Sunday Protist - Phytomonas: plant trypanosomatids!

While I was trying to come up with something quick to blog about, got a couple updates in Google Reader from J. Eukaryotic Microbiol, among them a paper on... trypanosomatids living in coconut tree phloem! Somehow, you don't typically think of plants being invaded by motile, flagellate things, but on a second thought: why not? The phloem is a vessel, and while perhaps there's no need to run away from macrophages or anything, there's no particular harm in retaining the ability to swim around. Especially if your other life happens in...insects!

Left: Phytomonas from coconut phloem. The arrow pointing to a transverse structure shows the sieve plate, which separate phloem tube cells. Note how the parasites congregate perpendicular to the plate. Kind of like salmon swimming upstream. The white round things in the middle image on the right are starch granules - food!(Keller & Miguens 2009 JEM; AOP)

For some idea of what these things are related to: (can I write a single post without showing or refering to a tree? Soon I'll get banished from organismal/cell biology...)

Phytomonas lives towards the bottom, amid monoxenous (single host) insect trypanosomatids. (Simpson et al. 2006 Trends Parasitol)

Trypanosomatids are worthy contenders for the Higher Parasite award (if ciliates are the higher eukaryotes, as we've established earlier, then why not have higher parasites as well?). In fact, they have a rather tight competition with the Apicomplexa, which are also a seriously effective bunch. Trypanosomatids and their brethren also have one of the most complicated mitochondrial genomes out there (if not THE most complicated), as alluded to towards the bottom of the Diplonemid post. They also have this nasty habit of constantly changing their surface proteins, thus outsmarting the host's immune response.

They can also be considered the reason why sub-Saharan Africa isn't Muslim, or particularly white for that matter: tryps are very good at completely decimating livestock, transport animals and clueless foreigners. Thus, the Islamic expansion was stopped upon reaching the Land of Tryps, as their camels and horses provided some much-needed fresh flesh for parasites, and running an empire without horses and camels is, well, difficult. Furthermore, Plasmodium and Trypanosoma did a nice job ganging up on the European invaders later on, both in person and by destroying their attempts at cattle farming. This story was told by a protistology instructor, demonstrating that protists can, in fact, dramatically impact human history. Ethnomicrobiology, the study of the interactions between humans and microbial life, would be a really cool thing to compile (and study)! Especially since almost every human culture on the planet has figured out a way to make their food rot in a way that it tastes nice, or sends you on a nice psychological trip. Usually the latter.

Again, to put things into morphological perspective, tryps are actually quite complex, despite what their simple wiggly appearance in light microscopy:

Overview of Tryp morphology, much more complex than the first impressions from microscopy. The review this is from discusses peculiar organelles called acidocalcisomes, which are apparently conserved throughout Eukarya and prokaryotes (eg. Agrobacterium), and may have been inherited from the bacterial proto-eukaryote. Seems to be involved in a whole bunch of biochemical ion pumping action. I personally prefer fun subcellular structures, like the cytoskeleton or the endomembrane system =P (Docampo et al. 2005 Nat Rev Microbiol)

The tryp flagellar pocket is a story in itself, even getting its very own review. Don't let the single flagellum fool you -- tryps are bikonts! The homologue of the inner flagellum in euglenids (which is really short already) simply got lost. Apparently screwing with flagellar structure really messes up the trypanosome cytokinesis, resulting in these wonderful convoluted clumps of parasite. But that's getting way off-topic...

Back to our trippy tryps. Tryps are predominantly insect parasites, but several lineages have taken a liking to vertebrates or plants on the side:

Monoxenous (single host) tryps spend their entire parasitic careers in insects. Phytomonas is heteroxenous, and alternates between insect and plant hosts, while Leishmania and Trypanosoma alternate between insects and vertebrates. Some insect tryps can also be found in plants, but I'm not too sure what exactly they mean by that. (Santos et al. 2007 Microbes & Infection)

Apparently some of those tryps don't particularly care whether they're hanging out in vertebrate blood vessels, insect haemolymph (or other organs) or plant phloem.
Turns out the haemolymph is quite low in oxygen levels, and I'd assume phloem sap would similarly not be anywhere near as rich as vertebrate blood. But on a second thought, much of the oxygen in vertebrate blood should be attached to haemoglobin, and thus not make much of a difference. What Tryps and co. are really after is glucose, which all three environments are rich in. Among other nutrients, of course, but here's a 2009 paper on Tryp energy metabolism for anyone who's into that sort of thing. *shudder*

So why infect plants? There's plenty of opportunity to accidentally learn to infect a new host if you spend a significant portion of your life (in vast numbers) resting as a spore outside your primary host. Thus, if your primary host happens to have a fetish for vertebrate blood, there's a high chance of frequent contact with that environment, and presumably it's similar enough to something the tryp is already adapted to. Thus, this jump to a new host isn't as shocking as it first looks. Similarly, if the insect host dines on plants, there's enough contact with the plant vascular system to eventually figure out a way to use it. After all, you might as well, especially since you don't have to be good enough to reproduce there or anything. There's a high likelihood of being slurped back up by the original host. So while really cool, it's not too shocking that such relationships evolve.

It would be interesting to trace host interactions of heteroxenous parasites (including fungi and oomycetes and all the rest); perhaps this host jumping is driven by a very close interaction between the two hosts. I know very little about the evolution of parasites, but it does seem really cool: how do the parasites (and other symbionts) manage to move between different hosts? Perhaps most often they simply coevolve with their host and diverge with them, but presumably cases of jumping between host lineages aren't all too rare?

Where this stuff could come in handy is that perhaps the infected plants may have evolved a nasty defensive response to Phytomonas. Phytomonas is a relative of Trypanosoma and Leishmania, which are not particularly welcomed by us, as they can be rather unkind (deadly). One wonders if anything can be learned from those plants and their defense strategies, and perhaps applied to human medicine. Somebody's probably on it already, I just don't follow biomedical literature.

Anyway, plant flagellate parasites = pretty awesome and unexpected. Upcoming biochem final = really UNawesome and quite expected. Anyone wanna write it for me?

Also, I owe posts and revised posts and other stuff. I'm on it, I swear! (and I really didn't mean to do a long Sunday Protist this time, but it always happens! Academic literature is like a freaking black hole/horribly addictive drug: sucks you right in, for hours! Or maybe I'm just insane... anyone else get sucked in for hours reading random papers on obscure topics? And actually enjoy it? Anyone?)

References:
Docampo, R., de Souza, W., Miranda, K., Rohloff, P., & Moreno, S. (2005). Acidocalcisomes — conserved from bacteria to man Nature Reviews Microbiology, 3 (3), 251-261 DOI: 10.1038/nrmicro1097

KELLER, D., & MIGUENS, F. (2009). In Vitro Cultivation and Morphological Characterization of Phloemic Trypanosomatids Isolated from Coconut Trees Journal of Eukaryotic Microbiology DOI: 10.1111/j.1550-7408.2009.00454.x

Santos, A., d'Avila-Levy, C., Elias, C., Vermelho, A., & Branquinha, M. (2007). Phytomonas serpens: immunological similarities with the human trypanosomatid pathogens Microbes and Infection, 9 (8), 915-921 DOI: 10.1016/j.micinf.2007.03.018

SIMPSON, A., STEVENS, J., & LUKES, J. (2006). The evolution and diversity of kinetoplastid flagellates Trends in Parasitology, 22 (4), 168-174 DOI: 10.1016/j.pt.2006.02.006