Contrasting Responses to Stress Displayed by Tobacco Overexpressing an Algal Plastid Terminal Oxidase in the Chloroplast.

Niaz Ahmad,Giles N Johnson,Lorna Mcausland,Peter J Nixon,Muhammad Omar Khan,Tracy Lawson,Ejazul Islam,Zheng-Yi Wei

doi:10.3389/fpls.2020.00501

Abstract

The plastid terminal oxidase (PTOX) – an interfacial diiron carboxylate protein found in the thylakoid membranes of chloroplasts – oxidizes plastoquinol and reduces molecular oxygen to water. It is believed to play a physiologically important role in the response of some plant species to light and salt (NaCl) stress by diverting excess electrons to oxygen thereby protecting photosystem II (PSII) from photodamage. PTOX is therefore a candidate for engineering stress tolerance in crop plants. Previously, we used chloroplast transformation technology to over express PTOX1 from the green alga Chlamydomonas reinhardtii in tobacco (generating line Nt-PTOX-OE). Contrary to expectation, growth of Nt-PTOX-OE plants was more sensitive to light stress. Here we have examined in detail the effects of PTOX1 on photosynthesis in Nt-PTOX-OE tobacco plants grown at two different light intensities. Under ‘low light’ (50 μmol photons m–2 s–1) conditions, Nt-PTOX-OE and WT plants showed similar photosynthetic activities. In contrast, under ‘high light’ (125 μmol photons m–2 s–1) conditions, Nt-PTOX-OE showed less PSII activity than WT while photosystem I (PSI) activity was unaffected. Nt-PTOX-OE grown under high light also failed to increase the chlorophyll a/b ratio and the maximum rate of CO2 assimilation compared to low-light grown plants, suggesting a defect in acclimation. In contrast, Nt-PTOX-OE plants showed much better germination, root length, and shoot biomass accumulation than WT when exposed to high levels of NaCl and showed better recovery and less chlorophyll bleaching after NaCl stress when grown hydroponically. Overall, our results strengthen the link between PTOX and the resistance of plants to salt stress.

Highlights

Plastid or plastoquinol terminal oxidase (PTOX) is a nonheme diiron carboxylate protein found widely in oxygenic photosynthetic organisms that oxidizes plastoquinol (PQH2) to plastoquinone (PQ) and reduces molecular oxygen to water (Joët et al, 2002; Josse et al, 2003)
We have found that the Nt-plastid terminal oxidase (PTOX)-OE plants showed much higher germination rates under NaCl stress, better root length, and exhibited less chlorophyll bleaching compared to wild type
Recent work has suggested that PTOX activity might be regulated at the level of attachment of PTOX to the thylakoid membrane, promoted by increased alkalinity of the stroma induced by high light (Laureau et al, 2013; Feilke et al, 2016; Bolte et al, 2020)

Summary

Introduction

Plastid or plastoquinol terminal oxidase (PTOX) is a nonheme diiron carboxylate protein found widely in oxygenic photosynthetic organisms that oxidizes plastoquinol (PQH2) to plastoquinone (PQ) and reduces molecular oxygen to water (Joët et al, 2002; Josse et al, 2003). Upregulation of PTOX expression has been reported for several stress-tolerant plant species acclimated to harsh environments such as drought, high light and high temperature (Quiles, 2006; Ibanez et al, 2010), high salinity (Stepien and Johnson, 2009), low temperature (Streb et al, 2005; Ivanov et al, 2012), and high levels of UV light (Laureau et al, 2013). PTOX is believed to act as a stress-induced safety valve that keeps the acceptor side of PSII oxidized, thereby helping to protect PSII from photo-damage (Streb et al, 2005; Laureau et al, 2013; Johnson and Stepien, 2016; Krieger-Liszkay and Feilke, 2016). PTOX has been proposed as a potential candidate for engineering stress tolerance in crop plants (Laureau et al, 2013; Johnson and Stepien, 2016; Krieger-Liszkay and Feilke, 2016). Expression of PTOX1 made plant growth susceptible to high light; an observation that was at odds with its suggested photoprotective function

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Conclusion