Smart pipes: the bundle sheath role as xylem-mesophyll barrier

Area of Science:

• Plant Physiology
• Molecular Biology
• Plant Pathology

Background:

• Abiotic toxicity symptoms in plants typically manifest at leaf margins.
• The bundle sheath tissue is hypothesized to regulate solute transport, preventing excessive toxic ion entry into leaves.
• This tissue may direct toxic solutes towards hydathodes for exudation.

Purpose of the Study:

• To test the hypothesis that the bundle sheath acts as a selective barrier against toxic ion transport.
• To investigate the role of the bundle sheath in managing toxic boron (B) distribution within Arabidopsis leaves.

Main Methods:

• Examined toxic boron distribution in mutant Arabidopsis with flooded mesophyll.
• Compared boron toxicity symptoms in mutants versus control plants.
• Observed guttation drops from control leaves to assess solute exudation.

Main Results:

• Control plants exhibited classical boron toxicity symptoms, with necrosis starting at leaf margins.
• Mutant plants, unlike controls, showed necrosis initiating within the leaf interior.
• This differential symptomology suggests altered toxic solute transport in mutants.

Conclusions:

• The bundle sheath functions as a selective barrier, regulating radial transport of toxic solutes from xylem to leaf tissues.
• This barrier actively directs toxic ions, such as boron, towards hydathodes, mitigating damage to the mesophyll.
• The findings support the proposed mechanism of bundle sheath-mediated protection against abiotic toxicity.