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Drying to survive
Many plant species can now be propagated by generation of somatic embryos - embryos that arise not from a sexual fusion but from individual cells within a callus culture. However, although they are morphologically similar, somatic and sexual embryos differ significantly in their desiccation tolerance. Embryos inside seeds can exist for extended periods with moisture contents as low as 10 %. During natural embryogenesis and seed maturation, the abscisic acid (ABA) content of the embryo increases, and it is known that this prevents precocious germination. However, it is now clear from research with mutants of Arabidopsis that ABA is also important for acquisition of desiccation tolerance by the embryo. Somatic embryos, by contrast, have very poor desiccation tolerance, and very low germination percentages are observed after they have been dried down to the water contents of normal dry seeds. Germination percentages may be improved slightly if drying is rapid but are still much lower than those of normal embryos. The obvious question, and one of those addressed by Sreedhar and colleagues (Guelph and Wageningen; pp. 391-400), is whether ABA can induce desiccation tolerance in somatic embryos. Based on their results, the answer is a resounding 'Yes!' Transfer of 22 d somatic embryos to 20 micromoles ABA dramatically increases desiccation tolerance as measured by germination rate after desiccation. But what are the cellular effects of ABA? Using several sensitive biophysical techniques, the authors have obtained evidence that in desiccation-intolerant embryos, membranes are more damaged, lipid phase transitions are more chaotic (possibly resulting from oxidative damage) and proteins have more beta-sheet (perhaps resulting from partial proteolysis) than in tolerant embryos. Based on these and other data, the authors conclude that the role of ABA is to decrease metabolic activity in the embryo to a level at which desiccation damage is minimized. How ABA does this is another question.Professor J. A. Bryant
University of Exeter, UK
J.a.bryant{at}exeter.ac.uk
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