nitrogen, as well as various heterotrophic bacteria, were likely to be a part of the
ancient plant microbiome.
With the development of an ozone shield to keep out damaging ultraviolet
irradiation and enable changes in soil structure that are probably correlated with
plant colonization and decay, rhizomes grew larger and deeper over time. In
Zosterophyllum (Fig. 21.3a), the rhizomes were both vertically and horizontally
oriented, which is also true of the Rhynie Chert plants, but no obvious root-like
structures extend from the horizontal rhizome of Rhynia (b). The lycopsid
Drepanophyus (c) developed deeper (10 cm), vertical and highly branched rhizomes
that were longer than those found on the other two fossilized plants in Fig. 21.3. By
the Middle-Late Devonian, fossils of various progymnosperms (d), including
aneurophytes (fern-like organisms) (e), as well as rooted trees (f, g) have been
reconstructed (Fig. 21.3) from fossils (Xue et al. 2016). From the Mid-Devonian
onwards, reconstructed fossils of Archaeopteris (h) strongly resembled seed-plants
in that they produced an elaborate root system and also formed secondary vascular
tissue, well-developed leaves, and exhibited endogenous development of lateral
roots (Stein et al. 2019). The roots also penetrated more deeply into the soil, at
least 50 cm. The net impact of all these developmental changes eventually led to
forests that are likely to have resulted in profound changes in climate as well as other
environmental factors.
21.3 Microbiome-Mediated Evolution
Plant evolution, like many profound changes in behavior, is influenced by quid pro
quo. Photosynthates produced by green plants provide carbon for heterotrophic
microbes, which in turn fix nitrogen to ammonia, solubilize rock phosphate to P,
etc. to promote plant growth via photosynthesis, which promotes the transfer of
carbon compounds to the heterotrophs. The symbiotic microbes also protect plants
from pathogen attack and promote their growth by synthesizing various plant
hormones. We and others hypothesize that this interaction is a strong
co-evolutionary stimulus for the evolution of plant microbiomes and their effects
on plant evolution. As plants elaborated leaves, roots, other plant organs, as well as
secondary tissues, the plant microbiome also evolved because each plant organ had a
specific environment (root vs. leaf) or biochemistry (mineral nutrient/water
acquisition vs. photosynthesis) to which the microbes needed to adapt.
The hologenome concept of evolution (HCE), as defined by Zilber-Rosenberg
and Rosenberg (2008), explains the outcome of the interaction between plants and
microbes over time. HCE is a biological hypothesis that explains the evolution of
both plants and animals. Briefly sketched in Rosenberg et al. (2007), the hypothesis
was further developed in the 2008 publication (Zilber-Rosenberg and Rosenberg
2008). HCE proposes that biological individuality is due to a multispecies assemblage that results from the symbiotic union between an animal and/or a plant plus its
symbiotic microbiota. This union results in a biological individual that the authors
21 The Hologenome Hypothesis and Its Application to Plant-Microbe Interactions on. . .
433
ancient plant microbiome.
With the development of an ozone shield to keep out damaging ultraviolet
irradiation and enable changes in soil structure that are probably correlated with
plant colonization and decay, rhizomes grew larger and deeper over time. In
Zosterophyllum (Fig. 21.3a), the rhizomes were both vertically and horizontally
oriented, which is also true of the Rhynie Chert plants, but no obvious root-like
structures extend from the horizontal rhizome of Rhynia (b). The lycopsid
Drepanophyus (c) developed deeper (10 cm), vertical and highly branched rhizomes
that were longer than those found on the other two fossilized plants in Fig. 21.3. By
the Middle-Late Devonian, fossils of various progymnosperms (d), including
aneurophytes (fern-like organisms) (e), as well as rooted trees (f, g) have been
reconstructed (Fig. 21.3) from fossils (Xue et al. 2016). From the Mid-Devonian
onwards, reconstructed fossils of Archaeopteris (h) strongly resembled seed-plants
in that they produced an elaborate root system and also formed secondary vascular
tissue, well-developed leaves, and exhibited endogenous development of lateral
roots (Stein et al. 2019). The roots also penetrated more deeply into the soil, at
least 50 cm. The net impact of all these developmental changes eventually led to
forests that are likely to have resulted in profound changes in climate as well as other
environmental factors.
21.3 Microbiome-Mediated Evolution
Plant evolution, like many profound changes in behavior, is influenced by quid pro
quo. Photosynthates produced by green plants provide carbon for heterotrophic
microbes, which in turn fix nitrogen to ammonia, solubilize rock phosphate to P,
etc. to promote plant growth via photosynthesis, which promotes the transfer of
carbon compounds to the heterotrophs. The symbiotic microbes also protect plants
from pathogen attack and promote their growth by synthesizing various plant
hormones. We and others hypothesize that this interaction is a strong
co-evolutionary stimulus for the evolution of plant microbiomes and their effects
on plant evolution. As plants elaborated leaves, roots, other plant organs, as well as
secondary tissues, the plant microbiome also evolved because each plant organ had a
specific environment (root vs. leaf) or biochemistry (mineral nutrient/water
acquisition vs. photosynthesis) to which the microbes needed to adapt.
The hologenome concept of evolution (HCE), as defined by Zilber-Rosenberg
and Rosenberg (2008), explains the outcome of the interaction between plants and
microbes over time. HCE is a biological hypothesis that explains the evolution of
both plants and animals. Briefly sketched in Rosenberg et al. (2007), the hypothesis
was further developed in the 2008 publication (Zilber-Rosenberg and Rosenberg
2008). HCE proposes that biological individuality is due to a multispecies assemblage that results from the symbiotic union between an animal and/or a plant plus its
symbiotic microbiota. This union results in a biological individual that the authors
21 The Hologenome Hypothesis and Its Application to Plant-Microbe Interactions on. . .
433