Elucidating the mechanisms of senescent sweetening in stored potato tubers to improve storage regimes and identify candidate genes

Storage for processing is typically undertaken at relatively high temperatures (> 8^oC) in the presence of sprout suppressors to prevent low-temperature sweetening.  However, tubers can undergo the distinct physiological process of senescent sweetening after prolonged storage (> 5 months) leading to significant losses. Unlike cold-induced sweetening, senescent sweetening is irreversible and therefore stored crops need to be monitored and utilised prior to sugar accumulation. Furthermore, while mechanisms of low-temperature sweetening are well understood the biological causes of senescent sweetening remain unclear.

The objectives of the project were  to adopt biochemical and molecular approaches to elucidate the mechanisms of senescent sweetening and to identify potential genes underlying the sweetening trait. Such knowledge was intended to assist store managers in the prediction of senescent sweetening and to provide breeders with candidate genes for the development of markers to accelerate the production of senescent sweetening resistant cultivars.

Approach

Over the first two years of the project, a senescent sweetening resistant and a sweetening sensitive cultivar were stored for up to 11 months under industry standard conditions. Tubers were regularly sampled and sugar content and fry colour determined to identify the transition to senescent sweetening. Changes in primary metabolites associated with sugar and starch metabolism were determined using gas chromatography/mass spectrometry and changes in the expression of >40,000 genes were determined using microarray technologies. In the final year of the project nine cultivars were stored and tested for sugar accumulation and fry colour. Changes in expression of key genes identified in years 1 and 2 were determined using RT-PCR.

 Results

While the precise date of sugar accumulation varied year-to-year, indicating the influence of the seasonal growing environment, the resistant cultivar did not exhibit sugar accumulation in any year while the susceptible cultivar did exhibit sugar accumulation following a period of storage in every year indicating a strong genetic influence. With the exception of differences in sugar accumulation, very few other primary metabolites exhibited consistent significant differences between cultivars. Similarly, changes in gene expression over the storage period were similar between cultivars. However, the susceptible cultivar exhibited a marked decline in expression of a plastid localised glucose phosphate translocator (GPT2) and genes associated with starch synthesis at the onset of sugar accumulation. Starch is only synthesised in the plastid and the substrate for starch synthesis, glucose-6-phosphate is imported from the cytosol by GPT2. As the potato tuber goes through futile cycles of starch degradation and resynthesis, these data suggest that senescent sweetening is associated with a reduction in starch resynthesis caused by a reduced capacity to transport substrate back into the plastid that leads to sugar accumulation in the cytosol. Monitoring of GPT2 expression in a broader range of cultivars in year 3 suggests that the mechanism is common across a broad range of germplasm.