Chloroplast is the site for photosynthesis through which light energy is converted into chemical energy. Besides photosynthesis, chloroplast has a role in plant immunity through synthesis of secondary metabolites, amino acid and phytohormone such as jasmonic acid and salicylic acid (Trotta et al., 2014; Kretschmer et al., 2020). Chloroplasts play a key role in orchestrating cellular responses to stress, including pathogen infection. These autonomous organelles can regulate plant immune response by transmitting signals to the nucleus and other cell compartments through retrograde signaling pathways and they alter their stromule frequency in reaction to internal signal transduction pathway. Upon pathogen infection, chloroplasts either congregate around the nucleus or send out dynamic tubular extensions called stromules to establish a physical contact with the nucleus (Caplan et al., 2015; Kumar et al., 2018; Ding et al., 2019). Stromules are stroma filled tubules extending from surface of all type of plastids such as proplastids, chloroplast, etioplasts, leucoplast, amyloplast and chromoplast (Natesan et al., 2005). Microtubules and actin filaments facilitates perinuclear clustering of chloroplast during an innate immune response (Kumar et al., 2018). Stromules aid in the amplification and/or transport of pro-defense signals into the nucleus and other subcellular compartments during immunity (Caplan et al., 2015). A decline in photosynthesis is often accompanied by plant immunity. Stromules facilitates the direct transfer of chloroplast sourced hydrogen per oxide (H2O2) to the nucleus. Stromule formation increases with enhanced ROS generation in chloroplasts and its frequency is regulated in response to the chloroplast redox status (Brunkard et al., 2015; de Torres Zabala et al., 2015). Photosynthetic activity provides NADPH, ATP and carbon skeleton which fuels the initiation of responses against the external stress. The Arabidopsis thaliana chloroplast protein ACCELERATED CELL DEATH2 (ACD2) modulated programmed cell death (Yao and Greenberg 2006).
Hydrogen per oxide is produced via superoxide and superoxide dismutase, by electron transport in chloroplast and mitochondria, plasma membrane NADPH oxidases, peroxisomal oxidases, type III peroxidases and other apoplastic oxidases (Smirnoff and Arnaud 2018). Aquaporin facilitate H2O2 into guard cell, suggesting a signaling role of aquaporin in context involving H2O2 (Rodrigues et al., 2017). In plasma membrane NADPH oxidases (NOX) transfer electrons from NADPH to oxygen, leading to the generation of superoxide and hydrogen peroxide in the apoplast (Sagi and Fluhr 2006). In plant NOX homologs have been named respiratory burst oxidase homologs (Rboh) and are also involved in production of ROS in response to pathogens (Sagi and Fluhr 2001; Torres et al., 2002).
Chloroplast metabolism results in the production of reactive oxygen species and nitric oxide as defense molecules (Kretschmer et al., 2020). Changes in chloroplast redox signaling pathway and reactive oxygen species metabolism mediates local and systemic signal modulating plant resistance to disease and light stress (Trotta et al., 2014). The major source of ROS is plasma membrane localized NADPH/NADPH oxidases that generate superoxide or cell wall localized peroxidases that generate H2O2 or both system operating in tandem (Bolwell 1999; Daudi et al., 2012). The ROS H2O2 during hypersensitive response-programmed cell death (HR-PCD) can also originate from plasma membrane localized Rboh of NADPH oxidase (Zhou et al., 2019; Torres et al., 2006).
Chloroplast is a central component in plant immunity. On recognition of pathogen- or microbe-associated molecular patterns (PAMPs/ MAMPs) by the plasma membrane receptor kinases leading to activation of mitogen-activated protein kinase (MAPK), followed by changes in cytoplasmic calcium concentration, activating calcium-dependent protein kinase (CDPK) cascades leading to rapid NADPH oxidase-driven burst of reactive oxygen species in apoplast. Thus together these processes provide protection to plant against plant pathogens (Boudsocq et al., 2010; Trotta et al., 2014). CDPK and MAPK cascades act differentially in four MAMP-mediated regulatory program to control early genes involved in synthesis of defense peptides and metabolites, cell wall modification and redox signaling (Boudsocq et al., 2010).
Calcium-sensing receptor (CAS) is a calcium-binding protein located on the thylakoid membrane in the chloroplast. Recognition of PAMPs by pattern recognition receptor leads to PAMP- triggered immunity (PTI) (induces basal defense); while recognition by resistance (R) gene leads to effector- triggered immunity (ETI) (induces hypersensitive response leading to cell death). Once the PTI or ETI signal is relayed to the chloroplast, Ca2+ from thylakoid lumen, containing high concentration of Ca2+ are transported to the stroma by CAS and the signal is then transduced from the chloroplast to the nucleus through the singlet oxygen (1O2)-mediated retrograde signaling pathway (Nomura et al., 2012; Kim and Apel2013) regulating the defense response through transcriptional reprogramming of defense related genes (Nomura et al., 2012; Yang et al., 2021).
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