Breeding Willow for Short Rotation Coppice Energy Cropping - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 76

Modern commercial scale willow breeding has developed very much in line with
the latest biotechnology in support of plant breeding. Molecular genetics studies of
willow began in the 1990s in Sweden, the UK, and the United States. The first
European mapping family (K3) was created in 1999 by Sval ¨ f Weibull. Subse-
quently, the larger K1 and K8 families, which comprise around 1,000 progeny from
full sibling crosses in a S. viminalis and S. schwerinii background, were developed.
Initially created to study the inheritance of rust resistance, the K8 family, in
particular, has been the source of many QTLs acting as markers for a number of
valuable traits. Rothamsted Research has continued the work started at Long
Ashton Research Station (closed in 2002), and there are now three copies of K8
planted in differing environments plus a further 12 families of 240-550 progeny
created since at Rothamsted [ 12 ].
The Swedes also studied families of their own creation, and in 2009 Sval¨f
Weibull, SLU at Uppsala, and Rothamsted Research joined forces to create an
Association Mapping Population consisting of S. viminalis accessions from across
Europe. This population avoids some of the limitation associated with biparental
linkage mapping and thus gives the power to seek to identify QTL through two
primary routes.
Using these resources, QTLs have been mapped for a large number of traits in
willow including rust resistance [ 18 - 20 ]; insect resistance [ 21 ]; shoot height, stem
diameter, and stem number [ 12 , 22 ]; frost tolerance; phenology [ 19 , 23 ]; water-use
efficiency and drought tolerance [ 24 , 25 ]; and saccharification potential [ 26 ].
Initially, molecular genetics in willow relied upon transfer of information from
the closely related genus Populus , for which a full genome sequence has been
available since 2006. This was facilitated by direct alignment of the K8 genetic map
[ 27 ] to the poplar genome [ 28 ] and provided an efficient route to identifying the
genes residing in the QTL identified from the willow mapping families. More
recently Rothamsted Research has begun sequencing an accession of S. viminalis.
In the United States, a similar effort is underway using a Salix purpurea of interest
to their breeding program, and in China a genome sequence has been developed for
S. sachalinensis .
At the time of writing, there is no working transformation system for willow.
Research effort continues at Rothamsted and other institutions. Some confirmation
of genes identified in the willow genetics research has been achieved by using
willow sequences to rescue Arabidopsis mutants [ 29 ]. Transformation systems for
poplar are routinely deployed and may be used for willow genetics research in the
near future.
Seed Production
Willow is dioecious and therefore an obligate outcrossing genus. Uniformity of
cultivars is achieved by vegetative reproduction of the F 1 generation. The majority
of species readily produce roots from lenticels on the stem when placed in dark