Potato (Solanum tuberosum L.) is one of the world's most economically important food crops and holds major significance for future food security. Global potato production has increased steadily from 267 million tonnes in 1990 to 370 in 2019. Despite the importance of potato as a global crop there are still many gaps in knowledge concerning the complex processes involved in potato development from initial growth in the field to long term potato storage.
Aspects of the potato tuber life cycle have major impacts on yield and on how the crop can be utilised. The earliness of tuberisation dictates the time to crop maturity and so this trait is a crucial factor in both the agronomy and economic yield of potato. Varieties that reach maturity more quickly are essential where growing seasons are short due to climatic conditions and disease pressure. Control of the potato life cycle also impacts on tuber storage, a critical trait in achieving year-round availability for both the fresh and processing food sectors.
In the UK alone ca.4 million tonnes of potato tubers are stored annually, however a major problem is the accumulation of reducing sugars (which influence the appearance of potatoes when fried) in the tuber during cold storage as well as tuber premature dormancy release (sprouting). These physiological processes are accompanied by significant deterioration in tuber quality and pose several challenges for postharvest management. Our research has addressed these key questions in potato tuber development and quality to provide improvements in potato production.
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To address critical issues in potato production, an improved mechanistic understanding of tuber development and quality is required.
Potato tubers undergo a period of endodormancy (plant determined non-growing phase) after maturation during which the tuber will not sprout. This inherent feature of tuber biology is one of the key determinants of tuber postharvest storage life. The length of tuber dormancy, and sprout growth rate, differs among potato cultivars.
Tuber dormancy is important economically as it dictates, to a very large extent, how long varieties can be stored before processing or for sale in fresh markets. Premature dormancy release in tubers during storage is accompanied by significant deterioration in product quality. The potato processing industry currently controls sprouting by using chemical sprout suppressants and/or by storing tubers at low temperatures. However, in the EU, authorisation of products containing the most effective synthetic chemical inhibitor Chlorpropham (CIPC) for controlling sprouting was withdrawn from January 2020. Low temperatures do suppress tuber sprouting in storage, but this also results in an undesirable accumulation of reducing sugars, which in turn increases both acrylamide forming potential and the incidence of browning in processed products cooked at high temperature. Additionally, the consequential increase in energy use associated with low temperature storage has adverse impacts on the environment and, furthermore, may be prohibitively expensive in developing countries that are increasingii potato production.
Besides the need to develop new storage strategies for the potato, breeding potato cultivars with extended, or otherwise modified dormancy or sprout vigour’ is desirable. Although there have been several genetic studies on tuber dormancy there has been virtually no progress in identifying the genes that exert genetic control of potato tuber dormancy and sprouting, partly due to the poor resolution of genetic maps that were used in research performed prior to the availability of modern marker technology platforms. The development of new tools is based on improved knowledge of the potato genome through research in previous funded Strategic Research Programmes.
Therefore, the aim of this current investigation was to dissect the genetic architecture of tuber sprout elongation using genetic tools and potato populations we’d developed previously. A further aim was to investigate in detail the key genes (revealed by genetic analysis) particularly with respect to their role in tuber development. Finally, we also wished to investigate the molecular basis of a phenomenon known as ‘senescent sweetening’ that results in reducing sugar accumulation in tubers during extended storage. The latter process is distinct from low temperature reducing sugar accumulation and despite senescent sweetening being a major problem in some commercial varieties, has received little attention from researchers.
We have performed a detailed genetic analysis of tuber sprout growth in storage using a potato population developed at the James Hutton Institute. A genetic and genomic approach was developed that enabled the genetic regions that control tuber sprout growth in storage to be determined. This analysis revealed that genetic factors contributing to the control of sprouting were present on five of the potato’s twelve chromosomes. Most genetic effects were consistent across two growing seasons. Overall, our findings reveal a very complex genetic architecture for tuber sprouting and sprout growth which has implications both for the potato as well as other root, bulb, and tuber crops where long-term storage is a significant factor.
Additionally, we characterised a candidate gene associated with one of the genetic regions that had the largest effect on tuber sprout growth. In other species, similar genes have been shown to have roles in organ growth and development. In order to confirm the role of the candidate genes we developed transgenic tester lines with either increased or decreased expression levels of the gene in question. This approach demonstrated that the gene impacted on the rate of tuber sprouting providing strong evidence that this was the causative gene underlying the sprout growth genetic locus. Additionally, the gene was also associated with different levels of the plant hormones, abscisic acid and cytokinins, providing insights into how these plant growth factors are important for potato sprouting in storage.
Figure: demonstration of the early tuber initiation in the transgenic tester line (left) compared with non-transgenic control.
In the same set of transgenic tester lines, we observed a large effect not only on tuber sprouting in storage but also on tuber initiation, the start of tuber growth. Our detailed analysis unraveled how this gene acts at the molecular level and adds significantly to our knowledge regarding how potato tubers are formed. Importantly, it also reveals that naturally occurring genetic variation in this gene impacts on tuber initiation and development.
Previous studies have identified that stress, due to reduced levels of oxygen, is associated with the accumulation of reducing sugars in potato tubers over long-term storage. Interestingly, in a comparative analysis of varieties susceptible and resistant to sugar accumulation, we found no evidence for a correlation between this stress and the accumulation of reducing sugars. However, we did observe that large number of genes associated with sugar and starch metabolism were deactivated at the onset of senescent sweetening, a finding not reported previously. Some of these changes give important insights into what happens in a potato tuber during senescent sweetening. Importantly the changes were observed in many different potato varieties and identify how subtle changes in carbohydrate metabolism can bring about the undesirable process of senescent sweetening.
This research provides important new insights into the complex processes of tuber initiation and sprouting in potato, crucial factors in both the agronomy and economic yield of potato.
Control of the potato life-cycle impacts on tuber storage necessary to ensure year-round availability, for both fresh and processing sectors. Crucial in reducing food waste as in the UK alone, ca.4 million tonnes of potato tubers are stored annually.
Our research identified the mechanism underpinning senescent sweetening in potato tubers, a problem of commercial importance that has received relatively little research prior to our study.
In the longer-term, genetic markers will support breeding efforts to develop potato varieties with improved storage and tuberization characteristics. This will underpin the both the economic sustainability of production and likely make an important contribution to global food security.
Natural Resources Institute, University of Greenwich, UK
Aalborg University, Denmark
Potato field trials were conducted at the James Hutton Institute’s experimental farm, Balruddery.
Co-funding was provided by the BBSRC Biotechnology and Biological Sciences Research Council (2016) Horticulture and Potato Initiative (HAPI) (grant no BB/K020889/1), GCRF Foundation Awards for Global Agricultural and Food Systems Research funded by the BBSRC project BB/P022553/1 and the Agriculture and Horticulture Development Board (grant number 1100020). The research leveraged amounts to over £2 million and has supported collaborations nationally and internationally with both the academic and commercial sectors.
- A member of the TERMINAL FLOWER 1/CENTRORADIALIS gene family controls sprout growth in potato tubers
- TERMINAL FLOWER‐1/CENTRORADIALIS inhibits tuberisation via protein interaction with the tuberigen activation complex
- Senescent sweetening in potato (Solanum tuberosum) tubers is associated with a reduction in plastidial glucose-6-phosphate/phosphate translocator transcripts
- Combining Conventional QTL Analysis and Whole-Exome Capture-based Bulk-Segregant Analysis Provides New Genetic Insights into Tuber Sprout Elongation and Dormancy Release in a Diploid Potato Population