Wodala, Barnabás

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Wodala Barnabás

University of Szeged

Volume 50(3-4):185, 2006Acta Biologica Szegediensishttp://www.sci.u-szeged.hu/ABSDISSERTATION SUMMARYDepartment of Plant Physiology, University of Szeged, Szeged, HungaryCombined effects of nitric oxide and cyanide on the photosynthetic electron transport of intact leavesBarnabás WodalaSupervisor: László ErdeiE-mail: wodala.barnabas@foolmoon.hu185Plants not only respond to ambient levels of nitric oxide (NO), but also generate NO themselves (Neill et al. 2003). In the past years, this gaseous free radical has been implicated as an important signaling molecule in numerous physiological processes (Río et al. 2004). Still, the role of NO in photosyn-thesis is poorly understood, which is well indicated by the modest number of in vivo and in vitro experiments in this area with mixed results (Takahashi and Yamasaki 2002; Yang et al. 2004). In our study, we aimed to clarify the potential effects of various NO donor molecules on the photosynthetic electron transport in intact leaves by means of quenching analysis of chlorophyll a fluorescence.The youngest fully expanded leaves of 2-week-old Pisum sativum L. cv Rajnai Törpe plants were excised and the peti-oles were submerged in Petri dishes containing distilled water, NO donor molecules and scavenger chemicals with various concentrations. Chlorophyll fluorescence of PS II of pea leaves was measured with a PAM fluorometer (PAM-2000, Heinz Walz GmbH, Effeltrich, Germany). QA- reoxidation kinetics was followed by a double-modulation fluorometer (Photon Systems Instruments, Brno, Czech Republic). Xan-thophyll cycle pigments were determined by HPLC.QA- reoxidation kinetics in pea leaf discs were measured in the presence of the NO donor, sodium nitroprusside (SNP) in order to see whether NO can replace bound bicarbonate at different binding sites, e.g. the non-heme iron complex, in PSII as measured earlier (Petrouleas and Diner 1990). SNP slows down the electron transfer between the primary and secondary quinone electron acceptors in vivo in a concentra-tion dependent manner, which indicates that NO displaces the bicarbonate from the acceptor side non-heme iron. Results in the presence of DCMU, which prevents forward electron transport, indicate that NO inhibits charge recombination reactions of QA- with the S2 state of the water-oxidising complex, as well as interacting with the tyrosine radical YDÿ in PSII. Applying scavengers haemoglobin and cPTIO resulted in a complete or partial elimination of these changes, respectively, which indicates a distinct role for NO as well as cyanide in the process.The joint effect of cyanide and NO – both released by SNP – on photosynthetic processes was observed in the complex chlorophyll a slow fluorescence kinetics, and experiments in the presence of the NO-specific scavenger cPTIO allowed clear distinction between effects induced by cyanide or NO. The SNP-induced moderate decrease in Fv/Fm, an indicator of light harvesting capacity, implies a structural alteration of the light harvesting complex of PS II, and measurements with cPTIO hold cyanide responsible for this concentration dependent decline. Increasing amounts of SNP cause a more dramatic change in quenching parameters qP and NPQ, which provide information about the ratio of open reaction centres in PS II (ready for electron transport) and the ratio of absorbed energy dissipated as heat, respectively. Consistent with results from QA-reoxidation kinetics, dropping qP values indicate a slowing rate of electron transport – mediated chiefly by NO. Instead of a resulting NPQ (ÄpH) decrease, the ÄpH is in-creased due to cyanide which inhibits the Calvin cycle, thus proton loss from the lumen. Other measurements with nitrite and nitrate, and another specific NO donor (GSNO) confirm a specific – cyanide-independent – NO effect.ReferencesNeill SJ, Desikan R, Hanock JT (2003) Nitric oxide signalling in plants. New Phytol 159:11-35.Petrouleas V, Diner BA (1990) Formation by NO of nitrosyl adducts of redox components of the photosystem II reaction center. I. NO binds to the acceptor-side non-heme iron. Biochim Biophys Acta 1015:131-140.Río LA, Corpas FJ, Barroso JB (2004) Nitric oxide and nitric oxide synthase activity in plants. Phytochem 65:783-792.Takahashi S, Yamasaki H (2002) Reversible inhibition of photophosphoryla-tion in chloroplasts by nitric oxide. FEBS Lett 512:145-148.Yang JD, Zhao HL, Zhang TH, Yun JF (2004) Effects of exogenous nitric oxide on photochemical activity of photosystem II in potato leaf under non-stress condition. Acta Bot Sin 46:1009-1014.

Combined effects of nitric oxide and cyanide on the photosynthetic electron transport of intact leaves

http://acta.bibl.u-szeged.hu/22809/1/50185.pdf

Combined effects of nitric oxide and cyanide on the photosynthetic electron transport of intact leaves

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