Dear Colleagues,

Light harvesting is the first step of photosynthesis that is the basis for most life on earth. In the light reactions of photosynthesis, the absorbed light, as photons, by the light-harvesting complexes (LHCs) is transferred to the photochemically active traps of the photosystems, the reaction centers (RCs). In linear electron transport, electrons are transferred through a series of redox carriers, beginning from the oxygen evolving complex (OEC) of photosystem II (PSII) (which oxidizes H₂O and releases O₂ and protons), through the plastoquinone (PQ) pool, the cytochrome b6f complex and plastocyanin (PC), and finally through PSI they are transferred to ferredoxin (Fd), which, in turn, reduces NADP+ to NADPH via ferredoxin: NADP+ oxidoreductase (FNR). The result of this process is the formation of ATP and reducing power (reduced ferredoxin and NADPH) that needs to be coordinated with the activity of metabolic processes for carbohydrate synthesis. Following the absorption of photons by LHCs, the transfer of excitons to RCs and the initiation of electron transfer must be well regulated to prevent “over-excitation” of the photosystems which favours the formation of reactive oxygen species (ROS) and photoinhibition. Alternatively, electrons can be transferred from the acceptor side of PSI to oxygen, the Mehler reaction that it contributes significantly to ROS generation and oxidative stress in leaves. In addition, a cyclic electron transport pathway occurs, involving the return of electrons from the acceptor side of PSI to the donor side.

In the light reactions of photosynthesis, ROS such as superoxide anion radical (O₂^{• –}), hydrogen peroxide (H₂O₂), and singlet oxygen (¹O₂) are continuously produced at basal levels that are incapable to cause damage, as they are being scavenged by different antioxidant mechanisms. Under most biotic or abiotic stresses, the absorbed light energy exceeds what it can be used and thus it can damage the photosynthetic apparatus, with PSII being particularly exposed to damage. In general, overexcitation of PSII is prevented largely by dissipation of excess excitation energy as heat. These processes are collectively termed non-photochemical quenching (NPQ) and are typically measured by chlorophyll a fluorescence quenching. If this excess excitation energy is not quenched by NPQ, increased production of ROS occurs that can lead to oxidative stress.

ROS in chloroplasts play dual roles as they generate oxidative stress and also confer essential biological function as redox signaling in response to biotic and abiotic stress conditions. The role of chloroplast antioxidants is not to totally eliminate ROS, but rather to achieve a suitable balance between production and removal so that to counterpart photosynthetic function, permitting an effective diffusion of signals to the nucleus and adjusting a plethora of physiological functions. ROS–antioxidant interaction provides essential information for the redox state that influences gene expression associated with biotic and abiotic stress responses, modulating photosynthetic acclimation or cell death to maximize defense to the stress factors. For this reason, one of the most impelling research challenges concerns the characterization of the molecular mechanisms of photoprotection, upon exposure to abiotic and biotic stresses, associated with plant environmental acclimation.

We encourage original research submissions, as well as review/mini review articles, concerning basic aspects and future research directions in the field. Research works submitted to this Special Issue should report high-novelty results, increasing existing knowledge on photoprotection mechanisms involved in the acclimation or plant defense to different biotic and abiotic stresses.

Prof. Dr. Michael Moustakas
Guest Editor

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• Reactive oxygen species (ROS)
• Photoinhibition
• Non-photochemical quenching (NPQ)
• Photoprotection
• Antioxidants
• Abiotic stress
• Biotic stress
• Chlorophyll a fluorescence
• Oxidative stress
• Superoxide anion radical (O₂^{• –})
• Hydrogen peroxide (H₂O₂)
• Singlet oxygen (¹O₂)