A hydrogenase-like activity associated with mitochondrial Complex I under hypoxia in vascular plants

For decades, biologists have known that plants, like animals, rely on a complex internal network to turn food into energy. This process usually requires oxygen, acting as the final step that allows cells to burn fuel efficiently. However, when oxygen is scarce, such as when a plant's roots are submerged in water, this standard system stalls. Without oxygen to accept the spent electrons, the cell's energy machinery risks backing up, creating a dangerous traffic jam of chemical signals that can damage the plant. For more than sixty years, scientists have observed that under these low-oxygen conditions, higher plants release tiny amounts of molecular hydrogen gas. Yet, the source of this gas remained a mystery. The question was not just where the gas came from, but how a plant cell, which lacks the specialized enzymes found in bacteria that produce hydrogen, could generate it at all. Understanding this mechanism is crucial because it reveals how plants survive stress and manage their internal chemical balance when the air they need is unavailable.

A team of researchers has now traced this elusive hydrogen production to a specific location inside the plant cell: the mitochondria, the powerhouses that generate energy. By studying young, dark-grown seedlings of the mung bean, the scientists found that when these tissues were placed in low-oxygen environments with slightly acidic conditions, they began to accumulate hydrogen gas. This activity was not random; it was tightly linked to a massive protein machine known as Complex I, a central component of the plant's respiratory system. When the researchers added a chemical that blocks Complex I, the hydrogen production stopped almost immediately. Further tests showed that the process depended on the flow of electrons through other parts of the energy system, specifically involving a pool of molecules that shuttle electrons and the final steps where oxygen usually enters the chain. The data indicated that the plant was using specific fuel sources, such as those that generate a molecule called NADH, to drive this reaction.

The most striking discovery was the sheer scale of the hydrogen being produced. The amount of hydrogen gas generated by the plant tissue was far greater than the total amount of the fuel molecule, NADH, available in the cell at any single moment. In fact, the cumulative hydrogen output was about one hundred times larger than the measured pool of NADH. This discrepancy suggested that the plant was not simply burning through a static reserve of fuel. Instead, the cell was constantly recycling its resources. The researchers propose a model where two versions of the Complex I machine work together. One version, the mature form, helps regenerate the fuel supply by moving electrons in a reverse direction, a process that usually requires energy but here is driven by the buildup of other chemicals. The second version, a slightly different or remodeled state of the same machine, acts as a safety valve. It takes the regenerated fuel and uses it to produce hydrogen gas, effectively clearing the chemical traffic jam and preventing damage.

This mechanism offers a new explanation for how plants handle stress without generating harmful byproducts. In animals, forcing electrons to move backward through the energy machinery often creates reactive oxygen species, which are unstable molecules that can damage cells. The researchers suggest that by diverting these electrons into hydrogen gas, the plant avoids this dangerous buildup. The hydrogen production acts as an outlet, allowing the cell to keep its internal chemical cycles running smoothly even when oxygen is missing. While the exact steps of how the hydrogen is formed at the atomic level are still being mapped, the evidence points to a specific branch within the Complex I machine that has the capacity to perform this chemistry. This finding suggests that the evolutionary history of these energy machines is deeper than previously thought, retaining a latent ability to produce hydrogen that is only revealed under specific conditions. The study does not claim to have solved every detail of the process, but it firmly establishes that this hydrogen production is a real, regulated function of the plant's mitochondrial system, providing a vital escape route for electrons when the normal path is blocked.