Modulation by phytochrome of the blue light‐induced extracellular acidification by leaf epidermal cells of pea (Pisum sativum L.): a kinetic analysis

Summary Blue light induces extracellular acidification, a prerequisite of cell expansion, in epidermis cells of young pea leaves, by stimulation of the proton pumping‐ATPase activity in the plasma membrane. A transient acidification, reaching a maximum 2.5–5 min after the start of the pulse, could be induced by pulses as short as 30 msec. A pulse of more than 3000 μmol m−2 saturated this response. Responsiveness to a second light pulse was recovered with a time constant of about 7 min. The fluence rate‐dependent lag time and sigmoidal increase of the acidification suggested the involvement of several reactions between light perception and activation of the ATPase. In wild‐type pea plants, the fluence response relation for short light pulses was biphasic, with a component that saturates at low fluence and one that saturates at high fluence. The phytochrome‐deficient mutant pcd2 showed a selective loss of the high‐fluence component, suggesting that the high‐fluence component is phytochrome‐dependent and the low‐fluence component is phytochrome‐independent. Treatment with the calmodulin inhibitor W7 also led to the elimination of the phytochrome‐dependent high‐fluence component. Simple models adapted from the one used to simulate blue light‐induced guard cell opening failed to explain one or more elements of the experimental data. The hypothesis that phytochrome and a blue light receptor interact in a short‐term photoresponse is endorsed by model calculations based upon a three‐step signal transduction cascade, of which one component can be modulated by phytochrome.