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.
|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