Genetic Resources for Developing C4 Perennial Grasses as Dedicated Bioenergy Crops.

Northern-adapted warm-season C₄ perennial grasses are among the most promising candidates for developing sustainable dedicated bioenergy crops for North America. Here, we highlight how genetic resources will play an essential role in increasing biomass yield of miscanthus (Miscanthus spp.), switchgrass (Panicum virgatum) and prairie cordgrass (Spartina pectinata). These three crops share a number of key features. The outcrossing nature of these crops suggests that considerable genetic variation exists within current cultivars, breeding populations and even selected clones. However, little breeding work has been done on these crops, with switchgrass having received the most attention as a forage crop. Given that these crops are in the early stages of development, germplasm resources will likely play an especially important role. Miscanthus is perhaps a poster-child for the critical need to use germplasm resources to improve yield and reduce risk, as the current industry is based on a single sterile triploid clone that was imported from Japan to Denmark in the 1930s. The natural geographic distribution of the germplasm base for these three crops is large, especially from north to south. Flowering time and overwintering ability are natural adaptation-traits that are closely associated with location of origin. If northern-adapted plants are brought south, flowering is usually early, resulting in shorter, lower-yielding plants. If southern-adapted plants are brought north, flowering is delayed and yield may be improved but overwintering ability may be insufficient. Thus, there will be a theme of using germplasm resources to balance the optimization of flowering time for yield with the timing of dormancy for adequate overwintering ability. To the extent that these traits can be decoupled, great advances will be made. The rhizomatous nature of these crops will allow plant breeders to use germplasm resources to select for rhizome lengths that optimize tiller mass and density for yield and persistence. For yield stability, breeders will use germplasm resources to select against the doughnut-hole effect, which is common among perennial grasses. All three crops have a range of ploidies in the germplasm base and it appears that exploiting existing ploidy resources and manipulating ploidy will likely be an effective strategy for increasing yield. Lastly, considerable population differentiation is present in the germplasm base of all three crops, which is an opportunity to exploit heterosis for yield improvement.