Eucalypt as trap plant to capture associative fungi in soil samples from great depth
Abstract
There are only very few papers reporting on mycorrhizal and endophytic fungi along a whole soil profile in great de pth. The objective of our study was to find propagules of arbuscular mycorrhizal (AMF), ectomycorrhizal (ECM) and dark septate endophytic fungi (DSE) using Eucalyptus grandis as trap plant. We sampled soil of the surface to eight m down in a four -years -old eucalypt plantation. The sampled soil was used as seeding substrate in 300 ml -pots in the greenhouse at a temperature of 28ºC. Plants were grown for 100 days before harvest and evaluation of fungal presence in the plantlets. In most of the treatments (soi l layers of increasing depth) we found root colonization of plantlets by different fungi. In some of the treatments, even in some originating from very deep soil layers, we detected the synthesis of ectomycorrhizal fruiting bodies. Some of the fungi in the plants could be isolated and identified. Therefore, we conclude that in every one of the different soil layers there was the presence of one or more potentially associative fungi with this tree.
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Introduction
Most plants can associate with symbiotic mycorrhizal fungi, a feature that promotes increased nutrient and water absorption, producing vigorous plants resistant to environmental stresses (Smith & Read 2008). Generally, every plant presents only one type of mycorrhizal interaction, however, in a few kinds, as Eucalyptus, one can find a simultaneous colonization with AMF and ECM (Campos et al. 2011).
DSE may also coexist on the same host plant with either kind of mycorrhizal fungi (Wagg et al. 2008; Newsham, 2011; Bonfim et al. 2016). Surveys on the structure of mycorrhizal fungi in plants are normally restricted to the uppermost soil layer (zero to 20 cm) where we find the greatest root biomass (Brundrett 1991). Only a few reports include a survey of more profound soil layers (Oehl et al. 2005; Becerra et al. 2014). Nevertheless, there is a direct relation between the height of a tree and the deepness of its root in the soil. For example, forest plants can have very deep root systems, down to 8 m or more (Pereira 2014). Therefore, several questions may be asked, as: are there associative fungi with Eucalyptus roots in deep soil layers?
To respond to this question our research group followed down the growth and vertical distribution of four year-old Eucalyptus roots and evidenced the presence of a rich and varied microbiota in all soil layers, including ecto and endomycorrhizal fungi, besides other endophytes, even at very deep sites (PEREIRA, 2014).
However, during this survey we could not find out if these fungal communities had the potential to infect Eucalyptus roots. The fact of the fungi being detected at those locations does not necessarily indicate their activity. Therefore, our hypothesis was that there are fungal propagules able to associate with these tree roots down to the greatest soil depth (to 8 m deep). Therefore, we set up a greenhouse experiment in which the sampled soil was transferred into plastic pots to serve as seeding substrate for Eucalyptus grandis seeds to obtain trap plants for these propagules.
This is a pioneer research because it shows the activity of fungi present in soil layers down to 8 m deep. It is of surmount importance to understand the life cycle of the fungal community and the functionality of the fine roots which are essential in furnishing water and nutrients to the trees, especially during scarceness of water in tropical soils in Brazil.
Conclusion
The trap plant method worked quite well to detect the type of fungal propagules present in increasing soil depth layers down to 8 m. Since we took great care not t o contaminate our soil samples between layers, we can affirm that all the fungal structures that were detected originated from propagules found in each of the soil layers. However, it is not possible to guarantee that all of them would have colonized plant roots in the soil profile in the field, since the physical and chemical soil attributes in great depth are quite different from those of the more superficial soil. There are variations in texture, aerat ion, pH, mineral nutrients, etc. In our greenhouse ex periment we modified those attributes to turn them more similar to those of the soil surface, just to allow for plant growth in the sampled soil.
