mainly would relate to assimilating carbon-containing nutrients which the tree prepares through its chlorophyll-containing organs. On the other hand, the fungus
would take its own mineral nutrients from the soil.
I easily can understand Frank’s opinion that the fungus was acting parasitically.
When I was a Boy Scout, long ago, during a camping trip I once noticed with a sense
of horror that the roots of a fallen tree were absolutely covered with white fungal
hyphae. My presumption was that the fungus had killed the tree. I finally realized,
more than 50 years later while writing my essays for this book, that the fungus which
surrounded those tree roots was the visible part of an ectomycorrhiza.
26.7.2 Endomycorrhizae
A second category of symbiosis between plant roots and fungi has been identified
and given the name endomycorrhiza. As suggested by that name, hyphae of the
involved fungi are most obviously internal to the plant roots.
The term arbuscular mycorrhiza describes a category of endomycorrhizal associations in which the hyphae penetrate root cells, grow in the space between the plant
cells, and connect the internal compartments of neighboring plant cells. Nutrient
exchange between the fungus and plant then occurs through an interface consisting
of branching fungal masses called arbuscles that develop inside of the root. The
development of an arbuscular mycorrhiza can include formation of arbuscules and
vesicles. Arbuscular mycorrhiza may help plants adapt to abiotic stresses including
drought, heat and sailinity (Begum et al. 2019). They also help plants to capture
necessary elements such as nitogen, phosphorus, and sulfur from the soil.
Arbuscular mycorrhiza are formed by members of the phylum Mucoromycota,
subphylum Glomeromycotina. Figure 26.32 shows arbuscular mycorrhiza in root
cortical cells of flax (Linum usitatissimum) and the arbuscules shown are described
as being paired.
Legume roots develop both rhizobial symbioses and arbuscular mycorrhizal
symbioses (Scheublin et al. 2004). The rhizobial symbiosis involves a bacterial
partner, as mentioned above, and that association characteristically but not necessarily results in the formation of root nodules. Rhizobial symbioses quite obviously
serve by providing fixed nitrogen for the plant.
Plants have developed endophytic interactions with many types of symbiotic
microbes (Strobel 2018; Zhang et al. 2019). Arbuscular mycorrhiza fungi can
interact either synergistically or antagonistically with the plants other endosymbiont
microbes, including those symbionts that are growing in aerial parts of the plant.
It has been suggested that arbuscular mycorrhizal associations appeared when
plants began colonizing the land, perhaps 400–460 million years ago. Rimington
et al. (2018) have written an article about the possible early establishment of
arbuscular mycorrhizae. Arbuscular mycorrhizal symbioses thus may have
represented an important step in the development of terrestrial plants. By now,
arbuscular mycorrhizal symbioses have become established in perhaps 85% of the
26 The Game of Evolution Is Won by Competitive Cheating
583
would take its own mineral nutrients from the soil.
I easily can understand Frank’s opinion that the fungus was acting parasitically.
When I was a Boy Scout, long ago, during a camping trip I once noticed with a sense
of horror that the roots of a fallen tree were absolutely covered with white fungal
hyphae. My presumption was that the fungus had killed the tree. I finally realized,
more than 50 years later while writing my essays for this book, that the fungus which
surrounded those tree roots was the visible part of an ectomycorrhiza.
26.7.2 Endomycorrhizae
A second category of symbiosis between plant roots and fungi has been identified
and given the name endomycorrhiza. As suggested by that name, hyphae of the
involved fungi are most obviously internal to the plant roots.
The term arbuscular mycorrhiza describes a category of endomycorrhizal associations in which the hyphae penetrate root cells, grow in the space between the plant
cells, and connect the internal compartments of neighboring plant cells. Nutrient
exchange between the fungus and plant then occurs through an interface consisting
of branching fungal masses called arbuscles that develop inside of the root. The
development of an arbuscular mycorrhiza can include formation of arbuscules and
vesicles. Arbuscular mycorrhiza may help plants adapt to abiotic stresses including
drought, heat and sailinity (Begum et al. 2019). They also help plants to capture
necessary elements such as nitogen, phosphorus, and sulfur from the soil.
Arbuscular mycorrhiza are formed by members of the phylum Mucoromycota,
subphylum Glomeromycotina. Figure 26.32 shows arbuscular mycorrhiza in root
cortical cells of flax (Linum usitatissimum) and the arbuscules shown are described
as being paired.
Legume roots develop both rhizobial symbioses and arbuscular mycorrhizal
symbioses (Scheublin et al. 2004). The rhizobial symbiosis involves a bacterial
partner, as mentioned above, and that association characteristically but not necessarily results in the formation of root nodules. Rhizobial symbioses quite obviously
serve by providing fixed nitrogen for the plant.
Plants have developed endophytic interactions with many types of symbiotic
microbes (Strobel 2018; Zhang et al. 2019). Arbuscular mycorrhiza fungi can
interact either synergistically or antagonistically with the plants other endosymbiont
microbes, including those symbionts that are growing in aerial parts of the plant.
It has been suggested that arbuscular mycorrhizal associations appeared when
plants began colonizing the land, perhaps 400–460 million years ago. Rimington
et al. (2018) have written an article about the possible early establishment of
arbuscular mycorrhizae. Arbuscular mycorrhizal symbioses thus may have
represented an important step in the development of terrestrial plants. By now,
arbuscular mycorrhizal symbioses have become established in perhaps 85% of the
26 The Game of Evolution Is Won by Competitive Cheating
583