Is There Commonality Facilitating Desiccation Tolerance Among Organisms

Besides being a trait of orthodox seeds, DT occurs in bacteria, terrestrial micro-algae, lichens, bryophytes, resurrection angiosperms and five animal phyla (Alpert, 2005).

Resurrection plants

Desiccation-tolerant resurrection plants are able to remain viable despite considerable dehydration, resuming metabolic activity when water once again becomes available. As is the case for orthodox seeds, DT of the vegetative tissues is based on a spectrum of relatively complex protection mechanisms that accompany dehydration (Illing et al., 2005).

Late embryogenesis abundant (LEA) proteins, sucrose and certain oligosaccha-rides accumulate coincidently with the acquisition of DT during orthodox seed development (Buitink et al., 2002), and particular antioxidant enzymes become prominent (Bailly, 2004). The expression of at least 16 different LEA genes (identified from a survey of only 425 cDNAs) (J.M. Farrant, 2005, University of Cape Town, South Africa, personal communication) has been found to occur in the leaves of the xero-tolerant resurrection plant, Xerophyta humilis (Baker) Dur. and Schinz during dehydration (Collett et al., 2004). The antioxidant 1-cys-peroxiredoxin, which had previously been considered to be seed-exclusive, was found to be abundantly expressed in tissues of the resurrection plants X. humilis and X. viscosa (Baker). Illing et al. (2005) also reported that sucrose accumulates only in the tissues of the desiccation-tolerant Eragrostis nindensis (Ficalho & Hiern), and not in related sensitive Eragrostis species indicating a further commonality between desiccation-tolerant seeds in general and vegetative tissues of resurrection plants.

Lichens

The result of the lichen-forming symbiosis between a fungus (i.e. the mycobiont) and a green alga or cyanobacterium (i.e. the photobiont) is that neither of the partners remains constrained to the cryptic habitats that would be obligatory for either one alone (Kranner et al., 2005). In the lichen Cladonia vulcani (Savicz), those same authors have shown this to be the outcome of significant upregulation of both photo-protective and antioxidant mechanisms, resulting in the lichen thallus being desiccation-tolerant (which is a prerequisite for its occurring above ground).

Desiccation-tolerant animals

DT has been documented for nematodes (Browne et al., 2002) and bdelloid rotifers (Lapinski and Tunnacliffe, 2003) and, classically, for the encysted embryos of the brine shrimp, Artemia species (see Clegg, 2005). In nematodes, accumulation of a 'novel heat-stable protein' was reported as a response to desiccation (Solomon et al., 2000), as was the upregulation of a gene coding for a LEA protein in response to desiccation stress in the desiccation-tolerant nematode, Aphelenchus avenae (Bastian) (Browne et al., 2002). In parallel, the latter authors found that A. avenae accumulates trehalose in response to dehydration, leading to the suggestion that the LEA-trehalose combination might act synergistically in the formation of a glass, as is currently favoured for LEA-sucrose-based glasses in plant cells (see later). A protein-trehalose synergism has also been demonstrated in vitro in the highly desiccation-tolerant encysted embryos of Artemia (Viner and Clegg, 2001; Clegg, 2005). However, neither of two species of bdelloid rotifers expressing a LEA-type protein were found to accumulate trehalose, or other simple sugars other than glucose (Lapinski and Tunnacliffe, 2003), the implications of which will be considered later.

Microorganisms

Prokaryotes in soil, which might periodically become very dry, need to be able to protect against the consequences of dehydration, and survival of bacteria in the dry state has important implications for health issues (Billi and Potts, 2002). In the stationary phase under salt-stress conditions, Bacillus subtilis is characterized by considerable accumulation of a protein, GsiB, which bears a marked similarity to LEA Group 1 proteins (Stacy and Aalen, 1998).

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