Homologs of vacuolar invertase inhibitor INH2 in tuber-bearing wild potato species and Solanum tuberosum: gene polymorphism and co-expression with saccharolytic enzyme genes in response to cold stress

Wild tuber-bearing species are commonly used as a source of valuable traits to develop new potato cultivars. One of the trends in potato breeding is elimination of cold-induced sweetening, which depends on the balance between activities of saccharolytic enzymes (vacuolar invertase PAIN-1 and sucrose synthase SUS4 that hydrolyze sucrose) and vacuolar invertase inhibitor INH2 that regulates the PAIN-1 activity. To clarify the mechanisms underlying cold resistance, we investigated INH2 polymorphism and tissue-specific expression, and co-expression profiles of INH2, PAIN-1, and SUS4 in wild tuber-bearing potatoes and Solarium tuberosum cultivars. Novel INH2 homologs were identified in eight wild species and four cultivars. Structural analysis predicted an N-terminal signal peptide and showed that nucleotide and amino acid polymorphisms (13.09% and 18.91%, respectively) did not affect the INH2 tertiary structure, suggesting that the INH2 function may be conserved in potato both at the intra- and inter-specific levels. INH2 and PAIN-1 mRNA were detected in buds, flowers, leaves, stems, roots, and tubers of S. tuberosum cv. Nadezhda, showing the highest (INH2) and lowest (PAIN-1) levels in the roots. During cold storage, tubers of four cultivars accumulated glucose and fructose, while the sucrose content changed insignificantly; INH2, PAIN-1, and SUS4 co-expression patterns in freshly harvested and cold-stored tubers differed among the cultivars. Cold exposure also caused distinct effects on INH2, PAIN-1, and SUS4 co-expression patterns in the leaves of four wild species differing in cold resistance. These data indicate that cold response in potato may depend on specific combinations of INH2, PAIN-1, and SUS4 activities.