Sugarcane is a unique plant because it accumulates carbohydrates in the form of sucrose in its stem. The sucrose is extracted and used in the food industry or fermented to produce alcohol. Sugarcane is the most produced crop worldwide – more than 1,800 million metric tons per year.
A few factors influence the amount and rate of sucrose accumulation such as environmental conditions, agricultural management, and biochemistry.
“Previous models of phloem flow have usually approximated the sieve tube as a cylinder with loading of solute at the one end and unloading at the other,” according to corresponding author Prof. Johann Rohwer.
The new version combined (1) loading and unloading along the entire length of the sieve tube and (2) a coupled reaction network where metabolites could be involved in enzyme-catalyzed reactions, transported between compartments and/or be carried over long distances in a fluid medium.
The first step in building the model was to describe the geometry. The sugarcane stalk was divided into a number of finite volumes to include nodes and internodes. On each of these volumes, a number of compartments were defined: source/leaves (only at the nodes), phloem, cytosol of the storage parenchyma (symplast), and vacuole.
Sucrose transport between the compartments was described using advection-diffusion-reaction (ADR) behavior modelled with partial differential equations (PDEs). This allowed molecule movement to be described as advection (with solutes carried by the bulk flow, which is generated by an osmotic pressure gradient) and/or diffusion (due to a concentration gradient). Detailed kinetic modelling of biochemical reactions describes the interconversion of metabolites in the various compartments.
Despite the simplicity of the updated model, several experimentally observed features of plant metabolism could be reproduced. The model showed sucrose accumulation in the vacuoles of stalk parenchyma cells, and was moreover able to demonstrate the up-regulation of photosynthesis in response to a change in sink demand.
This rigorous, quantitative framework that can form the basis for future modelling and experimental design.
The code, data, and instructions for setting up the computational environment are available as supplementary material.