# Chloroplasts at the Crossroad of Photosynthesis, Pathogen Infection and Plant Defense
## 8 Chloroplasts are Targeted by Pathogen Effectors
### HopO1-1, HopO1-2, and HopR1  
According to LOCALIZER, HopO1-1, HopO1-2, and HopR1 also contain an N-terminal cleavable chloroplast transit peptide [310]. However, discrepancy exists in the experimental data. Confocal microscopic analysis of transiently expressed fluorescent protein-tagged effectors showed that HopO1s and HopR1 localize to the plasma membrane and the cytoplasm of plant cells, respectively [305; 329]. However, [PERSON] et al. [35] demonstrated that in vitro translated HopO1-2 and HopR1 could be efficiently imported into pea chloroplasts. It is possible that HopO1-1, HopO1-2, and HopR1 localize to multiple plant subcellular compartments.  
HopO1-1 and HopO1-2 contain an ART (Arg ADP-ribosyltransferase) domain [329]. _Pst_ DC3000 mutant strains deficient in HopO1-1 or HopO1-2 displayed reduced _in planta_ growth, indicating that these two T3 Es are required for full virulence [329]. HopO1-1 and HopO1-2 variants with mutations in the ART domain were unable to suppress PTI and ETI, suggesting that the ART domain is essential for virulence [329]. Using a yeast-two-hybrid (Y2H) approach, HopO1-2 was found to interact with several non-chloroplastic plant proteins, including APC8 (anaphase-promoting complex subunit 8, an E3 ubiquitin ligase subunit), CSN5A (constitutive photomorphogenesis 9 signalosome 5A, a molecular regulator of E3 ubiquitin ligase), and OBE1 (oberon 1, a nuclear-targeted plant homeodomain finger protein) [318]. The biological significance of these interactions is not yet known. Furthermore, the _in planta_ targets of HopO1-1 and the chloroplast targets of HopO1-2 are not yet identified.  
HopR1 contains an AvrE (Avirulence protein E) effector domain and a SMC_N (N-terminus of structural maintenance of chromosomes proteins) domain [331]. Using the Y2H approach, HopR1 was found to interact with a number of nuclear-targeted proteins, such as JAZ3 (jasmonate-zim-domain protein 3), LSU1 (response to low sulfur 1), and TOE2 (target of early activation tagged 2) [35; 318]. As a repressor in JA signaling, JAZ3 inhibits MYC2, a transcriptional activator of JA signaling [337]. However, JAZ3 appeared to be a positive regulator of plant defense, as the _Arabidopsis jazz_ mutant displayed enhanced susceptibility to _P. syringae_ and the oomycete _Hyaloperonospora arabidopsis_[318]. Loss-of-function _Arabidopsis_ mutants of LSU2, a homolog of LSU1, also displayed enhanced susceptibility to _P. syringae_ and _H. arabidopsisidis_[318]. TOE2 is a transcription factor capable of interacting with a subset of JAZ repressors (JAZ1/3/4/9) [338]. These observations suggest that HopR1 may manipulate plant nuclear gene expression to assist pathogenicity.  
HopR1 also interacted with the chloroplastic protein PTF1 in the Y2H assay [35; 318]. PTF1 is able to bind to the blue light-responsive promoter region of the _psbD_ gene [67], which encodes the D2 protein of PSII. In the _Arabidopsis ptf1_ mutant, the accumulation of the _psbD_ transcript is significantly reduced; consequently, the _ptf1_ mutant displayed pale green cotyledons and retarded growth [67]. Because of the essential role of PTF1 in regulating photosynthesis, it is tempting to hypothesize that HopR1 modulates photosynthesis via interaction with PTF1. Another chloroplastic protein that interacted with HopR1 in the Y2H assay was CBSX2, a cystathionine \(\beta\)-synthase domain-containing protein [35; 318]. CBSX2 is highly homologous to CBSX1, a protein involved in the activation of plasidial thioredoxins [339]. Plasidial thioredoxins are thiol-based redox regulators of many chloroplastic processes, including chlorophyll biosynthesis, light and carbon fixation reactions of photosynthesis, and H\({}_{2}\)O\({}_{2}\) scavenging [339; 340]. Therefore, it is possible that HopR1 may localize to the chloroplast, interact with chloroplastic proteins such as PTF1 and CBSX2, and manipulate chloroplastic processes to assist pathogenicity.