Role of Plastid Terminal Oxidase (PTOX) as a safety valve for electrons in Hordeum vulgare (Barley) plants.

Abstract

Photosynthesis is a primary target of different abiotic stresses. Under sub-optimal conditions, the imbalance between a low-efficiency carbon fixation and excessive light absorbed can generate an overburdening of the photosynthetic apparatus. This can lead to the production of reactive oxygen species (ROS), which can damage cellular components. Plastid Terminal Oxidase (PTOX) has been proposed to play a role in some higher plants, acting as a safety valve for electrons, resulting in protection of the plastoquinone pool from over-reduction under environmental stress conditions. Due to PTOX ability to transport electrons from plastoquinol to molecular oxygen, generating water. The regulation of the enzyme is still unclear, however, it has been hypothesised to be sensitive to stromal pH variation and recent findings suggest that a translocation of the protein to the grana membranes could be involved. The protective role of PTOX as alternative electron sink in Hordeum vulgare plants was explored, in particular during cold acclimation and against water limitation and high salinity conditions. Additionally, the regulation of the activity of this enzyme was studied. Plastid terminal oxidase (PTOX) seems to account for the diversion of up to 25% of electrons to oxygen in control plants, acting as an important safety valve for electron transport potentially protecting barley leaves from an overreduction of the plastoquinol pool. In cold acclimated plants, PTOX does not appear to be important, however PTOX protein and transcript were still detected. In water restricted and salt treated plants, evidence of PTOX acting as a safety valve for electrons was seen. This plastoquinone oxidase seem to be diverting up to 31% of the total amount of electron transported by PSII at 21% O2 in water restricted plants and up to 45% in salt-treated leaves. This was supported by the presence of increased PTOX protein in both cases and also an increase in PTOX transcript either transiently, in water restricted plants, or in a sustained way in salt treated plants. Additionally, a dynamic and reversible response to O2 concentration was seen in the photosystem (PSII) electroon transport chain (ETR) and non-photochemical quenching (NPQ). Therefore it seems to be unlikely that PTOX is inhibited or in some way regulated by a previous oxygen concentration. It was not possible to confirm the translocation of PTOX to the grana in barley plants, however, direct or indirect variation in the pH has an effect on PTOX protein recovery in the salt-treated plants, with increasing detection of PTOX at more alkaline pH.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Giles Johnson (Supervisor) & Jennifer Rowntree (Supervisor)