The enigma of dual reproduction in potato : casting light on tuberization and flowering

Potato plants reproduce sexually through the formation of flowers, berries and seeds, and asexually through the formation of tubers. Environmental conditions can be used as a flexible switch to control reproduction in potato, steering it towards flowering when seeds are required for breeding or propagation, or towards tuberization when tubers are required for propagation or potato production. Light is a convenient environmental switch, as spectrum, photoperiod, light intensity and the daily light integral (DLI) can be manipulated. In this thesis the effect of light on tuberization and flowering time was quantified and the underlying molecular regulation was explored. Finally it was determined if tuberization and flowering compete and if so, how this is regulated. Tuberization is a short-day process. In long days, tuberization is delayed or even inhibited (depending on the genotype). In this thesis it was determined whether an external coincidence model can explain the photoperiodic effect on tuberization in potato. In the model plant Arabidopsis, photoperiodic flowering is controlled by coincidence of light and CONSTANS (CO) expression. In potato, a similar model was hypothesized to explain photoperiodic tuberization. Coincidence of the potato CO (StCOL1) expression and light in long days would induce StSP5G, which in turn would lead to the inhibition of StSP6A, the gene encoding for the tuberization signal StSP6A. Thus coincidence of StCOL1 and light is expected to repress tuberization. By using night breaks (30 minutes of light applied in the night) that coincided with StCOL1 expression in the night or not, it was demonstrated that coincidence between light and StCOL1 expression does not necessarily repress tuberization. In addition, it was shown that although flower bud appearance time is not affected by photoperiod, the number of leaves formed before the inflorescence in the genotype S. andigena decreases under shorter photoperiods, indicating an effect of photoperiod on potato flowering as well as tuberization. Not only day-length, but also light spectrum was found to affect the tuberization time. By applying far-red light throughout a long day, tuberization time was accelerated. However, far-red was not able to induce tuberization in genotypes that could not tuberize in long days. It was also demonstrated that the addition of blue light throughout a short day, delayed tuberization in some genotypes. However, when blue light was used to extend the day length to simulate a long day, tuberization was not as strongly repressed as in treatments where the day length was extended with white light. Neither far-red light, blue light nor day-length extension was found to affect the flower bud appearance time. Alternatively, a high DLI did accelerate flowering time in both short and long days. Under high DLI the number of the days until flower bud appearance was reduced, and less leaves were formed before the inflorescence. No correlation between high DLI-accelerated flowering and plant carbohydrate concentration was observed. Interestingly, it was found that accelerated flowering under high DLIs was controlled independently of the flowering time gene StSP3D. Finally it was tested if potato flowering and tuberization compete. Potato plants formed flower buds in both long and short days, but in short days where tuberization was strongly induced, the flower buds failed to develop. It was demonstrated that removing the tuber sink (by grafting or removing stolons) did not improve the flower development in tuber inducing conditions, while silencing the tuberization signal StSP6A did. It was also demonstrated that increasing StSP6A in long days impaired flower bud development, indicating that the tuberization signal StSP6A represses flower bud development rather than the competition for assimilates. This thesis discusses how potato flowering and tuberization are controlled by light on a molecular level and demonstrates how light can be used as a flexible regulator to manipulate potato reproduction.