Auxin binding to corn coleoptile membranes: Kinetics and specificity

Area of Science:

• Plant Physiology
• Molecular Biology
• Biochemistry

Background:

• Auxin, a key plant hormone, regulates growth and development.
• Understanding auxin binding sites is crucial for deciphering plant hormone signaling pathways.
• Previous research indicated auxin binding, but detailed characterization of binding sites was limited.

Purpose of the Study:

• To identify and characterize high-affinity binding sites for auxins in Zea mays L. coleoptiles.
• To investigate the specificity of these binding sites for auxin analogues.
• To determine the role of different binding sites in auxin perception.

Main Methods:

• Membrane preparations from Zea mays L. cv Kelvedon 33 coleoptiles were used.
• Binding assays were performed using radiolabeled 1-naphthylacetic acid (NAA) and 3-indolylacetic acid (IAA).
• Detergent-solubilized preparations were utilized to retain binding activity.
• Double reciprocal plots were employed to analyze the interactions of various analogues with binding sites.

Main Results:

• Two distinct high-affinity binding sites for NAA were identified, with dissociation constants (Kd) of 1.8×10⁻⁷ M (Site 1) and 14.5×10⁻⁷ M (Site 2).
• Similar binding characteristics were observed for 3-indolylacetic acid (IAA), indicating comparable site concentrations.
• Site 2 demonstrated specificity for active auxins and antiauxin transport inhibitors, consistent with an auxin receptor.
• Site 1 exhibited lower specificity, binding a wider range of tested compounds, including inactive analogues.

Conclusions:

• Zea mays L. coleoptile membranes contain at least two high-affinity auxin binding sites.
• Site 2 likely represents a functional auxin receptor due to its specific binding profile.
• Site 1 may represent a different type of binding protein or a less specific interaction site for auxins.