# Chloroplasts at the Crossroad of Photosynthesis, Pathogen Infection and Plant Defense
## 1 Introduction  
Photosynthesis, pathogen infection, and plant defense are three important biological processes that have been investigated separately for decades [1]. In algae and plants, photosynthesis occurs in the chloroplast. Photosynthesis could be divided into light reactions and carbon fixation reactions. Photosynthetic light reactions require the participation of four protein complexes in thylakoid membranes (Photosystem II [PSII], cytochrome \(b_{\textrm{sf}}\) complex, Photosystem I [PSI], and ATP synthase), and mobile electron carriers plastoquinone, plastocyanin, and ferredoxin. The end products of photosynthetic light reactions are ATP and NADPH; in oxygen photosynthetic organisms, molecular oxygen (O\({}_{2}\)) is also produced by PSII at this stage, as a water-splitting product. ATP and NADPH produced from photosynthetic light reactions are consumed by photosynthetic carbon fixation in a series of stromal reactions that reduce CO\({}_{2}\) to triose phosphates. These reactions are catalyzed by ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco). The carbohydrates (i.e., triose phosphates) produced by carbon fixation reactions serve as carbon skeletons for the synthesis of primary metabolites such as amino acids [2] and fatty acids [3], phytohormones such as abscisic acid (ABA) [4; 5], ethylene (ET) [6], jasmonic acid (JA) [7; 8], and salicylic acid (SA) [9; 10; 11], antimicrobial compounds such as camalexin [12; 13], and cell wall reinforcing polymers such as callose and lignin [14]. Synthesis ofthese primary and specialized metabolites and polymers often requires the consumption of ATP and NADPH, and sometimes, O\({}_{2}\), the three end products of photosynthetic light reactions. Key steps in the synthesis of defense-related hormones or their precursors occur in the chloroplast. PSII and PSI are also primary generators of singlet oxygen (\({}^{1}\)O\({}_{2}\)) and superoxide (O\({}_{2}\)\({}^{-}\)), respectively [15; 16; 17]. Reactive oxygen species (ROS) production by the photosynthetic electron transport chain has a protective role over the photosynthetic apparatus when the absorbed excitation energy exceeds the energy consumed during photosynthetic electron transport [15]. On the other hand, excess amounts of ROS damage proteins, lipids, and nucleic acids, and are therefore toxic to many cellular processes, including photosynthesis [15; 16; 17; 18; 19; 20; 21; 22; 23; 24]. All plants grown naturally are attacked by pathogens, such as bacteria, fungi, oomycetes, and nematodes [25]. Plant pathogens have two different lifestyles: necrootrophs and biotrophs [25; 26]. Necrotrophs kill plant tissues and gain nutrients from dead tissues; biotrophs keep plant tissues alive and gain nutrients from living cells [25; 26]. Hemibiotrophs, such as _Pseudomonas syringae_, a rod-shaped gram-negative bacterium with one to several polar flagella [27], are characterized by an initial biotrophic phase and a later recrotrophic phase [25; 26]. Bacterial pathogens enter host tissues through natural openings (e.g., stomata) or wounds [28; 29]. Upon invasion, bacterial pathogens utilize a combination of different virulence factors, such as type III effector proteins (T3 Es) and phytotoxins, to suppress host defense and promote pathogenicity [30; 31; 32]. T3 Es released by the type III secretion system (T3 SS) in gram-negative bacteria are predicted to collectively suppress plant basal defense and reprogram plant photosynthesis and metabolism, to assist pathogen proliferation and nutrition [33; 34; 35; 36; 37]. One example of phytotoxins is coronatine (COR), a non-host-specific polyketide produced by many strains of _P. syringae_[30; 38]. COR interferes with plant JA signaling by mimicking bioactive JA-isoleucine (JA-Ile) and \"fooling\" the JA receptor COR-insensitive 1 (COII) [39; 40; 41; 42]. By modulating plant JA signaling, COR causes stomatal reopening, chlorophyll degradation, and inhibition of SA-mediated defense responses [38].