# Chloroplasts at the Crossroad of Photosynthesis, Pathogen Infection and Plant Defense
## 8 Chloroplasts are Targeted by Pathogen Effectors
### HopBB1 and HopM1  
Another two possibly chloroplast-targeted T3 Es are HopBB1 and HopM1 (Table 1). Although the subcellular location of HopBB1 in the plant cell has not been experimentally investigated, HopBB1 was found to interact with some nuclear and chloroplastic proteins [35,318]. Therefore, HopBB1 may localize to multiple plant subcellular compartments. Discrepancy exists in experimental data on the localization of HopM1 in the plant cell. Using confocal microscopic analysis of fluorescent protein-tagged effector, [PERSON] et al. [324] showed that HopM1 localizes to the trans-Golgi network/early endosome compartment. However, [PERSON] et al. [305] showed that HopM1 localizes to the chloroplast, with a similar approach. The cause of this discrepancy is not clear. It is possible that HopM1 localizes to multiple plant subcellular compartments and different localizations trigger different responses. HopBB1 is a T3E with a putative AvrPphF-ORF2 domain (AvrPphF is a homolog of HopF2 in _P. syringae_ pv. _phaseolicola_; ORF stands for open reading frame) [331,343]. The AvrPphF-ORF2 domain is structurally homologous to the catalytic domain of bacterial ADP-ribosyltransferases [343], although purified AvrPphF-ORF2 did not show ADP-ribosyltransferase activity [344]. Using the Y2H approach, HopBB1 was found to interact with a number of nuclear proteins, including JAZ3, TCP14 and TCP15 (teosinte branched/cycloidea/PCF 14 and 15) [35]. Similar to other JAZ repressors, JAZ3 is a direct target of the SCFCOII E3 ubiquitin ligase during JA signaling [337]. TCP14 is a repressor for the transcription of a subset of JA response genes [319]. The interactions of HopBB1 with JAZ3 and TCP14 were confirmed with multiple independent approaches [319,345]. More importantly, HopBB1 was found to \"glue\" together JAZ3 and TCP14, two repressors of JA signaling, and target them for degradation by the SCFCOII-dependent ubiquitination [319]. HopBB1 was also found to interact with PTF1 in the Y2H assay [318]. Because of the essential role of PTF1 in regulating photosynthesis, it is tempting to hypothesize that HopBB1 modulates photosynthesis via interaction with PTF1. Further studies are needed to investigate the biological significance of the HopBB1-PTF1 interaction. HopM1 was found to interact with MIN7 (HopM1 interactor 7), an ADP-ribosylation factor-guanine nucleotide exchange factor, in the Y2H assay [325]. MIN7 is a key controller of vesicle trafficking and it is involved in PTI, ETI, and SA-mediated immunity [324,325]. The interaction between HopM1 and MIN7 was confirmed by the pull-down assay [325]. HopM1 targets MIN7 via the N terminus, promotes ubiquitination and destruction of MIN7 via host cell 26S proteasomes, and thus suppresses vesicular trafficking during plant defense [325]. In line with these results, MIN7 was found to co-localize with HopM1 to the trans-Golgi network/early endosome compartment of plant cells and ETI was found to block the degradation of MIN7 in resistant plants [324]. HopM1 also interactedwith MIN10, a 14-3-3 protein, in the Y2H assay [325]. MIN10 and other 14-3-3 proteins are required for early PTI responses, namely, stomatal immunity and PAMP-triggered ROS production [326]. HopM1 preferably eliminated endomembrane-associated MIN10, suggesting that HopM1 may target and destabilize MIN10 in plant endomembranes [325]. In agreement with this hypothesis, HopM1 was found to suppress early PTI responses and this effect is independent of MIN7 [326]. Therefore, HopM1 may suppress early PTI responses in a MIN7-independent manner, by destabilizing MIN10 and other 14-3-3 proteins [326]. HopM1 was also proposed to target and promote the destruction of _Arabidopsis_ response regulator 2 (ARR2) [327], a transcription factor capable of binding to TGA3 (TGA1a-related gene 3) and activating \(PR\) gene expression [346; 347]. Consistent with this hypothesis, HopM1 was found to suppress the expression of \(PR\) genes and this HopM1-mediated suppression was absent in the loss-of-function mutant of TGA3 [327].