Molecular adaptation and the origin of land plants
Introduction
The origin of land plants was one of the most important events in biological evolution and has been compared in significance to the “Cambrian explosion” (Bateman et al., 1998). Fossil evidence places the origin and diversification of land plants at a minimum of 450 million years ago (Graham, 1993; Kenrick and Crane, 1997a). The colonization of land by plants and the subsequent diversification of land plants modified the terrestrial environment and played an important role in the establishment of all modern terrestrial ecosystems. The land plant radiation is marked by a long list of morphological innovations and adaptations which include the alteration of generations with gametophytes and sporophytes, female and male sexual organs (antheridia and archegonia), a cuticle, and desiccation resistant haploid spores (Graham, 1993; Kenrick and Crane, 1997a). The first land plants faced numerous challenges that included but were not limited to, desiccation, temperature stress and increased exposure to UV radiation. It is then likely that the land plant radiation must have also involved numerous molecular and physiological adaptations.
The study of the molecular and physiological adaptations associated with the origin of land plants is a relatively new field (Graham, 1993; Graham and Wilcox, 2000; Raven, 2000). However a greater awareness of the importance of this field and the availability of genomic data and tools has helped to stimulate more research in this area. A major resource is the Physcomitrella patens (a moss) EST (expressed sequence tags) project (Machuka et al., 1999; Rensing et al., 2002). These EST sequences and those from the complete Arabidopsis thaliana and Oryza sativa genomes have been used in recent comparative plant genomics studies. An increasing awareness of the importance of phylogenetics in genomics studies (Pryer et al., 2002) will hopefully lead to new EST and genomics projects of basal streptophytes and basal embryophytes. Here I review physiological and molecular innovations associated with the early radiation of land plants and outline new research opportunities in this area.
Section snippets
Streptophyte phylogeny
The study of evolutionary adaptations requires robust phylogenetic trees that provide an evolutionary framework with which to analyze traits of interest. One of the past limitations on the study of plant adaptations to land has been the uncertainty surrounding the relationships of the basal land plant groups and their algal relatives. The recent progress in this area has greatly lessened this limitation and has enabled searchers to interpret their molecular and physiological data in an
Molecular evolution and physiological adaptations
In the announcement of the A. thaliana genome it was noted that many of the genes and gene families are plant-specific, i.e., they have no known homologs outside land plants (The Arabidopsis Genome Initiative, 2000). In the analysis of the A. thaliana genome it was found that approximately 50% of the genes associated with protein translation and only 20% of the genes associated with transcription have non-land plant homologs (The Arabidopsis Genome Initiative, 2000). That these functional
Future directions
There are a few patterns that are beginning to emerge from the studies mentioned above. The most striking is that of “evolution in stages” (Graham et al., 2000). In the cytokinin, auxin, phenolics, and heat shock studies we saw instances where a process, molecule or gene family maybe present in algae and basal land plants but becomes modified or more complicated in later diverging plant groups. New members of protein families may be adaptations to life on land but these proteins have not
Acknowledgments
I am grateful to Mike Simpson and two anonymous reviewers for helpful comments that greatly improved this manuscript. This work was supported by a grant from the National Science Foundation (IBN-0313900). I would also like to thank Linda Graham and Elizabeth Vierling for many stimulating conversations on the origin of land plants and the evolution of the heat shock proteins.
References (62)
- et al.
Mosses as model systems
Trends Plant Sci.
(1997) - et al.
Heat shock proteins and molecular chaperones: mediators of protein conformation and turnover in the cell
Cell
(1994) - et al.
Molecular evolution and structure-function relationships of the superoxide dismutase gene families in angiosperms and their relationship to other eukaryotic and prokaryotic superoxide dismutases
Arch. Biochem. Biophys.
(2002) - et al.
The chloroplast heat shock protein undergoes oxidation dependent conformational changes and may protect plants against oxidative stress
Cell Stress Chaperon
(1999) - et al.
The plant cell cycle
Curr. Opin. Plant Biol.
(1999) - et al.
Phylogenetic relationships of liverworts Hepaticae, a basal embryophyte lineage, inferred from nucleotide sequence data of the choloroplast gene rbcL
Mol. Phylogenet. Evol.
(1997) Green algal phylogeny
Trends Ecol. Evol.
(1995)- et al.
Deciding among green plants for whole genome studies
Trends Plant Sci.
(2002) - et al.
Phylogeny of early land plants: insights from genes and genomes
Trends Plant Sci.
(1999) - et al.
Moss transcriptome and beyond
Trends Plant Sci.
(2002)