A key molecule that enables plants to adapt to variations in ambient light intensity
Green plants produce organic compounds by the process called photosynthesis. Two photosynthetic systems, called photosystems I and II, are involved. The operation of photosystem II is associated with release of oxygen, while photosystem I produces strongly reducing soluble compounds (NADPH). After absorption of light energy by chlorophyll pigments exposed in collector arrays, this excitation energy is transferred to the reaction centres of photosystems I or II. The smooth operation of photosynthesis requires an optimal distribution of the excitation energy to these two photosystems, which relies on the ability of some of the collector arrays to associate with one or the other of the photosystems.
Researchers have succeeded in identifying the key molecule in this process, a kinase (STN7). Excessive stimulation of photosystem II relative to photosystem I causes the reduction of certain molecules (plastoquinones) and the activation of this kinase, resulting in the phosphorylation of the collector array. The phosphorylated arrays migrate from photosystem II to photosystem I, thereby balancing the excitation energy between the two photosystems.
In a mutant of Arabidopsis thaliana lacking kinase STN7, the phosphorylation of the arrays does not take place, and their migration is blocked. The absence of this process slows growth when incident light intensity varies. The protein kinase STN7 thus plays an essential role in the response of plants to environmental changes by enabling them to adapt efficiently to frequent changes in ambient light intensity.