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.

Obtaining good accuracy and reliability of estimated breeding values is essential to increase the efficiency of a plant breeding program. Genetic variation was assessed for categorical (Virc) and binary (Virb) mosaic (caused by Panicum mosaic virus), dry matter (DMY) and predicted ethanol (Etoh) yields, and lignin content (Klason or KL, and acid‐detergent or ADL) in a Summer–Kanlow switchgrass (Panicum virgatum L.) population. Breeding values were predicted with the restricted maximum likelihood–best linear unbiased prediction (REML‐BLUP) approaches using a multivariate phenotypic (PBLUP) and animal (ABLUP) models, integrating a three‐generation pedigree (1,622 half‐sibs) in ABLUP and not in PBLUP. Models were compared in their precision (accuracy and reliability) in assessing genetic parameters and estimating breeding values. The models were similar in most aspects, allocating the highest heritability (hi2) values to DMY (.38 ± .035 vs. .41 ± .035), Etoh (.46 ± .031 vs. .42 ± .033), and Virc (.43 ± .046 vs. .37 ± .047) and the lowest (.17 ± .032 to .30 ± .044) to KL, ADL, and Virb. Genetic correlations were always larger than residual and phenotypic correlations. Intermediate or strong additive genetic control suggest that selecting for high‐biomass genotypes will slightly increase lignin content and simultaneously impart mosaic tolerance. Mitigating an increase in lignin content will require including Etoh in a selection index based on its much stronger negative correlation (rG = −.63) with lignin. In this population, accuracy values ranged from .06 to .94 (PBLUP) and from .26 to .92 (ABLUP) and corresponding reliability ranged from .004 to .89 and from .07 to .87. However, ABLUP improved average reliability of DMY and Etoh by 11% and of other traits by 4–5% over the PBLUP model. The ABLUP was a better model over PBLUP, which is a valid analysis in the absence of a pedigree.

Maintaining low levels of rust incidence (caused by Puccinia novopanici) in switchgrass (Panicum virgatum L.) breeding populations is a priority for the USDA-ARS program engaged in improving cultivars for high biomass yield and quality. Essential to this goal is the unbiased and accurate estimation of genetic parameters to predict the merits of parents and progeny. Spores of the fungus were inoculated in greenhouse-grown seedling progeny of 31 half-sib families in generation 2 (Gen 2) of a composite Summer × Kanlow population for evaluation of rust incidence on the leaves with a 0–9 rating scale. Two parents were later chosen to cross and develop a linkage mapping population as Gen 3. The Gen 2, 3, and Kanlow seedlings were transplanted into the field located near Mead, NE, in early June 2020 and laid out as a replicated row–column design with six blocks of single-row plots of five plants each. The field trial was rated in September 2021 and 2022 with a 0–4 scale. Lab and field data were subjected to univariate linear mixed models via the restricted maximum likelihood to extract the variance components needed to predict the breeding values. The additive genetic variation was substantial (p < 0.01), enough to result in high heritability estimates ranging from 0.42 ± 14 to 0.73 ± 0.09 at the individual and family mean levels. This result implies that rust resistance is under strong genetic control to use mass selection for obtaining satisfactory gains. A possible rust incidence x year interaction was detected with a Spearman correlation of breeding values of −0.38, caused by significant rank changes of the Gen 3 genotypes in 2022 (a high heat and drought year). Genetic gains were predicted to reduce rust incidence scores by at least two points on the rating scale when selecting backwards, and by one point when selecting individual candidates as parents of the next generation. Faster gains (31 and 59%) were realized relative to the second generation by respectively selecting the top 10% of the families in Gen 3 or the top 10% of genotypes within this group. Based on these results, strategies for controlling the incidence of rust will be developed to optimize gains in the other traits of economic importance.

Panicum mosaic virus (PMV), the type species 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 co-infected millets with exacerbated disease phenotype and increased accumulation of PMV, compared to plants infected only by PMV. 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 against PMV. At 24°C, PMV was readily detected in inoculated leaves but not in upper non-inoculated leaves of Kanlow, suggesting that resistance to PMV was likely mediated by abrogation of long-distance virus transport. Co-infection 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 only by PMV. 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 by PMV+SPMV caused an enhanced asymptomatic infection, suggesting a role for SPMV in enhancing symptomless infection in a tolerant cultivar. These data suggest that enhanced asymptomatic infections in virus-tolerant switchgrass cultivar could serve as a source for virus spread and play an important role in panicum mosaic disease epidemiology under field conditions. Our data revealed that cultivars, co-infection with SPMV, and temperature influenced the severity of symptoms elicited by PMV in switchgrass.

Panicum mosaic virus (PMV), the type species 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 co-infected millets with exacerbated disease phenotype and increased accumulation of PMV, compared to plants infected only by PMV. 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 temperaturesensitive resistance against PMV. At 24°C, PMV was readily detected in inoculated leaves but not in upper non-inoculated leaves of Kanlow, suggesting that resistance to PMV was likely mediated by abrogation of long-distance virus transport. Co-infection 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 only by PMV. 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 by PMV+SPMV caused an enhanced asymptomatic infection, suggesting a role for SPMV in enhancing symptomless infection in a tolerant cultivar. These data suggest that enhanced asymptomatic infections in virus-tolerant switchgrass cultivar could serve as a source for virus spread and play an important role in panicum mosaic disease epidemiology under eld conditions. Our data revealed that cultivars, co-infection with SPMV, and temperature in uenced the severity of symptoms elicited by PMV in switchgrass.