Genetic and molecular approaches to valorise protein and fibre in potato

On a global scale, potato is an important crop for both consumption and the starch industry. At present, the starch industry is keen to create economic value and to reduce its impact on the environment by valorising all resources in potatoes. One way to contribute towards this goal is to develop new potato varieties with elite traits, i.e. a high level of tuber protein content and a reduced level of fibre hydration. Improving these traits is however challenging due to their complex nature. Therefore, a better understanding of the genetics and biological processes underlying tuber protein content and fibre hydration are relevant. The objectives of this study were to shed light on the genetic and molecular architectures of tuber protein content and fibre hydration and to pinpoint key factors (i.e. biological processes, molecular structures, genes and alleles) that are involved in modulation of these traits. Chapter 1 provides a description of the potato crop and the starch industry. We present protein and fibre as key resources, where improvement of their quantity and quality will lead to a better valorisation of the potato crop. The principles of potato genetics, potato breeding and modern tools for trait improvement are described. In Chapter 2 we studied the genetics of tuber protein content in a panel of tetraploid potato varieties. We estimated a moderate level of trait heritability, identified marker-trait associations (QTLs), haplotypes and candidate genes. Our findings showed that alleles of StCDF1 were associated with tuber protein content. The results provide resources for genomics-enabled breeding. In Chapter 3 we performed a multi-allelic QTL analysis of tuber protein content in a large bi-parental population of tetraploid potato. We estimated a moderate level of trait heritability and identified QTLs. The alleles underlying the QTLs provided both positive and negative effects on the level of tuber protein content. Our results showed that tuber protein content is a complex trait in potato. In Chapter 4 we studied the overexpression effect of a putative nitrate transporter gene (StNPF1.11) on tuber protein content in potato. Overexpression of StNPF1.11 increased tuber protein content, leaf chlorophyll content and plant height of young potato plants. A pleiotropic effect on tuber dry matter content (a proxy for starch content), suggests that the nitrogen status may affect tuber starch accumulation in potato in vivo. In Chapter 5 we studied the role of pectic rhamnogalacturonan (RG-I) galactan side-chains on the water-binding capacity (WBC) of potato cell walls. Both in-vivo and in-vitro truncation of RG-I β-(1→4)-D-galactan side-chains altered the WBC, but with contrasting effects. Our results reinforce the view that RG-I galactan side-chains play a role in modulating the WBC of potato cell walls. In Chapter 6 the insights that were generated in the experimental chapters are evaluated and discussed in a broader context. Finally, implications and prospects for future research are presented