Quantification of the brassinosteroid insensitive1 receptor in planta

Area of Science:

• Plant molecular biology
• Cellular imaging and quantification
• Plant hormone signaling

Background:

• Green fluorescent protein (GFP) is a standard tool for visualizing protein localization in plant cells.
• Accurate quantification of protein levels in living plant cells is crucial for understanding cellular processes.
• The Brassinosteroid Insensitive1 (BRI1) receptor plays a key role in brassinosteroid signaling in plants.

Purpose of the Study:

• To establish a confocal imaging-based method for approximating the number of GFP-labeled protein molecules in living Arabidopsis root cells.
• To quantify the abundance and distribution of the BRI1 receptor-GFP fusion protein in different root cell types.
• To correlate BRI1 receptor density with root cell sensitivity to brassinosteroids.

Main Methods:

• Developed a quantitative confocal microscopy technique calibrated with soluble GFP standards.
• Applied the method to measure BRI1-GFP fusion protein levels in various Arabidopsis root epidermal cells.
• Calculated BRI1-GFP molecular counts and plasma membrane receptor densities.

Main Results:

• Quantified BRI1-GFP molecules per cell, revealing significant differences (22,000–130,000) across root zones (meristem, elongation, maturation).
• Determined that plasma membrane BRI1-GFP receptor density is largely uniform (12 receptors/μm²) in meristem and elongation zones, with exceptions in quiescent center and columella cells (5–6 receptors/μm²).
• Observed a correlation between uniform meristem receptor density and brassinosteroid sensitivity.

Conclusions:

• The developed confocal imaging method provides a reliable approach for quantifying protein molecules in living plant cells.
• BRI1 receptor levels vary significantly between Arabidopsis root cell types, but plasma membrane density is remarkably conserved in key growth zones.
• Uniform receptor density in the meristem may be critical for consistent brassinosteroid signaling and root development.