Candidate effector proteins from the maize tar spot pathogen Phyllachora maydis localize to diverse plant cell compartments

Authors: Helm, Matthew; SINGH, RAKSHA; HILES, RACHEL - PURDUE UNIVERSITY; JAISWAL, NAMRATA; MYERS, ARIANA - ORISE FELLOW; IYER-PASCUZZI, ANJALI - PURDUE UNIVERSITY; Goodwin, Stephen - Steve

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2022
Publication Date: 11/17/2022
Citation: Helm, M.D., Singh, R., Hiles, R., Jaiswal, N., Myers, A., Iyer-Pascuzzi, A., Goodwin, S.B. 2023. Candidate effector proteins from the maize tar spot pathogen Phyllachora maydis localize to diverse plant cell compartments. Phytopathology. 112(12):2538-2548. https://doi.org/10.1094/PHYTO-05-22-0181-R.
DOI: https://doi.org/10.1094/PHYTO-05-22-0181-R

Interpretive Summary: Fungal plant pathogens need to circumvent the plant immune system in order to promote their growth and sustain colonization. To do this, fungi often inject proteins termed 'effectors' directly inside plant cells where they primarily function to “shut-off” the plant immune system, thereby allowing the pathogen to colonize the host. One such fungal pathogen, Phyllachora maydis, uses effector proteins to cause a disease called tar spot on corn leaves. Importantly, tar spot disease can significantly reduce overall corn yields and, in turn, induce severe financial penalties to farmers. Therefore, there is an urgent need to investigate how this fungal pathogen uses its effector proteins to infect corn leaves. Using live-cell imaging, we show that several effector proteins from Phyllachora maydis specificlaly localize to various plant organelles where they likely interfere with and manipulate host immune responses. Our work presented here provide valuable insights into the host processes potentially manipulated by this fungal pathogen and lays the foundation for generating testable hypotheses for future work.

Technical Abstract: 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 Phyllachora maydis, which causes tar spot disease on leaves of maize (Zea mays). Sequencing of the P. maydis genome revealed 462 putatively secreted proteins of which 40 contain expected effector-like sequence characteristics. However, the subcellular compartments targeted by P. maydis effectors candidate (PmECs) proteins remain unknown and it will be important to prioritize them for further functional characterization. We, therefore, sought to determine the subcellular compartments targeted by the super Yellow Fluorescent (sYFP)-tagged P. maydis effector candidate proteins using a Nicotiana benthamiana-based heterologous expression system. Immunoblot analyses showed that most of the PmEC-fluorescent protein fusions accumulated protein in N. benthamiana, indicating the candidate effectors could be expressed in dicot leaf cells. Laser-scanning confocal microscopy of N. benthamiana epidermal cells revealed most of the P. maydis putative effectors localized to the nucleus and cytosol. One candidate effector, PmEC01597, localized to multiple subcellular compartments including the nucleus, nucleolus, and plasma membrane while 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, our data presented herein suggest effector candidate proteins from P. maydis target diverse cellular organelles and may thus provide valuable insights into their putative functions as well as host processes potentially manipulated by this fungal pathogen.