The concept for a book providing an introduction to the biology of bryophytes emanates from our passion for these land plants and the lack of recent manuals offering general insights into their fascinating diversity and evolutionary history. Throughout its history, bryological research has contributed significantly to the field of plant sciences, starting with the discovery of sex chromosomes in plants and culminating most recently with the assembly of the Physcomitrella genome, fundamental for the study of the evolution of genes and their function during the diversification of land plants. Bryophytes are, in fact, pivotal in land plant evolution, and the recent advances in molecular phylogenetics and genomics have allowed for a clearer picture of land colonization and subsequent evolution to emerge. Ecologically, the significance of bryophytes for the regulation of global biogeochemical cycles, especially carbon, has long been acknowledged and has gained much attention in the present context of global changes. Finally, there has been an increasing concern for the conservation biology of cryptogams, and bryophytes in particular.

The aim of the present book is to offer a stepping-stone to anyone interested in discovering the fundamentals of the biology of bryophytes, making the bridge with more comprehensive treaties such as Goffinet and Shaw's Bryophyte Biology (2009) and Glime's Bryophyte Ecology (2007a). In Introduction to Bryophytes, we make an attempt at summarizing and explaining the recent advances made across the various aspects of bryophyte biology at a level that would be palatable to beginners.

In the previous chapter, we focused on the mechanisms that explain how bryophyte species are able to ‘travel’ across oceans and continents. The success in colonizing a new area, or the ability to persist within the same area in the long-term, depend, of course, on the ability of the species to cope with local environmental conditions. Bryophyte distributions are influenced by a variety of factors operating over a range of temporal and spatial scales (Rydin 2009) (Fig. 7.1), which are the focus of the present chapter.

Global ecology

A central thesis to plant ecology is that climate exerts the dominant control on the distribution of plants at the continental scale. In bryophytes, changes in communities depending on major climatic shifts during the Quaternary Era are evident from the succession of macro-remains preserved in peat. Each climatic phase had its characteristic range of habitats and these appear to have been colonized by bryophytes at a remarkable speed as soon as they developed and became available (Jonsgard & Birks1995). Two macrofossil taxa are of particular significance in the interpretation of macro-climatic changes: Sphagnum spp. and Racomitrium lanuginosum.

The local distribution of R. lanuginosum is controlled by its preference for cold, wet climates (Tallis 1995). By contrast, the occurrence of Sphagnum largely depends on water availability. In the Holarctic, for example, Sphagnum bogs are strictly restricted to areas experiencing an annual moisture balance > 0 (Gignac et al. 2000).

Vascular plants, particularly seed plants, dominate vegetation throughout much of the world today, from the lush rainforests of the tropics harbouring a vast diversity of angiosperms, to the boreal forests of coniferous trees, draping the northern latitudes of the globe. This dominance in the landscape is the result of a long evolutionary history of plants conquering land.

Evolution is the result of a suite of incessant attempts to improve fitness and take advantage of opportunities, such as escaping competition and occupying a new habitat. Pilgrims, fleeing the biotic interactions in the aquatic habitat, faced severe abiotic selection forces on land. How many attempts were made to conquer land is not known, but at least one of them led to the successful establishment of a colony. At least one population of one species had acquired a suite of traits that allowed it to complete its life cycle and persist on land. The ancestor to land plants was born. It may have taken another 100 million years for plants to overcome major hurdles, but by the Devonian Period (approximately 400 mya), a diversity of plants adapted to the terrestrial environment and able to absorb water and nutrients, and transport and distribute them throughout their aerial shoots, occupied at least some portions of the land masses. Soon thereafter, plants were freed from the necessity of water for sexual reproduction, by transporting their sperm cells in pollen grains carried by wind or insects to the female sex organs, and seeds protected the newly formed embryo.

Mosses (Division Bryophyta) are generally seen as small plants confined to humid habitats, avoiding exposure to direct sunlight. Yet, an alert naturalist will quickly notice their presence in virtually every ecosystem. In parts of the world where short growing seasons limit plant growth, mosses may dominate the vegetation. Similarly, in temperate and tropical rain forests, mosses compose luxuriant epiphytic communities that play important ecological functions, especially in terms of water and nutrient flow. Approximately 12 000 species of mosses are currently recognized, reflecting a broad morphological diversity. In fact, a unifying macroscopic definition is impossible due to both fundamental architectural differences among major lineages and extensive reduction, and hence character loss, across the phylogenetic tree of mosses. A sporophyte composed of a robust stalk elevating the capsule diagnoses most mosses, but is lacking in peat mosses and various ephemeral taxa inhabiting seasonally dry ecosystems. In many cases, closer examination further reveals one or two rings of teeth lining the capsule mouth. The vegetative (gametophyte) body always consists of a terete axis with sessile leaves, rather than a flattened thallus as in hornworts and some liverworts. Although associations with fungi are common, evidence for a symbiotic nature of the relationship benefiting the moss is lacking.

