Trends in Plant Science
Physcomitrella and Arabidopsis: the David and Goliath of reverse genetics
Section snippets
Genetic approaches for assessing gene function
As mutant isolation is a laborious task, strategies to manipulate the level of gene products by overexpression or antisense approaches are widely used. Unfortunately, in many cases these latter two approaches do not produce reliable data: transgenes integrate into the genome at unpredictable sites, thereby possibly generating new mutations. Moreover, expression of these randomly inserted transgenes suffers from position effects. Transformation with overexpression constructs also often results
Homologous recombination
Holger Puchta[5]has discussed the molecular basis of homologous recombination and the progress being made in understanding it in Physcomitrella patens[6]and Arabidopsis[7]. Physcomitrella is a moss, a tiny seedless plant; by contrast, Arabidopsis, the model species for plant genetics, is a seed-producing plant that is several times larger (Fig. 1). As targeting events of up to 90% were reported for Physcomitrella[6], Puchta recommended moss as an appropriate species for plant gene targeting
Reverse genetics with Physcomitrella
Recent experiments have allowed the isolation of two previously unknown Physcomitrella genes, and the reverse genetics approach has successfully been used to elucidate their biological function. In the lab of Ernst Heinz, a PCR approach employing primers deduced from widely conserved histidine boxes of acyl-lipid desaturases was used to isolate a homologous sequence from Physcomitrella. The corresponding full-length cDNA shared <27% sequence identity at the protein level with known acyl-lipid
Differences between Physcomitrella and Arabidopsis
It remains an open question as to why Physcomitrella is, as far as is known, the only plant with efficient homologous recombination. The simplistic view is that the moss is too distantly related to higher plants to be a comparable system. However, this objection can be overruled, as recent EST sequence data reveal a high degree of homology between Physcomitrella and seed plants. With respect to codon usage, the moss is more closely related to dicots such as Arabidopsis than the dicots are
Conclusions
From a physiological and genetic viewpoint it is clear that many fundamental processes can be studied in Physcomitrella as in a higher plant[14]. A variety of genes has now been cloned from Physcomitrella and these are remarkably homologous to their cognate higher plant genes11, 14. All transformation experiments performed to date have demonstrated that there are no significant differences in promoter usage or codon usage between Physcomitrella and angiosperm dicots such as Arabidopsis or
Acknowledgements
I am grateful for helpful comments from Eberhard Schäfer, Gunther Neuhaus, Rainer Hertel and Randall Cassada. I am indebted to the Deutsche Forschungsgemeinschaft for a Heisenberg Fellowship (Re837/3-1). Most of the work in my lab has been supported by the Deutsche Forschungsgemeinschaft (Re837/2, Re837/4 and Re837/5) and by the Commission of the European Community (PTP-project EUROMOSS).
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