The moss Physcomitrella patens is becoming the model of choice for functional genomic studies at the cellular level. Studies report that Physcomitrella survives moderate osmotic and salt stress, and that desiccation tolerance can be induced by exogenous ABA. Our goal was to quantify the extent of dehydration tolerance in wild type moss and to examine the nature of cellular damage caused by desiccation. We exposed Physcomitrella to humidities that generate water potentials from -4 (97% RH) to -273 MPa (13% RH) and monitored water loss until equilibrium. Water contents were measured on a dry matter basis to determine the extent of dehydration because fresh weights (FW) were found to be variable and, therefore, unreliable. We measured electrolyte leakage from rehydrating moss, assessed overall regrowth, and imaged cells to evaluate their response to drying and rehydration. Physcomitrella did not routinely survive water potentials \-13 MPa. Upon rehydration, moss dried to water contents[0.4 g g dm -1 maintained levels of leakage similar to those of hydrated controls. Moss dried to lower water contents leaked extensively, suggesting that plasma membranes were damaged. Moss protonemal cells were shrunken and their walls twisted, even at -13 MPa. Moss cells rehydrated after drying to -273 MPa failed to re-expand completely, again indicating membrane damage. ABA treatment elicited tolerance of desiccation to at least -273 MPa and limited membrane damage. Results of this work will form the basis for ongoing studies on the functional genomics of desiccation tolerance at the cellular level.

In seeds, the subcellular storage oil bodies have a matrix of oils (triacylglycerols) surrounded by a layer of phospholipids embedded with abundant structural proteins called oleosins. We used two maize (Zea mays L.) strains having diverse kernel (seed) oil contents to study the effects of varying the oil and oleosin contents on the structure of the oil bodies. Illinois High Oils (IHO, 15% w/w oils) and Illinois Low Oils (ILO, 0.5%) maize kernels were the products of breeding for diverse oil contents for about 100 generations. In both maize strains, although the genes for oil synthesis had apparently been modified drastically, the genes encoding oleosins appeared to be unaltered, as revealed by Southern blot analyses of the three oleosin genes and sodium dodecyl sulfate-polyacrylamide gel electrophoresis with immunoblotting of the oleosins. In addition, both strains contained the same three oleosin isoforms of a defined proportion, and both accumulated oils and oleosins coordinately. Oleosins in both strains were restricted to the oil bodies, as shown by analyses of the various subcellular fractions separated by sucrose-density-gradient centrifugation. Electron microscopy of the embryos and the isolated organelles revealed that the oil bodies in IHO were larger and had a spherical shape, whereas those in ILO were smaller and had irregular shapes. We conclude that in seeds, oleosin genes are expressed independent of the oil contents, and the size and shape of the oil bodies are dictated by the ratio of oils to oleosins synthesized during seed maturation. The extensive breeding for diverse oil contents has not altered the apparent mechanism of oil-body synthesis and the occurrence of hetero-dimer or -multimer of oleosin isoforms on the oil bodies.

The results confirm that leaf mechanical properties are affected by both water status and position along the lamina, and suggest a positive correlation between leaf internal architecture, tensile strength, cell wall chemistry and tolerance to dehydration for grasses.

Communicated by Andre T. Jagendorf, August 11, 1988 (received for review June 3, 1988 ABSTRACTThe freezing tolerance of protoplasts isolated from nonacclimated rye leaves (Secale cereale L. cv Puma) was significantly altered by using a pH-induced protoplastliposome fusion technique to modify the lipid composition ofthe plasma membrane. The increase in freezing tolerance was elicited by fusion with liposomes composed of either the total phospholipid fraction isolated from the plasma membrane of cold-acclimated leaves or single mono-or diunsaturated species of phosphatidylcholine (PtdCho). Of the PtdCho species tested, dilinoleoylphosphatidylcholine ([Lin2]PtdCho) and dilinolenoyl

Egerton-Warburton, L. M., Balsamo, R. A. and Close, T. J. 1997. Temporal accumulation and ultrastructural localization of dehydrins in Zea mays. -Physiol. Plant. 101: 545-555.Immunolocalization using polyclonal antibodies raised against a conserved dehydrin amino acid sequence was used to establish the temporal and spatial patterns of dehydrin accumulation in embryo tissue of Zea mays L. (var. Ohio 43) kernels imbibed in the presence of abscisic acid. The temporal pattem of accumulation indicated an increase in dehydrins over time (particularly between 15 &nd 30 h) and with maximum levels detected 48 h after the onset of imbibition. Dehydrins were first evident, and also the most concentrated, in the cytosol throughout the accumulation period suggesting that the primary function of dehydrins involves the cytosol and the structures contained therein. Only after an accumulation of dehydrins in the cytosol was there an increase in the abundance of nuclear dehydrins. In addition, dehydrins were also observed in association with the proteinaceous matrix of protein bodies and menibranes of protein and lipid bodies; these findings have not been reported previously. The observed localization at a number of sites indicates that the specific biochemical roles of dehydrins are likely to be diverse.