Light plays a fundamental role in the development of higher plants. It influences the activity of many genes and is sensed through at least three different photoreceptor systems (phytochromes, blue/UV‐A‐ and UV‐B photoreceptors). In contrast to UV‐B and blue/UV‐A photoreceptors, phytochromes absorb both in the long‐wavelength (red and far‐red) and short‐wavelength region of light, making it extremely difficult to differentiate the exact function of different photoreceptors in the blue/UV region. The availability of phytochrome mutants in combination with a transgenic approach has enabled clarification of the question whether or not phytochromes are a prerequisite for the action of blue/UV‐A photoreceptors on gene expression. This paper reports that the light‐responsive element of a gene encoding chalcone synthase, the key enzyme of the flavonoid pathway, mediates blue‐ and UV‐A‐induced transcription in the absence of phytochrome A and phytochrome B.

SummaryPhotoregulation of chalcone synthase (CHS) mRNA accumulation was analysed in parsley (Petroselinum crispum) and mustard (Sinapis alba) plants at different developmental stages. In both species, CHS mRNA accumulation in young etiolated seedlings was primarily under phytochrome control. In leaves of adult reetiolated plants, a UV-B photoreceptor was predominantly involved in photocontrol. The reduced red light control in mature leaves was not due to the absence of immunoreactive phytochrome. The apparent dependence of photoreceptor usage on the developmental state of the cell or organism was in accordance with observations on the photoregulation of fusion constructs between CHS promoters from parsley or mustard and the P-glucuronidase reporter gene (GUS). When tested in the parsley protoplast transient expression system, both constructs yielded the same type of photoregulation as observed for the endogenous CHS gene.

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