Jill M. Farrant

Plants - the invasion of dry land

Early land plants evolved from aquatic algal ancestors millions of years ago [1]. One of the first and most formidable obstacles to the successful adaptation to terrestrial environments is desiccation [1]. Rapid drying owing to heat, sunlight or wind can cause desiccation to occur in sensitive vegetative tissues within minutes of exposure. These desiccation-induced stresses are visible at all intertidal shore zones where algae are exposed to rapid changes in water availability throughout a normal day. Chlorophytic algae, specifically the Choleochetales group, which gave rise to Charophyceaen species were the precursors to the Bryophyte and Tracheophyte lineages of terrestrial plants and must have been the first groups to acquire desiccation tolerance [2]. Bryophytes widely evolved the ‘gametophytic’ solution to desiccation tolerance and these vegetative tissues are termed ‘fully’ desiccation-tolerant as they can withstand very rapid drying, on the order of seconds to minutes [3]. The main mechanisms responsible for the DT of mosses are believed to be related to cellular repair strategies upon re-hydration. The lack of vascular tissues and sophisticated tissue patterning ensures a rapid and uniform drying rate upon desiccation in mosses [4]. The molecular processes related to DT in mosses have been most intensively studied in Tortula ruralis (Figure 1A). One of the key adaptations in Tortula ruralis is the packaging of desiccation-related mRNA transcripts into mRNPs (messenger ribonucleic protein complexes) with polysomes so that upon rehydration these transcripts would become immediately available for the effective repair of desiccation- associated damage [5]. Pteridophytes have been suggested to show a mixed form of DT, intermediate between ‘full’ and ‘modified’ DT [2]. A recent study has shown that DT is widespread among fern gametophytes and related to habitat preference [6]. The simple morphology of the fern gametophytes governs the water-holding capacity of the thallus and probably the DT[6]. Hence it is likely that moss and fern gametophyte DT is of a similar, if not identical, nature. In the case of some ferns the sporophyte is stages, as well as the spores, are desiccation-tolerant, the nature of which is largely similar to that of the gametophytic tissue. In these tissues several of the mechanisms of DT reported to occur in angiosperms (which have ‘modified’ DT) are present [7,8,9], confirming an intermediate status of DT. In the case of the fern Mohria caffrorum (Figure 1C) the DT of the sporophyte is seasonal, being DT in the dry and desiccation sensitive (DS) in the wet season [9] and it would be interesting to know if genetic programming of the gametophyte is responsible for this switching process.

Extract from: Programming desiccation-tolerance: from plants to seeds to resurrection plants

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