Spinach is said to be good for preventing eye aging because it is rich in vitamin A. Now a path has opened for spinach to go directly into the eye and even treat ophthalmic diseases. The idea is to place spinach chloroplasts into the human eye to block disease-causing substances using the principle of photosynthesis.
A team led by Professor David Tai Wei Leong of the Department of Chemical and Biomolecular Engineering at the National University of Singapore said on the 15th (local time) in the journal Cell that it "successfully treated dry eye by injecting chloroplast components from spinach into animal corneal cells and harvesting light energy."
◇ Chloroplasts suppress reactive oxygen species that damage cells
Dry eye is an eye disease in which a lack of tears or tears that evaporate too quickly cause inflammation in the corneal sclera and the conjunctiva of the white of the eye. The medical term is keratoconjunctivitis sicca. When inflammation occurs in the eye, reactive oxygen species that are harmful to cells are produced. Healthy eyes produce antioxidants with NADPH (nicotinamide adenine dinucleotide phosphate) to neutralize reactive oxygen species, but eyes with dry eye cannot do so.
The researchers focused on the fact that NADPH is synthesized when plant leaves photosynthesize. If the human eye could photosynthesize like a plant, it could be seen as treating inflammation on its own. To verify this, they extracted thylakoids that synthesize NADPH from commercially available spinach. Thylakoids are coin-shaped structures inside chloroplasts, and their stacked formations are called grana.
Thylakoids receive light and split water to produce ATP (adenosine triphosphate), an energy molecule, and NADPH, which carries hydrogen and electrons. This is the light reaction. In the next step, the dark reaction, chloroplasts use the two substances synthesized in the light reaction to convert carbon dioxide into glucose. The researchers decided to place only the part that performs the light reaction into the eye. They extracted thylakoids from spinach and made them into nanoparticles 400 nm (nanometers; one-billionth of a meter) in size.
The team administered the particles into the eyes of mice with induced dry eye like eye drops. Chloroplasts are green, but the nanoparticles were transparent because they were so small. In the experiment, mice that received only saline in the eye had corneal thickness thin by 30%, while mice that received chloroplast components showed increased tear secretion and recovered corneal thickness within 5 days of treatment. The researchers said the effect was better than existing treatments. No side effects appeared for 2 months.
The same effect appeared in human corneal cells. Within 30 minutes, NADPH levels returned to normal, and reactive oxygen species were suppressed. In addition, when thylakoid nanoparticles were added to tears collected from patients with dry eye, NADPH levels increased 20-fold, and hydrogen peroxide, which damages cells, fell by more than 95%.
◇ Economic effect in the trillions of won, low production cost
More than 1.5 billion people worldwide suffer from dry eye. Beyond discomfort and poor vision, it can lead to depression and anxiety. As a result, treatment costs are high and work productivity declines, causing serious economic losses. In the United States alone, it is estimated to impose an annual economic burden of $3.84 billion (5.76 trillion won). If a therapy that induces photosynthesis in the eye is commercialized, a tremendous economic effect is expected.
The researchers said they are also preparing clinical trials in actual patients. Leong said, "Using the principle of plant photosynthesis, we directly restored the molecules depleted by dry eye," and noted, "Because spinach-derived substances are administered like eye drops and ordinary lighting is used, the likelihood of applying this to patient care is very high." Moreover, a chloroplast-component therapy costs little to produce. The team said that with a handful of spinach costing 300 won, they could make enough nanoparticles to administer to more than 50 people twice a day for a month.
It is not impossible for animals to photosynthesize. The sea slug Elysia chlorotica can live for months on photosynthesis alone without eating, thanks to absorbing chloroplasts from photosynthetic microalgae. A research team at the University of Tokyo in 2024 succeeded in transplanting plant chloroplasts into human cells and running them for two days. There have also been cases of achieving therapeutic effects using the same approach. In 2022, a research team at Zhejiang University School of Medicine in China reported in Nature that injecting particles containing chloroplast components into the knee joints of mice with arthritis slowed cartilage destruction.
However, before using it in actual patients, the therapeutic mechanism needs to be clarified further. Mary Ann Stepp of the George Washington University School of Medicine said in Science that "the efficacy confirmed this time could be seen as a result of promoting energy production rather than suppressing inflammation."
Plants also produce ATP, an energy molecule, in photosynthesis. Scientists think ATP may have helped treat dry eye. When a person has dry eye, neurons that help produce energy in corneal cells die. Thylakoids in chloroplasts produce ATP and could compensate for this.
References
Cell (2026), DOI: https://doi.org/10.1016/j.cell.2026.04.034
Proceedings of the Japan Academy, Series B (2024), DOI: https://doi.org/10.2183/pjab.100.035
Nature (2022), DOI: https://doi.org/10.1038/s41586-022-05499-y