propagules by sending out new hyphae (Ainsworth and Sussman 1966). In the field
of phytopathology, there have been many studies of sclerotia of Rhizoctonia solani
and Athelia rolfsii as primary sources of plant infections. Many of these studies have
focused on factors related to sclerotial formation and germination as a means of
reducing infection (Hausner and Reid 1999; Moromizato et al. 1991). At the same
time, research on the relationship between sclerotia and bacteria has focused on
microbial antagonism and volatile substances produced by sclerotia as defensive
responses (Fravel et al. 2002). When sclerotia of R. solani and A. rolfsii are placed in
soil, bacteria colonize and propagate on the sclerotial surface. The resulting
increased microbial activity in the mycosphere is referred to as “enhanced microbial
activity” (Naiki and Ui 1969; Gilbert and Linderman 1971).
The morphological characteristics of sclerotia suggest that they are formed by
dark septate endophytic (DSE) fungi including Cenococcum geophilum (Cg).
Cenococcum geophilum is a ubiquitous endophytic fungus that is widely distributed
from polar to tropic regions (LoBuglio 1999). Accordingly, Cg sclerotia are also
found ubiquitously but have not been studied to the same degree as sclerotia of plant
pathogens. Thus, many aspects of their ecology remain unknown. There is a report
of detection of antibiotic substances in the soil surrounding Cg mycorrhiza and
sclerotia (Krywolap et al. 1964).
Microbial carrier substrates have also been shown to support the effective functioning of beneficial microorganisms in soil. When beneficial microorganisms are
inoculated directly to soil, they are often rendered ineffective by competition with
indigenous microorganisms. However, the activity of beneficial microorganisms can
be maintained if the microorganisms are first attached to a carrier substrate. There
have been many studies of agricultural inoculation and the use of porous substrates
such as charcoal and ceramics as bacterial carriers to control disease or odor and to
promote composting (Noguchi 1997).
Although the mycosphere is known to have high microbial activity and fungi are
known to produce antibiotics to protect against colonization by microorganisms, the
internal spaces of sclerotia also serve as a habitat for other soil microorganisms,
resulting in the formation of distinct microbial communities inside and outside
sclerotia (Nonoyama 2010). Ohta et al. (2003) examined the bacterial community
identified from sclerotia grains in forest soils from Mt. Myoko, Japan and revealed
the predominance of alphaproteobacteria, represented by Sphingomonas spp. (currently Sphingobium). Although none of the representative isolates tested utilized
their model polycyclic aromatic hydrocarbon (PAH; naphthalenesulfonic acid),
many used p-hydroxybenzoic, vanillic, p-coumaric, and ferulic acids for growth.
An interesting observation that deserves further discussion is that the biochemical
characteristics of Sphingomonas isolates that showed activity inside the sclerotia
grains were not adaptive to the soil environment where the sclerotia were found.
These findings suggest endofungal bacteria as the possible origin of the bacterial
colonies inside sclerotia grains (e.g., Sato et al. 2010; Arendt et al. 2016). Nevertheless, the composition of the microbial community within sclerotia is likely
influenced by sclerotial structure, factors, and other sclerotial characteristics.
64
Y. S. Nonoyama and K. Narisawa
of phytopathology, there have been many studies of sclerotia of Rhizoctonia solani
and Athelia rolfsii as primary sources of plant infections. Many of these studies have
focused on factors related to sclerotial formation and germination as a means of
reducing infection (Hausner and Reid 1999; Moromizato et al. 1991). At the same
time, research on the relationship between sclerotia and bacteria has focused on
microbial antagonism and volatile substances produced by sclerotia as defensive
responses (Fravel et al. 2002). When sclerotia of R. solani and A. rolfsii are placed in
soil, bacteria colonize and propagate on the sclerotial surface. The resulting
increased microbial activity in the mycosphere is referred to as “enhanced microbial
activity” (Naiki and Ui 1969; Gilbert and Linderman 1971).
The morphological characteristics of sclerotia suggest that they are formed by
dark septate endophytic (DSE) fungi including Cenococcum geophilum (Cg).
Cenococcum geophilum is a ubiquitous endophytic fungus that is widely distributed
from polar to tropic regions (LoBuglio 1999). Accordingly, Cg sclerotia are also
found ubiquitously but have not been studied to the same degree as sclerotia of plant
pathogens. Thus, many aspects of their ecology remain unknown. There is a report
of detection of antibiotic substances in the soil surrounding Cg mycorrhiza and
sclerotia (Krywolap et al. 1964).
Microbial carrier substrates have also been shown to support the effective functioning of beneficial microorganisms in soil. When beneficial microorganisms are
inoculated directly to soil, they are often rendered ineffective by competition with
indigenous microorganisms. However, the activity of beneficial microorganisms can
be maintained if the microorganisms are first attached to a carrier substrate. There
have been many studies of agricultural inoculation and the use of porous substrates
such as charcoal and ceramics as bacterial carriers to control disease or odor and to
promote composting (Noguchi 1997).
Although the mycosphere is known to have high microbial activity and fungi are
known to produce antibiotics to protect against colonization by microorganisms, the
internal spaces of sclerotia also serve as a habitat for other soil microorganisms,
resulting in the formation of distinct microbial communities inside and outside
sclerotia (Nonoyama 2010). Ohta et al. (2003) examined the bacterial community
identified from sclerotia grains in forest soils from Mt. Myoko, Japan and revealed
the predominance of alphaproteobacteria, represented by Sphingomonas spp. (currently Sphingobium). Although none of the representative isolates tested utilized
their model polycyclic aromatic hydrocarbon (PAH; naphthalenesulfonic acid),
many used p-hydroxybenzoic, vanillic, p-coumaric, and ferulic acids for growth.
An interesting observation that deserves further discussion is that the biochemical
characteristics of Sphingomonas isolates that showed activity inside the sclerotia
grains were not adaptive to the soil environment where the sclerotia were found.
These findings suggest endofungal bacteria as the possible origin of the bacterial
colonies inside sclerotia grains (e.g., Sato et al. 2010; Arendt et al. 2016). Nevertheless, the composition of the microbial community within sclerotia is likely
influenced by sclerotial structure, factors, and other sclerotial characteristics.
64
Y. S. Nonoyama and K. Narisawa