Photosynthesis, pathogen infection and plant defense are biological processes. Photosynthesis generates adenosine triphosphate (ATP), nicotinamide adenine dinucleotide phosphate (NADPH) and carbohydrate. The defense responses are conditioned by the nutritional and signaling status and that plant need to regulate pathways to control the pathogen. Biotrophic fungi often reduce photosynthesis and alters the fluxes of carbon within the infected leaf (Swarbrick et al., 2006). Light energy and photosynthetic flux play an important role in the plant immune response (Muhlenbock et al., 2008). Plant resistance to pathogen differ between light and dark reaction and functional chloroplast is needed for defense responses (Kangasjarvi et al., 2012).  In darkness as well as in dim light and independent of a carbohydrate source, SA-induced pathogenesis-related (PR) gene expression and hypersensitive response (HR) to pathogen are reduced (Genoud et al., 2002). Functional chloroplast is necessary for HR and induction of PR proteins are dependent on light but independent of functional chloroplasts (Genoud et al., 2002). All living cell use ATP as energy source. This universal energy currency drives the biological reaction that allow cell to function (Khakh and Burnstock 2009). ATP is a critical molecule that allows cells and tissues to communicate with one another. Chloroplast can regulate plant immune response by transmitting signals to the nucleus and to other cell compartments. Diverse pathogen target the chloroplast and affect its function to promote colonization (Yang et al., 2021).

 Chloroplast has a role in plant immunity as they are the site for production of salicylic acid (SA), jasmonic acid (JA). Light induced signaling pathway interact with the pathogen/SA-mediated signal transduction route (Genoud et al., 2002; Griebel and Zeier 2008). In addition, chloroplast also generates reactive oxygen species (ROS) and nitric oxide (NO) and a site for calcium signaling (Lu and Yao 2018). SA acts as an endogenous signal responsible for inducing systemic acquired resistance (SAR) in plants (Gaffney et al., 1993; Delaney et al., 1994). The pathogen-associated molecular pattern (PAMP) are relayed to chloroplast evoking stromal calcium (Ca2+). The chloroplast-localized protein known as calcium-sensing receptor (CAS) is involved in stromal Ca2+ transient, in PAMP-induced basal resistance and R-gene-mediated hypersensitive cell death (Nomura et al., 2012). CAS is a thylakoid membrane associated Ca2+-binding protein involved in the regulation of cytoplasmic Ca2+ oscillation and extracellular Ca2+-induced stomatal closure (Nomura et al., 2008). Nomura et al.(2012) hypothesize that CAS may be involved in chloroplast-to-nucleus retrograde signaling to control the expression of nuclear-encoded defense genes including SA biosynthesis genes. CAS is essential for induction of PAMP induced defense gene, including SA biosynthesis gene through singlet oxygen-mediated retrograde signaling. Thus SA-mediated immune response are dependent on CAS.

Chloroplast carry out their interconversion by chemiosmotic mechanism in a similar manner as mitochondria do. Chloroplast envelop is comprised of  a highly permeable outer membrane, less permeable inner membrane in which membrane transport proteins are embedded and a narrow intermembrane space is between them. The inner membrane surrounds the large space stroma, which is analogous to the mitochondrial matrix and contains metabolic enzymes. Like mitochondria the choloroplast has its own genome. The stroma contains ribosomes, RNAs and chloroplast DNA (Alberts et al., 2002). The difference between the organization of mitochondria and the chloroplast is that the inner membrane of the chloroplast is not folded into cristae and does not contain electron-transport chains. Instead the thylakoid membrane forming a flattened disc like sac contains the electron-transport chain, photosynthetic light-capturing systems and ATP synthase. The lumen of each thylakoid may be connected with the lumen of other thylakoids thereby defining a third internal membrane distinct from the stroma that surrounds it (Alberts et al., 2002). The chloroplast ATP synthase, where ATP is made protrudes from the thylakoid membrane into the stroma, whereas, it protrudes into matrix from the inner mitochondrial membrane. The electron-transport process occur in the thylakoid membrane to produce ATP. The ATP synthase located in the thylakoid membrane , synthesizes ATP from adenosine diphosphate (ADP) and inorganic phosphate at the expense of the electrochemical proton (H+) gradient formed by light-dependent electron flow (McCarty et al., 2000).

In the photosynthetic electron-transfer reaction (Light reaction), the energy derived from sun light energizes an electron in chlorophyll that enables electron to move along electron-transport chain in the thylakoid membrane. The chlorophyll obtains its electron from water (H2O) producing oxygen O2 as a by product (water is oxidized and oxygen is released). During electron-transport process, H+ is pumped across the thylakoid membrane resulting in electron proton gradient that drives the synthesis of ATP in the stroma. Finally a high-energy electrons (together with H+) are loaded onto NADP+, converting it to NADPH (Alberts et al., 2002).

The carbon-fixation reaction is also known as dark reaction,  the ATP and NADPH produced by the photosynthetic electron-transfer reaction serve as the source of energy and reducing power that converts carbon dioxide  (CO2) to carbohydrate (Alberts et al., 2002).

The local and systemic acclimation in Arabidopsis thaliana leaves, to excess excitation energy (EEE) is associated with cell death and is regulated by redox changes of plastoquinone (PQ) pool. These redox changes cause a rapid decrease of stomatal conductance, global induction of ASCORBATE PEROXIDASE2 and PATHOGENESIS RESISTANCE1 , as well as enhanced production of reactive oxygen species (ROS) and ethylene that signals through ETHYLENE INSENSITIVE2 (EIN2) (Muhlenbock et al., 2008). The study suggests LESION SIMULATING DISEASE1 (LSD1), PHYTOALEXIN DEFICIENT4 (PAD4) and EIN2 regulate signaling of programmed cell death, light acclimation and holistic defense responses that are initiated by redox changes of PQ pool (Muhlenbock et al., 2008).

When the cell is unable to dissipate excess excitation energy, chloroplasts generate ROS such as singlet oxygen (1O2) or hydrogen peroxide (H2O2), activating distinct sets of genes. Nomura et al.(2012) propose CAS-dependent chloroplast-derived signal (possibly 1O2) may modulate the defense responses through transcriptional reprogramming of defense-related genes. The study on light-to-dark transition, Sai and Johnson (2002) darkness stimulates Ca2+ flux into the stroma affects a transient change in cytosolic Ca2+ levels in tobacco chloroplasts. PAMP-induced stromal Ca2+ transients precede CAS-dependent defense gene expression and SA biosynthesis, suggesting role for chloroplast Ca2+ in activation of defense gene expression (Nomura et al., 2012).


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