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Downloaded from orbit.dtu.dk on: Oct 04, 2021 Microfluidics of sugar transport in plant leaves and in biomimetic devices Rademaker, Hanna Publication date: 2016 Document Version Publisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA): Rademaker, H. (2016). Microfluidics of sugar transport in plant leaves and in biomimetic devices. Department of Physics, Technical University of Denmark. General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Ph.D. thesis Microfluidics of sugar transport in plant leaves and in biomimetic devices Hanna Rademaker 14 September 2016 Supervised by Tomas Bohr and Kaare Hartvig Jensen Cover image: Light microscopy image of a Coleus blumei leaf. The image shows the natural color. Microfluidics of sugar transport in plant leaves and in biomimetic devices Copyright ➞ 2016 Hanna Rademaker. All rights reserved. Typeset using LATEX and TikZ. Abstract The physical mechanisms underlying vital plant functions constitute a research field with many important, unsolved problems. Some of these research topics gaining at- tention in recent years are concerned with the fluid transport in plants. Plants photosynthesize sugars in their leaves for energy production and growth. These sugars are taken up into the veins of the leaf and then transported efficiently to other parts of the plant via the vascular system. The vascular system of plants has two types of tissue: the xylem brings water from the roots up to the leaves, and the phloem transports sugars from the sources in the leaves to the sinks in roots, fruits and regions of growth. The transport of sugar solution in the phloem is driven by an osmotic pumping mechanism. The uptake of water from the xylem into the phloem generates a hydrostatic pressure difference between sources and sinks which results in a bulk flow of sugar solution. It is not clear where in the leaf this bulk flow starts, especially in plants that have intercellular connections (plasmodesmata) between the phloem and the cells surrounding the veins. In these plants bulk flow could be involved in the process of sugar loading into the veins. We studied the physics of two basic mechanisms for sugar loading: the polymer trap and passive loading. The polymer trap is an active mechanism, which is characterized by an elevated concentration of sugars inside the veins compared to the rest of the leaf. This is achieved with the help of enzymes combining sucrose molecules entering the vein through plasmodesmata into larger sugar molecules. These molecules are then too large to move back out of the vein. In order for this system to work, the plasmodesmata have to act as extremely precise filters. Microscopy studies show that these plasmodesmata are very small, in fact they are too small to resolve their exact cross section available to transport. In our theoretical model, we approximated the plasmodesmata as cylindrical slit pores and investigated whether the pores could be small enough to fulfill the filtering function, and at the same time large enough to allow for sufficient transport of sucrose. We found that this mechanism is indeed feasible. We could further conclude that sugar is not only transported by diffusion, but is partly advected through the plasmodesmata by a bulk flow. This bulk flow is actually enough to drive the export from the leaf, meaning that no additional water has to be taken up into the phloem in order to drive the flow. In plants that use passive loading instead, the concentration of sugars inside the veins is lower than in the surrounding tissue, and the plasmodesmata connecting the phloem with the cells surrounding the veins are larger than in the polymer trap case. We suspected, that in passive loading the advective transport contribution to sugar loading could be even more important than in the polymer trap. We demonstrated advective loading of sugars in experiments with biomimetic devices, modeling the leaf as a system of three compartments: phloem, xylem and sugar producing tissue. We further developed a theoretical model of passive loading, enabling us to identify the key parameters that determine the sugar uptake into the phloem. Assuming values typically found in plants for the three key parameters sugar concentration, interface areas between the three compartments and pore size of the plasmodesmata, the uptake i of sugar can be dominated by either advection or diffusion. Of these key parameters, the pore size has the largest influence on the ratio of advective to diffusive loading. The next step in the transport of sugars is the export from the leaf. We studied the case of conifer needles, which are linear leaves with unbranched venation. Most