Plant Cuticles Are Polyelectrolytes with Isoelectric Points around Three

Area of Science:

• Plant Biology
• Biochemistry
• Materials Science

Background:

• Plant cuticles act as a protective barrier, regulating water and solute transport.
• Understanding the surface charge properties of cuticles is essential for plant physiology and agricultural applications.

Purpose of the Study:

• To determine the isoelectric points (pI) of isolated plant cuticles.
• To characterize the charge and ion permselectivity of cuticular membranes at different pH values.
• To identify the sources of fixed charges within the cuticular structure.

Main Methods:

• Isoelectric points determined using membrane potential measurements.
• Ion permselectivity assessed using radioactive tracers (82Br- for anions, 24Na+ for cations).
• Exchange capacity quantified through ion binding studies (45Ca2+ for cations).

Main Results:

• Isoelectric points for Citrus aurantium, Prunus armeniaca, and Pyrus communis leaves were 3.15, 3.45, and 2.90, respectively.
• Cuticles showed net positive charge and anion permselectivity below pI, and net negative charge and cation permselectivity above pI.
• Absence of asymmetry potentials indicated no fixed charge gradients. Basic amino acid residues contribute positive charges, while acidic amino acids, polygalacturonic acid, and cutin contribute negative charges.

Conclusions:

• Plant cuticles possess fixed charges derived from amino acids and polymers, dictating their pH-dependent surface charge and ion selectivity.
• The determined isoelectric points and charge characteristics are fundamental to understanding cuticle function in ion and water transport.
• Exchange capacities for basic and acidic groups were quantified, providing insights into the cuticle's chemical composition and barrier properties.