a, Comparison of spatial resolution and accuracy of the custom-made Clark-type microsensor used here (red, orange and pink triangles) alongside the commercially available sensor OX10, Unisense A/S (turquoise, cyan and cobalt-blue circles). Oxygen-consuming solid medium was prepared by sodium ascorbate oxidation. The dataset shows that, in 2% agar with a steep gradient in dissolved oxygen from atmospheric equilibrium to anoxia, the two sensors show identical responses. Five independent profiles were recorded for each type of microsensor. b, Depiction of the experimental setup adopted to measure the oxygen concentration profiles in the apical-to-basal direction (left) and radial direction (right). c, Oxygen concentration profile in the apex of a four-day-old Arabidopsis seedling. The experiment was repeated twice with similar results. d, Tracking of the sensor insertion pattern reconstructed after the oxygen profile in c was taken. The SAM was visualized by confocal microscopy after FM4-64 staining of plasma membranes. e, Oxygen concentration profile obtained by inserting the Clark-type microsensor laterally through the SAM of seven-day-old Arabidopsis plants in the shoot apical region (profile 1), at the junction of cotyledon vasculature (profile 2) and below the junction (profile 3). The experiment was repeated three times with similar results. A photograph of the experimental setup is shown within the plot frame (bottom right inset). f, Position of the sensor insertion points as identified by optical microscopy (top) and localization of the CLAVATA3 (CLV3) expression domain, reported by GUS staining of plants that express pCLV3:GUS in four-day-old plants (bottom).