THE PLANT CELL, Vol 5, Issue 4 451-463, Copyright © 1993 by American Society of Plant Biologists
Electron Transport Regulates Cellular Differentiation in the Filamentous Cyanobacterium Calothrix
D. Campbell, J. Houmard and N. T. de Marsac
Unite de Physiologie Microbienne, Centre National de la Recherche Scientifique, Unite de Recherche Associee 1129, Institut Pasteur, 28 rue du Docteur Roux, 75724 Paris Cedex 15, France
Differentiation of the filamentous cyanobacteria Calothrix sp strains PCC
7601 and PCC 7504 is regulated by light spectral quality. Vegetative
filaments differentiate motile, gas-vacuolated hormogonia after transfer to
fresh medium and incubation under red light. Hormogonia are transient and
give rise to vegetative filaments, or to heterocystous filaments if fixed
nitrogen is lacking. If incubated under green light after transfer to fresh
medium, vegetative filaments do not differentiate hormogonia but may
produce heterocysts directly, even in the presence of combined nitrogen. We
used inhibitors of thylakoid electron transport
(3-[3,4-dichlorophenyl]-1,1-dimethylurea and
2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone) to show that the opposing
effects of red and green light on cell differentiation arise through
differential excitations of photosystems I and II. Red light excitation of
photosystem I oxidizes the plastoquinone pool, stimulating differentiation
of hormogonia and inhibiting heterocyst differentiation. Conversely, net
reduction of plastoquinone by green light excitation of photosystem II
inhibits differentiation of hormogonia and stimulates heterocyst
differentiation. This photoperception mechanism is distinct from the light
regulation of complementary chromatic adaptation of phycobilisome
constituents. Although complementary chromatic adaptation operates
independently of the photocontrol of cellular differentiation, these two
regulatory processes are linked, because the general expression of
phycobiliprotein genes is transiently repressed during hormogonium
differentiation. In addition, absorbance by phycobilisomes largely
determines the light wavelengths that excite photosystem II, and thus the
wavelengths that can imbalance electron transport.
