Abstract
In spite of the very significant role that fungi are called to play in agricultural production and climate change over the next two decades, very little is known at this point about the parameters that control the spread of fungal hyphae in the pore space of soils. Monitoring of this process in 3 dimensions is not technically feasible at the moment. The use of transparent micromodels simulating the internal geometry of real soils affords an opportunity to approach the problem in 2 dimensions, provided it is confirmed that fungi would actually want to propagate in such artificial systems. In this context, the key objectives of the research described in this article are to ascertain, first, that the fungus Rhizoctonia solani can indeed grow in a micromodel of a sandy loam soil, and, second, to identify and analyze in detail the pattern by which it spreads in the tortuous pores of the micromodel. Experimental observations show that hyphae penetrate easily inside the micromodel, where they bend frequently to adapt to the confinement to which they are subjected, and branch at irregular intervals, unlike in current computer models of the growth of hyphae, which tend to describe them as series of straight tubular segments. A portion of the time, hyphae in the micromodels also exhibit thigmotropism, i.e., tend to follow solid surfaces closely. Sub-apical branching, which in unconfined situations seems to be controlled by the fungus, appears to be closely connected with the bending of the hyphae, resulting from their interactions with surfaces. These different observations not only indicate different directions to follow to modify current mesoscopic models of fungal growth, so they can apply to soils, but they also suggest a wealth of further experiments using the same set-up, involving for example competing fungal hyphae, or the coexistence of fungi and bacteria in the same pore space.
Highlights
An estimated 1.5 million species of fungi are present in terrestrial ecosystems (Hawksworth, 2001) where they fulfill a wide array of essential ecological functions, in particular in the global carbon cycle (Cromack and Caldwell, 1992)
Close to the entrance of the microporous portion of the micromodel (Figure 2b), whenever Rhizoctonia does not grow along the wall of the cavity, the branching pattern is very similar to what was observed earlier in the Potato dextrose agar (PDA) agar plates, which itself was in line with accounts published in the literature
The research described above corresponds to a first attempt to use a soil-like micromodel to identify the parameters that control the growth of fungal hyphae in the confining pore space of soils
Summary
An estimated 1.5 million species of fungi are present in terrestrial ecosystems (Hawksworth, 2001) where they fulfill a wide array of essential ecological functions, in particular in the global carbon cycle (Cromack and Caldwell, 1992) Their role in soil-plant feedback processes in the rhizosphere is widely regarded as key to achieving the estimated 100% increase in overall food production that will be needed in the 25 years, amidst decreasing availability of suitable land and already overexploited surface- or groundwater resources (e.g., Sposito, 2013; Baveye, 2015; Baveye et al, 2018). Fungal hyphae absorb and mineralize stable biomolecules like cellulose or lignin Since they can access organic matter and nutrients located in much tinier pores than those typically accessible to plant roots, fungi are able to provide sustenance that otherwise would be difficult for over 90% of vascular plants to take up on their own (Boddy, 1993). Last in this quick overview, but certainly not least, fungi play a crucial role in stabilizing the architecture of soils (e.g., Miller and Jastrow, 2000)
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