Cold-triggered induction of ROS- and raffinose metabolism in freezing-sensitive taproot tissue of sugar beet
Sugar beet (Beta vulgaris subsp. vulgaris) is the exclusive source of sugar in the form of sucrose in temperate climate zones. There, sugar beet is grown as an annual crop from spring to autumn because of the damaging effect of freezing temperatures to taproot tissue. Natural and breeded varieties display variance in the degree of tolerance to freezing temperatures and genotypes with elevated tolerance to freezing have been isolated. Here we compare initial responses to frost between genotypes with either low and high winter survival rates. The selected genotypes differed in the severity of frost injury. We combined transcriptomic and metabolite analyses of leaf- and taproot tissues from such genotypes to elucidate mechanisms of the early freezing response and to dissect genotype- and tissue-dependent responses. Freezing temperatures induced drastic downregulation of photosynthesis-related genes in leaves but upregulation of genes related to minor carbohydrate metabolism, particularly of genes involved in raffinose metabolism in both, leaf and taproot tissue. In agreement with this, it has been revealed that raffinose and the corresponding intermediates, inositol and galactinol, increased markedly in these tissues. We found that genotypes with improved tolerance to freezing, showed higher accumulation of raffinose in a defined interior region within the upper part of the taproot, the pith, representing the tissue most susceptible to freeze damages. This accumulation was accompanied by specific upregulation of raffinose synthesizing enzymes in taproots, suggesting a protective role for raffinose and its precursors for freezing damage in sugar beet.