a, Oxygen concentration profiles obtained inserting the Clark-type microsensor in the apical-to-basal direction through the inflorescence meristem of 5-week-old Arabidopsis plants. The inset shows a photo of the actual experimental setup with the insertion of the microsensor inside the inflorescence meristem. The experiment was repeated independently three times with similar results. b, Green fluorescent signal in inflorescence and floral meristems of plants transformed with the hypoxia reporters pPCO1:GFP-GUS and pPCO2:GFP-GUS that were characterized as specifically hypoxia responsive in a previous publication24. The experiment was repeated twice with similar results.