# Chloroplasts at the Crossroad of Photosynthesis, Pathogen Infection and Plant Defense
## 2 The Chloroplast is a Major Synthesis Site for Many Plant Hormones  
### The Chloroplast is a Major Site of ABA Biosynthesis  
ABA is a 15-carbon terpenoid synthesized via 40-carbon carotenoid intermediates, such as zeaxanthin, violaxanthin, and neoxanthin [4; 5]. Early steps of ABA biosynthesis, i.e., conversions among different 40-carbon carotenoid intermediates, occur in the chloroplast. It is worth mentioning that the conversion from zeaxanthin to violaxanthin requires the participation of NADPH and O\({}_{2}\)[54]. 9\({}^{\prime}\)_-cis_-neoxanthin and 9\({}^{\prime}\)_-cis_-violaxanthin are cleaved into 15-carbon xanthoxin, by 9-_cis_-epoxycarotonoid dioxygenase in the chloroplast. Xanthoxin is then transported to the cytosol and converted into abscisic aldehyde by a short-chain alcohol dehydrogenase. Abscisic aldehyde is oxidized into the final product ABA by an abscisic aldehyde oxidase. This step requires the participation of O\({}_{2}\)[4; 5]. After synthesis, ABA may undergo glycosylation or hydroxylation and become inactive [5; 55]. Upon stress, ABA is released from the ABA-glucose conjugate by \(\beta\)-glucosidase [56]. ABA is an important regulator of plant growth and development, biotic stresses, and abiotic stresses [55; 57]. ABA induces stomatal closure in response to drought and high salinity [58; 59; 60]. Stomatal closure limits gas exchange, which is required for photosynthetic carbon fixation reactions [61; 62; 63]. Consistent with relationships among ABA, stomata, and photosynthesis, exogenous ABA application was found to cause stomatal closure and reduced photosynthesis [61; 62; 63]. ABA treatment was found to repress the transcription of many plastid genes by both plastid-encoded RNA polymerase and nuclear-encoded plastid RNA polymerase [64]. However, the transcription of _psbD_ (_PSII reaction center protein D2_; psb stands for PSII), _psbA_ (_PSII reaction center protein D1_), and a few other genes did not respond to ABA treatment [64]. Furthermore, ABA treatment increased the transcription of four nuclear-encoded genes: _RSH2_ (_RelA/SpoT homolog 2_), _RSH3_ (_RelA/SpoT homolog 3_), _PTF1_ (_plastid transcription factor 1_), and _SIG5_ (_sigma factor 5_) [64]. RSH2 and RSH3 catalyze the synthesis of guanosine-3\({}^{\prime}\)-5\({}^{\prime}\)-bisdiphosphate (ppGpp) [65], an inhibitor of the plastid-encoded plastid RNA polymerase (PEP) [66]. Therefore, ABA may inhibit gene expression in the chloroplast by stimulating ppGpp synthesis [64]. PTF1 [67] and SIG5 [68] are chloroplast-targeted transcription factors required for the transcription of _psbD_. ABA may activate the transcription of _psbD_ by promoting transcription initiation at the blue light responsive promoter, which requires the participation of PTF1 and SIG5 [64]. The roles of ABA in plant defense against pathogens are multifaceted. ABA induces stomatal closure in response to pathogen attacks; therefore, ABA is important in blocking the entry of bacterial pathogens via stomata [28; 29]. The core components of ABA-mediated stomatal immunity include the regulatory component of ABA receptor (RCAR), 2C-type protein phosphatase (PP2C), and serine/threonine (Ser/Thr) protein kinase OST1 (open stomata 1) [69]. ABA-mediated stomatal closure involves other signaling molecules. For example, ABA application induced ROS production in guard cells, mediated by ABA-activated OST1 [70; 71]. OST1 promotes hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) production by phosphorylating the respiratory burst oxidase homologue NADPH oxidase F (RbohF) located on the guard cell plasma membrane [72]. ABA treatment also resulted in increased _Rbb1_ (a tomato homolog of _Arabidopsis thaliana RbohF_) gene expression, increased NADPH oxidase activity, and increased apoplastic and chloroplastic H\({}_{2}\)O\({}_{2}\) concentrations, at the whole seedling scale [73]. It should be noted that, although ABA promotes ROS production in non-seed tissues, this hormone suppresses ROS production in imbibed seeds [74; 75].