A molecular analysis of desiccation tolerance mechanisms in the anhydrobiotic nematode Panagrolaimus superbus using expressed sequenced tags

<p>Abstract</p> <p>Background</p> <p>Some organisms can survive extreme desiccation by entering into a state of suspended animation known as anhydrobiosis. <it>Panagrolaimus superbus </it>is a free-living anhydrobiotic nematode that can survive rapid environmental desiccation. The mechanisms that <it>P. superbus </it>uses to combat the potentially lethal effects of cellular dehydration may include the constitutive and inducible expression of protective molecules, along with behavioural and/or morphological adaptations that slow the rate of cellular water loss. In addition, inducible repair and revival programmes may also be required for successful rehydration and recovery from anhydrobiosis.</p> <p>Results</p> <p>To identify constitutively expressed candidate anhydrobiotic genes we obtained 9,216 ESTs from an unstressed mixed stage population of <it>P. superbus</it>. We derived 4,009 unigenes from these ESTs. These unigene annotations and sequences can be accessed at <url>http://www.nematodes.org/nembase4/species_info.php?species=PSC</url>. We manually annotated a set of 187 constitutively expressed candidate anhydrobiotic genes from <it>P. superbus</it>. Notable among those is a putative lineage expansion of the <it>lea </it>(late embryogenesis abundant) gene family. The most abundantly expressed sequence was a member of the nematode specific <it>sxp/ral-2 </it>family that is highly expressed in parasitic nematodes and secreted onto the surface of the nematodes' cuticles. There were 2,059 novel unigenes (51.7% of the total), 149 of which are predicted to encode intrinsically disordered proteins lacking a fixed tertiary structure. One unigene may encode an exo-β-1,3-glucanase (GHF5 family), most similar to a sequence from <it>Phytophthora infestans</it>. GHF5 enzymes have been reported from several species of plant parasitic nematodes, with horizontal gene transfer (HGT) from bacteria proposed to explain their evolutionary origin. This <it>P. superbus </it>sequence represents another possible HGT event within the Nematoda. The expression of five of the 19 putative stress response genes tested was upregulated in response to desiccation. These were the antioxidants <it>glutathione peroxidase, dj-1 </it>and <it>1-Cys peroxiredoxin</it>, an <it>shsp </it>sequence and an <it>lea </it>gene.</p> <p>Conclusions</p> <p><it>P. superbus </it>appears to utilise a strategy of combined constitutive and inducible gene expression in preparation for entry into anhydrobiosis. The apparent lineage expansion of <it>lea </it>genes, together with their constitutive and inducible expression, suggests that LEA3 proteins are important components of the anhydrobiotic protection repertoire of <it>P. superbus</it>.</p

Ecophysiology of desiccation/rehydration cycles in mosses and lichens

Although both lichens and bryophytes are all poikilohydric the groups seem to behave very differently. Bryophytes also show a clear preference for wetter areas and this seems to be a result of the different structures of the organisms. A lichen is algae (or cyanobacteria) suspended in a mycobiont with excess water often having a negative effect on photosynthesis. Bryophytes, in contrast, are true multicellular plants and can construct photosynthetic tissues that can effectively separate their photosynthetic and water storage functions. Under dry atmospheric conditions lichens and bryophytes will desiccate to low water contents and they become dormant. Ability to tolerate desiccation varies considerably both between and within the groups. Somewhat surprisingly, lichens appear to show less ability to tolerate long periods of desiccation than bryophytes, and even some vascular plants. Actual mechanisms of desiccation have been best studied in bryophytes and appear to be constitutive, no protein synthesis is required on rehydration to enable the commencement of metabolism and the necessary protection appears to be always present. Consistently high sucrose levels, for instance are reported from bryophytes. Cellular structure is often maintained when desiccated. Recovery from dryness also differs between the groups with bryophytes generally hydrating more slowly but there are large species differences. In general, rate of recovery may be related to the length of the hydrated activity period, species that hydrate and then dry rapidly, as on rock surfaces, recover rapidly. Species in habitats that remain wet for long periods once hydrated appear to recover more slowly from dryness. In addition to a photosynthetic response to light and temperature, the poikilohydric lichens and bryophytes also have a photosynthetic response to thallus water content. Starting with a dry thallus, addition of water will both increase the thallus water content and also allow photosynthesis and respiration to commence. Both processes increase almost linearly with further hydration at low water contents. Photosynthesis reaches a maximum at an optimal thallus water content (WCopt) that is strongly species dependant. In both groups this photosynthetic optimum represents full cellular turgor. At water contents above this optimum surface or external water can interfere with carbon dioxide uptake and can severely limit photosynthetic rates, especially in lichens. When thallus water contents are normalised to WCopt = 1, then the net photosynthesis (NP) response curves at water contents below WCopt are very similar for liverworts, mosses and higher plants, suggesting a common mechanism in controlling NP. It is suggested that this might be an inhibitor acting on Rubisco activity. In contrast to vascular plants both groups can carry out photosynthesis at lower, suboptimal thallus water contents and very low water potentials but the contribution that this makes to total carbon budget appears to be a major difference between the groups. Bryophytes seem to pass rapidly through this water content range when both drying and hydrating for tens of minutes are often enough. In contrast, it is now apparent that lichens are often active at low thallus water contents. They can not only hydrate from humid air alone, or from dew and fog, but can use these water sources very effectively, often achieving a major part of their annual carbon gain. Information on when the lichens and bryophytes are actually active is only recently starting to appear but, again, the groups seem to differ. Bryophytes strongly prefer wetter habitats and can be active and fully hydrated for long periods and seem to have excellent capacity to tolerate high light and UV radiation when wet. In contrast many lichens, in particular those with green algal symbionts, rarely seem to be hydrated for long periods, especially in high light conditions, and rapidly dry out. Lichens seem to be active mainly under suboptimal conditions one of which is suboptimal water content