By introducing an extra ion channel into the stomata of mustard plants, researchers have developed a new a way to speed up the stomatal response in their leaves. The speedier stomata of the optogenetically enhanced plants improved their photosynthetic efficiency and water use - producing more than twice the amount of biomass expected in the fluctuating light typical of outdoor growing conditions. Stomata are the tiny pores that cover the surface of a plant's leaves, allowing for the uptake of CO2 for photosynthesis and for the transpiration of water, by opening and closing in response to environmental conditions. However, these dual roles are often conflicting. While open stomata may allow a plant to assimilate large amounts of carbon for photosynthesis, this happens at the cost of increased water loss. What's more, stomata respond slowly to changing conditions. In environments where natural light fluctuates - due to passing clouds, for example - stomata could stay open or closed for longer than they need to. As a result, photosynthesis is generally not as efficient as it could be and too much water is lost from the plant. The ability to circumvent the carbon:water trade-off provides a promising avenue to improve crop productivity. To address this challenge, Maria Papanatsiou and colleagues used the optogenetic tool BLINK1 (Blue Light-Induced K+ channel 1) to engineer an extra ion channel into the stomata of the mustard plant Arabidopsis. According to Papanatsiou et al., the channel, which is triggered by exposure to blue light, causes the stomata to open or close more rapidly. According to the results, the increased speed improved the plant's water use efficiency without a penalty to CO2 uptake.