The existence of phospholipase A2 activity in plant mitochondria and its activation by hyperosmotic stress in durum wheat (Triticum durum Desf.)

• Published in: Plant science (Limerick) 2013-02, Vol.199, p.91-102
• Main Authors: Trono, Daniela, Soccio, Mario, Laus, Maura N, Pastore, Donato
• Format: Article
• Language: English
• Subjects: barley; calcium; corn; Durum wheat; enzymes; etiolation; free fatty acids; germination; Hyperosmotic stress; mannitol; Mitochondria; Phospholipase A2; Plant mitochondrial potassium channel; Plant uncoupling protein; potassium channels; potatoes; reactive oxygen species; seedlings; sodium chloride; tomatoes; Triticum turgidum subsp. durum; tubers
• ISSN: 0168-9452; 1873-2259
• DOI: 10.1016/j.plantsci.2012.11.002

The activity of mitochondrial phospholipase A₂ (PLA₂) was shown for the first time in plants. It was observed in etiolated seedlings from durum wheat, barley, tomato, spelt and green seedlings of maize, but not in potato and topinambur tubers and lentil etiolated seedlings. This result was achieved by a novel spectrophotometric assay based on the coupled PLA₂/lipoxygenase reactions using 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphatidylcholine as substrate; the mitochondrial localisation was assessed by checking recovery of marker enzymes. Durum wheat mitochondrial PLA₂ (DWM-PLA₂) showed maximal activity at pH 9.0 and 1mM Ca²⁺, hyperbolic kinetics (Kₘ=90±6μM, Vₘₐₓ=29±1nmolmin⁻¹mg⁻¹ of protein) and inhibition by methyl arachidonyl fluorophosphonate, 5-(4-benzyloxyphenyl)-4S-(7-phenylheptanoylamino)pentanoic acid and palmityl trifluoromethyl ketone. Reactive oxygen species had no effect on DWM-PLA₂, that instead was activated by about 50% and 95%, respectively, under salt (0.21M NaCl) and osmotic (0.42M mannitol) stress imposed during germination. Contrarily, a secondary Ca²⁺-independent activity, having optimum at pH 7.0, was stress-insensitive. We propose that the activation of DWM-PLA₂ is responsible for the strong increase of free fatty acids recently measured in mitochondria under the same stress conditions [Laus, et al., J. Exp. Bot. 62 (2011) 141–154] that, in turn, activate potassium channel and uncoupling protein, able to counteract hyperosmotic stress.