Control of seed size and yield by vernalisation

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A major problem with UK winter oilseed rape (WOSR) cultivation is the large inter-annual fluctuation in on-farm yields, varying from around 3.0 to 3.9 tonnes per hectare. Weather and climate are known to be critical factors in this yield instability, and climate change may be a factor in the current 'yield plateau' in UK agriculture. For this reason breeding and selection of new varieties suited to changing environments is widely appreciated to be a key long-term goal for UK agriculture. Targeting this effort requires an understanding of the precise biological mechanisms through which weather variation influences yield on farms. It is here that this proposal breaks new ground.In the UK and Northern Europe high yielding so-called 'winter' crops are distinguished from spring crops by their requirement for a period of prolonged winter cold before flowering, a process known as vernalisation. While the effect of vernalisation on flowering has been known for many years, here we show for the first time that vernalisation has a further major role in the control of seed size. We show that seeds from more strongly vernalised plants are around 30% larger than seeds from more weakly vernalised plants, and this size increase comes without any penalty in fruit or seed number. These effects are large enough to account for a substantial fraction of the 30% inter-annual variation in yield observed on farms in the UK. In addition, we provide new evidence that winter cold during a very precise 23-day period is highly correlated to UK rapeseed yields, suggesting that on a landscape scale vernalisation may be important for WOSR crop yield performance.The molecular mechanism underlying the vernalisation process is understood in detail. A key transcription factor gene, FLOWERING LOCUS C (FLC), is highly expressed before winter and is gradually silenced during the experience of cold weather by the accumulation of repressive chromatin marks. FLC expression in leaves and the shoot apical meristem controls flowering, but FLC is also strongly expressed in seeds where it has previously been implicated in seed vigour. Seed size on the other hand is controlled primarily by the expansion of the seed coat that occurs after fertilisation, creating the space into which the embryo and endosperm grow. Our preliminary study shows that in rapeseed both FLC and a second gene AUXIN RESPONSE FACTOR 2 (ARF2), a known seed size regulator in Arabidopsis, can both be genetically linked to the control of seed size by vernalisation. Furthermore, online genomic scale datasets show that ARF2 is likely a direct target of FLC.In this proposal we will elucidate the mechanism by which vernalisation controls seed size and examine the importance of this process in on-farm yield variation in WOSR. Our preliminary analysis has revealed a very strong candidate for this mechanism and our experimental programme is designed to investigate this in detail. Using the JIC experimental farm we will test whether winter cold affects WOSR yield by affecting seed size, and how the key molecular events of vernalisation and seed development unfold in a real field scenario. By taking lines carrying genetic variation at FLC and ARF2 loci through to maturity we will directly test whether temperature in early December affects rapeseed yield parameters through a mechanism involving these two genes. One goal is to identify new combinations of of FLC and ARF2 that maintain large seed size despite only weak vernalisation. Markers for these alleles can then be used by breeders to select new varieties with a more robust yield performance, ameliorating an important negative effect of warmer winters on UK agricultural performance.