Team:TU-Munich/Project/Physcomitrella

General description

The moss Physcomitrella patens belongs to the land plant division Bryophyta, which are one of the earliest representatives of the land plants (Embryophyta) having evolved from green algae about 470 million years ago during the early Paleozoic. Hence mosses have a much simpler anatomy than higher land plants such as trees and flowering plants, which in particular means that they have not yet developed a vascular system, i.e an internal transport system for water and nutrients. Since they also lack a complex waterproofing system to prevent absorbed water from evaporating they need a moist environment to grow. Their main habitats are therefore shady and damp places such as woods and edges of streams but they are also found to be resistant to periods of drought and therefore can be found widely spread around the world, from the tropics to tundra regions, from coastal sand dunes up to high mountains.

The general organization of plant tissue into roots, stem and leaves is found in a much more basic version in mosses. They show a differentiated stem with simple leaves, usually only a single layer of cells thick and lacking veins, that are used to absorb water and nutrients. Instead of roots they have similar threadlike rhizoids. These have a primary function as mechanical attachment rather than extraction of soil nutrients. Due to not having a vascular system bryophytes are doomed to stay small throughout their life-cycle typically stretching about 1-10 cm.

However different mosses and vascular plants are because of the early diverge of the evolutionary lineages, they share fundamental genetic and physiological processes. Hence a good approach to studying the complexity of higher land plants is to look at the bryophytes with their much simpler phenotype. Here researchers chose Physcomitrella patens as a model organism with a genome size of about 450 Mb along 27 chromosomes that is highly similar to other land plants in both exon-intron-structure and codon usage.

Life cycle

Figure 3:

Generally land plants show an alternation of generations, the haploid (1n) gametophyte produces sperm and eggs which fuse and transform into the diploid (2n) sporophyte. This then forms haploid spores which become new gametophytes. Besides having no vascular system, bryophytes also differ from higher land plants in the fact that the gametophyte is the dominant phase of their life cycle, whereas in vascular plants the principal generation is the sporophyte.

The life cycle of P. patens only takes about 3 months and starts with the spore developing into a filamentous structure, the juvenile, transitory stage of the gametophyte, called protonema, which is composed of two types of cells. The chloronema cells with large and numerous chloroplasts mostly perform photosynthesis and thus supply the photoautotrophic plant with energy while the task of the caulonema cells is fast growth. The adult stage of the gametophyte, called gametophore ("gamete-bearer") has a more complex structure bearing leafs, stem and rhizoids. The transition from juvenile to adult gametophyte is started by initial cells in the protonema filament that differentiate into buds. The budding is therefore a single-cell-event, greatly stimulated by the plant hormone cytokinin, which promotes cell division.

The sex organs of the moss develop from the tip of the gametophore. P. patens is monoicous, meaning that male and female organs are produced in one plant. When liquid water is surrounding the tip, flagellate sperm cells can swim from the male sex organ to the female organ and fertilize the egg within. A zygote then develops into the sporophyte, which in turn produces thousands of haploid spores by meiosis. Sporophytes are typically physically attached to and dependent on supply from the dominating gametophyte.

General

• P. patens stands out among the whole plant kingdom as the sole exception where gene targeting is feasible as an easy and fast routine procedure, even with an efficiency similar to S. cerevisiae, due to highly efficient homologous recombination. For that reason it is very easy to create knockout mosses by precise mutagenesis following the approach of reverse genetics in order to study the function of genes. Performing functional genomics in higher organisms is very important to understand biological functions of proteins in a multicellular context, e.g. in the context of cell-cell-contacts.
• As mosses mainly are in a haploid stage during their life cycle they are very straight-forward objects for genetics because complex backcrosses to determine changes in the genotype are not necessary
• Moss development starts with a filamentous tissue, the protonema, which is growing by apical cell division and, therefore is perfectly suitable for cell-lineage analysis since development of the plant can be pinpointed to the differentiation of a single cell. Also the simple life cycle makes P. patens a very useful item for developmental biology
• P. patens is increasingly used in biotechnology as a study object with implications for crop improvement or human health. Moss bioreactors can be used as an alternative to animal cell cultures (e.g. CHO cells) for the easy, inexpensive and safe production of complex biopharmaceuticals. For example it is a successful tool to produce asialo-EPO, a specific variant of Erythropoetin, which can perform its protective role by inhibiting apoptosis but has lost the potential doping activity. This safe drug is hard to produce in animal cell culture but easy to produce in the moss without impacting its growth or general performance. [Decker et al. (2012)]
• (plant evolution by comparing with results from higher plants such as Arabidopsis thaliana)

As a new chassis for iGEM

• As a plant P. patens offers interesting opportunities for application as it is self sustaining, renewable and a natural part of our environment. Therefore it is much easier to implement it into real world scenarios than bacteria or yeast. And although there is the disadvantage of having to wait about two weeks after transformation until experiments can be done, this is still very short considering the high complexity of the organism and can easily be done in the timeframe of the competition, by preparing the DNA constructs in bacteria.
• P. patens is a well studied model organism which means that besides having its full genome sequenced in 2006 there are well equipped databases. Furthermore there exists an International Moss Stock Center (IMSC) in which many ecotypes, mutants and transgenics of P. patens are stored and freely accessible to the scientific community. So there is enough knowledge and material to work on for synthetic biologists.
• At the same time the moss offers access to very exciting new physiological processes since it is a much more complex multicellular eukaryotic organism than yeast.
• P. patens is an easy plant to work with and requires neither expensive maintenance facilities nor large laboratory space. Most of the basic tools for high precision mutagenesis have been tested on this plant, were found to work, and are easily available.

SERK: The Template from P. patens

The Blueprint for our Synthetic Moss Receptor

SERK: The Template from P. patens

The Blueprint for our Synthetic Moss Receptor

[Edens et al., 1984]

1. [Edens et al., 1984] Edens, L., Bom, I., Ledeboer, A. M., Maat, J., Toonen, M. Y., Visser, C., and Verrips, C. T. (1984). Synthesis and processing of the plant protein thaumatin in yeast. Cell, 37(2):629–33.