Lack of water leads to leaf lipid losses
When beset by potentially harmful environmental factors, plants are unable to uproot themselves and move away. Instead, they have evolved a remarkable range of tolerance and resistance mechanisms. This is well illustrated in the work of Gigon et al. (University of Paris XII, pp. 345-351) on the effects of drought stress in Arabidopsis thaliana. We do not think of this species as being especially drought-tolerant and yet, as shown by these authors, it can recover from periods of severe water deficit. Plants were subjected to water deficit by withholding irrigation; at intervals for the next 14 days, relative water content (RWC) and lipid content and composition were determined. As RWC declined so did leaf lipid content, reaching as low as 22.5 % of control levels at 17 % RWC (reached on day 14). However, different lipid classes were lost at different rates. In particular, the proportion of total lipids represented by phospholipids (PL) changed very little while the proportion represented by monogalactosyl (MG) lipids declined and that of digalactosyl (DG) lipids increased. Since galactosyl lipids only occur in plastids, this suggests that modification of plastid membrane lipids may occur during water deficit. The authors followed this up by assaying lipolytic activity and by monitoring expression of genes encoding enzymes involved in lipid metabolism. The ability of leaf extracts to degrade all three lipid classes increased with increasing water deficit, but MG-degrading activity increased the most whilst the increase in DG-degrading activity occurred later than the other two. There were also increases in the transcript levels of genes encoding lipolytic and phospholipase enzymes and, rather intriguingly, of at least one gene (digalactosyldiacylglycerol synthase) involved in DG synthesis. By contrast, expression of monogalactosyldiacylglycerol synthase, involved in MG synthesis, declined. Finally, and perhaps surprisingly, even the most drought-stressed of these plants, with a RWC of 17 %, recovered when re-watered and all these changes in lipid content and metabolism were reversed.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
