THE CIRCADIAN OSCILLATOR IS REGULATED BY A VERY-LOW FLUENCE RESPONSE OF PHYTOCHROME IN WHEAT

Expression of genes encoding the light-harvesting chlorophyll a/b bind ing proteins of photosystem II (Cab) in etiolated wheat seedlings is c ontrolled by phytochrome and a circadian clock. Even photoconversion o f <1% of phytochrome to its active form, which can be achieved by moon light, induces the expression of the Cab genes, particularly that of t he Cab-1 gene, in circadian fashion. Thus, this reaction shows the cha racteristics of a low and a very low fluence response. A single far-re d light pulse given to an etiolated seedling is sufficient for a persi stence of the circadian oscillation of the Cab-1 mRNA level for at lea st 100 h. Subsequent red (R) or long-wavelength far-red (RG9) light ir radiations alter the free running rhythm. These observations indicate a change in sensitivity to phytochrome and/or a control by stable phyt ochrome. The latter hypothesis is supported by the observation that th e level of Cab-1 mRNA is increased or decreased by a second R or RG9 l ight pulse, respectively.