Accelerating breeding for biomass yield in short rotation coppice willow by exploiting knowledge of shoot development in Arabidopsis

Over three-quarters of our energy in the UK is derived from burning fossil fuels. However, there are two problems with this practice. Firstly, the associated emission of carbon dioxide and other greenhouse gases is causing global warming and, secondly, UK fossil fuel supplies are being depleted. Renewable sources of energy which are carbon-neutral, (i.e. no net release of carbon as carbon dioxide) are urgently needed. In 1999, renewable energy represented ~3% of total electricity generated in the UK. The government has set challenging targets for this to rise to 60% by 2050 and it recognised that the contributions from all renewable energy sources will need to be significantly boosted, including wind, solar and hydro-power and energy from growing 'biomass crops'. Biomass crops are perennial, fast growing species that are able rapidly to accumulate combustible material (e.g. stems and wood). The biomass is harvested and burnt for heat or electricity. Since the carbon dioxide released on combustion is re-incorporated during the following years' growth, they are carbon-neutral and also require few chemical inputs. Willows are among the most advanced biomass crops in temperate regions. They are grown in short-rotation coppice (SRC) cycles, in which planted cuttings are cut back after the first year of growth and the cut stumps (stools) are allowed to re-sprout to provide multiple shoots. These coppice shoots are harvested 3 years later to provide the first biomass harvest and the SRC cycle is continued for a further 20 years. High yields are crucial for SRC willow to be economical and although high yielding varieties are available, a doubling of yield has been proposed in order to produce the biomass required for UK targets. Improving biomass yield is thus a major goal of current UK willow breeding. The coppicing ability of willow is key to biomass production. Willow species differ in their coppicing ability but the genetic control of these differences is not yet known. Moreover, willows with more (thin) stems and willows with few thick stems can both produce high yield. This makes it difficult for breeders to select for high yielding biomass types or predict the outcome of crosses between different willows. Some developmental studies have attempted to understand the nature of these differences and have suggested that they relate to the number, position and outgrowth of pre-existing buds kept dormant by apical dominance. Little is known about the genetic control of these processes in willows, but in the model plant Arabidopsis there is a substantial body of knowledge on the regulation of bud formation and bud activity. There is also clear evidence that these regulatory mechanisms are conserved across higher plants, including trees. In this collaborative project we will take advantage of the advanced knowledge in Arabidopsis, to investigate the genetic regulation of coppicing in willow. In preliminary work we have shown that genes affecting branching in Arabidopsis are present in willow and that these genes co-associate in inheritance with biomass yield. We will now test the hypotheses that they are involved in determining bud behaviour and thus coppicing potential in willow. We will start by focussing on genes controlling branching in Arabidopsis, e.g. the more axillary branching (MAX) family but we will also investigate teosinte-branched (TCP family) a key gene determining branching in the crop plant maize, which has also been investigated in Arabidopsis. We will use our knowledge in Arabidopsis to test the hypotheses that bud number and bud behaviour may be under different genetic control in willow and to help identify other genes that can affect coppicing. Our overall goal will be to build up a model of the genetic regulation of shoot number and shoot outgrowth in coppiced willows and to provide markers for the genes involved, and thus facilitate the selection of improved biomass willows in breeding.

The UK Government is committed to increasing renewable energy and reducing greenhouse gas emissions and the contribution of biomass crops has been recognised. Willows are among the most advanced biomass crops. They are grown as short-rotation coppice (SRC), in which planted cuttings are cut back after one year and the cut stumps (stools) allowed to re-sprout. The coppice shoots are harvested 3 years later and the SRC cycle continued for 20 years. To be economic, sufficiently high yields are essential from only minimal inputs and yield improvement is the main objective of the Defra-funded SRC genetic improvement network BEGIN. Robust yield QTL have been identified but there is little understanding of the genetic control of underlying developmental processes. Diversity occurs among willows in coppicing ability but the genetic basis of this is not known. Willows with more thin stems and with few thick stems can both produce high yield. Yield-related traits of stem diameter and height do not co-associate with the same QTL as shoot number, suggesting these may be under separate control. Limited studies indicate that coppicing ability relates to the number, position and outgrowth of pre-existing buds kept dormant by apical dominance. In Arabidopsis the regulation of bud formation and activity is well investigated and evidence exists for conservation of regulatory mechanisms. Our preliminary research showed that MAX genes, which affect shoot branching in Arabidopsis, and TCP which affects branching in maize, map to willow yield QTL which co-associate with stem diameter and height. Using genetics and developmental studies, this project will test: (i) that MAX & TCP are involved in regulating bud behaviour in coppicing (ii) that bud number and behaviour are under separate control (iii) that, through Arabidopsis, other genes influencing coppicing can be identified and (iv) that the identification of genes for coppicing can accelerate breeding for high biomass yield.