Endosymbiotic relationships are prevalent among many marine organisms, ranging from parasitic to beneficial mutualism.
The author of this paper illustrates how this relationship is capable of evolving, particularly through developing resistance to antagonistic behaviour exhibited by the endosymbiont. The particular relationship in focus is the fungal host Rhizopus microsporus and its bacterial endosymbiont Burkholderia rhizoxinica.
The author of this paper illustrates how this relationship is capable of evolving, particularly through developing resistance to antagonistic behaviour exhibited by the endosymbiont. The particular relationship in focus is the fungal host Rhizopus microsporus and its bacterial endosymbiont Burkholderia rhizoxinica.
Scherlach, et al (2006), discuss an antimitotic substance produced by these bacteria which are then further processed by the host fungi into a phytotoxin known as rhizoxin. (Scherlach, et al, 2012) This toxin is targeted at the rice plant acting as a virulence factor which causes swelling and eventual degradation resulting in nutrition for the fungi and the bacteria. The bacteria also benefits from protection in the fungus cytosol. The bacterial antimitotic secretion was originally hazardous to the fungi causing breakdown which the bacteria would then uptake for nutrition. However, R. microsporus has developed a resistance to the toxin as Schmitt, et al, (2008) describe due to a β-tubulin binding site mutation causing a parasitism-mutualism shift.
This relationship provides an interesting twist as uptake of the bacteria is achieved in an entirely new method unlike the usual way such as endocytosis. The authors have found that Burkholderia rhizoxinica uses secretions of chitinolytic enzymes from a type 2 secretion system (T2SS) ascertained through deletion of the chitinase (chi) gene. Sample colonies lacking the gene were unable to break through the fungal hyphae. The enzyme softens a spot in the fungal cell wall which creates a small hole for the bacteria to slip through and only a small amount of enzyme is secreted, providing evidence that the relationship is mutualistic rather than antagonistic.
In another study done by Lackner, et al, in 2011, they found what they dubbed a type 3 secretion system (T3SS). Similar to the T2SS, mutants that lacked the T3SS were unable to initiate sporulation.
In fact the relationship has progressed so far that the bacteria is unable to sporulate without the bacteria present at all. This is perhaps indicative of a reduced genome where bacterial functions surpassed the need for the fungi to have its own genes for certain processes such as those that trigger sporulation. Another suggestion would be that sporulation only occurs under prevalent nutrient conditions, conditions provided by the toxin produced by the bacterial-fungi symbiosis. The author comments in a podcast reviewing the paper that any time the fungus germinates, the bacteria is already present in an example of vertical gene transfer.
At any rate it poses the questions of at what point the relationship changed in such a way that the fungi became dependant on the bacteria and whether the bacteria itself will become less of an independent organism and more like an organelle of sorts similar to the reduction of cyanobacteria to chloroplasts in prokaryotes.
This relationship provides an interesting twist as uptake of the bacteria is achieved in an entirely new method unlike the usual way such as endocytosis. The authors have found that Burkholderia rhizoxinica uses secretions of chitinolytic enzymes from a type 2 secretion system (T2SS) ascertained through deletion of the chitinase (chi) gene. Sample colonies lacking the gene were unable to break through the fungal hyphae. The enzyme softens a spot in the fungal cell wall which creates a small hole for the bacteria to slip through and only a small amount of enzyme is secreted, providing evidence that the relationship is mutualistic rather than antagonistic.
In another study done by Lackner, et al, in 2011, they found what they dubbed a type 3 secretion system (T3SS). Similar to the T2SS, mutants that lacked the T3SS were unable to initiate sporulation.
In fact the relationship has progressed so far that the bacteria is unable to sporulate without the bacteria present at all. This is perhaps indicative of a reduced genome where bacterial functions surpassed the need for the fungi to have its own genes for certain processes such as those that trigger sporulation. Another suggestion would be that sporulation only occurs under prevalent nutrient conditions, conditions provided by the toxin produced by the bacterial-fungi symbiosis. The author comments in a podcast reviewing the paper that any time the fungus germinates, the bacteria is already present in an example of vertical gene transfer.
At any rate it poses the questions of at what point the relationship changed in such a way that the fungi became dependant on the bacteria and whether the bacteria itself will become less of an independent organism and more like an organelle of sorts similar to the reduction of cyanobacteria to chloroplasts in prokaryotes.
Original reference:
Dijksterhuis, J., Hertweck, C., Lackner, G., Moebius, N. & Üzüm, Z. (2014) Active invasion of bacteria into living fungal cells. Elife, 2(3).
doi: 10.7554/eLife.03007#sthash.q3IizmtY.dpuf
http://elifesciences.org/content/3/e03007#ref-47
Additional references:
Busch, B., Hertweck, C., Lackner, G., Paszkowski, U. & Scherlach, K. (2012) Symbiotic cooperation in the biosynthesis of a phytotoxin. Angew Chem Int Ed 2012. 51, 9615–8.
http://onlinelibrary.wiley.com/doi/10.1002/anie.201204540/abstract
Dahse, H.M., Hertweck, C., Partida-Martinez, L.P.& Scherlach, K. (2006) Antimitotic rhizoxin derivatives from a cultured bacterial endosymbiont of the rice pathogenic fungus Rhizopus microsporus. J Am Chem Soc. 128(35), 11529-36.
http://www.ncbi.nlm.nih.gov/pubmed/16939276?access_num=16939276&link_type=MED&dopt=Abstract
http://www.ncbi.nlm.nih.gov/pubmed/16939276?access_num=16939276&link_type=MED&dopt=Abstract
Dölz, F., Einax, E., Hertweck, C., Partida-Martinez, L.P., Schmitt, I., Telle, S., Voigt, K., Winkler, R. & Wöstemeyer, J. (2008) Evolution of host resistance in a toxin-producing bacterial-fungal alliance. The ISME Journal. 2, 632–641.
http://www.nature.com/ismej/journal/v2/n6/full/ismej200819a.html
Hertweck, C., Lackner, G. & Moebius, N. (2011) Endofungal bacterium controls its host by an hrp type III secretion system. The ISME Journal. 5, 252–261.http://www.nature.com/ismej/journal/v5/n2/full/ismej2010126a.html
http://www.nature.com/ismej/journal/v2/n6/full/ismej200819a.html
Hertweck, C., Lackner, G. & Moebius, N. (2011) Endofungal bacterium controls its host by an hrp type III secretion system. The ISME Journal. 5, 252–261.http://www.nature.com/ismej/journal/v5/n2/full/ismej2010126a.html
Hi Bekki. Thanks for the post on an interesting topic - evolution of endosymbionts is a topic we will explore in the lectures on vents and seeps. However, everyone should keep the posts for this blog focussed on marine microbial examples.
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