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

Area of Science:

• Plant Physiology
• Biophysics

Background:

• Vascular plants transport sugars via Münch pressure flow, driven by osmotic pressure differences.
• Phloem loading, the transfer of sugars to the phloem, is critical for initiating this flow.
• While herbaceous plants use active loading, trees often rely on passive symplastic diffusion, despite long transport distances.

Purpose of the Study:

• To investigate the dynamics of passive phloem loading using a synthetic microfluidic model.
• To determine the capacity of passive loading to power long-distance sugar transport in plants.
• To explore the conditions under which passive loading is effective, particularly in tree species.

Main Methods:

• Development of a synthetic microfluidic device modeling passive phloem loading.
• Analysis of the balance between diffusive loading resistance and convective phloem export.
• Simulation of Münch transport dynamics under varying hydraulic resistances and xylem pressures.

Main Results:

• Phloem concentration is determined by the interplay of diffusive loading and convective export resistances.
• Passive loading can generate hydrostatic pressures up to 10 bars, relevant for plant transport.
• Convection-limited export, prevalent in plants with high resistance and low xylem pressure, supports efficient passive loading.

Conclusions:

• Passive phloem loading is sufficient to drive long-distance sugar transport in large plants.
• Trees, with their extensive transport systems, are well-adapted to benefit from passive loading strategies.
• The study validates the role of passive loading in plant physiology and long-distance transport mechanisms.