Cold-induced sweetening in potato (Solanum tuberosum L.): genetic analysis of the apoplastic invertase inhibitor gene

Potatoes are increasingly consumed in the form of processed foodstuffs such as French fries and crisps. After harvest, potatoes need to be kept at low temperature to prevent sprouting during storage for round-the-year processing. However, tubers accumulate reducing sugars at low temperatures, a phenomenon referred to as cold-induced sweetening (CIS). The processing of these high sugar potatoes into crisps or fries leads to a dark brown to black product that renders them unfit for human consumption and causes a great loss to the processing industry. To prevent sprouting and diseases, chemical treatments have been applied to tubers in storage. The recent withdrawal of these chemicals has increased the reliance on cold storage for potato tubers and highlighted the importance of CIS. Extensive research is required to produce a cultivar resistant to cold induced sweetening (CIS) along with good processing quality. The present work focused on increasing the understanding of the biological processes (physiological, biochemical and molecular) contributing to the initiation and/or controlling of CIS in potato tubers. The genetic basis of this trait was examined in progeny from a cross between potato cultivars with poor and excellent CIS response. This included molecular markers for candidate genes of known position on potato chromosomes to assess the role of different alleles involved in carbohydrate metabolism. Association studies between marker alleles and the phenotype of the progeny were performed. Among all the candidate genes, allele diversity for apoplastic invertase inhibitor gene was further studied from resistant and susceptible potato cultivars. In total five alleles were identified. Polymorphism was observed in both exon and intron regions. Three alleles had a unique substitution at the predicted junction of the signal peptide and mature protein. In order to identify the specific alleles that may play a role in resistance to CIS, transgenic potato plants have been produced with overexpression and antisense repression of apoplastic invertase inhibitor alleles. The results revealed that there were no consistent differences in CIS traits among the transgenic lines with the various alleles. However, the results of the transcript analysis showed much higher and stable transcripts levels of the apoplastic invertase inhibitor in the 1021/1 derived transgenic lines. This greater accumulation or stability of the apoplastic invertase inhibitor transcripts in 1021/1 may be a key factor contributing to the CIS trait of this cultivar. A key difference from previous studies involves the use of 1021/1, a potato cultivar known to have the very high resistance to CIS. This study provided to help identify key genes for the future genetic improvement of tuber properties with respect to long-term storage and processing characteristics. The development of a potato that does not sweeten in the cold will revolutionize the potato industry as this problem currently contributes 20 percent losses after crop harvest. Once identified, clones resistance to CIS could be used in potato breeding programmes for the development of cold-resistant processing cultivars. Eliminating the need for chemical application of sprout inhibitors will be helpful to develop sustainable approaches to benefit both mankind and potato industries. These biotechnological tools will be used to identify elite potatoes with improved properties of the tuber with respect to long-term storage and processing characteristics (cold-sweetening and after-cooking darkening).