than a Swiss army knife

Plant peroxidases (class III peroxidases) are present in all land plants. They are members of a large multigenic family. Probably due to this high number of isoforms, and to a very heterogeneous regulation of their expression, plant peroxidases are involved in a broad range of physiological processes all along the plant life cycle. Due to two possible catalytic cycles, peroxidative and hydroxylic, peroxidases can generate reactive oxygen species (ROS) (*OH, HOO*), polymerise cell wall compounds, and regulate H2O2 levels. By modulating their activity and expression following internal and external stimuli, peroxidases are prevalent at every stage of plant growth, including the demands that the plant meets in stressful conditions. These multifunctional enzymes can build a rigid wall or produce ROS to make it more flexible; they can prevent biological and chemical attacks by raising physical barriers or by counterattacking with a large production of ROS; they can be involved in a more peaceful symbiosis. They are finally present from the first hours of a plant's life until its last moments. Although some functions look paradoxical, the whole process is probably regulated by a fine-tuning that has yet to be elucidated. This review will discuss the factors that can influence this delicate balance.

Diversity and classification of mycorrhizal associations

Most mycorrhizas are 'balanced' mutualistic associations in which the fungus and plant exchange commodities required for their growth and survival. Myco-heterotrophic plants have 'exploitative' mycorrhizas where transfer processes apparently benefit only plants. Exploitative associations are symbiotic (in the broad sense), but are not mutualistic. A new definition of mycorrhizas that encompasses all types of these associations while excluding other plant-fungus interactions is provided. This definition recognises the importance of nutrient transfer at an interface resulting from synchronised plant-fungus development. The diversity of interactions between mycorrhizal fungi and plants is considered. Mycorrhizal fungi also function as endophytes, necrotrophs and antagonists of host or non-host plants, with roles that vary during the lifespan of their associations. It is recommended that mycorrhizal associations are defined and classified primarily by anatomical criteria regulated by the host plant. A revised classification scheme for types and categories of mycorrhizal associations defined by these criteria is proposed. The main categories of vesicular-arbuscular mycorrhizal associations (VAM) are 'linear' or 'coiling', and of ectomycorrhizal associations (ECM) are 'epidermal' or 'cortical'. Subcategories of coiling VAM and epidermal ECM occur in certain host plants. Fungus-controlled features result in 'morphotypes' within categories of VAM and ECM. Arbutoid and monotropoid associations should be considered subcategories of epidermal ECM and ectendomycorrhizas should be relegated to an ECM morphotype. Both arbuscules and vesicles define mycorrhizas formed by glomeromycotan fungi. A new classification scheme for categories, subcategories and morphotypes of mycorrhizal associations is provided.

Two genetically related strains of Tuber borchii produce Tilia mycorrhizas with different morphological traits

Two genetically related strains of Tuber borchii Vittad. (1BO and 43BO) produce mycorrhizas with Tilia platyphyllos Scop. with a different degree of efficiency. The aim of this work was to characterize the morphology of the fungal symbiotic structures in order to examine potential relationships between the anatomical traits of the mycorrhiza, the mycorrhizal capacities of the fungal strains and their effect on the host plants. Some morphological features of mantle hyphae (small size, intense staining, vacuolization, abundance of mitochondria) led to a mantle with morphological features that were isolate-specific. There were unexpected differences, at least under our experimental conditions: 1BO strain mantle cells were larger, less reactive to staining, more highly vacuolated and poorer in mitochondria than those of 43BO. These features were found throughout the mantle in 1BO, while the inner mantle hyphae of 43BO were significantly smaller and more intensely stained than the outer cells. In the 43BO strain there was a positive relation between these features and higher infectivity (evaluated as percentage of mycorrhizal tips) as well as a slightly more effective stimulation of plant growth. These observations suggest that genetically related truffle strains produce mycorrhizas with different morphologies, which may be related to a more efficient response of the host plant to inoculation.

Multitrophic interactions between a Rhizoctonia sp. and mycorrhizal fungi affect Scots pine seedling performance in nursery soil

•  Interspecific variation in ectomycorrhizal fungal (ECMF) control of a root pathogenic uninucleate Rhizoctonia sp. (UnR) was identified in vitro and in planta. •  Fungal-fungal and host-fungal interactions were assessed in direct confrontation and cell-free assays, the rhizosphere of Scots pine (Pinus sylvestris) seedling radicles and seedling mycorrhizospheres developed in N-limited nursery soil. •  Isolates of Suillus bovinus inhibited UnR growth although no agar-diffusable fungicidal activity was detected. Presence of nonsymbiotic ECMF mycelia did not prevent UnR colonization of radicle apices and the onset of damping-off symptoms. Seedlings hosting S. bovinus mycorrhizas and extensive extramatrical mycelium showed vigorous and healthy shoot growth after a 168-d UnR challenge. Root biomass of Wilcoxina mikolae and Paxillus involutus colonized seedlings were negatively affected by both low soil nutrient status and UnR exposure. However, UnR was isolated from long and mycorrhizal short roots in all ECMF coinoculation treatments. •  The differential responses highlight multitrophic host-fungal interaction dynamics that require further characterization in the development of 'efficient' UnR biological control solutions utilizing mycorrhizal fungal inoculants.