Biogeography is the science that aims to describe the spatial distributions of biota (a pattern) and understand the means by which these distributions were achieved (a process). Biogeography is a field that existed long before evolutionary biology and indeed helped in founding the evolutionary ideas of Charles Darwin and Alfred Wallace, among others (Humphries & Parenti 1999). Biogeography and evolutionary biology therefore interface with each other, as the discovery of the mechanisms regulating species distributions involves an understanding of species dispersal ability, evolutionary rates and diversification mode, which are among the main foci of the sciences of evolution.

In general, bryophyte species have broad geographic ranges that often span more than one continent (e.g. Figs. 6.1–6.3). Some, termed as ‘cosmopolitan’, are even widespread across all continents. Bryophyte species thus tend to show wider distributions than vascular plants. In fact, many bryophyte species exhibit the same disjunctions that are well known in flowering plants at the generic level. For example, 43% of the moss species found in North America are also found in Europe, while 70% of the species found in Europe also occur in North America (Frahm & Vitt 1993). By contrast, 48% of the genera, but only 6.5% of the species, are shared between the North American and European vascular flora (Qian 1999). Two competing hypotheses, namely repeated intercontinental dispersal and continental drift, have traditionally been proposed to explain the broad and highly disjunctive distributions typical of bryophyte species.

The natural corollary of the narrow relationship between bryophytes and their environment is that a great deal of variation in community composition and physiological behaviour is to be expected whenever changes in environmental conditions occur. In the current context, global change brought about by human activities has increasingly been an area of concern. Bryophytes, perhaps more than many other organisms, are highly sensitive. Their response to climate change is obvious from the reconstruction of paleovegetation based on the analysis of macrofossils (see Section 7.1), so that a great deal of changes in species distribution and community composition is to be expected in the current context of global warming (Box 7.1). The response of bryophytes to an increasingly polluted world is no less dramatic. This is because bryophytes, being poikilohydric, mostly lack a protective, highly hydrophobic cuticle and are, therefore, directly exposed to pollutants. As a result, pollutants may bind on cell walls, eventually pass through the cell membrane and affect metabolism.

In this chapter, we review how pollution affects bryophyte ecophysiology and community composition. We focus on airborne pollution, which arguably constitutes the main threat to ecosystems (Lee 1998). We then assess how understanding the response of bryophytes to these changes can be used as one indicator to monitor the quality of the environment. Finally, we examine the mechanisms by which bryophytes are able to adapt and survive in a changing environment.

Hornworts (Division Anthocerotophyta) compose the least diverse lineage of bryophytes. Their name refers to the horn-like sporophyte that lacks a seta and dehisces along one or two vertical lines (Fig. 5.1a). They further differ from mosses by the lack of leaves. Their vegetative body is indeed thalloid and rosette or ribbon-shaped and thereby resembles that of some liverworts. Also like liverworts, hornworts lack conspicuous and constant diagnostic gametophytic features and assigning a thallus to one lineage or another relies on microscopic characters. Although morphologically rather simple in architecture, hornworts may be physiologically elaborate organisms. Many species establish and maintain intimate symbiotic associations with endophytic nitrogen-fixing cyanobacteria and endomycorrhizal fungi. Furthermore, hornworts have carbon-concentrating mechanisms lacking in other bryophytes. Phylogenetically, hornworts appear more derived than mosses and liverworts, too: recent studies suggest that they alone share a unique common ancestor with vascular plants. Although easily overlooked in the field because of their often thin and when dry, dark thallus and neglected by biologists for their low global diversity, hornworts are drawing increasing attention for their significance in the evolution of land plants and their partnerships with endosymbionts.

The correlations between species distribution patterns and environmental factors that were analyzed in the previous chapters at different spatial scales are a vital starting point in making hypotheses about causes and effects of biotic interactions, historical factors, reproductive characteristics and immediate physiological requirements. It appears that most bryophyte species tend to occupy well-defined niches, as if they had evolved discrete physiological optima for factors such as humidity, light, temperature and nature of the substratum. Water availability, in particular, seems to play a major role in bryophyte distributions. In fact, bryophytes are believed to have colonized the land from a fresh water origin (Mishler & Churchill 1985). This major ecological shift must have required the evolution of adaptive mechanisms to survive such harsh drying treatments in order to have successfully exploited terrestrial habitats (Oliver et al. 2005).

