Light-dependent osmoregulation in pea stem protoplasts. Photoreceptors, tissue specificity, ion relationships, and physiological implications

Light-induced changes in the volume of protoplasts bathed in a medium of co nstant osmolarity are useful indications of light-dependent cellular osmore gulation. With this in mind, we investigated the effect of light on the vol ume of protoplasts isolated from the elongating stems of pea (Pisum sativum ) seedlings raised under red light. The protoplasts were isolated separatel y from epidermal peels and the remaining peeled stems. Under continuous red light, the protoplasts of peeled stems swelled steadily, but those of epid ermal peels maintained a constant volume. Experiments employing far-red lig ht and phytochrome-deficient mutants revealed that the observed swelling is a light-induced response mediated mainly by phytochromes A and B with a li ttle greater contribution by phytochrome A. Protoplasts of epidermal peels and peeled stems shrank transiently in response to a pulse of blue light. T he blue light responsiveness in this shrinking response, which itself is pr obably mediated by cryptochrome, is under the strict control of phytochrome s A and B with equal contributions by these phytochromes. We suggest that t he swelling response participates in the maintenance of high tissue tension of elongating stems and that the shrinking response is involved in stem gr owth inhibition. Other findings include the following: The swelling is caus ed by uptake of K+ and Cl-. The presence of Ca2+ in the bathing medium is r equired for phytochrome signaling in the swelling response, but not in the response establishing blue light responsiveness. Phytochrome A mediates the two responses in a totally red/far-red light reversible manner, as does ph ytochrome B.