Quantification of the BRI1 receptor in planta

Esse, G.W. van; Westphal, A.H.; SURENDRAN, R.P.; Albrecht, C.; Veen, M.B. van; Borst, J.W.; Vries, S.C. de


In plants, green fluorescent protein (GFP) is routinely used to determine the subcellular location of fusion proteins. Here, we show that confocal imaging can be employed to approximate the number of GFP-labeled protein molecules present in living Arabidopsis (Arabidopsis thaliana) root cells. The technique involves calibration with soluble GFP to provide a usable protein concentration range within the confocal volume of the microscope. As a proof of principle, we quantified the Brassinosteroid Insensitive1 (BRI1) receptor fused to GFP, under control of its own promoter. The number of BRI1-GFP molecules per root epidermal cell ranges from 22,000 in the meristem and 130,000 in the elongation zone to 80,000 in the maturation zone, indicating that up to 6-fold differences in BRI1 receptor content exist. In contrast, when taking into account differences in cell size, BRI1-GFP receptor density in the plasma membrane is kept constant at 12 receptors µm-2 in all cells throughout the meristem and elongation zone. Only the quiescent center and columella cells deviate from this pattern and have 5 to 6 receptors µm-2. Remarkably, root cell sensitivity toward brassinosteroids appears to coincide with uniform meristem receptor density