ON THE ELUSIVE PRESENCE OF PTP IN PLANT MITOCHONDRIA

The presence of the permeability transition pore (PTP) in plant mitochondria has been often suggested on the basis of the cyclosporine A (CsA) inhibition of programmed cell death. Permeability transition in isolated plant mitochondria has been observed in few cases, although only in mitochondria from potato tuber (Arpagaus et al., 2002) all the typical features of a PTP similar to the mammalian one have been shown. In particular, PTP opening induced by Ca2+ and Pi caused a Ψ decrease and a sucrose-mediated swelling, inhibited by CsA. As recently proposed for mammalian mitochondria (Giorgio et al., 2013), PTP machinery is formed by a dimer of ATP synthase involving also cyclophilin-D (Cyp-D) as regulator. Whereas it has been demonstrated that ATP synthase dimers are formed also in plants and yeasts (Davies et al., 2011), only Cyp-D gene has been reported in plants, being its expression in mitochondria not verified yet. The present work aims at comparing the PTP characteristics in plant mitochondria from different species and tissues and investigating the presence of Cyp-D. The mitochondrial PT was shown in numerous plant species only as Ca2+- and Pi-stimulated Ψ collapse, without any concomitant matrix swelling. The effect of several well-known PTP modulators on the Ψ collapse was performed in pea mitochondria. This evidence shows a scenario similar to that already suggested for low permeability transition in plant mitochondria, where PT is mediated by an unspecific K+ channel. On the contrary, the appearance of swelling was observed only in potato; this phenomenon was insensitive to CsA and was evident even in the absence of respiratory substrates. The presence of Cyp-D was shown by immunochemical analysis in mitochondria isolated from pea, potato and arum lily. Nevertheless, CsA-inhibition of PT was not found in these species. From these results, we suggest that plant mitochondria possess the components of a Ca2+- dependent PTP machinery, but the function of this putative pore could be masked by the activity of an unspecific influx of monovalent ions. This work is supported by project PRIN 2010CSJX4F. Arpagaus, S. Rawyler A., Braendle R. (2002) Occurrence and characteristics of the permeability transition pore in plant. Journal of Biological Chemistry, 18: 1780-1787 Giorgio V., von Stockum S., Antoniel M., Fabbro A., Fogolari F., Forte M., Glick G.D, Petronilli V., Zoratti M., Szabó I., Lippe G., Bernardi P. (2013) Dimers of mitochondrial ATP synthase form the permeability transition pore. PNAS, 110: 5887–5892 Davies K.M., Strauss M., Daum B., Kief J.H., Osiewacz H.D., Rycovska A., Zickermann V., Kühlbrandt W. (2011) Macromolecular organization of ATP synthase and complex I in whole mitochondria. PNAS, 108: 1421- 1426.