Plant Photochemistry, Reactive Oxygen Species and Photoprotection

Dear Colleagues,

Plant photochemistry is a highly regulated process, in which the solar energy absorbed as photons by the light-harvesting complexes (LHCs) is transferred to the reaction centres (RCs), where through charge separation the electrons flow from photosystem II (PSII) through cytochrome b6f and diffusible electron carriers to photosystem I (PSI). PSII and PSI  work co-ordinately for efficient electron transfer and are located in the photosynthetic membranes of chloroplasts (i.e., the thylakoids). The result of the light reactions is the formation of ATP and reducing power (reduced ferredoxin and NADPH) that need to be coordinated with the activity of metabolic processes for carbohydrate synthesis. However, in the light reactions of photosynthesis, reactive oxygen species (ROS) such as superoxide anion radical (O₂^{• –}), hydrogen peroxide (H₂O₂), and singlet oxygen (¹O₂) are continuously produced at basal levels but are scavenged by different antioxidant mechanisms. Thus, in plant photochemistry if the absorbed light energy exceeds that which can be used, this excess excitation energy must be quenched by the photoprotective mechanism of non-photochemical quenching (NPQ), so as not to damage the photosynthetic apparatus by the increased ROS production that can lead to oxidative stress.

ROS produced in chloroplasts not only generate oxidative stress, but also confer important biological function (e.g., redox signalling). As the ROS formed by energy transfer (¹O₂) and electron transport (O₂^{• –}, H₂O₂) are produced simultaneously, it seems likely that their action in signalling pathways interferes with the signalling pathways occasionally to antagonize each other. The role of chloroplast antioxidants, which often have overlying or interrelating functions, is not to totally eliminate ROS, but rather to achieve an appropriate balance between production and subtraction so as to match with the operation of photosynthesis, permitting an efficient spreading of signals to the nucleus. Consequently, ROS not only provide cells with tools to monitor electron transport and thus prevent over-reduction or over-oxidation, but also produce redox regulatory networks that enable plants to sense and respond to biotic and abiotic stress conditions. ROS activate the plant’s defence mechanisms in order to cope with the oxidative stress and are essential as signalling molecules for the regulation of a plethora of physiological functions tightly, accomplishing plant function and development.

This Special Issue of Photochem will contribute to a better understanding of plant photochemistry mechanisms and ROS interference as a signalling molecule for photoprotection, but also for generating oxidative stress under different environmental conditions.

We encourage original research submissions, as well as review/mini-review articles, concerning basic aspects and future research directions in the field.

Prof. Dr. Michael Moustakas
Guest Editor

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