Rutin, the phenolglycoside from eucalyptus root exudates, stimulates Pisolithus hyphal growth at picomolar concentrations

•  Ectomycorrhizal hyphal growth is shown to be stimulated by a phenol compound isolated from Eucalyptus globulus ssp. bicostata root exudates, highlighting the importance of phenolics in host-fungal interaction. •  HPLC analysis allowed separation and identification of phenolic compounds from Eucalyptus seedling tissues and root exudates. The activity of the flavonol, rutin, was tested on a range of mycorrhizal and saprophytic fungi. •  Rutin stimulated Pisolithus hyphal growth by more than twofold, and the fungus responded significantly to concentrations as low as 1 pM; only a few strains responded. •  Rutin from Eucalyptus globulus ssp. bicostata root exudates is a flavonoid signal for Pisolithus, and is the first such flavonoid signal identified. A rutin gradient could contribute to orientating hyphal elongation toward the root tip thereby favouring mycorrhizal infection, and might also influence the interaction between fungi in the rhizosphere.

Hypaphorine from the ectomycorrhizal fungus Pisolithus tinctorius counteracts activities of indole-3-acetic acid and ethylene but not synthetic auxins in eucalypt seedlings

Very little is known about the molecules regulating the interaction between plants and ectomycorrhizal fungi during root colonization. The role of fungal auxin in ectomycorrhiza has repeatedly been suggested and questioned, suggesting that, if fungal auxin controls some steps of colonized root development, its activity might be tightly controlled in time and in space by plant and/or fungal regulatory mechanisms. We demonstrate that fungal hypaphorine, the betaine of tryptophan, counteracts the activity of indole-3-acetic acid (IAA) on eucalypt tap root elongation but does not affect the activity of the IAA analogs 2,4-D ((2,4-dichlorophenoxy)acetic acid) or NAA (1-naphthaleneacetic acid). These data suggest that IAA and hypaphorine interact during the very early steps of the IAA perception or signal transduction pathway. Furthermore, while seedling treatment with 1-amincocyclopropane-1-carboxylic acid (ACC), the precursor of ethylene, results in formation of a hypocotyl apical hook, hypaphorine application as well as root colonization by Pisolithus tinctorius, a hypaphorine-accumulating ectomycorrhizal fungus, stimulated hook opening. Hypaphorine counteraction with ACC is likely a consequence of hypaphorine interaction with IAA. In most plant-microbe interactions studied, the interactions result in increased auxin synthesis or auxin accumulation in plant tissues. The P. tinctorius / eucalypt interaction is intriguing because in this interaction the microbe down-regulates the auxin activity in the host plant. Hypaphorine might be the first specific IAA antagonist identified.

Host plant stimulates hypaphorine accumulation in Pisolithus tinctorius hyphae during ectomycorrhizal infection while excreted fungal hypaphorine controls root hair development

The hypaphorine concentration in Pisolithus tinctorius Coker & Couch hyphae colonizing Eucalyptus roots was 3 to 5 times higher than in adjacent parts of the fungal colony. This phenomenon, observed 24 h after inoculation, was also recorded in several-month-old, well-established ectomycorrhizas. Accumulation was controlled by specific root-derived diffusible molecules: it can be induced through a membrane, but not by non-host plants. In pure culture, high hypaphorine concentration was found only in the youngest mycelium, i.e. the outer 2 mm of the colony. Fungal hypaphorine had no IAA-like activity on Eucalyptus root development and therefore could not be considered as an auxin analogue; instead, a strong reduction of root hair elongation was recorded.

Peroxidase and laccase activities in mycorrhizal and non-mycorrhizal fine roots of Norway spruce (Picea abies) and larch (Larix decidua)

Peroxidase (EC 1.11.1.7) and laccase (EC 1.10.3.1) activities were determined in mycorrhizal and non-mycorrhizal main and lateral roots of Picea abies (L.) Karst. (Norway spruce) and Larix decidua Mill, (larch) and in mycelia of the ectomycorrhizal fungus Laccaria amethystea (Bull.) Murr. grown under axenic conditions. Peroxidase isozyme patterns were identified after isoelectric focussing. In both tree species, mycorrhizae contained the lowest, and laterals of noninoculated plants the highest, peroxidase activities. Pure mycelia of Laccaria amethystea contained considerable laccase activity but no peroxidase activity. Laccase activity was barely detected in noninoculated laterals of spruce, but was present in noninoculated laterals of larch and in main roots of Norway spruce and larch. Highest laccase activities were found in mycorrhizae of both tree species, indicating that most of the activity was derived from the fungus. Laterals of Norway spruce contained eight, and those of larch five, acidic peroxidase isozymes. In mycorrhizae of Norway spruce and larch, specific peroxidase isozymes with pI values of 4.5 and 6.2 and 5.8 and 6.0, respectively, were almost completely suppressed. The specific suppression of peroxidase suggests that the fungal symbiont is able to modify the host defence response in mature mycorrhizae. Key words: defence mechanism, laccase, mycorrhiza, peroxidase (isozymes), plant–fungus interaction.