Phloem loading through plasmodesmata: a biophysical analysis

TL;DR

This paper develops biophysical models to analyze passive and active symplastic phloem loading via plasmodesmata, predicting how oligomerization and segregation influence sugar transport efficiency and plant export rates.

Contribution

It introduces an integrated model combining transport kinetics and plasmodesmata physics to elucidate mechanisms and advantages of symplastic phloem loading.

Findings

Oligomerization reduces leaf sugar content needed for export.

Segregation and inverted sugar gradients are feasible with realistic parameters.

Higher export rates are possible if polymers diffuse back into mesophyll.

Abstract

In many species, sucrose en route out of the leaf migrates from photosynthetically active mesophyll cells into the phloem down its concentration gradient via plasmodesmata, i.e., symplastically. In some of these plants the process is entirely passive, but in others phloem sucrose is actively converted into larger sugars, raffinose and stachyose, and segregated (trapped), thus raising total phloem sugar concentration to a level higher than in the mesophyll. Questions remain regarding the mechanisms and selective advantages conferred by both of these symplastic loading processes. Here we present an integrated model - including local and global transport and the kinetics of oligomerization - for passive and active symplastic loading. We also propose a physical model of transport through the plasmodesmata. With these models, we predict that: 1) relative to passive loading, oligomerization of sucrose in the phloem, even in the absence of segregation, lowers the sugar content in the leaf required to achieve a given export rate and accelerates export for a given concentration of sucrose in the mesophyll; and 2) segregation of oligomers and the inverted gradient of total sugar content can be achieved for physiologically reasonable parameter values, but even higher export rates can be accessed in scenarios in which polymers are allowed to diffuse back into the mesophyll. We discuss these predictions in relation to further studies aimed at the clarification of loading mechanisms, fitness of active and passive symplastic loading, and potential targets for engineering improved rates of export.