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Rhizoids are thin filaments of cells that anchor leafy moss plants onto their growth surface, which can be soil, rock, or trees, just to name a few. Rhizoids also function in water uptake. They help by creating many capillary spaces in which water can be move from the soil to the plant. However, rhizoids are not the only structures that are able to take up water in mosses. The leafy gametophyte plants can absorb water through many parts of their body including leaves and stems.
The water uptake structures that you are probably more familiar with are roots. They are underground organs that function in water uptake and anchor the sporophytes of vascular plants into the soil. Near the tips of each root there are elongated, filamentous cells (root hairs) that increase the surface area through which the roots can take in water.
Though root hairs and rhizoids have similar functions and they both start with the letter 'R', these two structures have completely independent evolutionary origins. By that I mean that root hairs are not rhizoids that have been changed and modified over evolutionary time. Another piece of evidence that points to them being evolutionary independent is that rhizoids are only present on the gametophytes, whereas root hairs are only on the sporophytes. Having structures that are exclusive to opposite generations typically indicates that have evolved independently.
So, root hairs and rhizoids have similar functions, structurally they are both filamentous in shape, but what about the genes that control their development. Might they be using the same or similar parts of their genetic toolkit to build these two structures?
Scientists examined this by figuring out the genes that are important for forming the root hairs in flowering plants, then looking to see if these same genes are also important for root hairs in mosses (Menand et al 2007; Pires et al 2013). The figure below shows some of their results. Let me walk you through it. On the left are mosses will brown rhizoids growing from the base. On the right are flowering plant roots with thin root hairs sticking out of the sides. WT and Col0 are what the plants look like naturally with no changes to the genes.
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| Part of Figure 4 from Menand et al 2007 |
They found a group of related genes in mosses and flowering plants that influence both rhizoid and root hair formation. Pprsl1, Pprsl2, and rhd6-3 are the names of three members of this group of genes.
What we see on the left is that they knockout/turn off Pprsl1 = rhizoids still form, they knockout/turn off Pprsl2 = rhizoids still form, but when they turn both of them off = no to only a few rhizoids form.
On the left, center panel they turn off the gene rhd6-3 and the root does not make any root hairs. The coolest part of the study is that they are able to knock out the gene that makes root hairs, then use the moss gene to control the formation of root hairs. They are using a moss gene to control the production of root hairs in a flowering plant. Pretty wild!
This is just a small part of the story where they show that root hairs and rhizoids are controlled by the same network of genes. I think that it is a great example of plants using the genetic tools at their disposal to build similar structures on completely different parts of the plant in distantly related species.
Menand B, Yi K, Jouannic S, Hoffmann L, Ryan E, Linstead P, Schaefer DG, &; Dolan L (2007). An ancient mechanism controls the development of cells with a rooting function in land plants. Science (New York, N.Y.), 316 (5830), 1477-80 PMID: 17556585
Pires ND, Yi K, Breuninger H, Catarino B, Menand B, &; Dolan L (2013). Recruitment and remodeling of an ancient gene regulatory network during land plant evolution. Proceedings of the National Academy of Sciences of the United States of America, 110 (23), 9571-6 PMID: 23690618
It is a very interesting story and I like your presentation of it. The homology question is something I think about a lot, and I know that my opinion is colored by my having a background more in genetics than in comparative morphology.
ReplyDeletePersonally, I am not currently comfortable saying that rhizoids and root hairs are non-homologous. They emerge from different structures, so they are technically non-homologous structures, but is that a meaningful way of thinking if the same genetic cascade is just triggered in a different place? If 1) the last common ancestor had rhizoids and 2) such a filamentous structure is adept at nutrient absorption, it would not be surprising that it was repositioned as the body plans were altered. In this case, both this family of bHLH proteins and auxin is upstream of both rhizoids and root hairs, but two datapoints don’t prove that they are conserved regulatory networks beyond any doubt.
As for the gametophyte/sporophyte difference, I sense that people often overestimate the difficulty in moving some aspect from one to the other generation and others (usually molecular biologists) tend to dismiss the difficulty too easily. Clearly, many developmental programs have switched generations, but it was probably only possible as part of a dramatic, concerted change that shifted whole networks to being active in the other generation. Given what we know about genetic cascades, perhaps a few specific regulatory gene changes facilitated the developmental change. (I believe some of Stefan’s transcriptome papers discussed conservation of generation-specific gene expression patterns and found that more homologs were either non-generation-specific or had different specificities in moss versus Arabidopsis than were same-generation-specific in both.)
Thanks for the good read!
Michael says it much better than I was going to.
ReplyDeleteThanks for the comments. I'm glad you enjoyed the post! I also like thinking about homology and it always seems to result in an engaging discussion. When I think about structural homology the criterion I use is that structures in the present organisms have evolved from a common ancestor that had an ancestral structure that gave rise via descent with modification to both of the present structures. I acknowledge that it is a bit short-sighted of a definition because it does not take into account any of the underlying genetics. Overall I think that definitions of homology really depend on the level at which they are examined and the criteria that are used. If we used a genetic cascade criterion, we could define homologous structures as those controlled by the same genetic network, which would shake up my evaluation of the rhizoid/root hair homology.
ReplyDeleteDespite the alternative, I would still say that rhizoids and root hairs are non-homologous structures, since they did not evolve from a common ancestor with a structure that resulted in the two via descent with modification. That being said, considering it is the same genetic cascade expressed in a different location controlling their development, they do have quite a lot in common. What could we call that? Maybe genetic homology, rather than structural homology? Or loosen up the homology definition so that both types of criteria could fit in? I think that the lack of consensus is one reason why homology is still a topic for debate.
I agree that the co-opting of genetic networks from the gametophyte to the sporophyte generation is probably more common of a phenomenon than people might think. On one hand it is not very surprising that the genetic programs and the structures that they control could be moved from one generation or one location to the other, but I always find it amazing when science can actually demonstrate their commonalities. I bet you are right that it depends on you speciality science background whether you think of these processes as more likely or not.
Thanks for your thoughts!