Relationship between invertase gene expression and sucrose concentration in the tuberous roots of sweet potato (Ipomoea batatas L. Lam.) during cold storage

Low sweetness in freshly-harvested tuberous roots is an undesirable characteristic of the sweet potato (Ipomoea batatas L. Lam.) cultivar, 'Kokei 14'. In the present study, we sought to characterise the biochemical and molecular aspects of sucrose accumulation in the tuberous roots of 'Kokei 14' during cold storage in comparison with another sweet potato cultivar, 'Beniazuma'. The tuberous roots of both cultivars were harvested and stored at 4 degrees C for 30 d. The sucrose, fructose, and glucose concentrations, the activities of two sugar-metabolising enzymes, and the expression of six sucrose-cleaving acid invertase (AI) genes were monitored. Exposure to chilling temperatures (4 C) for up to 30 d increased the sucrose concentration in the tuberous roots of 'Kokei 14', but not in those of 'Beniazuma'. In line with this phenomenon, we found that the activity of AT was suppressed to approx. 0.4-times the pre-treatment value in 'Kokei 14', but increased to 2.7-times the pre-treatment value in Beniazuma' during cold storage for 30 d. The patterns and levels of expression of six genes encoding AIs indicated that the protein encoded by IbAINV1 was the main AI that degraded sucrose in the tuberous roots of both cultivars. IbAINV1 gene expression increased transiently and reached a maximum (2.3-times the pre-treatment value) at 20 d in Beniazuma', whereas it decreased to approx. 0.14-times the pre-treatment value by 10 d in 'Kokei 14' and remained low for the duration of the experiment. Sucrose concentrations did not change in the tuberous roots of 'Kokei 14' stored at 13 C for 20 d. Therefore, we conclude that the accumulation of sucrose in the tuberous roots of 'Kokei 14' was caused mainly by suppression of IbAINV1 gene expression in response to chilling.