82
the cell walls and, in this way, can block the dead conducting cells to counteract the
spreading of phytopathogenic organisms. They may be filled with various substances (tannins, gums, resins, etc.) as a result of physical damage or parasitic activity (Collins and Parke 2008; Feng et al. 2013; for an extensive review, see De Micco
et al. 2016). Harper et al. (2012) hypothesize that the Jurassic tyloses formed a
physical barrier to prevent the fungus from spreading. The morphology and pattern
of colonization suggest that the fossil shares similarities with various extant
Ascomycota, including sap-stain, blue-stain, and dark-stain fungi that are pathogens
of various conifers (see Ballard et al. 1982; Hessburg and Hansen 1987). Last, in
some extant gymnosperms, including Pinus, the number of resin ducts in the xylem
may increase as a result of a fungal infection (Martín-Rodrigues et al. 2013). This is
certainly a structural feature that can also be recognized in fossil wood. In addition,
certain types of tissue disruption are caused by parasitic plants invading stem tissue,
but may also be the result of fungal infection (Gomes and Fernandes 1994; do
Amaral and Ceccantini 2011). However, no evidence of such tissue disruptions in
fossil wood has been produced to date, which may be due in part to the fact that
most investigators of fossil wood lack a search image for such structures, or perhaps
attribute the disruptions to a different cause.
3.3.1.4 Host Plant Preservation and Fungal Distribution
While the evidence used to infer fungal parasitism in the fossils surveyed in the
preceding sections largely consists of fossilized host responses, there is one example of a fossil fungus-land plant interaction that deserves special mention because in
this case host plant preservation and fungal distribution within the host have been
used to infer the nutritional mode of the fungus (Barthel et al. 2010; Krings et al.
2017b). This fungus occurs in a silicified Early Permian Psaronius root mantle from
Germany, and displays a consistent pattern of host cell colonization that includes
the formation of swellings effective in pushing a hyphal tip through the host cell
wall and multi-branched structures remotely resembling arbuscules and certain
haustoria that probably served in nutrient extraction or exchange (Fig. 3.3g). The
different tissues of the host root mantle, including the fragile root aerenchyma, are
exquisitely preserved, suggesting that the roots were intact, and thus probably alive
at the time of fossilization. Moreover, the strictly intracellular growth pattern of the
fungus seems implausible for a saprotroph that extends through moribund or dead
and decaying plant tissue. However, the fungus did apparently not trigger any host
response or disease symptom, suggesting it may have been a harmless endophyte or
mild parasite, which extracted some nutrients, but not in an amount sufficient
enough to cause notable damage. It is also possible, however, that the fungus was
well adapted to its mode of life, rendering it “invisible” to the immune response of
the plant, although this is virtually impossible to test in fossils.
C. J. Harper and M. Krings
the cell walls and, in this way, can block the dead conducting cells to counteract the
spreading of phytopathogenic organisms. They may be filled with various substances (tannins, gums, resins, etc.) as a result of physical damage or parasitic activity (Collins and Parke 2008; Feng et al. 2013; for an extensive review, see De Micco
et al. 2016). Harper et al. (2012) hypothesize that the Jurassic tyloses formed a
physical barrier to prevent the fungus from spreading. The morphology and pattern
of colonization suggest that the fossil shares similarities with various extant
Ascomycota, including sap-stain, blue-stain, and dark-stain fungi that are pathogens
of various conifers (see Ballard et al. 1982; Hessburg and Hansen 1987). Last, in
some extant gymnosperms, including Pinus, the number of resin ducts in the xylem
may increase as a result of a fungal infection (Martín-Rodrigues et al. 2013). This is
certainly a structural feature that can also be recognized in fossil wood. In addition,
certain types of tissue disruption are caused by parasitic plants invading stem tissue,
but may also be the result of fungal infection (Gomes and Fernandes 1994; do
Amaral and Ceccantini 2011). However, no evidence of such tissue disruptions in
fossil wood has been produced to date, which may be due in part to the fact that
most investigators of fossil wood lack a search image for such structures, or perhaps
attribute the disruptions to a different cause.
3.3.1.4 Host Plant Preservation and Fungal Distribution
While the evidence used to infer fungal parasitism in the fossils surveyed in the
preceding sections largely consists of fossilized host responses, there is one example of a fossil fungus-land plant interaction that deserves special mention because in
this case host plant preservation and fungal distribution within the host have been
used to infer the nutritional mode of the fungus (Barthel et al. 2010; Krings et al.
2017b). This fungus occurs in a silicified Early Permian Psaronius root mantle from
Germany, and displays a consistent pattern of host cell colonization that includes
the formation of swellings effective in pushing a hyphal tip through the host cell
wall and multi-branched structures remotely resembling arbuscules and certain
haustoria that probably served in nutrient extraction or exchange (Fig. 3.3g). The
different tissues of the host root mantle, including the fragile root aerenchyma, are
exquisitely preserved, suggesting that the roots were intact, and thus probably alive
at the time of fossilization. Moreover, the strictly intracellular growth pattern of the
fungus seems implausible for a saprotroph that extends through moribund or dead
and decaying plant tissue. However, the fungus did apparently not trigger any host
response or disease symptom, suggesting it may have been a harmless endophyte or
mild parasite, which extracted some nutrients, but not in an amount sufficient
enough to cause notable damage. It is also possible, however, that the fungus was
well adapted to its mode of life, rendering it “invisible” to the immune response of
the plant, although this is virtually impossible to test in fossils.
C. J. Harper and M. Krings