81
matter interpreted as resin that might represent a host response. In addition, on the
inner surface of the cells are swellings suggestive of some type of wall apposition.
Although fungi today frequently target the nutritional density in reproductive structures of plants (Vujanovic et al. 2009), the preceding is one of the few persuasive
fossil examples of this fungal strategy.
3.3.1.3 Host Responses in Woody Plants
Woody plants have a long evolutionary and antagonist history with fungi (Schwarze
et al. 2000; Vacher et al. 2008). Wood-degrading fungi encompass a heterogeneous
assemblage of basidiomycetes and ascomycetes, and constitute one of the major
drivers of carbon cycling in forest ecosystems today (Lindahl et al. 2002; van der
Heijden et al. 2008). Some investigators have suggested that wood-rotting fungi
begin their life cycle as parasites, but then, once the host is dead, switch to saprotrophism (Garrett 1970; Lewis 1973). Evidence of decay attributable to fungi is frequently encountered in fossil wood; however, studies focusing on fossil fungal
wood degradation are rare (see Harper et al. 2016 for a review; Wan et al. 2017), and
documented examples of (partially) decayed fossil wood containing well preserved
fungal remains are even rarer. Harper et al. (2017a) report on decaying glossopterid
wood from the Permian of Antarctica that contain fungal remains, symptoms of
white pocket-rot decay, arthropod remains, and host-responses in the form of appositions (Fig. 3.3e). Appositions that occur at sites of infection or attempted penetration by a fungus (Pearce 1996) are composed of material and components not
normally present in cell walls (e.g., phenolic compounds, callose, silicon) plus normal cell wall components, especially suberin (Pearce and Holloway 1984), that can
partially to fully occlude cell lumina to contain or prevent further spreading of the
intruder (Aist 1976, 1983). The lumina of some of the tracheids in the glossopterid
wood are completely sealed by some opaque matter (Harper et al. 2017a: fig. 2F, J),
while the cell walls of other tracheids are swollen and partially occlude the lumen
(Harper et al. 2017a: fig. 2I). Both types of cell lumen occlusion might represent
strategies of passive defense against antagonistic fungal expansion within the wood.
Conspicuous swellings in extant wood have been interpreted as a reaction or barrier
zone to penetration by delignifying fungi (Schwarze and Baum 2000). Similar
appositions have also been documented in other Permian woods from Antarctica
(Stubblefield et al. 1985; Stubblefield and Taylor 1986; Weaver et al. 1997). Other
structures in fossil woods believed to represent host responses to fungi include
ergastic substances and resin (Stubblefield et al. 1985; Gnaedinger et al. 2015).
However, it is difficult to specifically attribute these formations to fungal parasitism
because they are also known to be produced in response to damages caused by fire
or mechanical injury (e.g., Shrimpton 1973; Blanchette and Biggs 1992). In rare
cases, such as in a Jurassic conifer wood from Antarctica, the fungus is in direct
contact with a possible host response in the form of tyloses (Fig. 3.3f) (Harper et al.
2012). Tyloses are bladder- or sac-like outgrowths (protoplasmic bulges) on parenchyma cells that extend into adjacent conducting cells (tracheids, vessels) via pits in
3 Fungi as Parasites: A Conspectus of the Fossil Record
matter interpreted as resin that might represent a host response. In addition, on the
inner surface of the cells are swellings suggestive of some type of wall apposition.
Although fungi today frequently target the nutritional density in reproductive structures of plants (Vujanovic et al. 2009), the preceding is one of the few persuasive
fossil examples of this fungal strategy.
3.3.1.3 Host Responses in Woody Plants
Woody plants have a long evolutionary and antagonist history with fungi (Schwarze
et al. 2000; Vacher et al. 2008). Wood-degrading fungi encompass a heterogeneous
assemblage of basidiomycetes and ascomycetes, and constitute one of the major
drivers of carbon cycling in forest ecosystems today (Lindahl et al. 2002; van der
Heijden et al. 2008). Some investigators have suggested that wood-rotting fungi
begin their life cycle as parasites, but then, once the host is dead, switch to saprotrophism (Garrett 1970; Lewis 1973). Evidence of decay attributable to fungi is frequently encountered in fossil wood; however, studies focusing on fossil fungal
wood degradation are rare (see Harper et al. 2016 for a review; Wan et al. 2017), and
documented examples of (partially) decayed fossil wood containing well preserved
fungal remains are even rarer. Harper et al. (2017a) report on decaying glossopterid
wood from the Permian of Antarctica that contain fungal remains, symptoms of
white pocket-rot decay, arthropod remains, and host-responses in the form of appositions (Fig. 3.3e). Appositions that occur at sites of infection or attempted penetration by a fungus (Pearce 1996) are composed of material and components not
normally present in cell walls (e.g., phenolic compounds, callose, silicon) plus normal cell wall components, especially suberin (Pearce and Holloway 1984), that can
partially to fully occlude cell lumina to contain or prevent further spreading of the
intruder (Aist 1976, 1983). The lumina of some of the tracheids in the glossopterid
wood are completely sealed by some opaque matter (Harper et al. 2017a: fig. 2F, J),
while the cell walls of other tracheids are swollen and partially occlude the lumen
(Harper et al. 2017a: fig. 2I). Both types of cell lumen occlusion might represent
strategies of passive defense against antagonistic fungal expansion within the wood.
Conspicuous swellings in extant wood have been interpreted as a reaction or barrier
zone to penetration by delignifying fungi (Schwarze and Baum 2000). Similar
appositions have also been documented in other Permian woods from Antarctica
(Stubblefield et al. 1985; Stubblefield and Taylor 1986; Weaver et al. 1997). Other
structures in fossil woods believed to represent host responses to fungi include
ergastic substances and resin (Stubblefield et al. 1985; Gnaedinger et al. 2015).
However, it is difficult to specifically attribute these formations to fungal parasitism
because they are also known to be produced in response to damages caused by fire
or mechanical injury (e.g., Shrimpton 1973; Blanchette and Biggs 1992). In rare
cases, such as in a Jurassic conifer wood from Antarctica, the fungus is in direct
contact with a possible host response in the form of tyloses (Fig. 3.3f) (Harper et al.
2012). Tyloses are bladder- or sac-like outgrowths (protoplasmic bulges) on parenchyma cells that extend into adjacent conducting cells (tracheids, vessels) via pits in
3 Fungi as Parasites: A Conspectus of the Fossil Record