et al. 2011). It is important to note that AMF can also reduce N 2 O gas emissions and
nutrient leaching from soil (Bender et al. 2014; Bender et al. 2015).
Nitrogen-fixing bacteria, associated with roughly 7% of plant species, represent another widespread group of rhizosphere symbionts (Steidinger et al. 2019).
Instead of extracting nutrients from the soil, nitrogen fixers convert abundant, but
unusable N 2 gas in the atmosphere into forms that plants can assimilate. These are
common in legumes (Fabaceae) and other dicotyledonous angiosperms occurring in
nitrogen-poor soils. Nitrogen-fixing symbionts appear limited by temperature and
soil pH to warm, arid regions (e.g., tropical savannas and xeric shrublands) and with
continued warming are expected to increase in abundance in regions such as the arid
southwest of North America (Liao et al. 2017). These expansions could partially
offset the massive losses of evergreen trees also predicted for this region (McDowell
et al. 2016). Unfortunately, an expansion of nitrogen-fixing symbioses has the
potential to severely worsen climate change if soil N 2 O emissions outweigh carbon
sequestration, but uncertainty surrounds model estimates (Kou-Giesbrecht and
Menge 2019).
In addition to rhizosphere-associated symbionts, plants are routinely infected
with aboveground endophytes, usually fungi or bacteria, that spend at least part of
their life cycle inhabiting plant tissues (Hardoim et al. 2015). Often with blurred
boundaries, endophyte lifestyles vary from opportunistic to obligate, with variable
routes of transmission. Many interactions are commensal, but others are beneficial or
conditionally beneficial, often improving plant fitness by conferring tolerance
against biotic or abiotic stress. A meta-analysis across 42 host plants and 94 endophyte strains found that endophytes increased host plant biomass during nitrogen
deficiency, drought, and high salinity (Rho et al. 2018). Fungal endophytes, in
particular, are known for roles in mediating abiotic and biotic stress and hence
may play a prominent role in mitigating climate stress (Rodriguez et al. 2009;
Kivlin et al. 2013). The clavicipitaceous endophytes (Class 1) are associated with
about 25% of grass species and confer protection against herbivores and drought
stress (Rudgers et al. 2009; Clay and Schardl 2002). Those that promote drought
tolerance can expand host ranges into drier habitats (Afkhami et al. 2014). The more
diverse and taxonomically widespread non-clavicipitaceous endophytes (Classes
2–4) are harder to generalize, though infection phenotypes include drought and
thermal tolerance (Rodriguez et al. 2008; Redman et al. 2002; Arnold et al. 2003).
16.5 Conclusions
The aggregate actions of humans have created prominent evolutionary forces that
will, to varying degrees, impact ecosystems across the planet. The ability of
multicellular eukaryotes to relocate, acclimate, or adapt to these changes will often
depend on their microbial symbionts. In some cases, symbionts will provide a muchneeded assist, but for many highly specialized associations, they will serve as an
“Achilles heel” exposing acute vulnerabilities.
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K. M. Oliver and C. H. V. Higashi
nutrient leaching from soil (Bender et al. 2014; Bender et al. 2015).
Nitrogen-fixing bacteria, associated with roughly 7% of plant species, represent another widespread group of rhizosphere symbionts (Steidinger et al. 2019).
Instead of extracting nutrients from the soil, nitrogen fixers convert abundant, but
unusable N 2 gas in the atmosphere into forms that plants can assimilate. These are
common in legumes (Fabaceae) and other dicotyledonous angiosperms occurring in
nitrogen-poor soils. Nitrogen-fixing symbionts appear limited by temperature and
soil pH to warm, arid regions (e.g., tropical savannas and xeric shrublands) and with
continued warming are expected to increase in abundance in regions such as the arid
southwest of North America (Liao et al. 2017). These expansions could partially
offset the massive losses of evergreen trees also predicted for this region (McDowell
et al. 2016). Unfortunately, an expansion of nitrogen-fixing symbioses has the
potential to severely worsen climate change if soil N 2 O emissions outweigh carbon
sequestration, but uncertainty surrounds model estimates (Kou-Giesbrecht and
Menge 2019).
In addition to rhizosphere-associated symbionts, plants are routinely infected
with aboveground endophytes, usually fungi or bacteria, that spend at least part of
their life cycle inhabiting plant tissues (Hardoim et al. 2015). Often with blurred
boundaries, endophyte lifestyles vary from opportunistic to obligate, with variable
routes of transmission. Many interactions are commensal, but others are beneficial or
conditionally beneficial, often improving plant fitness by conferring tolerance
against biotic or abiotic stress. A meta-analysis across 42 host plants and 94 endophyte strains found that endophytes increased host plant biomass during nitrogen
deficiency, drought, and high salinity (Rho et al. 2018). Fungal endophytes, in
particular, are known for roles in mediating abiotic and biotic stress and hence
may play a prominent role in mitigating climate stress (Rodriguez et al. 2009;
Kivlin et al. 2013). The clavicipitaceous endophytes (Class 1) are associated with
about 25% of grass species and confer protection against herbivores and drought
stress (Rudgers et al. 2009; Clay and Schardl 2002). Those that promote drought
tolerance can expand host ranges into drier habitats (Afkhami et al. 2014). The more
diverse and taxonomically widespread non-clavicipitaceous endophytes (Classes
2–4) are harder to generalize, though infection phenotypes include drought and
thermal tolerance (Rodriguez et al. 2008; Redman et al. 2002; Arnold et al. 2003).
16.5 Conclusions
The aggregate actions of humans have created prominent evolutionary forces that
will, to varying degrees, impact ecosystems across the planet. The ability of
multicellular eukaryotes to relocate, acclimate, or adapt to these changes will often
depend on their microbial symbionts. In some cases, symbionts will provide a muchneeded assist, but for many highly specialized associations, they will serve as an
“Achilles heel” exposing acute vulnerabilities.
282
K. M. Oliver and C. H. V. Higashi