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Resistance and resistance breaking mechanisms in the melon/Melon necrotic spot virus interaction

Miguel Aranda: CEBAS-CSIC

<div><em data-mce-fragment="1">Melon necrotic spot virus</em> (MNSV; genus <em data-mce-fragment="1">Carmovirus</em>, family <em data-mce-fragment="1">Tombusviridae</em>) is a pathogen endemic in cucurbit crops worldwide. In melon, the recessive <em data-mce-fragment="1">nsv </em>gene confers immunity to MNSV. Resistant (<em data-mce-fragment="1">nsv</em>/<em data-mce-fragment="1">nsv</em>) melon hybrids have been developed and are widely cultivated. <em data-mce-fragment="1">Nsv</em> codes for the eukaryotic translation initiation factor (eIF) 4E, which together with the scaffolding protein eIF4G forms the eIF4F cap-binding complex. Although <em data-mce-fragment="1">nsv</em> resistance seems to be durable, at least two resistance-breaking MNSV isolates have been described: MNSV-264 has the peculiar ability of infecting not only resistant melons, but also <em data-mce-fragment="1">Nicotiana benthamiana</em> and <em data-mce-fragment="1">Gomphrena globosa</em> plants, which are normally non-hosts for MNSV. MNSV-N infects resistant melons but not <em data-mce-fragment="1">N. benthamiana</em> or <em data-mce-fragment="1">G. globosa</em> plants. For both isolates, this capacity depends on the 3´untranslated region of the virus, which contains an RNA structural element that functions as cap-independent translation enhancer (CITE) needed for efficient translation of viral mRNAs. Avirulent MNSV isolates contain an I-shaped 3´-CITE of approximately 50 nucleotides that specifically interacts with eIF4E; thus, when critical nucleotide or amino acid substitutions are introduced in either partner, the interaction does not occur, abolishing translation of viral mRNAs and, consequently, virus multiplication. Virulent MNSV isolates harbor different 3´-CITEs that confer the virus the ability to translate its mRNAs in the presence of the resistance eIF4E allele or even in eIF4E absence. Interestingly, both virulent isolates have acquired their new 3´-CITEs by recombination, suggesting that 3´-CITEs are modular, interchangeable RNA elements with important adaptive significance.</div>

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