Effect of cultivar and temperature on the synergistic interaction between panicum mosaic virus and satellite panicum mosaic virus in switchgrass.

Panicum mosaic virus (PMV), the type member of the genus Panicovirus in the family Tombusviridae, naturally infects switchgrass (Panicum virgatum L.). PMV and its molecular partner, satellite panicum mosaic virus (SPMV), interact synergistically in coinfected millets to exacerbate the disease phenotype and increase the accumulation of PMV compared to plants infected with PMV alone. In this study, we examined the reaction of switchgrass cvs. Summer and Kanlow to PMV and PMV+SPMV infections at 24°C and 32°C. Switchgrass cv. Summer was susceptible to PMV at both temperatures. In contrast, cv. Kanlow was tolerant to PMV at 24°C, but not at 32°C, suggesting that Kanlow harbors temperature-sensitive resistance to PMV. At 24°C, PMV was readily detected in inoculated leaves, but not in upper uninoculated leaves of Kanlow, suggesting that resistance to PMV was likely mediated by abrogation of long-distance virus transport. Coinfection by PMV and SPMV at 24°C and 32°C in cv. Summer, but not in Kanlow, caused increased symptomatic systemic infection and mild disease synergism with slightly increased PMV accumulation compared to plants infected with PMV alone. These data suggest that the interaction between PMV and SPMV in switchgrass is cultivar-dependent, manifested in Summer but not in Kanlow. However, co-inoculation of cv. Kanlow with PMV+SPMV caused an enhanced asymptomatic infection, suggesting a role of SPMV in enhancement of symptomless infection in a tolerant cultivar. These data suggest that enhanced asymptomatic infections in a virus-tolerant switchgrass cultivar could serve as a source of virus spread and play an important role in panicum mosaic disease epidemiology under field conditions. Our data reveal that the cultivar, coinfection with SPMV, and temperature influence the severity of symptoms elicited by PMV in switchgrass.

Transcriptome divergence during leaf development in two contrasting switchgrass (Panicum virgatum L.) cultivars.

The genetics and responses to biotic stressors of tetraploid switchgrass (Panicum virgatum L.) lowland cultivar 'Kanlow' and upland cultivar Summer are distinct and can be exploited for trait improvement. In general, there is a paucity of data on the basal differences in transcription across tissue developmental times for switchgrass cultivars. Here, the changes in basal and temporal expression of genes related to leaf functions were evaluated for greenhouse grown 'Kanlow', and 'Summer' plants. Three biological replicates of the 4th leaf pooled from 15 plants per replicate were harvested at regular intervals beginning from leaf emergence through senescence. Increases and decreases in leaf chlorophyll and N content were similar for both cultivars. Likewise, multidimensional scaling (MDS) analysis indicated both cultivar-independent and cultivar-specific gene expression. Cultivar-independent genes and gene-networks included those associated with leaf function, such as growth/senescence, carbon/nitrogen assimilation, photosynthesis, chlorophyll biosynthesis, and chlorophyll degradation. However, many genes encoding nucleotide-binding leucine rich repeat (NB-LRRs) proteins and wall-bound kinases associated with detecting and responding to environmental signals were differentially expressed. Several of these belonged to unique cultivar-specific gene co-expression networks. Analysis of genomic resequencing data provided several examples of NB-LRRs genes that were not expressed and/or apparently absent in the genomes of Summer plants. It is plausible that cultivar (ecotype)-specific genes and gene-networks could be one of the drivers for the documented differences in responses to leaf-borne pathogens between these two cultivars. Incorporating broad resistance to plant pathogens in elite switchgrass germplasm could improve sustainability of biomass production under low-input conditions.