There are four major groups of modern land plant: non-vascular land plants that include mosses , liverworts and hornworts; seedless vascular plants including ferns and horsetails; gymnosperms including conifers and cycads that are characterised by the protection conferred upon the embryo by a hardened seed casing; and angiosperms such that protect and disperse their seeds by flowering or fruiting, reviewed by (Raven et al., 2005), (Mader, 2007).
The evolution of modern land plants from unicellular prokaryotes that acquired the ability to photosynthesise will be discussed here. The evidence in favour of modern land plants sharing a relatively recent common ancestor with algae will also be presented. Additionally, the sequence of evolutionary divergences that have produced modern land plants from their prokaryotic ancestors will be discussed and the adaptations that modern plant groups have evolved in order to successfully exist on land will also be described.
The evolution of plants from algae
Modern land plants have evolved from the first plants to inhabit the land, that themselves originated from freshwater-borne algae that had in turn evolved the ability to utilise chloroplasts in order to take energy in the form of ultraviolet light from the sun. Such algae originated from endosymbiosis with cyanobacteria with primitive bacteria, reviewed by (Mauseth, 1991), (Mader, 2007). Figure 1 shows the evolutionary lineage of plants within the kingdom protista from a prokaryotic ancestor and the divergence of plants from the other taxonomic kingdoms. It should be noted that Protista is not a “true” kingdom based on recent molecular studies but is a classification of convenience used by evolutionary biologists, reviewed by (Raven et al., 2005).
Eukaryotic cells are believed to have arisen from prokaryotes by the process of endosymbiosis, whereby primitive prokaryotic cells were engulfed by larger prokaryotic cells, reviewed by (Raven et al., 2005), (Mader, 2007). Endosymbiotic cyanobacteria that obtain their energy from sunlight through the process of photosynthesis were utilised for energy production within the host cell and have now evolved into the chlorophyll-containing chloroplasts, reviewed by (Douglas, 1998), (Gould et al., 2008). The earliest land plants capable of photosynthesis converted carbon dioxide to oxygen, thus producing the atmosphere in which aerobic land organisms survive today, reviewed by (Gould et al., 2008).
Evidence for the evolution of plants had been traditionally described in terms of physiological characteristics, but with the advent of modern molecular biology, genome screening techniques and the resulting construction of gene trees, researchers can now be more confident about the evolutionary origins of modern land plants, reviewed by (Gould et al., 2008), (Douglas, 1998), (Bock and Timmis, 2008), (Moreira and Philippe, 2001).
Based on this phylogenetic evidence, modern land plants currently belong to the phylum Streptophyta within the kingdom Protista that also includes Chlorophyta (green algae), Rhodophyta, Euglenozoa, Alveolata and Stramenopila, reviewed by (Raven et al., 2005) The evolutionary tree of the kingdom Protista is shown in Figure 2. Again, the classification within the protists is a matter for scientific debate, but grouping of modern land plants within the same kingdom as green algae by genome analysis suggests a close common ancestor of plants and algae.
The evidence for the close evolutionary origins of modern land plants and green algae is provided by recent studies that have identified and compared the genome sequences of species of both modern plants and algae. Chara vulgaris, a species of Charales green algae is also classified within the phylum Streptophyta, rather than within Chlorophyta and has been identified as the closest algal species to modern land plants in terms of genetic homology (Turmel et al., 2006, Turmel et al., 2003). These studies described Charales as an evolutionary “sister” to modern land plants by identifying the genomic sequences of mitochondria (Turmel et al., 2003) and chloroplasts (Turmel et al., 2006) of different species of Streptophyta, suggesting a relatively recent divergence from a common ancestor.
Following their divergence from green algae within the phylum Streptophyta, land plants subsequently evolved into four modern groups: the nonvascular, seedless vascular, gymnosperms and angiosperms.
In addition to being phylogenetically separate in terms of their evolutionary history and genomic DNA sequences, the four groups are notable for their differences in their vascular and reproductive physiology and prior to the use of modern molecular biological techniques, were grouped on this basis. Non-vascular plants do not have seeds or a vascular system, as their name suggest, seedless vascular plants possess a vascular system but no seeds, while both gymnosperms and angiosperms possess both seeds and a vascular system, but characteristic differences involve how these two groups have evolved separate methods of seed protection and dispersal, reviewed by (Raven et al., 2005), (Mader, 2007).
Adaptations to life on land
When plants first moved out of the water and on to the land, it was essential to their success that plants evolved a number of physiological and molecular mechanisms for maximising their survival in their new environment. New challenges that were not relevant to water plants such as reduced water availability, temperature variations and increased exposure to sunlight were encountered by the first land plants, reviewed by (Waters, 2003).
