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Skeleton holds key to cell reconstruction
The ability of many mosses to recover from severe dehydration is astonishing and there has been extensive research on the effects of desiccation and re-wetting on cellular membranes and on key biochemical processes. However, as pointed out by Pressel et al. (Queen Mary, London and Caserta, Italy, pp. 67–76), little work has been done on transport systems. Focusing specifically on Polytrichum formosum, they have turned their attention to leptoids, food-conducting cells, generally considered to be the functional equivalent of phloem sieve elements in vascular plants. Shoots of plants collected from the field were allowed to dry over a period of 18 days, reaching a water content of only 7.5 %. In this state, they were able to survive for several weeks. Rehydration was achieved by immersing the shoots in distilled water. Throughout the dehydration–rehydration procedure, samples were taken and fixed for light and electron microscopy. In the hydrated state, leptoid cells have an ‘elaborate cytoplasmic architecture’ with a very obvious polarity along the long axis of the cell; this polarity is largely lost during desiccation. Microtubules disappear, as do the stacks of ER, the latter being replaced with ‘membranous tubules’ aligned across the cell; the elongated shapes of nucleus, plastids and mitochondria convert to more rounded forms; small vacuoles accumulate in the cytoplasm; plasmodesmata become plugged. On rehydration, the start of the reversal of these changes is visible within 30 min and is well under way by 2 h. Recovery proceeds in an orderly way with some events being completed early and others taking 12–24 h. One of the early events is the re-establishment of the microtubular cytoskeleton. Indeed, it is likely that this event plays a key role in the re-ordering of cellular structure during recovery: in shoots rehydrated in the presence of oryzalin, a disrupter of microtubules, leptoids failed to regain their normal structure.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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