EXPRESSION OF TOBACCO GENES FOR LIGHT-HARVESTING CHLOROPHYLL A B BINDING-PROTEINS OF PHOTOSYSTEM-II IS CONTROLLED BY 2 CIRCADIAN OSCILLATORS IN A DEVELOPMENTALLY-REGULATED FASHION/

Light-induced expression of genes encoding the light-harvesting chloro phyll alb binding proteins of photosystem II (Cab) was shown to be con trolled by a circadian oscillator coupled to the red-light-apsorbing p lant photoreceptor phytochrome, Here we show that a red-light-insensit ive oscillator is also involved in regulating the expression of the Ca b genes, We provide evidence that germination leads, in light-independ ent manner, to the setting and/or synchronization of endogenous oscill ators and that it induces the expression of Cab genes in a circadian f ashion. This circadian oscillator is not coupled to phytochrome, as it cannot be reset by red light for at least 44 h after sowing, Short re d light pulses given between 12 and 44 h after sowing, however, induce new rhythms without perturbing the already free-running red-light-ind ependent circadian oscillation, At this Stage of development, the phyt ochrome-coupled and uncoupled circadian rhythms coexist. Both circadia n rhythms are expressed and exhibit period lengths close to 24 h but a re phased differently. At later stages of development (60 h or later a fter sowing), red light treatments synchronized these free-running rhy thms and led to the appearance of a single new circadian oscillation. These data indicate that during early development the expression of si ngle single tobacco Cab genes, particularly expression of the Cab21 an d Cab40 genes, is controlled in a developmentally dependent manner by two circadian oscillators.