The evolution of a root and vascular system has been necessary within three of the modern land plant groups, to collect and transport the water necessary for photosynthesis from the soil to the leaves in the absence of an aquatic habitat. Cooksonia, a now extinct species of vascular seedless plant, were the first plants to evolve a mechanism for the transport of water and nutrients, known as the xylem. The phloem also evolved to facilitate the movement of sugars, amino acids, hormones and other molecules important for the nutrition of land plants. The xylem and phloem form the major passages of the plant vascular system, reviewed by (Raven et al., 2005). Water-borne plants had not evolved such a system because of the high availability of water, so early land plants were lacking this complex mechanism of transport. The loss of water through transpiration must also be controlled by land plants, so thick or waxy cuticles to prevent evaporation, particularly in dry environments and stomata to control water loss have evolved, reviewed by (Mader, 2007), (Raven et al., 2005).
The leaves themselves have evolved for the specific requirements of individual species and can be large in some species to maximise the surface area and number of chloroplasts to increase the rate of photosynthesis in a biological trade-off against the rate of transpiration. Plants that inhabit wet conditions therefore tend to favour larger leaves, while those in arid conditions, such as cacti, possess spines rather than leaves in order to minimise water loss, reviewed by (Mauseth, 2006, Raven et al., 2005, Mader, 2007). Leaves can also be involved in reproduction by attracting pollinating insects or by functioning to regenerate a whole plant from the leaf alone. Some plants in nutrient deficient environments have evolved insectivorous leaves in order to obtain the nutrients that they cannot take up through their roots from insects, reviewed by (Raven et al., 2005, Mader, 2007).
There are a number of types of root system that have evolved depending on the needs of particular species. The primary function of roots is to collect water by osmosis, but can also function as anchorage for protection and stability. Roots can also be contractile, to move the plant to more hospitable temperatures, while roots can also be used for storage of water and nutrients, reviewed by (Raven et al., 2005, Mader, 2007). Like the roots, the stems of land plants have also evolved mechanisms to maximise the success of the plant in protection, nutrient and water storage also for reproduction by vegetative propagation, reviewed by (Raven et al., 2005)
Seeded plants have evolved mechanisms of seed dispersal that are suited to the land. The fruiting and flowering mechanisms of angiosperms allow seed dispersal and pollination by animals while conversely, gymnosperms have evolved mechanisms in order to protect their seeds from animals, reviewed by (Mader, 2007).
Throughout natural history, symbiosis has been a trait that confers evolutionary success upon plant species (Selosse et al., 2004). While endosymbiosis allowed land plants to evolve by providing the mechanism of photosynthesis through interaction with chloroplasts, some modern vascular plant species that do not have ability to photosynthesise rely on symbiotic relationships with other organisms such as fungi, algae or cyanobacteria to produce energy from sunlight, reviewed by (DePriest, 2004).
Modern land plants also utilise symbiosis in order to aquire nutrients. Around 90% of modern plants form a symbiosis with the fungus mycorrhizae that provides a benefit for both the plants and the fungus, reviewed by (Raven et al., 2005). In mutually beneficial mycorrhizal symbiosis, plants provide organic nutrients to the fungus as a product of photosynthesis from the carbon dioxide substrate, while the fungus, that interacts with the plants roots, aids the uptake of nutrients required by the plant, reviewed by (Raven et al., 2005).
In addition to the physiological modifications that plants evolved to successfully inhabit the land, changes on the molecular level have also been identified in terms of growth hormones and heat shock proteins, among other molecules, reviewed by (Waters, 2003).
The most successful plant group?
All modern organisms can be described as successful, in terms of their evolution over billions of years from simple beginnings to their current species while avoiding extinction from competition, predation or un-hospitable environments. As discussed above, all modern plants have evolved adaptations to survive and reproduce within the conditions in which they inhabit.
In terms of the success of land plant groups, each group must be well adapted to the environment that it inhabits in order to survive the evolutionary process. In terms of species diversity, angiosperms of the phylum Anthrophyta contain around 250,000 species compared to around 25,000 species of bryophytes (that include the non vascular mosses, liverworts and hornworts) reviewed by (Raven et al., 2005), but in terms of the number of different environmental conditions they inhabit, the non-vascular plants could be described as being the most successful plant group and due to their adaptations in temperature resistance and in maximising small volumes of water, reviewed by (Mader, 2007). Lichens are known to survive extreme desiccation and inhabit a wide range of conditions on Earth, including arctic, boreal and desert environments, reviewed by DePriest, (2004).
Modern land plants have evolved over billions of years from a unicellular prokaryotic ancestor that underwent endosymbiosis with cyanobacteria and gained the ability to photosynthesise. Divergence from freshwater green algae and movement onto land was followed by the evolution of early land plants into the four modern groups that exist today. Each group of modern land-plants possesses evolutionary adaptations appropriate to the environmental conditions in which they inhabit and each can be described as successful within its own environment.