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!