Rice seedlings reveal a 24-hour anatomical switch that controls oxygen leakage into flooded soils

Rice is one of the few staple crops that can germinate and establish itself in waterlogged, oxygen-starved soils, and the secret to that resilience lies in how its young roots manage oxygen. A new study published in Plant and Soil by Shotaro Tamaru, Nanami Fujiwara, Hinako Shiba, Tomonori Tsunoda, Ronnie N. Glud, Katsuhiro Shiono and colleagues has now mapped, hour by hour and millimeter by millimeter, exactly when and where rice seedlings begin leaking oxygen into their surroundings during the critical first days of seedling establishment. The findings reveal a striking developmental program: the ability of a root to oxygenate the soil around it is dictated not by light or photosynthesis, but by the rapid construction of internal air channels and external sealing barriers within roughly a day of root emergence.

The question the team set out to answer is deceptively simple but agronomically vital. When rice seeds germinate underwater, the emerging seedling must quickly produce roots that can function in an anaerobic medium where oxygen diffusion is slow and microbial demand for oxygen is intense. Successful seedling establishment under these conditions is crucial for subsequent growth and yield formation. Roots obtain oxygen by transporting it internally from the shoot down through the root, and some of that oxygen escapes radially into the surrounding soil, a process known as radial oxygen loss, or ROL. This leakage is a double-edged sword: it oxygenates the rhizosphere, alleviating anaerobic stress and detoxifying reduced compounds, but it also depletes the oxygen supply needed to sustain respiration at the growing root tip.

Despite decades of research on root aeration in rice, the developmental regulation of ROL during the earliest phase of seedling establishment had remained unclear. In particular, no one had systematically tracked how ROL changes in real time as individual roots emerge, elongate and mature, nor how light availability, which powers photosynthesis in the shoot and could in principle boost the oxygen supply delivered downward, affects this process. The new study addressed both gaps by combining continuous spatiotemporal oxygen imaging with anatomical analysis of the two root types that appear during establishment: seminal roots, which emerge first from the seedling, and adventitious roots, which arise later from the stem base.

The technical centerpiece of the work was a planar oxygen optode, a transparent sensor film that reports oxygen concentration across a two-dimensional surface with high spatial and temporal resolution. Seedlings were grown against the optode in a stagnant, deoxygenated agar medium, a setup designed to mimic the absence of convective water movement that characterizes waterlogged soils. As roots grew along the sensor surface, the researchers could watch oxygen distributions unfold around each root in real time, distinguishing oxygen that accumulated in front of the root apex from oxygen released along the root axis behind it. This imaging approach, originally developed for studying oxygen dynamics in marine sediments and seagrass rhizospheres, allowed the team to link visible patterns of oxygen leakage directly to the developmental state of each root.

The results exposed a fundamental difference between the two root types. Fewer than half of the seminal roots released oxygen immediately after they emerged, whereas most adventitious roots began releasing oxygen the moment they appeared. This early leakage from adventitious roots occurred before any detectable formation of a barrier to radial oxygen loss and before exodermal suberization, the deposition of waxy suberin in the outermost cortical cell layer that later seals the root surface. In other words, young adventitious roots were initially wide open, venting internally transported oxygen into the surrounding medium almost as fast as it arrived.

Anatomical examination explained why. These short, newly emerged adventitious roots contained well-developed cortical intercellular spaces, continuous gas-filled channels through the root cortex that offer little resistance to oxygen diffusing downward and then escaping sideways. Crucially, however, they lacked lysigenous aerenchyma, the programmed cell death structures that in mature rice roots create large open cavities for long-distance gas transport. Within 24 hours of emergence, that picture changed dramatically: lysigenous aerenchyma developed, and a barrier to radial oxygen loss formed. At this stage, detectable oxygen accumulation in front of the root apex was reduced, and the oxygenated zone shifted to the region behind the apex, consistent with the root having sealed its surface everywhere except near the tip, where oxygen is deliberately reserved to fuel extension growth.

This 24-hour transition is the study’s most striking finding, because it shows that the aeration behavior of a rice root is rewritten within a single day of its emergence. The timing suggests a tightly choreographed developmental sequence in which the root first prioritizes rapid establishment, accepting the cost of oxygen leakage while its internal plumbing is still rudimentary, and then invests in sealing and specialization once the aerenchyma network is in place. The barrier to radial oxygen loss, which previous work has linked to suberin and glycerol ester deposition in the exodermis under the influence of signals such as abscisic acid, low oxygen and iron, thus appears remarkably early in the life of an adventitious root, well before the root has grown to any substantial length.

The light experiments delivered a surprise. Because photosynthesis in the shoot generates the oxygen that travels down to the roots, the team expected light to increase both total oxygen release from whole roots and oxygen concentrations around the growing tips. Instead, light only slightly promoted root elongation and did not significantly increase total oxygen release from whole roots or the oxygen concentration around the adventitious root tip. Nevertheless, root-tip oxygen concentration was positively correlated with root elongation rate under both light and dark conditions, confirming that oxygen supply at the apex remains a direct constraint on how fast a root can push through anaerobic soil. The conclusion drawn by the authors is that root aeration during early rice seedling establishment under waterlogged conditions is regulated primarily by anatomical development, whereas clear coordination with light availability was not observed.

That anatomical primacy has practical implications for rice breeding and cultivation. If the rate-limiting factors for seedling establishment are the speed of aerenchyma formation and the induction of the ROL barrier rather than the photosynthetic output of the seedling, then breeding efforts aimed at improving stand establishment in flooded fields should target the genetic and hormonal regulation of these anatomical traits. Prior studies have shown that the ROL barrier protects roots not only against oxygen loss but also against the intrusion of toxic reduced compounds such as hydrogen sulfide and iron, and that environmental triggers including organic acids and low nitrate can induce barrier formation. Understanding exactly when this barrier appears during seedling establishment, as the new study now documents, provides a temporal framework for identifying the molecular switches that control it.

The study also showcases the power of planar optode imaging to resolve processes that conventional methods, such as polarographic electrodes or whole-root gas flux measurements, average out or miss entirely. By capturing the spatiotemporal dynamics of oxygen around individual roots as they develop, the technique revealed heterogeneity among roots of the same seedling, a transient leaky phase in adventitious roots, and a spatial shift in oxygen release from the apex to the mature zone that would be invisible to bulk measurements. As climate change intensifies flooding pressure on rice systems worldwide, tools and knowledge of this kind, linking real-time rhizosphere chemistry to root anatomy, will be essential for developing varieties that establish reliably in oxygen-depleted soils. The work was supported by the Japan Society for the Promotion of Science KAKENHI, Fukui Prefectural University, and the Danish National Research Foundation’s Danish Center for Hadal Research.

Subject of Research: Spatiotemporal dynamics of radial oxygen loss and root aeration during rice seedling establishment in waterlogged conditions

Article Title: Spatiotemporal dynamics of radial oxygen loss during rice seedling establishment in a stagnant deoxygenated agar medium

Article References: Tamaru, S., Fujiwara, N., Shiba, H., Tsunoda, T., Glud, R. N., & Shiono, K. (2026). Spatiotemporal dynamics of radial oxygen loss during rice seedling establishment in a stagnant deoxygenated agar medium. Plant and Soil. https://doi.org/10.1007/s11104-026-09018-0

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09018-0

Keywords: rice, radial oxygen loss, aerenchyma, planar optode, seedling establishment, root anatomy, waterlogging, rhizosphere oxygenation, exodermal suberization, Oryza sativa, plant physiology, oxygen imaging