# Chloroplasts at the Crossroad of Photosynthesis, Pathogen Infection and Plant Defense
## 9 Chloroplast Structure and Functions are Manipulated by Phytotoxins  
Plant pathogens also produce phytotoxins to suppress host defense and promote pathogenicity [30]. Among the five most extensively investigated _P. syringae_ phytotoxins, syringomycin and syringopeptin form ion channels in the plant cell plasma membrane, causing cytolysis and necrosis [30]. Although the subcellular distribution of tabtoxin, phaseolotoxin, and coronatine has not been experimentally investigated, these three phytotoxins were found to affect chloroplast structure and functions and cause chlorosis [30]. Tabtoxin inhibits cytosolic and chloroplastic glutamine (Gln) synthetase [348; 349]. The chloroplastic Gln synthetase re-assimilates photorespiratory ammonia. Free ammonia dissipates pH gradients across biological membranes, causing disruption of thylakoid membranes, uncoupling of photophosphorylation, and ultimately, chlorosis of plant tissues [350]. Phaseolotoxin inhibits ornithine carbamoyltransferase [351; 352; 353; 354], which converts ornithine and carbamoyl phosphate to citruline in the chloroplast [355]. Inhibition of ornithine carbamoyltransferase results in over-accumulation of ornithine, deficiency of Arg, blockage of translation, reduced chlorophyll synthesis, and chlorosis of plant tissues [353; 354]. Consequently, photosynthesis is limited by the very low level of chlorophyll at the site of infection [354]. COR activates JA signaling by mimicking JA-Ile, the endogenous bioactive jasmonate [356]. Like JA-Ile, COR is capable of binding to the JA co-receptor complex, which contains the JA receptor COI1 and a JAZ repressor [357]. The binding triggers SCFCOI1-mediated ubiquitination and 26S proteosome-mediated degradation of JAZ repressors [337; 42; 358]. Upon destruction of JAZs, positive transcription regulators, such as MYC2/3/4 and MYBs, are relieved from repression and activate the expression of JA-responsive genes [8]. One example of MYC-regulated genes is NAC (NAM, ATAF, and CUC) transcription factors (e.g., ANAC019/055/072), which are involved in COR-induced stomatal reopening and chlorophyll degradation [359; 360]. NAC transcription factors also suppress the accumulation of SA by repressing the SA biosynthetic gene _ICS1_ and activating the SA modifying gene _BSMT1_ (_benzoic/s salicylic acid carboxyl methyltransferase 1_) [359]. Chlorophyll catabolic genes (e.g., _SGR1_ [_stay-green 1_]) are also regulated by MYCs [360; 361]. COR suppressed plant defense by disabling stomatal defense [28], inducing _SGR1_ expression and causing chlorophyll degradation [361], and inhibiting SA accumulation [359], in COI1-dependent manners. COR treatment also caused leaf growth arrestment, repression of photosynthetic genes, and a transient reduction of PSII quantum yield at the following dawn, presumably due to delayed stomatal opening at the night-day transition [292]. Interestingly, COR was found to suppress callose deposition, enhance bacterial growth, and promote bacterial virulence in the COI-deficient mutant, suggesting that COR has other plant target(s) besides the COI1-JAZ complex [362]. The subcellular distribution of COR in the plant cell has been experimentally investigated. Because COR binds to the COI1-JAZ complex, it is conceivable that at least a fraction of COR localizes to the nucleus. Using electron microscopy and immunogold labeling, COR was also found to be associated with chloroplasts in plants infected by _P. syringae_[363]. Therefore, it was hypothesized that COR may translocate to the chloroplast and interact with chloroplast-associated proteins during JA/COR signaling [364]. Summary: The Chloroplast Plays a Central Role in the Interplay between Photosynthesis, Pathogen infection, and Plant Defense  
As the site of photosynthetic light reactions and carbon fixation reactions, the chloroplast is indispensible for photosynthesis. The chloroplast is also a major generator of defense-related signaling molecules or their precursors [365].