Abstract
Research Highlights: Mycorrhizal helper bacteria (MHB) promote mycorrhization processes and are commonly found in the mycorrhizosphere of fungi, such as the edible hypogeous fungus Tuber melanosporum Vittad. Background and Objectives: The effectiveness of MHB in promoting the mycorrhization process and the root development of Portuguese oak (Quercus faginea Lam.) seedlings destined for truffle plantations has not been determined. The main aim of this study was to shed light on the effect of bacterial inoculation on fungal root tip colonization and seedling root traits. Material and methods: We performed a co-inoculation trial using three bacteria naturally present in the T. melanosporum niche (i.e., Pseudomonas fluorescens, Pseudomonas putida, and Bacillus amyloliquefaciens) and two different bacterial inoculation times (one month and nine months after fungal inoculation) under glasshouse conditions. Results: Only P. fluorescens had a significant mycorrhizal promoter effect, increasing the truffle inoculation rates of root tips by more than 10% compared with seedlings that received non-bacterial inoculation treatments. Simultaneously, the co-inoculation of P. fluorescens with T. melanosporum improved seedling root growth parameters compared with those of seedlings that received non-bacterial inoculation treatments. The different bacterial inoculation times and applications of uninoculated bacterial growth media did not affect the root traits analyzed or the root mycorrhization rates. Conclusions: These results suggest that P. fluorescens bacteria have a potential commercial application as a treatment for truffle-inoculated seedlings to improve both seedling quality and mycorrhizal colonization under nursery conditions.
Highlights
The hypogeous fruiting bodies of fungal species in the Tuber genus form ectomycorrhizal (ECM)associations with angiosperms and gymnosperms worldwide [1]
The bacterial inoculation time did not appear to have any interactive effect on mycorrhization rates (F[1,129] value = 0.04, p = 0.84); we decided that mycorrhization data obtained for both bacterial inoculation times (T1 and T2) for each type of inoculation treatment should be grouped together for further analysis (Table 1)
Analysis of the effect of individual bacteria on T. melanosporum root tip colonization rates (Table 1) showed that colonization rates ranged from a minimum of 25.4% ± 2.7% of root tips colonized by T. melanosporum, which was recorded for control seedlings (CS), to a maximum of 35.1% ± 1.6% of root tips colonized by T. melanosporum, which was recorded for seedlings inoculated with P. fluorescens (F)
Summary
The hypogeous fruiting bodies of fungal species in the Tuber genus form ectomycorrhizal (ECM)associations with angiosperms and gymnosperms worldwide [1]. Truffle cultivation involves inoculating specific host-tree seedlings in the nursery with a truffle inoculum, and planting these seedlings in areas with suitable conditions for the development of the fungus [6]. Fungal spores need to make contact with the seedling’s fine roots so that when the spores germinate, they form mycorrhizae with emerging root tips, establishing symbiotic associations [7]. The use of high truffle inoculation rates is recommended on seedlings intended for planting, with the minimum percentage of 30% T. melanosporum mycorrhizal root tips indicated as a prerequisite that slightly varies depending on the methodology used for mycorrhizal seedling evaluation [8,9,10]. Different inoculation methods have been developed to increase the mycorrhization rate of black truffle in seedlings, such as the use of soil from productive trees [11]
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