Factors sustaining tissue hypoxia in the shoot apical meristem

The study of stress-dependent hypoxia has led to breakthroughs in the development of new crop varieties that are resistant to unfavorable environmental conditions such as flooding and waterlogging. However, a chronic hypoxic niche enclosing the shoot apical meristem (SAM) is present under non-stressful conditions and is necessary to maintain vital activity. In our research, we aim to visualize the internal oxygen distribution in the SAM and investigate how a hypoxic microenvironment may be passively generated or actively maintained in such a localized region of the plant. Oxygen profiling of the shoot apical meristem (SAM) using a Clark-type oxygen microsensor revealed a steep gradient, suggesting a potential diffusion limitation for oxygen reaching the SAM core. Here I set out to investigate three proposed factors contributing to hypoxia maintenance in SAM: (1) the presence of a putative diffusion barrier that inhibits oxygen uptake, (2) dense cell organization that restricts oxygen movement, and (3) high metabolic activity necessary for organ formation. Our findings show that a cuticle-like layer covers the SAM and leaf primordia, and disruption of this layer leads to elevated oxygen levels and a weakened hypoxia response. Additionally, the SAM revealed a compact structure compared with the surrounding tissues, which may further restrict oxygen diffusion. Metabolic activity also plays a key role, as metabolic arrest results in a higher oxygen state within the SAM. To monitor changes in oxygen levels, we developed a genetically encoded fluorescent oxygen biosensor that provides a clear and bright response to fluctuations in oxygen availability in plant tissues. This tool offers new opportunities for studying oxygen dynamics in the SAM and other plant tissues. This research unraveled how plants maintain chronic hypoxia but it also provides a genetically encoded oxygen biosensor to visualize internal oxygen levels at high spatial resolution for use by the scientific community.