Comparative transcriptome analyses between cultivated and wild grapes reveal conservation of expressed genes but extensive rewiring of co-expression networks.

KEY MESSAGE: The transcriptomes of wild and cultivated grapes consists of similar expressed genes but distinct wiring of co-expressed modules associated with environmental conditions. Grapevine is an important fruit crop worldwide, with high economic value and widespread distribution. Commercial production is based on Vitis vinifera, and, to a lesser extent, on hybrids with American grapes, such as V. labrusca. Wild grape relatives are important sources of resistance against biotic and abiotic factors; however, their global gene expression patterns remain poorly characterized. We associated genome-wide transcript profiling to phenotypic analyses to investigate the responses of cultivated and wild vines to vineyard conditions. The expressed genes in the Vitis reference transcriptome are largely shared by wild grapes, V. labrusca hybrids and vinifera cultivars. In contrast, significant differential regulation between wild and vinifera genotypes represents 80% of gene expression variation, regardless of the environment. In wild grapes, genes associated to regulatory processes are downregulated, whereas those involved in metabolic pathways are upregulated, in comparison to vinifera. Photosynthesis-related ontologies are overrepresented in the induced genes, in agreement with higher contents of chlorophyll in wild grapes. Co-regulated gene network analyses provide evidence of more complex transcriptome organization in vinifera. In wild grapes, genes involved in signaling pathways of stress-related hormones are overrepresented in modules associated with the environment. Consensus network analyses revealed high preservation within co-regulated gene modules between cultivated and wild grapes, but divergent relationships among the expression clusters. In conclusion, the distinct phenotypes of wild and cultivated grapes are underlain by differences in gene expression, but also by distinct higher-order organization of the transcriptome and contrasting association of co-expressed gene clusters with the environment.

Expression of disease resistance in genetically modified grapevines correlates with the contents of viral sequences in the T-DNA and global genome methylation.

Increased tolerance to pathogens is an important goal in conventional and biotechnology-assisted grapevine breeding programs worldwide. Fungal and viral pathogens cause direct losses in berry production, but also affect the quality of the final products. Precision breeding strategies allow the introduction of resistance characters in elite cultivars, although the factors determining the plant's overall performance are not fully characterized. Grapevine plants expressing defense proteins, from fungal or plant origins, or of the coat protein gene of grapevine leafroll-associated virus 3 (GLRaV-3) were generated by Agrobacterium-mediated transformation of somatic embryos and shoot apical meristems. The responses of the transformed lines to pathogen challenges were investigated by biochemical, phytopathological and molecular methods. The expression of a Metarhizium anisopliae chitinase gene delayed pathogenesis and disease progression against the necrotrophic pathogen Botrytis cinerea. Modified lines expressing a Solanum nigrum osmotin-like protein also exhibited slower disease progression, but to a smaller extent. Grapevine lines carrying two hairpin-inducing constructs had lower GLRaV-3 titers when challenged by grafting, although disease symptoms and viral multiplication were detected. The levels of global genome methylation were determined for the genetically engineered lines, and correlation analyses demonstrated the association between higher levels of methylated DNA and larger portions of virus-derived sequences. Resistance expression was also negatively correlated with the contents of introduced viral sequences and genome methylation, indicating that the effectiveness of resistance strategies employing sequences of viral origin is subject to epigenetic regulation in grapevine.

Systemic transport of Alfalfa mosaic virus can be mediated by the movement proteins of several viruses assigned to five genera of the 30K family.

We previously showed that the movement protein (MP) gene of Alfalfa mosaic virus (AMV) is functionally exchangeable for the cell-to-cell transport of the corresponding genes of Tobacco mosaic virus (TMV), Brome mosaic virus, Prunus necrotic ringspot virus, Cucumber mosaic virus and Cowpea mosaic virus. We have analysed the capacity of the heterologous MPs to systemically transport the corresponding chimeric AMV genome. All MPs were competent in systemic transport but required the fusion at their C terminus of the coat protein-interacting C-terminal 44 aa (A44) of the AMV MP. Except for the TMV MP, the presence of the hybrid virus in upper leaves correlated with the capacity to move locally. These results suggest that all the MPs assigned to the 30K superfamily should be exchangeable not only for local virus movement but also for systemic transport when the A44 fragment is present.

Genetic diversity of the movement and coat protein genes of South American isolates of Prunus necrotic ringspot virus.

Prunus necrotic ringspot virus (PNRSV) is distributed worldwide, but no molecular data have been previously reported from South American isolates. The nucleotide sequences corresponding to the movement (MP) and coat (CP) proteins of 23 isolates of PNRSV from Chile, Brazil, and Uruguay, and from different Prunus species, have been obtained. Phylogenetic analysis performed with full-length MP and CP sequences from all the PNRSV isolates confirmed the clustering of the isolates into the previously reported PV32-I, PV96-II and PE5-III phylogroups. No association was found between specific sequences and host, geographic origin or symptomatology. Comparative analysis showed that both MP and CP have phylogroup-specific amino acids and all of the motifs previously characterized for both proteins. The study of the distribution of synonymous and nonsynonymous changes along both open reading frames revealed that most amino acid sites are under the effect of negative purifying selection.