Candidate Effector Proteins from the Maize Tar Spot Pathogen Localize to Diverse Plant Cell Compartments.

Most fungal pathogens secrete effector proteins into host cells to modulate their immune responses, thereby promoting pathogenesis and fungal growth. One such fungal pathogen is the ascomycete , which causes tar spot disease on leaves of maize (). Sequencing of the genome revealed 462 putatively secreted proteins, of which 40 contain expected effector-like sequence characteristics. However, the subcellular compartments targeted by effector candidate (PmEC) proteins remain unknown, and it will be important to prioritize them for further functional characterization. To test the hypothesis that PmECs target diverse subcellular compartments, cellular locations of super yellow fluorescent protein-tagged PmEC proteins were identified using a -based heterologous expression system. Immunoblot analyses showed that most of the PmEC-fluorescent protein fusions accumulated protein in , indicating that the candidate effectors could be expressed in dicot leaf cells. Laser-scanning confocal microscopy of epidermal cells revealed that most of the putative effectors localized to the nucleus and cytosol. One candidate effector, PmEC01597, localized to multiple subcellular compartments including the nucleus, nucleolus, and plasma membrane, whereas an additional putative effector, PmEC03792, preferentially labelled both the nucleus and nucleolus. Intriguingly, one candidate effector, PmEC04573, consistently localized to the stroma of chloroplasts as well as stroma-containing tubules (stromules). Collectively, these data suggest that effector candidate proteins from target diverse cellular organelles and could thus provide valuable insights into their putative functions, as well as host processes potentially manipulated by this fungal pathogen.