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Over the last 470 million years, plant evolution has seen major evolutionary transitions such as the move from water to land and the origins of vascular tissues, seeds, and flowers[1]. These have resulted in the evolution of terrestrial flora that has shaped modern ecosystems, and the diversification of the Plant Kingdom, Viridiplantae, into over 374,000 described species[2]. Each of these transitions was accompanied by the gain and loss of genes in plant genomes. For example, whole genome duplications are known to be fundamental to the origins of both seed and flowering plants [3,4]. With the ever-increasing quality and quantity of whole genome data, evolutionary insight into origins of distinct plant groups using comparative genomic techniques is now feasible. Here, using an evolutionary genomics pipeline to compare 208 complete genomes, we analyse the gene content of the ancestral genomes of the last common ancestor of land plants and all other major groups of plant. This approach reveals an unprecedented level of fundamental genomic novelties in two nodes related to the origin of land plants, the first in the origin of streptophytes during the Ediacaran and another in the ancestor of land plants in the Ordovician. Our findings highlight the biological processes that evolved with the origin of land plants and emphasise the importance of conserved gene novelties in plant diversification. Comparisons to other eukaryotic studies suggest a separation of the genomic origins of multicellularity and terrestralisation in plants.

Publisher Summary This chapter examines the differences in physiology among embryophytes and their algal ancestors, with particular emphasis on their water relations. The embryophytes have very significant variations in water relations and the chapter considers their evolution within the embryophytes as well as the evolution of embryophyte water relations from those of their algal ancestors. The chapter also examines the relationship of the likely evolution of embryophyte water relations to cladistic analyses of embryophyte phylogeny and to the fossil record. The physiological changes that occurred in the evolution from algal ancestors to the different grades of organization of embryophytes has been determined from the physiology of extant plants in relation to their phylogeny as determined by cladistic analysis and from the order in which anatomical features appeared in the fossil record. The fossil record of embryophytes also reveals certain characteristics of organisms that are not found today. The fossil record is not helpful in providing some information such as desiccation tolerance or intolerance, except by applying an empirical correlation from extant plants that no embryophyte more than 1 meter in height is desiccation tolerant in the vegetative phase. Overall, lines of evidence indicate that the earliest embryophytes were desiccation tolerant and poikilohydric.

Publisher Summary This chapter explores the current understanding of the origin of the spore wall, the function(s) of the spore wall in early land plants, and mode of spore wall formation in early land plants. The embryophytes are one of the major kingdoms of eukaryotice life. The sporopollenin wall surrounding charophycean zygotes and embryophyte spores/pollen grains is homologous, and the spore/pollen wall is an embryophyte synapomorphy that evolved as an adaptive response to the invasion of the land. It is considered that the primary function of the spore/pollen wall involves protection in the harsh subaerial environment. Regarding spore wall development, ultrastructural studies have demonstrated that structural elements present in the spore walls of extant plants can be recognized in the fossil spores of early land plants, suggesting that spore wall development was similar in extant and ancient land plants. Studies of molecular genetics of spore wall formation are in their infancy but have the potential to solve many unanswered questions regarding spore wall homologies and developmental processes—particularly when such studies are commenced on more “primitive” land plants and their extant sister group.