like) in 2.7 Gya fossilized marine shales. Evidence for an early Silurian (440 Mya)
origin, based on a study of a consortium of bacteria and cyanobacteria as the
photosynthetic partner in a microbial mat, was also published (Tomescu et al.
2008), while other reports indicate that lichen fossils (with both cyanobacteria and
green algae) were present from the Lower Devonian (Honegger et al. 2013a, b). The
connections linking the lichen-like fossils of the Watanabe et al. studies and other
microbial mat-like consortia have not been made. In any cases, the increased O 2
brought about by cyanobacterial and unicellular green algal photosynthesis in the
ancient world most likely led to the quicker formation of an ozone shield, thereby
reducing ultraviolet irradiation and accelerating the rate of evolution.
Strong evidence for land plant evolution is based on fossils from the middle
Ordovician to the early Silurian. In mid-Ordovician (460 to 455 Mya) shales from
Saudi Arabia and Argentina, spores similar to bryophyte (mosses and liverwort)
spores (Strother et al. 1996; Rubenstein et al. 2010) were found. A plant similar to
Cooksonia, a leafless, rootless plant with terminal sporangia, might be a possible
candidate. That fungi in the Ordovician became associated with plants was demonstrated by finding fossilized non-septate hyphae and spores of Glomalean-like fungi
within bryophyte cells (Redecker et al. 2000). Additional evidence for the evolution
of land plants comes from the Rhynie Chert (Scotland) in the fossils of Algaophyton
(formerly Rhynia) major. Edwards (1986) described this plant based on a study of
fossils that had characters of both bryophytes and tracheophytes (vascular plants).
The dichotomously branched vertical axis of the plant was ca. 183 mm tall and
rootless, but on one side of the fossilized underground rhizomes, root hair-like
structures (rhizoids) were found. The vertical axis was externally characterized by
stomata on the epidermis and terminal sporangia, and sections of the axis show what
appears to be conducting cells, but no true vascular tissue, leading Edwards (1986) to
decide that the plant was a bryophyte. In addition, Edwards noted that cell walls of
the outer cortex contained fungal hyphae. Later, Remy et al. (1994) observed that
highly branched hyphae (arbuscules) were present in cortical cells, indicating that
even the most ancient bryophytes harbored mycorrhizal fungi. The correlation
between land colonization and mycorrhization suggests that the AM fungal association may have been the driver for plant survival in environments low in nutrients,
especially P. Indeed, Pirozynski and Malloch in their 1975 publication hypothesized
that the interaction with fungi enabled the evolution of land plants by opening a
completely new environment as early as 460 Mya.
Although early Devonian plants did not have true roots or leaves, true roots are
found in fossils from the Middle Devonian (410.8 to 407.6 Mya) (Fig. 21.3) on the
ventral surface of horizontal stems of Bathurstia denticulata (Zosterphyllopsida)
(Kotyk and Basinger 2000). The roots were unbranched, had a central vascular
strand, and were several cm long (1.7 mm average in width), but longer roots were
wider and reached lengths of 5 cm. Based on extant plants and fossils, it is also
highly likely that many microbes in the soil, especially AM fungi, were associated
with plants early on. We cannot be certain of the identities of these other microbes,
but many cyanobacteria, which in addition to undergoing photosynthesis fix
21 The Hologenome Hypothesis and Its Application to Plant-Microbe Interactions on. . .
431
origin, based on a study of a consortium of bacteria and cyanobacteria as the
photosynthetic partner in a microbial mat, was also published (Tomescu et al.
2008), while other reports indicate that lichen fossils (with both cyanobacteria and
green algae) were present from the Lower Devonian (Honegger et al. 2013a, b). The
connections linking the lichen-like fossils of the Watanabe et al. studies and other
microbial mat-like consortia have not been made. In any cases, the increased O 2
brought about by cyanobacterial and unicellular green algal photosynthesis in the
ancient world most likely led to the quicker formation of an ozone shield, thereby
reducing ultraviolet irradiation and accelerating the rate of evolution.
Strong evidence for land plant evolution is based on fossils from the middle
Ordovician to the early Silurian. In mid-Ordovician (460 to 455 Mya) shales from
Saudi Arabia and Argentina, spores similar to bryophyte (mosses and liverwort)
spores (Strother et al. 1996; Rubenstein et al. 2010) were found. A plant similar to
Cooksonia, a leafless, rootless plant with terminal sporangia, might be a possible
candidate. That fungi in the Ordovician became associated with plants was demonstrated by finding fossilized non-septate hyphae and spores of Glomalean-like fungi
within bryophyte cells (Redecker et al. 2000). Additional evidence for the evolution
of land plants comes from the Rhynie Chert (Scotland) in the fossils of Algaophyton
(formerly Rhynia) major. Edwards (1986) described this plant based on a study of
fossils that had characters of both bryophytes and tracheophytes (vascular plants).
The dichotomously branched vertical axis of the plant was ca. 183 mm tall and
rootless, but on one side of the fossilized underground rhizomes, root hair-like
structures (rhizoids) were found. The vertical axis was externally characterized by
stomata on the epidermis and terminal sporangia, and sections of the axis show what
appears to be conducting cells, but no true vascular tissue, leading Edwards (1986) to
decide that the plant was a bryophyte. In addition, Edwards noted that cell walls of
the outer cortex contained fungal hyphae. Later, Remy et al. (1994) observed that
highly branched hyphae (arbuscules) were present in cortical cells, indicating that
even the most ancient bryophytes harbored mycorrhizal fungi. The correlation
between land colonization and mycorrhization suggests that the AM fungal association may have been the driver for plant survival in environments low in nutrients,
especially P. Indeed, Pirozynski and Malloch in their 1975 publication hypothesized
that the interaction with fungi enabled the evolution of land plants by opening a
completely new environment as early as 460 Mya.
Although early Devonian plants did not have true roots or leaves, true roots are
found in fossils from the Middle Devonian (410.8 to 407.6 Mya) (Fig. 21.3) on the
ventral surface of horizontal stems of Bathurstia denticulata (Zosterphyllopsida)
(Kotyk and Basinger 2000). The roots were unbranched, had a central vascular
strand, and were several cm long (1.7 mm average in width), but longer roots were
wider and reached lengths of 5 cm. Based on extant plants and fossils, it is also
highly likely that many microbes in the soil, especially AM fungi, were associated
with plants early on. We cannot be certain of the identities of these other microbes,
but many cyanobacteria, which in addition to undergoing photosynthesis fix
21 The Hologenome Hypothesis and Its Application to Plant-Microbe Interactions on. . .
431