A Video on the Moss Life Cycle

Take a break from your regularly scheduled program to check out this video on the moss life cycle. I would definitely recommend this video to students learning about mosses or bryophytes in class or for anyone who wants to brush up on their plant life cycles.

Overall I think that it is a nice video with accurate information. There was only one typo that I saw. The maternal gametophyte cap covering the sporophyte apex during its development is called the calyptra. No e after the t. 

Hat tip to Dr. Juan Carlos Villarreal for sending me this video from YouTube. 

Two Structures, One Set of Genes

When presented with a problem it is typically easier to solve it with tools at your disposal rather than inventing something new. This is also the case for plants and animals. When presented with a developmental or evolutionary challenge, it is often easier for them to use genes that already exist in their genetic toolkit to respond to the challenge.

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. 

Check out the publications for more details about their experiments and findings. 

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

Mosses on Science Friday

While doing some lab work today I was listening to a podcast of Science Friday from NPR. They mentioned a new web-video that had been posted about moss landscaping. I have posted it below or you can click here to check out the video at the SciFri website.

It is a nice video and has some good information if you are interested in encouraging mosses to grow in your yard. I especially like the hand drawn graphics that illustrate moss rhizoids. All of the science they discussed sounded solid. I think that they did their fact-checking well, which is always nice to see.

Typically when I meet people and tell them that I study mosses people respond with, "Oh, I have moss growing in my yard. Do you know how I can get rid of it? " I could start to outline all the ways in which mosses are fabulous and why you would never want to eliminate them. However this usually does not sway people. Instead I say, " Yes I know what you will need to do. 1) You need to change the pH of the soil by adding lime, but it is hard to do that for any large area and you might then need a lot of lime. 2) You probably have a wet area with poor drainage, which you need to fix to make the soil drier. And 3) you should cut down all the trees in your yard. The mosses will not be able to handle the sunlight and the grass will grow better. " This last statement usually results in a jaw-dropping reaction from most people and a statement that they could not possibly cut down their trees. Then they are much more open to learning to love, like or tolerate the mosses. I then go on to tell them that moss landscaping is becoming more and more popular and they should join the trend.

They also make the same the three points in the video. That pH, water, and sunlight are the main things to consider when trying to convert your lawn into a moss covered area. I also second their point about the low-maintenance nature of a moss lawn. By not requiring a weekly mow a lot of fossil fuel energy can be saved. What do you think, is a moss lawn in your future?

Examining Moss Filaments: Protonema & Rhizoids

Every week members of the Goffinet Laboratory group meet to discuss a research journal article about bryophytes. The papers that we read range from morphological to molecular and may relate to either mosses, hornworts, or liverworts, all of which we study in the laboratory. Last week's paper focused on moss protonema and rhizoids.

Protonema are unicellular filaments of haploid/gametophyte tissue. In the moss life cycle a spore germinates to produce filamentous protonema that then develop into leafy gametophytes. At right is a photograph of the protonema of Funaria hygrometrica.

Pressel, S., Ligrone, R. and J. G. Duckett. 2008. Cellular Differentiation in Moss Protonemata: A Morphological and Experimental Study. Annals of Botany 102:227-245.

This research paper focuses on three types of bryophyte filaments: chloronema and caulonema (both types of protonema) and rhizoids. They define rhizoids as filaments that are produced only by the mature leafy gametophyte plants. They are often pigmented brown and function to attach the gametophyte to the substrate (soil, tree bark, or rock that they are growing on).

They had a number of goals for their research, but I am not going to go into all of them. The one that I found the most interesting was that they examined 200 moss species and determined the cellular changes that occur during differentiation of the caulonema and rhizoid filaments. (Differentiation is the process by which cells acquire all of the characteristics that they will have at maturity.) You may ask why just describe a maturation process inside of the cells. Well as the authors mention (and I wholeheartedly agree), it is important to describe the sequence of events that occur in these filaments because it lays the foundation for future experiments. Researchers have to know how structures develop normally, so that they have a control/baseline to compare to experiments.

Additionally, the paper is full of great images. There are light microscopy photos zoomed in to the point that you can see the nucleus inside of the cell. Some of the other images illustrate a feature that I had not heard of before. The rhizoids produce a mucilage sheath (i.e. slime) that covers the entire outside of the cell wall. The remainder of the images are transmission electron micrographs that show all sorts of cellular structures. You can see mitochondria, chloroplasts, golgi bodies and nuclei, just to name a few. In order to see these structures some serious magnification is needed. These organelles are probably magnified 10,000-50,000X. I think that it is just really fabulous that we can see all of these tiny biological features inside of the cells!