Studies model the local influence of phloem resistance on taproot and lateral root growth.

An efficient root system has an architecture that effectively takes up water and nutrients from the soil and transports them to the shoot. Root architecture is controlled by external aspects like soil water gradients and internal aspects like the quantity of carbohydrates supplied to each individual root tip.

Several key factors influence the amount of carbohydrates that reach the roots. These include the amount of carbohydrates being created and used, and how easily the carbohydrates can move through the plant. The ease of carbohydrate movement, which is measured as resistance, is regulated by the diameter of the phloem sieve tubes.

Experimentally determining the independent impacts of these factors on root growth poses a challenge. However, a recent publication in in silico Plants demonstrated how computational modelling can be used to quantify the impact of phloem resistance on root growth.

Research associate Xiao-Ran Zhou, from the Institute of Bio- and Geosciences at Forschungszentrum Jülich and colleagues used a combination of computational plant models (CPlantBox and PiafMunch) to help understand experimental findings about how local phloem anatomical features influence the root system architecture.

In their study, the authors conducted simulations manipulating the phloem resistance in the roots and examined how it affected the local distribution of carbohydrates within the roots. They specifically investigated the impact on both the primary taproot and the smaller lateral roots, which extend horizontally or diagonally from the taproot, analyzing them separately.

In one simulation, the researchers manipulated the taproot sieve-tube diameter in three plants, setting it at 50%, 100%, or 200%. In another simulation, they uniformly set the sieve-tube diameter of the lateral roots in three plants at 50%, 100%, or 150%. Their findings indicated that as the phloem resistivity of a root increased, both the primary and lateral roots experienced a reduction in carbohydrate availability, leading to a decrease in root growth.

To investigate the impact of phloem radius changes on root growth within the entire root system or locally, the researchers conducted a simulation of three plants having lateral roots with heterogeneous taproot sieve-tube diameters of 50%, 100%, and 150%. Their findings revealed that alterations in taproot sieve-tube resistivity affected each individual root locally, without influencing neighboring roots.

When mixing roots with different phloem resistivities within a single root system the final length is influenced by these resistivities alone, and not the resistivities of the neighbouring roots.

These experiments illustrate the direct correlation between phloem anatomy and the limitation of carbon flow as a sink-limiting factor within the plant. When the resistivity along the pathway of carbohydrate flow becomes excessively high, it leads to a decrease in carbohydrate flow, thereby impacting the growth of individual roots.

Zhou concludes, “these results provide a first example where the features of individual roots within the root system, such as their growth rate and final length, are not predetermined but rather arise as emergent properties of the system. This discovery underscores the significance of integrating more flexible representations of the root system into computational models.”

READ THE ARTICLE:

Xiao-Ran Zhou, Andrea Schnepf, Jan Vanderborght, Daniel Leitner, Harry Vereecken, Guillaume Lobet, Phloem anatomy restricts root system architecture development: theoretical clues from in silico experiments, in silico Plants, Volume 5, Issue 2, 2023, diad012, https://doi.org/10.1093/insilicoplants/diad012

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