Plastid terminal oxidase (PTOX) has the potential to act as a safety valve for excess excitation energy in the alpine plant species Ranunculus glacialis L.

Abstract

Ranunculus glacialis leaves were tested for their plastid terminal oxidase (PTOX) content and electron flow to photorespiration and to alternative acceptors. In shade-leaves, the PTOX and NAD(P)H dehydrogenase (NDH) content were markedly lower than in sun-leaves. Carbon assimilation/light and Ci response curves were not different in sun- and shade-leaves, but photosynthetic capacity was the highest in sun-leaves. Based on calculation of the apparent specificity factor of ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco), the magnitude of alternative electron flow unrelated to carboxylation and oxygenation of Rubisco correlated to the PTOX content in sun-, shade- and growth chamber-leaves. Similarly, fluorescence induction kinetics indicated more complete and more rapid reoxidation of the plastoquinone (PQ) pool in sun- than in shade-leaves. Blocking electron flow to assimilation, photorespiration and the Mehler reaction with appropriate inhibitors showed that sun-leaves were able to maintain higher electron flow and PQ oxidation. The results suggest that PTOX can act as a safety valve in R. glacialis leaves under conditions where incident photon flux density (PFD) exceeds the growth PFD and under conditions where the plastoquinone pool is highly reduced. Such conditions can occur frequently in alpine climates due to rapid light and temperature changes.

Variation in relative water content, proline accumulation and stress gene expression in two cowpea landraces under drought

“…We tested the effect of water stress on chloroplast and mitochondria electron transfer which can result in oxidative stress (Bartoli et al, 2005). Expression of the chloroplast PTOX gene is part of the stress response of some plant species (Johnson and Stepien, 2016) (Laureau et al, 2013). A low level of PTOX expression was detected in cowpea in control conditions (Fig.…”

“…PTOX is upregulated by stress in some plant species (Laureau et al, 2013) (Stepien and Johnson, 2009) (Johnson and Stepien, 2016). PTOX electron transfer to dioxygen, bypassing photosynthetic cytochrome B6f and photosystem I, may act as a safety valve for transfer of excess electrons from the photosynthetic electron transfer chain and ROS formation (Laureau et al, 2013)(Krieger-Liszkay and Feilke, 2015) (Nawrocki et al, 2015).…”

“…For example, mitochondrial alternative oxidase (AOX) isoforms can be induced by drought in plants, including cowpea, to limit mitochondrial ROS production (Vanlerberghe et al, 2016) (Costa et al, 2010) (Wang and Vanlerberghe, 2013). The plastid terminal oxidase PTOX, a chloroplast homologue of AOX, is induced in some plant species to limit excess electron transfer in the chloroplast during stress (Laureau et al, 2013) (Johnson and Stepien, 2016).…”

Diurnal variation in the coupling of photosynthetic electron transport and carbon fixation in iron-limited phytoplankton in the NE subarctic Pacific

“…The dissipation of excess excitation energy as thermal energy before reaching RCII, estimated as NPQ NSV , prevents excess electron transport and over-reduction of the ETC. After the initial charge separation in RCII, excess electron transport and over-reduction of the ETC can be alleviated by a number of alternative electron pathways; the upregulation of which will increase K c /n PSII (e.g., Bailey et al, 2008;Cardol et al, 2011;Laureau et al, 2013;Mackey et al, 2008;McDonald et al, 2011;Niyogi, 2000;Streb et al, 2005;Vass, 2011;Zehr and Kudela, 2009). Thus, both NPQ NSV and K c /n PSII respond strongly to excess excitation pressure, providing a possible mechanistic interpretation for their correlation.…”

Light acclimation and pH perturbations affect photosynthetic performance in Chlorella mass culture

“…reaction kinetics operate in the opposite direction and are therefore not likely to explain low oxygen evolution rates in acidified samples in the present study. It is still un known if PTOX activity can generally act as a safety valve under stress conditions in higher plants, partly due to open questions regarding reduction kinetics in this pathway (Trouillard et al 2012, Laureau et al 2013). Nevertheless, it is possible that this mechanism is employed under pH shock conditions in Chlo rella as electron redox systems can participate in homeostasis (Houyoux et al 2011).…”