Passive phloem loading and long-distance transport in a synthetic tree-on-a-chip

Abstract

Vascular plants rely on differences of osmotic pressure to export sugars from regions of synthesis (mature leaves) to sugar sinks (roots, fruits). In this process, known as Münch pressure flow, the loading of sugars from photosynthetic cells to the export conduit (the phloem) is crucial, as it sets the pressure head necessary to power long-distance transport. Whereas most herbaceous plants use active mechanisms to increase phloem concentration above that of the photosynthetic cells, in most tree species, for which transport distances are largest, loading seems to occur via passive symplastic diffusion from the mesophyll to the phloem. Here, we use a synthetic microfluidic model of a passive loader to explore the nonlinear dynamics that arise during export and determine the ability of passive loading to drive long-distance transport. We first demonstrate that in our device, phloem concentration is set by the balance between the resistances to diffusive loading from the source and convective export through the phloem. Convection-limited export corresponds to classical models of Münch transport, where phloem concentration is close to that of the source; in contrast, diffusion-limited export leads to small phloem concentrations and weak scaling of flow rates with the hydraulic resistance. We then show that the effective regime of convection-limited export is predominant in plants with large transport resistances and low xylem pressures. Moreover, hydrostatic pressures developed in our synthetic passive loader can reach botanically relevant values as high as 10 bars. We conclude that passive loading is sufficient to drive long-distance transport in large plants, and that trees are well suited to take full advantage of passive phloem loading strategies.

Phloem Loading through Plasmodesmata: A Biophysical Analysis

“…Here, we identify the characteristic magnitude of these rates and normalize the global advection of sugars within the transport phloem by the diffusive component of transport through plasmodesmata. We call the resulting nondimensional ratio the flushing number (f) and show, in the following sections and in a separate study (Comtet et al, 2017), that it provides a useful parameterization of the predicted behavior, even when we consider both advective and diffusive transport inside the plasmodesmata (see Supplemental Information S2).…”

“…For small values of f, loading is convection limited and sugar concentration in the MV-phloem is high, favoring gradient inversion. This number is relevant for both passive and active symplastic loaders (Comtet et al, 2017).…”

“…Insights into these transport processes may also suggest ways to design efficient synthetic systems to control chemical processes (Stroock et al, 2014;Comtet et al, 2017).…”

Designing a bioinspired synthetic tree by unidirectional freezing for simultaneous solar steam generation and salt collection

“…A natural vascular plant transports tons of water from soil to air per day using a specialized phloem network powered by sunlight. This continuous water transport is efficiently achieved with reduced pressure in the leaf because of water evaporation 28,29 . Herein, a free‐standing PVA‐based bioinspired synthetic tree with T‐shaped asymmetric structure was designed with a unidirectional freezing method.…”

“…This continuous water transport is efficiently achieved with reduced pressure in the leaf because of water evaporation. 28,29 Herein, a free-standing PVA-based bioinspired synthetic tree with T-shaped asymmetric structure was designed with a unidirectional freezing method. The PVA matrix has excellent mechanical strength, and can support a 200 g weights ( Figure S1).…”