Implication of phospholipase D in response of Hordeum vulgare root to short-term potassium deprivation

To verify the possible involvement of lipids and several other compounds including hydrogen peroxide (H2O2) and glyceraldehyde-3-phosphate dehydrogenase (G3PDH) in the response of Hordeum vulgare to early potassium deprivation, plants were grown in hydroponic conditions for 30 d with a modified Hewitt nutrient solution containing 3 mM K+. They were then incubated for increasing periods of time ranging from 2 to 36 h in the same medium deprived of K+. In contrast to leaves, root K+ concentration showed its greatest decrease after 6 h of treatment. The main lipids of the control barley roots were phospholipids (PL), representing more than 50% of the total lipids. PL did not change with treatment, whereas free sterols (FS) decreased following K+ deprivation, showing a reduction of approximately 17% after 36 h. With respect to the individual PL, 30 h K+ deprivation led to a reduction in phosphatidylcholine (PC), phosphatidylserine (PS), and phosphatidylinositol (PI) levels, whereas phosphatidylglycerol (PG), phosphatidylethanolamine (PE), and phosphatidic acid (PA) levels increased. The maximum PA accumulation and the highest phospholipase D (PLD) activation, estimated by an accumulation of phosphatidylbutanol (PtBut), were observed after 24 h of K+ starvation. At the root level, H2O2 showed the maximum value after 6 h of incubation in -K solution. In parallel, G3PDH activity reached its minimum. On the basis of a concomitant stimulation of PLD activity and, consequently, PA accumulation, enhancement of H2O2 production, and inhibition of G3PDH activity, we suggest a possible involvement of these three compounds in an early response to K+ deprivation.