Why don't plants have any chlamydial symbionts?
It seems that all known chlamydiae are obligate intracellular symbionts -- they can only reproduce inside eukaryotic cells, and remain metabolically inactive outside of their hosts (a virus-like lifestyle). Chlamydiae can infect different kinds of animals (mammals, birds, fishes, arthropods, crustaceans) and unicellular eukaryotes (such as environmental amoebae). Remarkably, chlamydiae have never been found in plants or in other plastid-containing organisms (red and green algae, plants and glaucophytes, together known as Plantae or Archaeplastida).
On the other hand, as I have explained in a previous post (A cell potpourri: eukaryotes and their organelles), an important number of Archaeplastida genes are derived from ancient chlamydiae, suggesting a long-term symbiosis between ancestors of chlamydiae and Archaeplastida. The bacterial endosymbiont was later lost, leaving some of its genes behind.
So, the question is: why don't present-day Archaeplastida have any chlamydial symbionts (parasitic, mutualistic, commensal)? Why can't they be infected, once more, by chlamydiae?
I may think of some possible answers:
(1) Actually, there are chlamyidial symbionts in Archaeplastida. We just haven't found them. (Have we looked for them?)
(2) Archaeplastida are not special: other eukaryotic lineages appear to lack chlamydial symbionts (just a hypothesis, I have no idea). In other words, chlamydiae are able to infect cells from only specific eukaryotic lineages. (How wide is chlamydial host-range? Has anybody tried to infect Archaeplastida cells with chlamydiae?)
(3) Archaeplastida are special: after being infected by chlamydia-like bacteria, the Archaeplastida lineage became resistant to over-infection by other chlamydiae. (If this is the case, it would be nice to know the molecular mechanisms responsible for the resistance. Can we make an Archaeplastida cell susceptible to infection by knocking-out specific genes? Are these genes derived from chlamyidiae? Or, the other way around, can we make an Archaeplastida cell susceptible to infection by adding specific genes from other eukaryotes? This knowledge could be useful to design new anti-chlamydial therapies.)
You can contribute to this discussion here (post a comment), or at the PLoS ONE site where a relevant article was published (Chlamydiae has contributed at least 55 genes to Plantae with predominantly plastid functions).
Related links:
Image:
Electron micrograph of a cell infected by Chlamydia trachomatis. © American Society for Microbiology. Reference: Beatty WL, Morrison RP, Byrne GI. Persistent chlamydiae: from cell culture to a paradigm for chlamydial pathogenesis. Microbiol. Rev. (1994) 58, 686-699.
