Viruses have to encapsidate their own genomes during the assembly process. For most RNA viruses, there are sequences within the viral RNA and virion proteins needed for high efficiency of genome encapsidation. However, the roles of host proteins in this process are not understood. Here we find that the cellular DEAD-box RNA helicase DDX6 is required for efficient genome packaging of foamy virus, a spumaretrovirus. After infection, a significant amount of DDX6, normally concentrated in P bodies and stress granules, re-localizes to the pericentriolar site where viral RNAs and Gag capsid proteins are concentrated and capsids are assembled. Knockdown of DDX6 by siRNA leads to a decreased level of viral nucleic acids in extracellular particles, although viral protein expression, capsid assembly and release, and accumulation of viral RNA and Gag protein at the assembly site are little affected. DDX6 does not interact stably with Gag proteins nor is it incorporated into particles. However, we find that the ATPase/helicase motif of DDX6 is essential for viral replication. This suggests that the ATP hydrolysis and/or the RNA unwinding activities of DDX6 function in moderating the viral RNA conformation and/or viral RNA-Gag ribonucleoprotein complex in a transient manner to facilitate incorporation of the viral RNA into particles. These results reveal a unique role for a highly conserved cellular protein of RNA metabolism in specifically re-locating to the site of viral assembly for its function as a catalyst in retroviral RNA packaging.

Fusarium wilt (FW), caused by Fusarium oxysporum f. sp. vasinfectum (Atk.) W.C. Snyder & H.N. Hans (FOV), is one of the most destructive diseases in cotton (Gossypium spp.) worldwide. FOV race 4 (FOV4) is a highly virulent nominal race of this pathogen and is a significant threat to cotton production in the western and southwestern US and potentially the entire Cotton Belt. A field survey in three southern counties of New Mexico was conducted in 619 cotton fields in 2018-2020 to identify FOV4. From 132 samples of cotton plants that exhibited wilt symptoms, Fusarium spp. were the most frequently isolated group of fungal species with an isolation frequency of 57.4%. Eighty-four Fusarium spp. isolates were subsequently characterized by a DNA sequence analysis of three genes, EF-1α, PHO and BT encoding for translation elongation factor, phosphate permease and β-tubulin, respectively. Forty-two isolates were identified to be FOV4 from 10 cotton fields and confirmed with a positive 500 bp fragment that is diagnostic for FOV4. Twenty-six (62%) of the 42 FOV4 isolates were T type and the remainder (38%) were Null type with and without a Tfo1 insertion in PHO, respectively; and each FOV4-infested field contained the same FOV4 genotype. Ten representative FOV4 isolates with one each from the 10 FOV4-infested fields were evaluated for their pathogenicity on resistant Pima PHY 841 RF and susceptible Upland PHY 725 RF at 7, 14, 21, and 28 days post inoculation under temperature-controlled conditions at 21-22°C. Based on disease severity rating (DSR), mortality rate (MR) and the area under the disease progress curve (AUDPC) value, all the 10 isolates were pathogenic to both cotton cultivars and differed in virulence in that four isolates of the T genotype as a whole were more virulent than the six isolates of the N genotype. PHY 841 RF had significantly higher levels of resistance than PHY 725 RF to all FOV4 isolates. The results provide the first comprehensive account of the occurrence, distribution and virulence of FOV4 in cotton production in New Mexico and will be useful in developing an effective strategy to manage FW in both the state of New Mexico and entire southwestern Cotton Belt.

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Sclerotinia sclerotiorum infects a broad range of plant hosts, and is typically identified by the production of white mycelium and black sclerotia on infected plants and culture. Isolates of S. sclerotiorum with darklypigmented mycelium have been reported on Valencia peanut in New Mexico and Texas. This study was conducted to determine the relationship between dark pigmentation in mycelium, oxalic acid production, and pathogenicity of S. sclerotiorum on Valencia peanut. A darkly-pigmented (SD) and a mutant, non-pigmented (SW) isolate of S. sclerotiorum were compared for vegetative growth on various growth media under different environmental conditions, and for pathogenicity on peanut. The SD isolate became darker in mycelium pigmentation as temperature increased beyond 20°C and at pH between 5.0 and 6.0. Pigmentation in the SW isolate remained unaffected. Oxalic acid production did not increase in either SD or SW isolates when grown on media amended with pure oxalic acid or its precursor arabinose. The SD isolate caused peanut plant death and necrotic lesions on leaflets, while the SW isolate did not cause any symptoms. The differential behavior of SD and SW isolates in pathogenicity does not appear to be associated with mycelium pigmentation.Key words: Arachis hypogaea, Sclerotinia sclerotiorum, melanin-inhibitors, oxalic acid, mycelium pigmentation.Résumé : Le Sclerotinia sclerotiorum attaque une gamme d'hôtes très large, et il est typiquement identifié par la production de mycéliens blancs et sclérotes noirs sur milieux de culture solide. L'occurrence d'isolats de S. sclerotiorum avec des mycéliens de pigmentation sombre a été reportée sur l'arachide de type Valencia au Nouveau-Mexique et au Texas. La présente étude a été conduite pour établir le rapport entre la pigmentation des mycéliens, la production de l'acide oxalique, et la pathogénicité de S. sclerotiorum sur l'arachide de type Valencia. Un isolat avec mycéliens de pigmentation sombre (SD) a été comparé avec un variant avec mycéliens blancs (SW) en terme de la croissance mycélienne sur milieux de culture solide sous conditions variées, et pathogénicité sur l'arachide. La pigmentation mycélienne chez l'isolat SD s'intensifia avec la température audelà de 20°C et avec le pH entre 5.0 et 6.0. Par contre, aucun changement en pigmentation n'a été enregistré chez l'isolat SW. La production en acide oxalique demeura invariante pour les deux isolats sur milieux de cultures solides amendés avec l'acide oxalique pur ou avec l'arabinose. L'isolat SD a causé la mort des plantes d'arachide et des lésions nécrotiques sur les feuillètes, alors que l'isolat SW n'a causé aucun symptômes. Le comportement différentiel entre les deux isolats, au niveau de la pathogénicité, ne semble pas être relaté à la pigmentation mycélienne.