Abstract
The biotrophic fungus Ustilago maydis secretes a plethora of uncharacterized effector proteins and causes smut disease in maize. Among the effector genes that are up-regulated during the biotrophic growth in maize, we identified vp1 (virulence promoting 1), which has an expression that was up-regulated and maintained at a high level throughout the life cycle of the fungus. We characterized Vp1 by applying in silico analysis, reverse genetics, phenotypic assessment, microscopy, and protein localization and provided a fundamental understanding of the Vp1 protein in U. maydis. The reduction in fungal virulence and colonization in the vp1 mutant suggests the virulence-promoting function of Vp1. The deletion studies on the NLS (nuclear localization signal) sequence and the protein localization study revealed that the C-terminus of Vp1 is processed after secretion in plant apoplast and could localize to the plant nucleus. The Ustilago hordei ortholog UhVp1 lacks NLS localized in the plant cytoplasm, suggesting that the orthologs might have a distinct subcellular localization. Further complementation studies of the Vp1 orthologs in related smut fungi revealed that none of them could complement the virulence function of U. maydis Vp1, suggesting that UmVp1 could acquire a specialized function via sequence divergence.
Highlights
Accepted: 21 July 2021Smut fungi rely heavily on delivering diverse effector proteins to suppress plant immunity and modulate plant metabolic pathways to facilitate their infection [1,2,3]
While some core effectors are present in smut fungi like Ustilago maydis, Sporisorium scitamineum, Sporisorium reilianum, Ustilago hordei, and Melanopsichium pennsylvanicum, accessory effectors exist in a subset of smut fungi [4,8]
This may indicate that UMAG00538 is one of the virulence factors required for fungal development inside plant cells
Summary
Accepted: 21 July 2021Smut fungi rely heavily on delivering diverse effector proteins to suppress plant immunity and modulate plant metabolic pathways to facilitate their infection [1,2,3]. In the genome of smut fungi, which infects most monocot grass species, approximately 7% of the coding genes encode proteins containing a signal peptide for secretion [4]. This number does not include the effectors that lack signal peptides and are predicted to be secreted via an unconventional secretion pathway [5,6]. Among the putative effector proteins, half of them have no functional domains [4]. This makes them novel and interesting candidates for studying their functional roles in fungal virulence. The previous studies on the U. maydis Pep, Sta, Cce, and Rsp showed that the orthologs from related smut fungi could successfully rescue the reduced phenotype of the deletion mutants in U. maydis [9,10,11,12], while the anthocyanin-induction function of Tin, or the virulence-promoting function of See and ApB73, could not be Published: 23 July 2021
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