Chloroplast is the site for the synthesis of plant defense molecules such as salicylic acid and jasmonic acid, reactive oxygen species (ROS) production and calcium oscillations which are plant defense strategy against the necrotrophic and biotrophic pathogens. ROS is a cytotoxic agent and participates in signaling network. Production of ROS in extracellular space the apoplast, can influence its generation in the chloroplast and both can regulate nuclear gene expression (Shapiguzov et al., 2012). NADPH oxidases (nicotinamide adenine dinucleotide phosphate oxidase) well known as respiratory burst oxidase homologs (RBOHs) are the key enzymes for ROS production in plant. They are transmembrane flavoproteins that oxidize cytoplasmic NADPH, translocate electrons across plasma membrane and reduce extracellular oxygen (triplet) to yield superoxide in the cell wall, because of the negative charge this short-lived ROS is not able to cross the lipid bilayer and therefore remains in the apoplast, where it is converted into hydrogen per oxide (Shapiguzov et al., 2012). ROS perturbs the redox state of the cell which can result in detrimental protein oxidation and severe cellular damage (Spoel and Loake 2011; Bleau and Spoel 2021). Evidence indicate that pathogen-induced changes in the cellular redox environment are sensed by the reactive cysteine residues of key regulatory proteins (Spoel and Loake 2011).
Patterns-triggered immunity (PTI) initiates defense response associated with ROS generation, stomata closure, activation of mitogen-activated protein kinases (MAPKs) and induction of defense genes. Pathogen target chloroplast in order to reduce the release of chloroplast-derived signals. Chloroplast can amplify the signal and transmit it to the nucleus via cytosolic signaling network to control gene expression /transcription and translation (Baier and Dietz 2005; Shaikhali and Wingsle 2017). The apoplastic signal can also reach the nucleus through cytosolic pathway directly. Due to the damaging nature of ROS, cells develop sensitive signaling cascades by which the nucleus can detect changes in chloroplast activity and respond to the signals. This phenomena of communication from the chloroplast to the nucleus is termed to as retrograde signaling (Padmanabhan and Kumar 2010). Retrograde signaling involves calcium sensing and a ROS signal that could be transferred to the nucleus via stromule bridges originating from chloroplast (Park et al., 2018; Kretschmer et al., 2020).
Apoplastic ROS accumulation is sensed in all cellular compartments via different redox –based mechanisms and ROS produced in apoplast and in chloroplast, mitochondria and peroxisomes are involved in interorganellar communication to trigger defense response (Mignolet-Spruyt et al., 2016; Kuzniak and Kopczewski 2020). The apoplastic and chloroplastic ROS-induced signal communicate to neighbouring cell (local signaling) or participate in long distance (systemic) signaling (Shapiguzov et al., 2012).
Redox imbalance in chloroplast could be transmitted into cytoplasm by metabolic coupling. Sensor then reside in cytoplasm. The cytosolic antioxidant system shields the nucleus from chloroplast ROS signals. The chloroplast to nucleus signaling is a redox signaling pathway triggered by redox imbalances in the cytosol. Photosynthetic ROS signals and redox imbalances are buffered by cytosolic antioxidants. Whether they reach the nucleus depends on the rates of ROS-formation and the strength of the cytosolic antioxidant system (Baier and Dietz 2005). The term regulation describes adjustment of activity i.e. of enzymes or transcription factors while signaling refers to transport of information from one site to another via signal molecule that may have additional function for example metabolic intermediate (Baier and Dietz 2005).
The chloroplast redox state is determined by electron flow through photosynthetic electron transport chain (PETC), the plastoquinone (PQ) is suggested to be redox regulator. The metabolic redox state of the chloroplast regulates the import of the preproteins by altering the activity or composition of transport component. It is with these redox changes that chloroplasts communicate with other plant cell compartments (Balsera et al., 2010). PQ is associated with regulation of accumulation of ROS and antioxidant activity that determines full expression of effective defense (Maciejewska et al., 2002). The redox signaling pathway involves receptors/acceptors, transmitters/transducers, responders and the redox network, involves various redox cues (Shaikhali and Wingsle 2017). The changes in ROS concentration mediated by enzymatic and non-enzymatic antioxidants are transferred into redox signals used by plants to activate various physiological processes (Kopczewski and Kuzniak 2013).
Photosynthesis is a high-rate redox metabolic process that is subjected to a rapid change in photon capture, electron fluxes and redox potential during photosynthesis causing reactive oxygen species (ROS) to be released. Thus photosynthesizing chloroplast function as source of redox and ROS information. This photosynthesis-related ROS and redox information feed in various pathway MAPK and OXI1 (Oxidative signal inducible 1) signaling and control gene expression and translation (Dietz et al., 2016). The release of ROS and oxidation product envelop permeabilization (for larger molecules) and metabolic interference with mitochondria and peroxisomes produce an intricate ROS and redox signature which controls acclimation processes (Dietz et al., 2016). Singlet oxygen trigger activation of OXI1 affect the jasmonate pathway (Shumbe et al., 2016). Singlet oxygen induced changes in gene expression may lead to programmed cell death or acclimation.
The phytotoxin coronatine (structural and functional similarity with jasmonic acid) produced by Pseudomonas syringae pv. tomato DC3000 in tomato induces reactive oxygen species leading to chlorosis and reduction in chlorophyll content (Ishiga et al., 2009). The effectors from fungi and oomycetes pathogen target chloroplast (Kretschmer et al., 2020). Non-host or incompatible host interactions induce defense-associated gene leading to localized cell death. The ROS generated in chloroplast during non-host interaction are essential for hypersensitive response but do not induce pathogenesis-related genes or signaling components of the response (Zurbriggen et al., 2009). Modulation of chloroplastic ROS levels by expression of a heterologous antioxidant protein may protect plant from necrotrophic fungus (Rossi et al., 2017).
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