Because plants are sessile, they have developed intricate strategies to adapt to changing environmental variables, including light. Their growth and development, from germination to flowering, is critically influenced by light, particularly at red (660 nm) and far-red (730 nm) wavelengths. Higher plants perceive red and far-red light by means of specific light sensors called phytochromes(A-E). However, very little is known about how light signals are transduced to elicit responses in plants. Here we report that nucleoside diphosphate kinase 2 (NDPK2) is an upstream component in the phytochrome signalling pathway in the plant Arabidopsis thaliana. In animal and human cells, NDPK acts as a tumour suppressor. We show that recombinant NDPK2 in Arabidopsis preferentially binds to the red-light-activated form of phytochrome in vitro and that this interaction increases the activity of recombinant NDPK2. Furthermore, a mutant lacking NDPK2 showed a partial defect in responses to both red and farred light, including cotyledon opening and greening. These results indicate that NDPK2 is a positive signalling component of the phytochrome-mediated light-signal-transduction pathway in Arabidopsis.

No abstract

Abstract-The photochemistry and photobiology of psoralens have been rcvicwed on the basis of selected literature. In addition to the well establishcd photoaddition of psoralens to DNA. photoreactions involving RNA. tRNA and proteins warrant further studies. Although thymine seems to be one of the predominant bases for the psoralen photocycloaddition in DNA. photoreactions of other bases such as adenine and the mechanisms other than cycloaddition should also bc investigated in ordcr to fully assess the specificity of psoralen photoreactions and their application BS a probe for biopolymcr structure and as a tool for photobiological studies of microorganisms. Thc relativc photobiological importance of monoaddition vs crosslinking of psoralens to nucleic acids also remains to be fully elucidated.

Several skin-sensitizing psoralens and their model coumarins fluoresce weakly and phosphoresce relatively strongly. The excited states of some of these molecules have been investigated in rigid matrices at 12 and 77 °K by means of luminescence and photoselection measurements. Coumarins and psoralens exhibit a relatively high phosphorescence to fluorescence ratio ( / > 1); however, in coumarylpyrone (benzo[l,2-6:4,5-6']dipyran-2,8-dione) and coumarins with strong charge transfer substituents, the ratio is significantly less than unity. In coumarylpyrone this is interpreted as being due to a decreased efficiency of the intersystem crossing from 8 ( , *) to ( , *) as a consequence of the lowering in energy of the ( , *) state below the 3( , *) state. For the series of molecules investigated, it was established that the 0-0 phosphorescence frequencies, ¡Vo, are essentially independent of substituents or the extent of conjugation. This result reinforces the model of energy localization in the 3( , *) state as proposed previously for coumarin and psoralen. The electronic structures of the excited states of selected compounds calculated by SCF-MO methods are consistent with this model. The MO data as well as the spectroscopic data may be correlated with the photobiological reactivity of these molecules toward DNA.Coumarins, particularly psoralens, are known to photosensitize skin erythema and skin cancer in mice and guinea pigs, though skin cancer in man is not unequivocally established.3•4 56The skin-sensitizing potency of psoralens has been correlated with their photoreactivity toward pyrimidine bases of DNA via cycloaddition.3•5-7 In an attempt to describe the electronic mechanism involved in the photocycloaddition of skin-sensitizing compounds to DNA bases, we have previously studied the excited states of coumarin as a model skin sensitizer.7 8 The present paper summarizes more complete studies, many at high resolution, of the excited states of a series of coumarins and psoralens.By far the most interesting molecule examined in this study is coumarylpyrone, benzo[ 1,2-6:4,5-6 'jdipyran-2,8-dione. The impetus for considering this particular system (and its analog benzo[l,2-6:4,5-6'Jdipyran-2,7-dione, which is yet to be synthesized) arose out of a previous work7 concerned with the photocycloaddition of the excited coumarins to DNA bases. In this work it was suggested that the triplet state of coumarin and its derivatives undergoes cycloaddition (at the 3,4 double bond) with pyrimidine bases of DNA. Two such double bonds within the molecular framework as found in coumarylpyrone would theoretically provide two active sites with an opportunity to crosslink both strands of the DNA double helix. Figure 1 shows the structures and numbering systems for coumarin, psoralen, isopsoralen, and coumarylpyrone.