conifer leaves are not longer than 6cm, which is rather short compared to broad leaves with sizes spanning from millimeters to meters. In order to understand this limitation we modeled the phloem conduits in linear leaves as cylindrical, osmotic pipes running from the tip to the base of the needle. Using a simple analytical model we calculated the sugar export rate from these conduits assuming a constant concentration of sugars along the pipe. We found that in needles longer than a characteristic length the fluid close to the tip becomes stagnant and sugars can no longer be exported efficiently. This means, that very little output can be gained from making a leaf longer than the efficient leaf length. Our prediction for an efficient leaf length matches well with the mean needle lengths from a data set comprising 519 of the 629 currently known conifer species. We further calculated the energy dissipated by the export of sugar solution from linear osmotic pipes. There are two main contributions to the dissipation of energy, one due to the resistance of membrane pores and one due to Poiseuille resistance inside the pipe. We found simple and general analytical solutions for flow rates and dissipation of energy for single pipes, generalizing the normal Poiseuille expression and showing that the driving force is not only the pressure, but the “water potential”, which is a combination of concentration and pressure. We also treated a system of coupled parallel pipes with a power law distribution of lengths, as found in linear leaves. The results for the system of coupled pipes are surprisingly similar to the single pipe solutions, and likewise show the emergence of the stagnant zone for leaves longer than the effective length. ii Resum´e(Danish) De fysiske mekanismer, som ligger til grund for de vitale plantefunktioner, er et forskn- ingsomr˚ade med mange vigtige uløste problemer. Nogle af disse problemer, som har f˚aet større opmærksomhed i de senere ˚ar, knytter sig til væsketransport i planter. Planter producerer sukker i deres blade ved hjælp af fotosyntese. Sukkeret, som bruges som energi- og vækstkilde optages i bladets ledningsstrenge, som s˚atrans- porterer dem effektivt til andre dele af planten via karsystemet. Planternes karsystem har to typer af væv: xylemet, der bringer vand fra rødderne op til bladene, og phloemet, der transporterer sukker fra kilderne i bladene til drænene i rødder, frugter og vækst- omr˚ader. Transporten af sukkeropløsningen i phloemet drives af en osmotisk pumpe- mekanisme: Optagelsen af vand fra xylemet ind i phloemet frembringer en hydrostatisk trykforskel mellem kilder og dræn, som medfører en strømning af sukkeropløsning. Det er ikke forst˚aet, hvor i bladet strømningen starter, især i de planter, der har intercellulære forbindelser (plasmodesmata) mellem phloemet og cellerne omkring led- ningsstrengene. I disse planter er strømningen sandsynligvis involveret i optagelsen (loading) af sukker ind i ledningsstrengene. Vi har undersøgt fysikken bag to grundlæggende mekanismer for loading af sukker: polymerfælden og passiv loading. Polymerfælden er en aktiv mekanisme, der er karak- teriseret ved en forøget koncentration af sukker i ledningsstrengene i forhold til resten af bladet. Dette opn˚as ved hjælp af enzymer, der sammensætter sukrosemolekyler, som kommer ind i ledningsstrengen gennem plasmodesmata, til større sukkermolekyler. Molekylerne er derefter for store til at komme ud af ledningsstrengen igen, de er fanget i en “fælde”. For at systemet kan virke, skal plasmodesmata fungere som ekstremt præcise filtre. Mikroskopiske undersøgelser viser, at disse plasmodesmata er meget sm˚a. De er faktisk s˚asm˚a, at det ikke har været muligt nøjagtigt at bestemme det tværsnitsareal, som kan bruges til transporten. Vi har modelleret plasmodesmata som cylindriske porer, blokeret i midten af en uigennemtrængelig cylinder. Vi un- dersøgte derefter med vores teoretiske model, om porerne kunne være sm˚anok til at udføre filtreringsfunktionen, og samtidig store nok til at tillade tilstrækkelig trans- port af sukrose. Vi fandt at mekanismen faktisk er gennemførlig. Derudover kunne vi konkludere at sukker, udover diffusion, transporteres ved hjælp af advektion gen- nem plasmodesmata. Strømningen er faktisk tilstrækkelig til at drive eksporten ud af bladet, som betyder, at der ikke behøves optagelse af ekstra vand i phloemet for at drive strømningen. I planter, der bruger passiv loading, er koncentrationen af sukker i ledningsstren- gene lavere end i det omliggende væv og plasmodesmata, som forbinder phloemet med cellerne omkring ledningsstrengene, er større end i planter, der anvender polymer- fælden.

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