The action spectrum for chloroplast movements and evidence for blue-light-photoreceptor cycling in the alga Vaucheria

Abstract

Local stimulation of the coenocytic alga Vaucheria sessilis D.C. by blue light resulted in accumulation of chloroplasts and other organelles. The photoresponse followed a well-defined, wavelength-and fluence-rate-dependent latency period (≧10 s), and could lead to a tenfold decrease in relative cellular transmittance to 675-nm light within 5 min. Light-induced aggregation of chloroplasts was examined at eight wavelengths of light between 385 and 528 nm. A fiber-optic microphotometer was employed and the response was quantitated on the basis of the rate of 675-nm transmittance change after correcting for changes in light scattering. Chloroplast aggregation exhibited a nearly identical quantum-flux-density dependence at all eight wavelenths tested; it showed an action spectrum with a sharp maximum near 470 nm, a trough at 430 nm, and action in the near-ultraviolet spectral region. Light at 454 nm was six times less effective than 473-nm light in stimulating aggregation, a difference which could not be accounted for by chlorophyll screening alone. Beyond the latency period reciprocity did not hold for chloroplast aggregation. Instead, aggregation could be fitted to a kinetic model involving steady-state photoreceptor cycling during continuous irradiation. Chloroplast aggregation in the light was compared with three growth-associated photoresponses in Vaucheria — phototropic bending, branching and apical expansion. Time course and kinetic similarities, and the presence of a cytoplasmic fiber network in growing tips of Vaucheria, indicate that these photoresponses may be related mechanistically.