Introduction
The colonization of land by plants in the Palaeozoic was a highly significant event in Earth’s history, both from an evolutionary point of view and because it fundamentally changed the ecology and environment of the planet (Beerling 2007). Land plants evolved to form crucial components of all modern terrestrial ecosystems through evolutionary adaptations involving changes in anatomy, physiology and life cycle (Waters 2003; Menand et al. 2007; Cronk 2009). Key adaptations include rooting structures, conducting tissues, cuticle, stomata, and sex organs such as gametangia and spores/pollen.
Development of a durable spore wall is essential for terrestrialization as it enables the spore to withstand physical abrasion, desiccation and UV-B radiation (Wellman 2004). As part of their life cycle, sexually reproducing embryophytes manufacture either spores, or their more derived homologues pollen. The major component of the spore/pollen wall proposed to be of primary importance in enabling resistance to the conditions described above is the highly resistant biopolymer sporopollenin (Ito et al. 2007; Cronk 2009).
It seems reasonable to hypothesize that colonization of the land by plants was not possible prior to the evolution of the sporopollenin spore wall, and this adaptation is considered to be a synapomorphy of the embryophytes. Additionally, spore walls are not present in the hypothesized embryophyte antecedents, the green algae (Wellman 2004). However, the production of sporopollenin is highly likely to be pre-adaptive as it is present in a number of different algal groups such as the charophyceans, which have been proposed as the sister group to the embryophytes. In certain charophyceans, sporopollenin occurs, but is located in an inner layer of the zygote wall (Graham 1993).
Phylogenetic studies and fossil evidence have shown that the most basal living land plants are the paraphyletic ‘bryophytes’ (Kenrick and Crane 1997; Qui et al. 2006) . They comprise the liverworts, mosses and hornworts, and their phylogenetic position should allow us to further elaborate the evolutionary changes that facilitated the conquest of land by plants (Rensing et al. 2008). The moss Physcomitrella patens is the first ‘bryophyte’ genome to be sequenced. This genome, through comparisons with angiosperm genomes, is proving to be a valuable tool in experimental studies that attempt to reconstruct genome evolution during the colonization of land (Reski and Cove 2004; Quatrano et al. 2007; Rensing et al. 2008).
Phylogenetic tree for land plant evolution derived from analysis by Qui et al. (2006). The bryophytes are a paraphyletic group comprising three separate lineages. Together with the vascular plants (which include the angiosperms), bryophytes form the embryophytes, which have a sister group relationship to the green algae.
In this review, we first outline the nature of spore/pollen wall development in the major plant groups, before considering emerging understanding of the molecular genetics of pollen wall development. The latter includes identification of genes involved in sporopollenin biosynthesis and exospore formation, callose wall formation and tetrad separation. We also report results from BLAST searches of the basal land plant physcomitrella and the clubmoss Selaginella moellendorfii using genes implicated in pollen wall development in arabidopsis.