Effector biology during biotrophic invasion of plant cells

Chaudhari, Prateek; Ahmed, Bulbul; Joly, David L; Germain, Hugo

Computer software, such as NOD, PSORT II, and WoLF PSORT, can predict the subcellular localization of various proteins, but that of very few candidate effectors has been verified experimentally [41] [42] [43] relative to the wealth of those from all plant pathogens. A number of plant pathogen-secreted effector proteins have been reported to localize in the nucleus, but most localization studies have been conducted with GFP-tagged assays. It should be noted that GFP fusion may abrogate proper effector localization, either by hiding a sorting signal or by inducing change in the 3D structure of native effectors which could prevent interaction with a protein involved in true effector localization. In addition, most of these experiments are transient assays and do not examine localization during infection. Therefore, although GFP represents a very powerful tool at our disposal to identify subcellular effector localization, care should be taken when analyzing the results. However, since GFP does not diffuse to the nucleolus, it is safe to assume that nucleolar localization is effector-driven. RXLR effectors, such as HaRxLL3b, HaAtr13 Emoy2 and HaRxL44 from Hyaloperonospora arabidopsidis, localize to the nucleolus of plant cells. 30 In Phytophtora capsici, CRN effectors all localize to the nucleus, and at least two have been found to accumulate in the nucleolus, suggesting that there might be subnuclear localization domains. 44 The nucleolus is a multifunctional subcellular organelle critically involved in ribosome biogenesis and protein synthesis. 45 Several DNA viruses and retroviruses are known to target the nucleolus. Umbravirus ORF3, potato leafroll virus capsid protein and influenza virus nucleoprotein are some examples of viral proteins localizing to the nucleolus. [46] [47] [48] [49] Given that viruses are entirely dependent on the host machinery to translate their genome into proteins, they are expected to target the nucleolus. However, one can wonder why biotrophic filamentous pathogens would target this subnuclear compartment. The effector HaRxL44 from the obligate biotrophic pathogen H. arabidopsidis was recently shown to target nucleolar (and nuclear) Mediator subunit 19a (MED19a). This interaction results in MED19a degradation in a proteasome-dependent manner. MED19a degradation appears to shift transcription from salicylic acid-responsive defense to jasmonic acid and ethylene-responsive transcription, thereby conning the host to enhance its susceptibility. 50