Sugar loading is not required for phloem sap flow in maize plants

Area of Science:

• Plant Biology
• Physiology
• Biochemistry

Background:

• Phloem transport of photoassimilates is vital for plant growth and yield.
• The pressure-flow mechanism, driven by sugar loading into the phloem, is the prevailing theory for phloem transport.
• The necessity of sugar import as the sole driving force for phloem sap flow remains to be fully elucidated.

Purpose of the Study:

• To investigate whether sugar import into the phloem is essential for driving phloem sap flow.
• To explore alternative mechanisms that might maintain phloem transport in the absence of significant sugar loading.

Main Methods:

• Utilized a maize sucrose transporter1 (sut1) loss-of-function mutant.
• Employed carbon-11 radiotracer techniques to measure carbon export from leaves.
• Analyzed phloem pressure and sap flow speeds.
• Measured potassium (K+) abundance in the phloem.
• Applied fluid dynamic modeling.

Main Results:

• The sut1 mutant exhibited severely reduced carbon export (~4% of wild type).
• Despite reduced sugar export, the mutant maintained near-wild-type phloem pressure (~100%) and significant sap flow speeds (50-75% of wild type).
• Elevated potassium (K+) abundance in the phloem of sut1 mutant leaves was observed.

Conclusions:

• Increased K+ loading can compensate for decreased sucrose loading, maintaining phloem pressure and transport via the pressure-flow mechanism.
• Phloem sap flow and transport of other nutrients/signals may be regulated independently of sugar loading.
• This suggests a more flexible regulation of carbon-nutrient homeostasis and signaling molecule distribution under varying environmental conditions.