Plastoquinone (PQ) and ubiquinone (UQ) are prenylquinones functioning as electron transporters in electron transport chain of oxygenic photosynthesis and the aerobic respiratory chain and are also involved in plant response to stress, regulate gene expression and cell signal transduction (Liu and Lu 2016). Plastoquinone are located on the thylakoids of chloroplasts while UQ is located in the inner membrane of mitochondria. Plant Solanum nigrum when treated with Phytophthora infestans derived elicitor showed enhanced reactive oxygen species (ROS) production, lipid peroxidation and lipoxygenase activity. These events are accompanied by increased PQ level (Maciejewska et al., 2002). Plastoquinone and UQ can scavenge free radicals and prevent lipid peroxidation, protein oxidation and DNA damage in plant in response to biotic and abiotic stresses (Liu and Lu 2016). PQ and UQ exert antioxidant activity in the reduced form plastoquinol and ubiquinol (Hundal et al., 1995). Superoxide dismutase activity depended on PQ pool. Nosek et al.(2015) studied changes in infection-induced catalase activity, plants develop response that is vital for hypersensitive response-like (HR-like) only when PQ pool generated signal was similar to that in light. When PQ pool generated signal was similar to darkness, catalase activity response remained stress independent. Suggesting that during abiotic or biotic stresses signal generated from PQ pool orchestrates plant response and carbon metabolism affects this regulatory pathway (Nosek et al., 2015). Light may modulate defense responses but sometime it may become essential for development of resistance (Roberts and Paul 2006; Delprato et al., 2015). Large fraction of PQ pool is located outside the thylakoid membrane in plastoglobules and chloroplast envelopes, reflecting its multiple function beyond electron transport (Havaux 2020). Level of PQ is significantly increased after pathogen attack (Maciejewska et al., 2002). Hypothesizing that PQ may maintain a controlled balance between accumulation of ROS and antioxidant activity that determines the full expression of effective defense (Maciejewska et al., 2002).

Bacteria and fungi trigger stomatal closure through pathogen-associated molecular patterns, preventing entry through these pores (Gudesblat et al., 2009). Hence stomata are considered part of plant innate immune response. The study of Wang et al.(2016) demonstrates that a number of hormone-and pathogen-related genes are involved in PQ pool signaling. The possibility is that these genes generate secondary signals which improves biotic and abiotic response in plant when the PQ pool is reduced. Over reduction of the PQ pool of guard cell chloroplast could be a potential way to regulate stomatal closure During calcium (Ca+)-induced stomatal closure, Ca+ may initiate stromal acidification by unidentified mechanism, mobilizing electron from the PQ pool to chloroplastic hydrogen peroxide (H2O2) synthesis in guard cells or in mesophyll.  Chloroplastic H2O2 of guard cell directly causes stomatal closure whereas, chloroplastic H2O2 of mesophyll may diffuse to the guard cells and aggravate the closure (Wang et al., 2016).

Plastoquinone acts as an electron carrier between photosystem II and the cytochrome b6f complex (Van Eerden et al., 2017). ROS acts as signaling molecule or mediates production of phytoalexins or serve as a source of further defense (Kovtun et al., 2000; Thoma et al., 2003). ROS signaling molecule in plant cell may control processes such as plant defense, programmed cell death and stomatal behaviour (Apel and Hirt 2004).  PQ has dual role, 1) reducing oxygen to superoxide by plastosemiquinone and 2) reduction of superoxide to hydrogen peroxide by plastohydroquinone (Mubarakshina and Ivanov 2010).

The reaction centre of photosystem 1 and II (PSI sand PSII) in chloroplast thylakoids are the major generation site of ROS (Asada 2006). Formation of superoxide and H2O2 is associated with PSI, the superoxide is converted to H2O2 by superoxide dismutase. Whereas, singlet oxygen is formed in PSII (Trotta et al, 2014; Kretschmer et al., 2020).  However, the other studies present PQ pool also generates H2O2 (Mubarakshina and Ivanov 2010). ROS caused by photosynthetic changesmay explain why early photosynthesis is involved in the development of plant disease resistance resulting in compatible or incompatible interaction between pathogens and plant (Yang and Luo 2021).

Insight into the function of plant genes is crucial. Muhlenbock et al.(2008) work suggest that balance activities of LESION SIMULATING DISEASE1, ENHANCED DISEASE SUSCEPTIBILITY1, PHYTOALEXIN DEFECIENT4 and ETHYLENE INSENSITIVE2 regulate signaling of programmed cell death, light acclimation and defense responses that are initiated by redox changes of the PQ pool. Karpinski et al.(1997) found a light-sensing mechanism in higher plants that regulates the expression of genes not directly involved in photosynthesis or protection of photosynthetic apparatus but in the scavenging of H2O2 in the cytosol. This mechanism is controlled by the redox status of plastoquinone pool.


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