a, The customized Clark-type microsensor was used to measure oxygen profiles throughout the shoot apex of one-week-old S. lycopersicum var. Micro-Tom plants, in the apical-to-basal direction. These measurements show the presence of an oxygen gradient in the shoot apex of this plant species. The experiment was repeated twice with similar results. b, Photograph showing the insertion of the micro-electrode inside the tomato SAM. c, d, Overlay of oxygen profiles (c and d shown as cyan and red, respectively, in a) and micrographs of SAM tissues after FM4-64 membrane staining that show the actual penetration of the sensor. The puncture in the centre of the meristem and the concomitant accumulation of FM4-64 shows the penetration of the sensor into the tissue. The experiment was repeated four times with similar results; two examples are shown in c and d. e, Similar to Arabidopsis, the typical hypoxia-marker genes ALCOHOL DEHYDROGENASE 2 (ADH2), pyruvate decarboxylases (PDC1 and PDC3), PLANT CYSTEINE OXIDASE 2 (PCO2) and PHYTOGLOBIN 1 (HB1a) are expressed at a higher level in SAM-enriched tissues than in juvenile leaves of two-week-old plants of S. lycopersicum var. Micro-Tom. PYRUVATE DECARBOXYLASE 4 (PDC4) does not exhibit the same pattern. These results indicate that SAM cells experience hypoxic conditions. RT–qPCR was used to measure the expression of hypoxia-inducible genes (two-sided t-test; n = 4 pools of 5 plants).