Abstract
Fungal pathogens secrete effector molecules into host plant cells to suppress host immunity to colonize plants. Ongoing efforts are being made to identify and characterize effector proteins in many fungal plant pathogens. Nevertheless, the precise biological and biochemical functions of many effectors, such as their trafficking from the pathogen to the host, have yet to be fully understood. In this study, we show that an effector candidate, matured PstCTE1 of Puccinia striiformis f. sp. tritici, localizes to chloroplasts when expressed in planta. It has no conserved transit signal region that can be detected by widely accepted prediction tools including TargetP and ChloroP, it must be carrying a unique localization signal. We have shown that N-terminal tagged red fluorescent protein has no effect on the chloroplast localization of PstCTE1, suggesting a new chloroplast translocation mechanism. We also observed the entrance of the candidate effector to the chloroplast even with the construct having the intact signal peptide on the N-terminus of the transit peptide region. Possibly due to overexpression of the protein in N. benthamina, accumulation in the ER (cytoplasm) was obvious. As previously reported, PstCTE1, similar to effector proteins, may either escape from the secretory pathway by retrograde transport, or translation may occur at alternative sites. This would result in a truncated and/or non-functional signal peptide at the N-terminus in a non-host model system (Nicotiana benthamiana), if it is not re-entering the cell from the apoplast. Our study adds PstCTE1 to the pool of few candidate effectors, experimentally shown to target the chloroplast.
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Acknowledgements
The research was funded by TUBITAK (Scientific and Technological Research Council of Turkey); KBAG-110 T445, COST-113Z350 research funds. TUBITAK (BIDEB-2211/C) supported AAO & ACO. TUBITAK (BIDEB-2215) supported BD. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. We are grateful to Dr. T. O. Bozkurt (Imperial College, London, UK) and Dr. C. D. Son (METU, Ankara, Turkey) for their microscopy resources. We thank the Kamoun Laboratory, Sainsbury Laboratory, Norwich, UK, for providing the plasmids. We thank to Dr. Bekir Ulker for the critical reading of this manuscript (he will always be remembered).
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The first author, AAO conducted the experiments and prepared manuscript draft as part of the PhD thesis study under supervision of principal investigator, MSA. All authors (AAO, ACO, BD, MSA) analysed the results and reviewed the manuscript.
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Dedicated to the memory of Dr. Bekir Ulker
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Unique transit peptide region of a Pst candidate effector efficiently guides targeting the chloroplasts.
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Andac, A., Ozketen, A.C., Dagvadorj, B. et al. An effector of Puccinia striiformis f. sp. tritici targets chloroplasts with a novel and robust targeting signal. Eur J Plant Pathol 157, 751–765 (2020). https://doi.org/10.1007/s10658-020-02033-6
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DOI: https://doi.org/10.1007/s10658-020-02033-6