Efficiently starting and preferentially filling a potato
Authors:   , van den Herik Bastiaan Mees 
Published: January 2024

In this thesis, a diverse range of models was employed to explore various aspects of source-sink dynamics in potato. The models allowed for an exploration of transport efficiency and sucrose allocation in the vasculature, the unloading potential in the tuber unloading zone, and the competition and allocation processes at the whole plant level.

In chapter 2 of the thesis, we substantiated the physiological relevance of SP6A-mediated SWEET11 blockage for long-distance sucrose transport and its ultimate delivery to tuber sinks. A biophysical transport model was developed that incorporated a single-source and a single-sink. The model revealed that high viscosity in the phloem is the major resistance factor in potatoes. Furthermore, we observed an increase in sucrose delivery to the sinks that surpassed the reduction in sucrose efflux caused by SP6A.This non-linear effect arises from an intricate interplay of sucrose and water dynamics, which recursively feeds back into itself. Recognizing that plants have more intricate architectures than a simple single-source and single-sink, in chapter 3 we extended the model to a single-source and two-sink architecture. We investigated the effects of plant architecture and sink physiology on resource allocation between the two sinks. The model was applied in a case study involving sink and source leaves, as well as tubers. This highlighted that tubers face significant disadvantages compared to sink leaves. Cumulative disadvantages experienced by tubers enabled an undirected, plant level reduction of sucrose efflux through SP6A to preferentially benefit tuber resource partitioning.

We shifted attention to the tuber unloading zone in chapter 4 We employed a combined bioinformatics and modeling approach to improve our understanding of the factors responsible for increased tuber sink strength. We found that tuberization was associated with a decline in callose deposition in tubers due to a decrease in callose synthesis as well as a coordinated change in sucrose metabolism towards starch production. Using a biophysical model of sucrose unloading we demonstrated how this coordinated switch to symplastic unloading with a concurrent metabolic switch enhances the sucrose gradient. Combined, this created the physiological conditions necessary to potentiate symplastic transport and enhance tuber sink strength. In chapter 5, a comprehensive dataset of above- and belowground potato growth was generated, focusing on wildtype (WT) and plants with silencing of florigen (SP3D), tuberigen (SP6A), and sucrose export (SWEET11). Silencing of SP6A resulted in a delayed onset of tuber formation by approximately 2 weeks. However, it also led to a significantly delayed senescence of the canopy by 4 to 5 weeks. As a consequence, tuber growth was prolonged, resulting in mildly increased final tuber fresh weight. This delay in tuber onset also correlated with reduced synchronization in tuber formation and increased variance in tuber sizes.
These results, combined with an ODE growth model, suggest that resource competition between tubers and leaves, plays a crucial role in determining growth dynamics,tuber synchronization and final size of the tubers. Collectively, this thesis provides valuable insights into the roles of SP6A in sucrose allocation within potato plants and have yielded new insights into sucrose allocation processes.