Phytochrome induces photoreversible calcium fluxes in a purified mitochondrial fraction from oats

Abstract

Whether phytochrome can regulate such fluxes across mitochondrial membranes was investigated, using the Ca2+-sensitive dye murexide to monitor the uptake and release of Ca2+ by mitochondria. Ca2+ fluxes in these organelles could be photoreversibly altered, red light diminishing the net uptake rate and far-red light restoring this rate to its dark control level. Treatment of the mitochondria with ruthenium red blocked their Ca2+ uptake. In the presence of this inhibitor, red light induced a net efflux of Ca2+ from the mitochondria, and subsequent far-red light reduced this efflux to nearly zero, the dark control level. Light-induced rate changes in Ca2+ flux, both with and without the inhibitor, persisted for several minutes in the dark and remained photoreversible through several irradiations for as long as 30 min. The purity of the mitochondrial preparation was judged to be .apprx. 80% by electron microscopic morphometry; most of the phytochrome present was localized on the mitochondria in the preparation by using immunocytochemical methods. Red light activation of phytochrome may initiate an increase in the cytosolic Ca2+ concentration. The results are integrated with the fact that calmodulin is a component of plant cell cytoplasms to construct a model postulating that phytochrome directs photomorphogenesis in part through its regulation of Ca2+ and calmodulin-controlled enzyme activities.