Identification of genes differentially expressed during the interaction between the plant symbiont Suillus luteus and two plant pathogenic allopatric Heterobasidion species

Beneficial mutualistic fungus Suillus luteus provided excellent buffering insurance in Scots pine defense responses under pathogen challenge at transcriptome level

BACKGROUND

Mutualistic mycorrhiza fungi that live in symbiosis with plants facilitates nutrient and water acquisition, improving tree growth and performance. In this study, we evaluated the potential of mutualistic fungal inoculation to improve the growth and disease resistance of Scots pine (Pinus sylvestris L.) against the forest pathogen Heterobasidion annosum.

RESULTS

In co-inoculation experiment, Scots pine seedlings were pre-inoculated with mutualistic beneficial fungus (Suillus luteus) prior to H. annosum infection. The result revealed that inoculation with beneficial fungus promoted plant root growth. Transcriptome analyses revealed that co-inoculated plants and plants inoculated with beneficial fungus shared some similarities in defense gene responses. However, pathogen infection alone had unique sets of genes encoding pathogenesis-related (PR) proteins, phenylpropanoid pathway/lignin biosynthesis, flavonoid biosynthesis, chalcone/stilbene biosynthesis, ethylene signaling pathway, JA signaling pathway, cell remodeling and growth, transporters, and fungal recognition. On the other hand, beneficial fungus inoculation repressed the expression of PR proteins, and other defense-related genes such as laccases, chalcone/stilbene synthases, terpene synthases, cytochrome P450s. The co-inoculated plants did not equally enhance the induction of PR genes, chalcone/stilbene biosynthesis, however genes related to cell wall growth, water and nutrient transporters, phenylpropanoid/lignin biosynthesis/flavonoid biosynthesis, and hormone signaling were induced.

CONCLUSION

S. luteus promoted mutualistic interaction by suppressing plant defense responses. Pre-inoculation of Scots pine seedlings with beneficial fungus S. luteus prior to pathogen challenge promoted primary root growth, as well as had a balancing buffering role in plant defense responses and cell growth at transcriptome level.

Effect of Deadwood on Ectomycorrhizal Colonisation of Old-Growth Oak Forests

Although the importance of coarse woody debris (CWD) for species diversity is recognized, the effects of coarse woody debris decay class on species composition have received little attention. We examined how the species composition of ectomycorrhizal fungi (ECM) changes with CWD decay. We describe ectomycorrhizal root tips and the diversity of mycorrhizal fungal species at three English oak (Quercus robur L.) sites. DNA barcoding revealed a total of 17 ECM fungal species. The highest degree of mycorrhizal colonization was found in CWDadvanced (27.2%) and CWDearly (27.1%). Based on exploration types, ectomycorrhizae were classified with respect to ecologically relevant soil features. The short distance type was significantly correlated with soil P2O5, while the contact type was correlated with soil C/N. The lowest mean content of soil Corg was found in the CWDabsent site. The difference in total soil N between sites decreased with increasing CWD decomposition, whereas total C/N increased correspondingly. In this study we confirmed that soil CWD stimulates ectomycorrhizal fungi, representing contact or short-distance exploration types of mycelium.

Comparative transcriptional and metabolic responses of Pinus pinea to a native and a non-native Heterobasidion species

Heterobasidion irregulare is a causal agent of root and butt-rot disease in conifers, and is native to North America. In 1944 it was introduced in central Italy in a Pinus pinea stand, where it shares the same niche with the native species Heterobasidion annosum. The introduction of a non-native pathogen may have significant negative effects on a naïve host tree and the ecosystem in which it resides, requiring a better understanding of the system. We compared the spatio-temporal phenotypic, transcriptional and metabolic host responses to inoculation with the two Heterobasidion species in a large experiment with P. pinea seedlings. Differences in length of lesions at the inoculation site (IS), expression of host genes involved in lignin pathway and in cell rescue and defence, and analysis of terpenes at both IS and 12 cm above the IS (distal site, DS), were assessed at 3, 14 and 35 days post inoculation (dpi). Results clearly showed that both species elicit similar physiological and biochemical responses in P. pinea seedlings. The analysis of host transcripts and total terpenes showed differences between inoculation sites and between pathogen and mock inoculated plants. Both pathogen and mock inoculations induced antimicrobial peptide and phenylalanine ammonia-lyase overexpression at IS beginning at 3 dpi; while at DS all the analysed genes, except for peroxidase, were overexpressed at 14 dpi. A significantly higher accumulation of terpenoids was observed at 14 dpi at IS, and at 35 dpi at DS. The terpene blend at IS showed significant variation among treatments and sampling times, while no significant differences were ever observed in DS tissues. Based on our results, H. irregulare does not seem to have competitive advantages over the native species H. annosum in terms of pathogenicity towards P. pinea trees; this may explain why the non-native species has not widely spread over the 73 years since its putative year of introduction into central Italy.

A nonnative and a native fungal plant pathogen similarly stimulate ectomycorrhizal development but are perceived differently by a fungal symbiont

The effects of plant symbionts on host defence responses against pathogens have been extensively documented, but little is known about the impact of pathogens on the symbiosis and if such an impact may differ for nonnative and native pathogens. Here, this issue was addressed in a study of the model system comprising Pinus pinea, its ectomycorrhizal symbiont Tuber borchii, and the nonnative and native pathogens Heterobasidion irregulare and Heterobasidion annosum, respectively. In a 6-month inoculation experiment and using both in planta and gene expression analyses, we tested the hypothesis that H. irregulare has greater effects on the symbiosis than H. annosum. Although the two pathogens induced the same morphological reaction in the plant-symbiont complex, with mycorrhizal density increasing exponentially with pathogen colonization of the host, the number of target genes regulated in T. borchii in plants inoculated with the native pathogen (i.e. 67% of tested genes) was more than twice that in plants inoculated with the nonnative pathogen (i.e. 27% of genes). Although the two fungal pathogens did not differentially affect the amount of ectomycorrhizas, the fungal symbiont perceived their presence differently. The results may suggest that the symbiont has the ability to recognize a self/native and a nonself/nonnative pathogen, probably through host plant-mediated signal transduction.

Understanding plant cell-wall remodelling during the symbiotic interaction between Tuber melanosporum and Corylus avellana using a carbohydrate microarray

A combined approach, using a carbohydrate microarray as a support for genomic data, has revealed subtle plant cell-wall remodelling during Tuber melanosporum and Corylus avellana interaction. Cell walls are involved, to a great extent, in mediating plant-microbe interactions. An important feature of these interactions concerns changes in the cell-wall composition during interaction with other organisms. In ectomycorrhizae, plant and fungal cell walls come into direct contact, and represent the interface between the two partners. However, very little information is available on the re-arrangement that could occur within the plant and fungal cell walls during ectomycorrhizal symbiosis. Taking advantage of the Comprehensive Microarray Polymer Profiling (CoMPP) technology, the current study has had the aim of monitoring the changes that take place in the plant cell wall in Corylus avellana roots during colonization by the ascomycetous ectomycorrhizal fungus T. melanosporum. Additionally, genes encoding putative plant cell-wall degrading enzymes (PCWDEs) have been identified in the T. melanosporum genome, and RT-qPCRs have been performed to verify the expression of selected genes in fully developed C. avellana/T. melanosporum ectomycorrhizae. A localized degradation of pectin seems to occur during fungal colonization, in agreement with the growth of the ectomycorrhizal fungus through the middle lamella and with the fungal gene expression of genes acting on these polysaccharides.