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A balance between water and energy
For a plant photosynthesis is of vital importance. The conversion of CO2 into glucose creates energy, for living, growing, and multiplying. CO2 enters the plant via stomata, pores, located at the underside of the leaf. Photosynthetic rest product oxygen, as well as water, leaves the plant via stomata. It is thus important that the stomata are open during the day. But in a drought, when there is a shortage of water, the stomata need to close. But then no photosynthesis can occur. Therefore, stomata opening and closing requires tight regulation. Researchers from Sweden discovered how the plant manages this.
When there is no drought, the clock protein ZEITLUPE regulates the opening of the stomata. In the evening, when there is lots of ZEITLUPE protein, the stomata close, and in the morning, when the amount of ZEITLUPE is reduced, the stomata open again. But during a drought, the drought protocol starts working, leaded by the manager: ABA, a plant hormone. ABA convinces ZEITLUPE to let the stomata close earlier in the day. ABA does this by inhibiting the breakdown of the stomata closing protein OST1. This enables OST1 to work together with ZEITLUPE, together they close the stomata earlier.
The researchers noticed that this was not the whole story. As clock protein ZEITLUPE works together with ABA to prevent evaporation, another clock protein, PRR5, tries to stop them, so photosynthesis can go ahead. PRR5 also interacts with OST1, but in contrast to ZEITLUPE, PRR5 prevents OST1 to do its job of closing the stomata. They stay open, so CO2 for photosynthesis can enter.
The amount of ZEITLUPE and PRR5 is changing in daily rhythm, each peaking at a different time of day. In this way photosynthesis can go on during a drought. But absence of PRR5 might cause a plant to die due to lack of energy during a drought. Absence of ZEITLUPE, on the other hand, increases dead through dehydration. With both ZEITLUPE and PRR5 there is a balance between energy production and water evaporation.
Jurca M, Sjölander J, Ibáñez C, Matrosova A, Johansson M, Kozarewa I, Takata N, Bakó L, Webb AAR, Israelsson-Nordström M and Eriksson ME (2022) ZEITLUPE Promotes ABA-Induced Stomatal Closure in Arabidopsis and Populus. Frontiers in Plant Science 13:829121. doi: 10.3389/fpls.2022.829121
Read more about stomata in: Light lets plants breathe