Membrane remodelling and triacylglycerol accumulation in drought stress resistance: The case study of soybean phospholipases A.

Agriculture is facing major constraints with the increase of global warming, being drought a major factor affecting productivity. Soybean (Glycine max) is among the most important food crops due to the high protein and lipid content of its seeds despite being considerably sensitive to drought. Previous knowledge has shown that drought induces a severe modulation in lipid and fatty acid content of leaves, related to alteration of membrane structure by lipolytic enzymes and activation of signalling pathways. In that sense, little is known on lipid modulation and lipolytic enzymes' role in soybean drought stress tolerance. In this work, we present for the first time, soybean leaves lipid content modulation in several drought stress levels, highlighting the involvement of phospholipases A. Moreover, a comprehensive analysis of the phospholipase A superfamily was performed, where 53 coding genes were identified and 7 were selected to gene expression analysis in order to elucidate their role in soybean lipid modulation under water deficit. Proportionally to the drought severity, our results revealed that galactolipids relative abundance and their content in linolenic acid decrease. At the same time an accumulation of neutral lipids, mainly due to triacylglycerol content increase, as well as their content in linolenic acid, is observed. Overall, PLA gene expression regulation and lipid modulation corroborate the hypothesis that phospholipases A may be channelling the plastidial fatty acids into extraplastidial lipids leading to a drought-induced accumulation of triacylglycerol in soybean leaves, a key feature to cope with water stress.

Study of the effects of salicylic acid on soybean mitochondrial lipids and respiratory properties using the alternative oxidase as a stress-reporter protein.

Biotic and abiotic stresses can lead to modifications in the lipid composition of cell membranes. Although mitochondria appear to be implicated in stress responses, little is known about the membrane lipid changes that occur in these organelles in plants. Besides cytochrome c oxidase, plant mitochondria have an alternative oxidase (AOX) that accepts electrons directly from ubiquinol, dissipating energy as heat. AOX upregulation occurs under a variety of stresses and its induction by salicylic acid (SA) has been observed in different plant species. AOX was also suggested to be used as a functional marker for cell reprogramming under stress. In the present study, we have used etiolated soybean (Glycine max (L.) Merr. cv Cresir) seedlings to study the effects of SA treatment on the lipid composition and the respiratory properties of hypocotyl mitochondria. AOX expression was studied in detail, as a reporter protein, to evaluate whether modifications in mitochondrial energy metabolism were occurring. In mitochondria extracted from SA-treated seedlings, AOX capacity and protein contents increased. Both AOX1 and AOX2b transcripts accumulated in response to SA, but with different kinetics. A reduction in external NADH oxidation capacity was observed, whereas succinate respiration remained unchanged. The phospholipid composition of mitochondria remained similar in control and SA-treated plants, but a reduction in the relative amount of linolenic acid was observed in phosphatidylcholine, phosphatidylethanolamine and cardiolipin. The possible causes of the fatty acid modifications observed, and the implications for mitochondrial metabolism are discussed.

Alternative oxidase involvement in cold stress response of Arabidopsis thaliana fad2 and FAD3+ cell suspensions altered in membrane lipid composition.

To investigate how the fatty acid composition of membrane lipids influences cell growth and mitochondrial respiration, in particular the expression and capacity of alternative oxidase (AOX), under cold stress, we used the Arabidopsis thaliana fad2 knockout and FAD3+ -overexpressing cultured cells lines affected in extrachloroplastic fatty acid desaturation activities. At 22 degrees C, fad2 mitochondria exhibited a low polyunsaturated fatty acid content and low protein to lipid ratio, while mitochondria from FAD3+ were enriched in linolenic acid and in total membrane protein. As a consequence, both mutants showed a higher membrane microviscosity than the wild type. After exposure to 9 degrees C, FAD3+ mitochondria exhibited lower microviscosity and lower rigidification upon a temperature downshift than fad2. Furthermore, the extent of reduction of cell growth and respiratiory rates in the phosphorylating state was positively related to the cold sensitivity of each cell line, being more pronounced in fad2 that in the wild type, whereas the stability of those parameters reflected the cold resistance of FAD3+. In contrast, an increase in AOX capacity was observed in the three cell lines at 9 degrees C. These inductions were correlated to AOX protein amounts and seem to result from an accumulation of AOX1c transcripts in the three cell lines and of AOX1a transcripts in wild-type and fad2 cells. The fact that there is no direct relationship between the degree of cold tolerance of each cell line and their ability to enhance their AOX capacity suggests that the participation of AOX in the response of Arabidopsis cells to cold stress does not necessarily favor cold tolerance.