Symbiogenesis is a word used to describe cooperations between species which increase
their mutual survival. Endosymbiotic theory is the idea that eukaryotic cells formed
from prokaryotic organisms and that organelles such as mitochondria and chloroplasts
formed from ingestion of other organisms. John Archibald has offered his opinion that
endosymbiosis may not have played a role in the origin of cytosolic compartments
other than mitochondria and the plastids (Archibald 2015). Chloroplasts are one group
of plastids.
At this point I would like to return to a mentioning of Helen Abbott’s presentation
(Abbott HC de 1887). Abbott said that sugar is not a reserve material for plants and
indeed for some few of the plants, with an example being sorghum-cane (genus
Sorghum), “. . . it [sugar] must be regarded as a waste product attending euthanasia
which marks decay of the plant after the maturity of growth.” Helen Abbott was a
very well noted plant chemist and later became a physician. She did not consider the
knowledge that carbohydates sometimes are produced as an inducement for
mutulistic symbiont animals to consume the plant in a way that supports dispersal
and fertilization of the seeds. Sugar production prior to maturation of the seed would
be counter productive, because the plant and its seed-bearing products might get
eaten too early, before the seeds are ready for dispersal. Humans are in fact
mutualistic symbionts of the plants that we grow agriculturally. Sugar cane and
humans are just one example of symbiont enticements and consequent ingestion!
The microbial interactions that connect plants and soil have very nicely been
discussed by Ahkami et al. (2017). One of the strongest areas of interest in agriculture is the symbiotic interactions of plants with the rhizospheric microbial communities, those microbes which live in the area of the plants roots. The interactions of
plants and microbes cover a broad spectrum, from mutualistic and commensal to
parasitic. Li et al. (2019) helpfully have mentioned our knowledge that colonization
of plant roots by endophytic fungi may aid plant survival during periods of water
stress caused by drought. Gage (2004) has presented a very nicely detailed review of
the initial signalling process which occurs between many plant hosts and potentially
symbiont Rhizobium bacteria, after which the rhizobia move from the root surface
into the inner root tissue, and then the Rhizobium populate cells of the plants
developing root nodules. The Rhizobium provide fixed nitrogen in exchange for
carbohydates received from the plant (Moore 1905).
I would like to also mention for you the symbioses that involve protists. Protists
are a diverse group of eukaryotic species considered as being neither animal, nor
plant, nor fungi. Nowack and Melkonian (Nowack and Melkonian 2010) very nicely
summarized information about endosymbionts of protists. The microbes known as
flagellates are included among the protists. Both prokaryotes and eukaryotes are
represented among the endosymbionts that are housed within protists. Those authors
(Nowack and Melkonian 2010) also included mention of the tripartite symbiotic
associations which exist between termites which house intestinal flagellates, and the
bacteria that reside endosymbiotically within those flagellates. Nowack and
Melkonian (2010) presumed that these tripartite symbioses might support the capability of termites to survive on recalcitrant plant matter as their sole nutrient source
8
C. J. Hurst
their mutual survival. Endosymbiotic theory is the idea that eukaryotic cells formed
from prokaryotic organisms and that organelles such as mitochondria and chloroplasts
formed from ingestion of other organisms. John Archibald has offered his opinion that
endosymbiosis may not have played a role in the origin of cytosolic compartments
other than mitochondria and the plastids (Archibald 2015). Chloroplasts are one group
of plastids.
At this point I would like to return to a mentioning of Helen Abbott’s presentation
(Abbott HC de 1887). Abbott said that sugar is not a reserve material for plants and
indeed for some few of the plants, with an example being sorghum-cane (genus
Sorghum), “. . . it [sugar] must be regarded as a waste product attending euthanasia
which marks decay of the plant after the maturity of growth.” Helen Abbott was a
very well noted plant chemist and later became a physician. She did not consider the
knowledge that carbohydates sometimes are produced as an inducement for
mutulistic symbiont animals to consume the plant in a way that supports dispersal
and fertilization of the seeds. Sugar production prior to maturation of the seed would
be counter productive, because the plant and its seed-bearing products might get
eaten too early, before the seeds are ready for dispersal. Humans are in fact
mutualistic symbionts of the plants that we grow agriculturally. Sugar cane and
humans are just one example of symbiont enticements and consequent ingestion!
The microbial interactions that connect plants and soil have very nicely been
discussed by Ahkami et al. (2017). One of the strongest areas of interest in agriculture is the symbiotic interactions of plants with the rhizospheric microbial communities, those microbes which live in the area of the plants roots. The interactions of
plants and microbes cover a broad spectrum, from mutualistic and commensal to
parasitic. Li et al. (2019) helpfully have mentioned our knowledge that colonization
of plant roots by endophytic fungi may aid plant survival during periods of water
stress caused by drought. Gage (2004) has presented a very nicely detailed review of
the initial signalling process which occurs between many plant hosts and potentially
symbiont Rhizobium bacteria, after which the rhizobia move from the root surface
into the inner root tissue, and then the Rhizobium populate cells of the plants
developing root nodules. The Rhizobium provide fixed nitrogen in exchange for
carbohydates received from the plant (Moore 1905).
I would like to also mention for you the symbioses that involve protists. Protists
are a diverse group of eukaryotic species considered as being neither animal, nor
plant, nor fungi. Nowack and Melkonian (Nowack and Melkonian 2010) very nicely
summarized information about endosymbionts of protists. The microbes known as
flagellates are included among the protists. Both prokaryotes and eukaryotes are
represented among the endosymbionts that are housed within protists. Those authors
(Nowack and Melkonian 2010) also included mention of the tripartite symbiotic
associations which exist between termites which house intestinal flagellates, and the
bacteria that reside endosymbiotically within those flagellates. Nowack and
Melkonian (2010) presumed that these tripartite symbioses might support the capability of termites to survive on recalcitrant plant matter as their sole nutrient source
8
C. J. Hurst