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Animal and plant microbial pathogens deploy effector proteins and virulence factors to manipulate host cell innate immunity, often using unconventional secretion routes that are poorly understood. Transfer RNA (tRNA) anticodon modifications occur across taxa, but few biological functions are known. Here, in the devastating blast fungus Magnaporthe oryzae, we find that unconventional protein secretion in living host rice cells depends on tRNA modification and codon usage. Using gene deletions, mass spectrometry and live-cell imaging, we characterized the M. oryzae Uba4-Urm1 sulfur relay system mediating tRNA anticodon wobble uridine 2-thiolation (s2U34), a conserved modification required for efficient decoding of AA-ending cognate codons. In M. oryzae, cytoplasmic effectors like Pwl2 and AVR-Pita are translocated into host cells via an unconventional secretion route; apoplastic effectors like Bas4 are secreted by the conventional ER-Golgi pathway. Loss of U34 thiolation abolished PWL2 and AVR-PITA (but not BAS4) mRNA translation in host cells. Paromomycin treatment, which increases near-cognate tRNA acceptance, restored Pwl2 and AVR-Pita production in U34 thiolation-deficient mutant strains. Synonymous AA- to ¬¬AG-ending codon changes remediated PWL2 mRNA translation in uba4; in UBA4+, expressing recoded PWL2 resulted in Pwl2 super-secretion that destabilized the microbe-host cell interface. Thus, wobble U34 tRNA thiolation and codon usage tune pathogen unconventional protein secretion in host cells.
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Paths of Least Resistance: Unconventional Effector Secretion by Fungal and Oomycete Plant Pathogens
Effector secretion by different routes mediates the molecular interplay between host plant and pathogen, but mechanistic details in eukaryotes are sparse. This may limit the discovery of new effectors that could be utilized for improving host plant disease resistance. In fungi and oomycetes, apoplastic effectors are secreted via the conventional endoplasmic reticulum (ER)-Golgi pathway, while cytoplasmic effectors are packaged into vesicles that bypass Golgi in an unconventional protein secretion (UPS) pathway. In Magnaporthe oryzae, the Golgi bypass UPS pathway incorporates components of the exocyst complex and a t-SNARE, presumably to fuse Golgi bypass vesicles to the fungal plasma membrane. Upstream, cytoplasmic effector mRNA translation in M. oryzae requires the efficient decoding of AA-ending codons. This involves the modification of wobble uridines in the anticodon loop of cognate tRNAs and fine-tunes cytoplasmic effector translation and secretion rates to maintain biotrophic interfacial complex integrity and permit host infection. Thus, plant-fungal interface integrity is intimately tied to effector codon usage, which is a surprising constraint on pathogenicity. Here, we discuss these findings within the context of fungal and oomycete effector discovery, delivery, and function in host cells. We show how cracking the codon code for unconventional cytoplasmic effector secretion in M. oryzae has revealed AA-ending codon usage bias in cytoplasmic effector mRNAs across kingdoms, including within the RxLR-dEER motif-encoding sequence of a bona fide Phytophthora infestans cytoplasmic effector, suggesting its subjection to translational speed control. By focusing on recent developments in understanding unconventional effector secretion, we draw attention to this important but understudied area of host-pathogen interactions. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .
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- Award ID(s):
- 2106153
- PAR ID:
- 10596494
- Publisher / Repository:
- American Phytopathological Society
- Date Published:
- Journal Name:
- Molecular Plant-Microbe Interactions®
- Volume:
- 37
- Issue:
- 9
- ISSN:
- 0894-0282
- Page Range / eLocation ID:
- 653 to 661
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1094/MPMI-12-23-0212-CR
