# Chloroplasts at the Crossroad of Photosynthesis, Pathogen Infection and Plant Defense
## 3 The Chloroplast is a Major Site of Free Radical Production
### The Photosynthetic Electron Transport Chain is a Major Site of ROS Production
However, in _Xcv_-inoculated tobacco plants with compromised chloroplastic ROS production, localized cell death was significantly reduced [215]. These tobacco plants expressed cyanobacterial flavodoxin, which prevents chloroplastic ROS production during pathogen infection [215]. The chloroplast has a number of enzymatic ROS scavenging systems, such as SOD, ascorbate peroxidase (APX), glutathione peroxidase (GPx), and the thioredoxin-peroxiredoxin (TRX-PRX) system [15; 16; 217]. Overexpression of thylakoid-bound APX resulted in delayed hypersensitive response and reduced symptoms [213]. On the contrary, silencing of _PRX_ genes resulted in enhanced spreading of _Pst_ wild-type strain DC3000-induced PCD and enhanced bacterial growth and disease susceptibility [216]. These symptoms were absent when the plants were inoculated with a COR-deficient _Pst_ strain DB29 [216]. Taken together, these results suggest that chloroplastic ROS scavenging is critical to a plant's hypersensitive response and that COR-producing bacterial strains may influence the homeostasis of chloroplastic ROS in a COR-dependent manner [216]. Chloroplastic ROS also up-regulate the expression of defense-related genes in the nucleus. By silencing thylakoid-bound APX, it was discovered that over-accumulation of chloroplastic H\({}_{2}\)O\({}_{2}\) resulted in up-regulation of nuclear genes involved in pathogen defense [218]. The retrograde transcriptional reprogramming induced by chloroplast-generated ROS could potentially be achieved by (1) diffusion of ROS from the chloroplast to other subcellular compartments (e.g., the cytosol and then the nucleus) [219; 220], (2) manipulating hormone (e.g., JA and SA) signaling [218], and (3) influencing the integrity of the chloroplast envelope [221; 222; 223]. ROS influences the production and signaling of phytohormones (Figure 1). H\({}_{2}\)O\({}_{2}\) acts as a secondary messenger in ABA signaling and mediates ABA-induced stomatal closure [224; 225]. Both H\({}_{2}\)O\({}_{2}\) and ABA treatments activated mitogen-activated protein kinase (MAPK) and up-regulated the expression and activities of antioxidant enzymes [226; 227; 228; 229]. Pretreating plants with ROS inhibitors (diphenylene iodonium and imidazole [NADPH oxidase inhibitors]) or scavengers (Tiron [O\({}_{2}\)\({}^{-}\) scavenger] and dimethylthiourea [H\({}_{2}\)O\({}_{2}\) scavenger]) blocked such enhancements [230]. These observations suggest that ROS are required in ABA-induced antioxidant defense [226; 227; 228; 229; 230]. Interestingly, H\({}_{2}\)O\({}_{2}\) was found to up-regulate ABA catabolic genes during _Arabidopsis_ seed imbibition, which resulted in a lower ABA level [231]. The antagonism between ROS and ABA in seed germination was also observed in monocots [74; 75]. ROS play a positive role in ET signaling. For instance, H\({}_{2}\)O\({}_{2}\) induced the transcription of ET synthetic genes in etiolated _Brassica oleracea_ seedlings [232]. O\({}_{2}\)\({}^{-}\) generators promoted ET synthesis in etiolated mung bean seedlings [233]. Furthermore, ET-induced stomatal closure was dependent on H\({}_{2}\)O\({}_{2}\) synthesis in _Arabidopsis_ guard cells [109]. ROS have a negative impact on JA signaling. ROS negated cell wall damage-triggered JA production in _Arabidopsis_ seedlings [150]. Over-accumulation of chloroplastic H\({}_{2}\)O\({}_{2}\) up-regulated the expression of transcription factors that negatively regulate JA signaling (e.g., jasmonate-zim-domain [JAZ] proteins) [234]. ROS play a positive role in SA signaling. H\({}_{2}\)O\({}_{2}\) over-accumulation in the chloroplast resulted in elevated SA levels and enhanced SA responses [218]. It was proposed that the elevated SA level assists the accumulation of H\({}_{2}\)O\({}_{2}\), to control the propagation of cell death [235]. ROS influences the production and signal transduction of other non-hormone signaling molecules (Figure 1).