Genetic strategies for dissecting complex traits in biomass willows (Salix spp.)

Willows are highly diverse catkin-bearing trees and shrubs of the genus Salix. They occur in many growth forms, from tall trees to creeping alpines, and successfully occupy a wide variety of ecological niches. Shrubby willows (sub-genus Vetrix) have many characteristics that render them suited to cultivation in much faster growth cycles than conventional forestry. They respond well to coppicing, can be propagated vegetatively as cuttings and achieve rapid growth with low fertilizer inputs. As a result, willows grown as short rotation coppice are now among the leading commercially grown biomass crops in temperate regions. However, although willows have a long history of cultivation for traditional uses, their industrial use is relatively recent and, compared with major arable crops, they are largely undomesticated. Breeding programmes initiated to improve willow as a biomass crop achieved a doubling of yields within a period of <15 years. These advances were made by selecting for stem characteristics (height and diameter) and coppicing response (shoot number and shoot vigour), as well as resistance to pests, diseases and environmental stress, with little or no knowledge of the genetic basis of these traits. Genetics and genomics, combined with extensive phenotyping, have substantially improved our understanding of the basis of biomass traits in willow for more targeted breeding via marker-assisted selection. Here, we present the strategy we have adopted in which a genetic-based approach was used to dissect complex traits into more defined components for molecular breeding and gene discovery.