In the present chapter, we describe adaptive strategies to drought and show how water relations interact with other physiological attributes and constraints to shape the ecological and geographical distribution of bryophytes.

Water relations

Adaptive strategies to drought

All the basic metabolic processes of life take place only in the aqueous medium of a hydrated protoplasm. Drying to equilibrium with even moderately dry air is, hence, instantly lethal to most animals and plants.

Plastids are a group of organelles present in the cells of all higher and lower plants, including algae, which function in a variety of different ways to enable plants to grow and function. Although different types of plastids which are found in different types of cells have modified roles, according to the type of cell in which they reside, the foremost function of plastids is carrying out the process of photosynthesis. Photosynthesis is a fundamental feature of plants and is facilitated by the presence of green, pigmented chloroplasts within plant cells. Indeed, photosynthesis is a defining feature of plants and enables them to fix carbon from the gaseous carbon dioxide in the air and synthesise a variety of complex organic molecules which allows them to increase in stature and mass. Photosynthesis is carried out by chloroplasts, which by virtue of containing the green pigment chlorophyll, defines the phenotype of green plants. Photosynthetic Eukaryotes have increased in their complexity dramatically since the first land plants, termed Embryophytes, evolved from freshwater multicellular green algae, around 450 million years ago. The current-day group of algae that are most closely related to these ancient algae are the Chlorophytes (Fig. 1.1). From these have evolved the lower plants, which includes the liverworts, mosses, hornworts and ferns (Fig. 1.1). Subsequently, the Gymnosperms and then the flowering plants, the Angiosperms, evolved and the Angiosperms, in particular, have been highly successful in conquering the planet such that much of the Earth is covered in green swathes of vegetation containing countless numbers of photosynthetic chloroplasts within their cells.

The revolution in genetics and molecular biology that occurred towards the end of the twentieth century inspired enormous progress in understanding many aspects of the molecular control of development of different organisms. It has also aided our understanding of how information contained in their respective genomes gives rise to large populations of different proteins, termed the proteome. In turn, these proteins interact with metabolites enabling the organism to develop its specific phenotype. Moreover, the use of molecular genetic techniques enabled genetic systems to be altered artificially in order to exploit aspects of molecular synthesis or developmental biology for improvement of agricultural species of plants and animals.

Early on in this revolution, plastids were seen as an attractive system in which plant biotechnology could be performed, since they have several properties that are advantageous in this area. Also, as we have seen throughout this book, plastids carry out many vital processes in plant cells, which have the potential to be manipulated for improvement or increased efficiency, leading to crop plants with more optimised phenotypes for specific environments. Because plastids are organelles, which define a distinct compartment within the plant cell, sequestering novel molecules into the plastid compartment presents a major advantage compared with allowing accumulation of novel molecules in the cytosol where they could be toxic. Since the relative proportion of the plastid compartment size compared to the cytoplasmic volume is fairly high, the plastid represents a significant compartment in which novel molecules can be accumulated.

Since the first endosymbiotic event that enabled a free-living photosynthetic organism to take up residence in a eukaryotic cell, the organisms that we know of as plants have evolved in a dramatic and highly varied way. In particular, plants have evolved from their single-celled ancestors into complex multicellular structures. A key characteristic of these multicellular plants is that they contain cells of different types, which are distinguishable from each other in the functions that they perform within the whole complex organism. With increasing complexity of form through the evolution of Bryophytes, Monilophytes and into the higher plants, Gymnosperms and Angiosperms, large numbers of different types of cells have been developed such that, in the more complex members of the flowering plants, the Angiosperms, there are over 50 different types of cells. As a result of this diversification of cell types in plants, the original endosymbiotic plastid found itself being manipulated by the host cell to take on a variety of different roles in these different types of cells. Plastids evolved from their original photosynthetic function after the original endosymbiosis to take up a key role in the cell as a whole, particularly in relation to biochemical interactions in the cell's metabolism. As a result, in modern-day higher plants, there are a variety of different types of plastids, which fulfil different roles in different types of plant cells. The situation, however, is not clear-cut. There is significant interaction between different plastid types in different types of cells in that the plastids can interconvert between different types according to molecular and environmental signals.

