8.1 Introduction
Sclerotia are durable structures formed in soil and on plants by various fungal
species (Willetts 1971, 1972), including crop pathogenic fungi such as Sclerotium
hydrophilum and Sclerotinia sclerotiorum (Hausner and Reid 1999; Cessna et al.
2000). Spherical black sclerotia (0.2–4 mm) formed by the ectomycorrhizal fungus
Cenococcum geophilum Fr. (Cg) have frequently been collected from forest soils
(Trappe 1969; Massicotte et al. 1992; Obase et al. 2014). Cg sclerotia are hard,
compact masses of fungal hyphae that contain preservatives such as glycogen
(LoBuglio 1999).
The biochemical properties of Cg sclerotia that allow them to resist degradation
are unclear. Various components, including melanin-like pigments, that might
contribute to the resistance of Sclerotium rolfsii Sacc. sclerotia to biological and
chemical degradation (Chet et al. 1967) have been discussed, and melanin deposited
in cell walls has also been implicated in the degradation resistance of Cg sclerotia
(Malik and Haider 1982). In addition, a particular fungal melanin,
dihydroxynaphthalene melanin, known as polyketide melanin, is believed to be
involved in the strong resistance of various fungi, including Cg, to irradiation,
enzymatic lysis, and environmental stresses such as water stress (Butler and Day
1998; Fernandez and Koide 2013). Most of the organic constituents of fungal cell
walls and sclerotia are believed to be labile compounds (Dallies et al. 1998). For
example, neutral saccharides account for 6.0–16% of the carbon content of Cg
sclerotia collected from soil on Mount Ontake, central Japan (Sugiura et al. 2017).
Melanic compounds present in tree root tips colonized by Cg, extramatrical mycelium, and sclerotia are likely to be large and stable pools of carbon in forest soils
(Dahlberg et al. 1997).
Sclerotia often contain Al, in several forms. Hodson and Wilkins (1991) reported
that in Paxillus involutus Fr. the aluminum is almost entirely confined to the cortical
cell walls and is not detectable inside the endodermis of the roots of Norway spruce
inoculated with P. involutus. Jentschke et al. (1991) performed an experiment with
spruce seedlings that were colonized with Lactarius rufus Fr. and exposed to 800 μM
Al(NO 3 ) 3 (pH 3.9) for 13 or 17 weeks and confirmed that the aluminum concentration in the cell walls of mycorrhizal tissues is high. These results suggest that Cg
sclerotia may also contain Al, in the cell walls of the Cg mycelia that formed the
sclerotia; Al is highly available in low-pH forest soils. However, the potential
phytotoxicity of aluminum–humus complexes due to their instability in volcanic
ash soil under low-pH conditions has been revealed by plant culture tests performed
using synthetic aluminum–humus complexes (Takahashi et al. 2007). Degradation
of aluminum–carbon complexes in sclerotia and the transformation of aluminum and
carbon in sclerotia are not well understood. The degradation resistance of Cg
sclerotia in soil and sediment is also of interest to researchers in the fields of
geoscience and geochemistry. For example, Hormes et al. (2004) used
14 C dating
to estimate that Cg “spores” found in buried soils in glacial sediments in northern
Sweden were 5000 years old, and this information was used to clarify the
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M. Watanabe and A. Genseki
Sclerotia are durable structures formed in soil and on plants by various fungal
species (Willetts 1971, 1972), including crop pathogenic fungi such as Sclerotium
hydrophilum and Sclerotinia sclerotiorum (Hausner and Reid 1999; Cessna et al.
2000). Spherical black sclerotia (0.2–4 mm) formed by the ectomycorrhizal fungus
Cenococcum geophilum Fr. (Cg) have frequently been collected from forest soils
(Trappe 1969; Massicotte et al. 1992; Obase et al. 2014). Cg sclerotia are hard,
compact masses of fungal hyphae that contain preservatives such as glycogen
(LoBuglio 1999).
The biochemical properties of Cg sclerotia that allow them to resist degradation
are unclear. Various components, including melanin-like pigments, that might
contribute to the resistance of Sclerotium rolfsii Sacc. sclerotia to biological and
chemical degradation (Chet et al. 1967) have been discussed, and melanin deposited
in cell walls has also been implicated in the degradation resistance of Cg sclerotia
(Malik and Haider 1982). In addition, a particular fungal melanin,
dihydroxynaphthalene melanin, known as polyketide melanin, is believed to be
involved in the strong resistance of various fungi, including Cg, to irradiation,
enzymatic lysis, and environmental stresses such as water stress (Butler and Day
1998; Fernandez and Koide 2013). Most of the organic constituents of fungal cell
walls and sclerotia are believed to be labile compounds (Dallies et al. 1998). For
example, neutral saccharides account for 6.0–16% of the carbon content of Cg
sclerotia collected from soil on Mount Ontake, central Japan (Sugiura et al. 2017).
Melanic compounds present in tree root tips colonized by Cg, extramatrical mycelium, and sclerotia are likely to be large and stable pools of carbon in forest soils
(Dahlberg et al. 1997).
Sclerotia often contain Al, in several forms. Hodson and Wilkins (1991) reported
that in Paxillus involutus Fr. the aluminum is almost entirely confined to the cortical
cell walls and is not detectable inside the endodermis of the roots of Norway spruce
inoculated with P. involutus. Jentschke et al. (1991) performed an experiment with
spruce seedlings that were colonized with Lactarius rufus Fr. and exposed to 800 μM
Al(NO 3 ) 3 (pH 3.9) for 13 or 17 weeks and confirmed that the aluminum concentration in the cell walls of mycorrhizal tissues is high. These results suggest that Cg
sclerotia may also contain Al, in the cell walls of the Cg mycelia that formed the
sclerotia; Al is highly available in low-pH forest soils. However, the potential
phytotoxicity of aluminum–humus complexes due to their instability in volcanic
ash soil under low-pH conditions has been revealed by plant culture tests performed
using synthetic aluminum–humus complexes (Takahashi et al. 2007). Degradation
of aluminum–carbon complexes in sclerotia and the transformation of aluminum and
carbon in sclerotia are not well understood. The degradation resistance of Cg
sclerotia in soil and sediment is also of interest to researchers in the fields of
geoscience and geochemistry. For example, Hormes et al. (2004) used
14 C dating
to estimate that Cg “spores” found in buried soils in glacial sediments in northern
Sweden were 5000 years old, and this information was used to clarify the
140
M. Watanabe and A. Genseki