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

Blue light induces extracellular acidification, a prerequisite of cell expa nsion, 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 induce d by pulses as short as 30 msec. A pulse of more than 3000 mu mol m(-2) sat urated this response. Responsiveness to a second light pulse was recovered with a time constant of about 7 min. The fluence rate-dependent lag time an d sigmoidal increase of the acidification suggested the involvement of seve ral reactions between light perception and activation of the ATPase. In wil d-type pea plants, the fluence response relation for short light pulses was biphasic, with a component that saturates at low fluence and one that satu rates at high fluence. The phytochrome-deficient mutant pcd2 showed a selec tive loss of the high-fluence component, suggesting that the high-fluence c omponent is phytochrome-dependent and the low-fluence component is phytochr ome-independent. Treatment with the calmodulin inhibitor W7 also led to the elimination of the phytochrome-dependent high-fluence component. Simple mo dels adapted from the one used to simulate blue light-induced guard cell op ening failed to explain one or more elements of the experimental data. The hypothesis that phytochrome and a blue light receptor interact in a short-t erm photoresponse is endorsed by model calculations based upon a three-step signal transduction cascade, of which one component can be modulated by ph ytochrome.