From what has been discussed in the preceding chapters, it should be clear that the plastid is a highly dynamic organelle in terms of its biochemistry, its molecular biological systems, its photochemistry and its molecular interaction with the nucleus. In addition to these aspects of plastid biology, one also needs to consider how the plastid resides within the cytoplasm of the cell, in many cases as a large closely packed population of individual organelles resulting from extensive divisions, and how the individual organelles move about in the cell and physically interact with other cellular organelles. Classic images of plastids from sectioned leaves and electron micrographs (see Figs. 2.4, 2.5) give the impression of a static organelle in which little changes physically with time. Modern imaging techniques, together with developments in the use of visible molecular markers that can be artificially incorporated into plastids, reveal a very different view of plastids: that of a highly dynamic organelle capable of rapid changes in morphology and movement within the cell. In this chapter, some of these dynamic aspects of plastid biology are described. In addition, we consider how different types of plastids differentiate in different types of cells and how they might influence plant development itself.

Plastid division

All plastids in a plant originate from proplastids in the cells of the meristem, which in turn are generally derived from the few proplastids in the egg cell in the flower. The fact that plastids need to divide to establish large populations of plastids in the large number of cells that make up a plant is obvious.

During the growth and development of a seedling plant, the various types of plastids which are found in different cell types develop from those proplastids found in the cells of the shoot and root meristems. These proplastids provide the source for all plastids in all the different types of cells within the plant. The way in which proplastids develop into different plastid types in different types of cells is poorly understood, but by far the best-researched plastid developmental pathway is that of the proplastid developing into a mature chloroplast. This developmental pathway occurs primarily in the mesophyll cells of leaves, resulting in large populations of mature green chloroplasts in these cells, although chloroplast development can also occur to varying degrees in all other green tissues of a plant. The development of a mature chloroplast in a leaf mesophyll cell or in other green tissues requires a huge synthesis of proteins, lipids and metabolites, the vast majority of which are imported from the cytosol into the plastid. In particular, an extensive array of proteins encoded in the nucleus and translated on cytosolic ribosomes are required to be imported using the import mechanisms described in Chapter 5.

Moreover, a major feature of this chloroplast developmental pathway is coordination. Not only is coordination required between the expression of nuclear genes encoding proteins destined for the chloroplast and the expression of those genes encoded on the chloroplast's own genome, but also coordination is required between the expression of these nuclear genes and the developmental status of the chloroplast, as well as its functional status in terms of photosynthetic activity and the degree to which the chloroplast is stressed by environmental conditions.

Plants are fundamental in enabling the planet Earth to function as a relatively stable system. They exert control over the biosphere by their interaction with their environment, a fundamental aspect of which is fixing carbon dioxide from the atmosphere and generating oxygen, in the process of photosynthesis. This critical process is carried out by specialised chloroplast organelles within green plant tissues, primarily leaves. However, chloroplasts are only one member of a family of organelles called plastids, which reside in all plant cells, and which take on different forms in relation to their cellular function, biochemistry and storage capacity. For many years, photosynthesis research overshadowed other aspects of plastid biology, but in the last two decades, much new knowledge about how plastids function and how they relate to their evolutionary past has become available from research. This book provides an overview of a wide range of aspects of modern plastid biology, including a consideration of different plastid types and how they relate to cell function, plastid genomes and how proteins are imported into plastids, photosynthesis and core aspects of plastid biochemistry, plastid signalling and functionality within a cellular context and plastid genetic manipulation. The modern era of molecular biology and molecular genetics has enabled much to be learnt about how plastids function and a picture is revealed of a highly complex organelle at the very core of plant cellular function. This information should be useful for final-year undergraduate students or Masters students interested in plant sciences and cell biology.

The fact that plastids contain their own DNA has been known since the 1960s and has been a central tenet in the construction of theories about plastid evolution and endosymbiosis. The genome of the plastid in higher plants contains approximately 120–160 genes, the products of which function in the plastid in a variety of ways, most importantly in the process of photosynthesis, but also in the translation machinery of the plastid itself. All of the proteins which are encoded by the plastid genome function within the plastid and are not exported. Thus the plastid genome is a distinct ‘in-house’ genome, which is replicated and transcribed to produce proteins crucial for plastid function. Many of the genes that were present on the genome of the original endosymbiont have migrated into the nuclear genome, leaving behind a residual number of genes which constitute the modern plastid genome that is found today in higher plants. In this chapter we consider how the plastid genome is structured and stored in the plastid, the sequence of the plastid genome and what it encodes and how mRNA molecules resulting from transcription of the plastid genome are translated within the plastid itself.

The architecture of the plastid genome

The DNA in the plastid (ptDNA) has long been considered to exist as a closed, circular molecule of double-stranded DNA of between 120 and 160 kb in size. The evidence for such a circular structure came largely from scanning electron micrographs of isolated ptDNA, which show distinct circular structures, along with shorter linear DNA molecules (Fig. 3.1).