# Chloroplasts at the Crossroad of Photosynthesis, Pathogen Infection and Plant Defense
## 8 Chloroplasts are Targeted by Pathogen Effectors
### AvrRps4 and HopK1  
According to LOCALIZER, AvrRps4 and HopK1 contain an N-terminal cleavable chloroplast transit peptide [310]. However, confocal microscopic analysis of GFP-tagged effectors, chloroplast import assays of radiolabeled effectors, and subcellular fractionation of _Arabidopsis_ plants expressing hemagglutinin-tagged effectors demonstrated that these two T3 Es localize to multiple plant subcellular compartments: chloroplasts, the nucleus, and the cytoplasm [308; 311]. The chloroplast fraction contains processed but not full-length forms of AvrRps4 and HopK1, the nuclear fraction contains full-length but not processed forms, and the cytoplasmic fraction mainly contains full-length forms [308]. While nuclear and cytoplasmic pools of AvrRps4 and HopK1 trigger immunity and the hypersensitive response, respectively [312], the chloroplastic pool is responsible for their virulence [308]. Consequently, _in planta_ N-terminal processing of AvrRps4 and HopK1 and their chloroplast localization are required for their full virulence function, but are not required for their ability to induce immunity or the hypersensitive response [308]. Consistent with this hypothesis, expression of the AvrRps4 variant lacking the N-terminal transit peptide failed to localize the protein to the chloroplast  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Name**} & **Full Length** & **Cleavable** & **Signature** & **Known and Potential** & **Function** & **References** \\  & **(sa)** & **Transit Peptide2  & **J2X3**, & **TCP14** & **TCP14** \\  & & & & & & 1A response \\  & & & & & & \\  & &  
however, the protein retained its ability to induce the hypersensitive response [308]. Another piece of evidence is that the AvrRps4 processing-deficient mutant strain displayed reduced growth and milder disease symptoms, but still induced the hypersensitive response and immunity in plants [309]. AvrRps4 contains a KRVY motif (amino acids 135-138) [309] and a putative SSM4 E3 ubiquitin-protein ligase domain [331]. Mutations in the KRVY motif resulted in the abolishment of AvrRps4-triggered hypersensitive response and immunity, demonstrating that the KRVY motif is required for the avirulence activity of AvrRps4 [309]. Although the function of the SSM4 E3 ubiquitin ligase domain has not been experimentally demonstrated, many bacterial effectors manipulate the ubiquitination pathway [177; 178; 179; 299]. Some act as deubiquitinases or E3 ligases and alter the function and stability of target proteins in the plant cell [177; 178; 179; 299]; others regulate their own function, stability, and final destination [332; 333; 334; 335]. AvrRps4 is detected by the R protein RPS4 (resistance to _P. syringae_ 4) [313; 316]. RPS4 may form complexes with EDS1 [312; 313; 314], a positive regulator of basal resistance and ETI. AvrRps4 targets EDS1 in the cytoplasm and the nucleus and disrupts the interactions of EDS1 with its partners such as RPS4 [312; 313]. RPS4 may form complexes with another R protein, RRS1 (resistance to _Ralstonia solanacearum_ 1) [316; 336]. In the presence of RRS1, the RPS4-RRS1 complex associates the EDS1-PAD4 (phytoalexin deficient 4) or EDS1-SAG101 (senescence-associated gene 1) complex in the nucleus, and AvrRps4 does not disrupt their association [336]. In the absence of RRS1, AvrRps4 forms nucleocytoplasmic aggregates with EDS1, and the association between the two complexes is disrupted [336]. AvrRps4 also targets the WRKY domain of RPS1, activating RPS4-RRS1-dependent ETI, and targets the WRKY domain of transcription factors WRKY33, WRKY41, WRKY60, and WRKY70, interfering with the WRKY-dependent defense [317]. Taken together, AvrRps4 and HopK1 probably localize to multiple plant subcellular compartments, with the processed form localizing to the chloroplast and the full-length form localizing to the nucleus and the cytosol (e.g., cytoplasmic membranes). The chloroplast targets of AvrRps4 and the signature domain and _in planta_ targets of HopK1 are not yet known.