SummaryThe interaction of phytochrome signalling with the SA signal transduction pathway has been investigated in Arabidopsis using single and multiple mutants affected in light perception (phyA and phyB de®cient) and light-signal processing (psi2, phytochrome signalling). The induction of PR1 by SA and functional analogues has been found to strictly correlate with the activity of the signalling pathway controlled by both phyA and phyB photoreceptors. In darkness as well as dim light, and independently of a carbohydrate source, SA-induced PR gene expression as well as the hypersensitive response to pathogens (HR) are strongly reduced. Moreover, the initiation of HR also exhibits a strict dependence upon both the presence and the amplitude of a phytochrome-elicited signal. The growth of an incompatible strain of bacterial a pathogen (Pseudomonas syringae pv. tomato) was enhanced in phyAphyB and decreased in psi2 mutants. While functional chloroplasts were found necessary for the development of an HR, the induction of PRs was strictly dependent on light, but independent of functional chloroplasts. Taken together, these data demonstrate that the light-induced signalling pathway interacts with the pathogen/SA-mediated signal transduction route. These results are summarized in a formalism that allows qualitative computer simulation.

The phytochrome (phy) family of photoreceptors is of crucial importance throughout the life cycle of higher plants. Light-induced nuclear import is required for most phytochrome responses. Nuclear accumulation of phyA is dependent on two related proteins called FHY1 (Far-red elongated HYpocotyl 1) and FHL (FHY1 Like), with FHY1 playing the predominant function. The transcription of FHY1 and FHL are controlled by FHY3 (Far-red elongated HYpocotyl 3) and FAR1 (FAr-red impaired Response 1), a related pair of transcription factors, which thus indirectly control phyA nuclear accumulation. FHY1 and FHL preferentially interact with the light-activated form of phyA, but the mechanism by which they enable photoreceptor accumulation in the nucleus remains unsolved. Sequence comparison of numerous FHY1-related proteins indicates that only the NLS located at the N-terminus and the phyA-interaction domain located at the C-terminus are conserved. We demonstrate that these two parts of FHY1 are sufficient for FHY1 function. phyA nuclear accumulation is inhibited in the presence of high levels of FHY1 variants unable to enter the nucleus. Furthermore, nuclear accumulation of phyA becomes light- and FHY1-independent when an NLS sequence is fused to phyA, strongly suggesting that FHY1 mediates nuclear import of light-activated phyA. In accordance with this idea, FHY1 and FHY3 become functionally dispensable in seedlings expressing a constitutively nuclear version of phyA. Our data suggest that the mechanism uncovered in Arabidopsis is conserved in higher plants. Moreover, this mechanism allows us to propose a model explaining why phyA needs a specific nuclear import pathway.

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Phototropin photoreceptors (phot1 and phot2 in Arabidopsis thaliana) enable responses to directional light cues (e.g., positive phototropism in the hypocotyl). In Arabidopsis, phot1 is essential for phototropism in response to low light, a response that is also modulated by phytochrome A (phyA), representing a classical example of photoreceptor coaction. The molecular mechanisms underlying promotion of phototropism by phyA remain unclear. Most phyA responses require nuclear accumulation of the photoreceptor, but interestingly, it has been proposed that cytosolic phyA promotes phototropism. By comparing the kinetics of phototropism in seedlings with different subcellular localizations of phyA, we show that nuclear phyA accelerates the phototropic response, whereas in the fhy1 fhl mutant, in which phyA remains in the cytosol, phototropic bending is slower than in the wild type. Consistent with this data, we find that transcription factors needed for full phyA responses are needed for normal phototropism. Moreover, we show that phyA is the primary photoreceptor promoting the expression of phototropism regulators in low light (e.g., PHYTOCHROME KINASE SUB-STRATE1 [PKS1] and ROOT PHOTO TROPISM2 [RPT2]). Although phyA remains cytosolic in fhy1 fhl, induction of PKS1 and RPT2 expression still occurs in fhy1 fhl, indicating that a low level of nuclear phyA signaling is still present in fhy1 fhl.

A new mutant called psi2 (for p hytochrome si gnaling) was isolated by screening for elevated activity of a chlorophyll a / b binding protein-luciferase ( CAB2-LUC ) transgene in Arabidopsis. This mutant exhibited hypersensitive induction of CAB1 , CAB2 , and the small subunit of ribulose-1,5-bisphosphate carboxylase ( RBCS ) promoters in the very low fluence range of red light and a hypersensitive response in hypocotyl growth in continuous red light of higher fluences. In addition, at high-but not low-light fluence rates, the mutant showed light-dependent superinduction of the pathogenrelated protein gene PR-1a and developed spontaneous necrotic lesions in the absence of any pathogen. Expression of genes responding to various hormone and environmental stress pathways in the mutant was not significantly different from that of the wild type. Analysis of double mutants demonstrated that the effects of the psi2 mutation are dependent on both phytochromes phyA and phyB. The mutation is recessive and maps to the bottom of chromosome 5. Together, our results suggest that PSI2 specifically and negatively regulates both phyA and phyB phototransduction pathways. The induction of cell death by deregulated signaling pathways observed in psi2 is reminiscent of retinal degenerative diseases in animals and humans. INTRODUCTIONBecause light provides the energy source for photosynthesis, plants must adapt to changes in ambient light quantity and quality to optimize the efficiency of this important process. The results of this adaptation are apparent: almost every step of plant development, from seed germination to fruit maturation, is dependent on light. Moreover, light regulates the circadian rhythm, the adaptative size modification of tissue and organs, and the expression of photosynthetic genes and many other known and unknown genes (Terzaghi and Cashmore, 1995).To date, what we know about light information processing and signal transduction concerns the nature and function of photoreceptors. A family of photoreceptors called phytochromes has been shown to be involved in the perception of red and far-red light. Their biochemical and molecular properties and physiological roles are now well described (reviewed in Quail et al., 1995;Smith, 1995). In addition, a longpostulated UV and blue light receptor (cryptochrome) was recently characterized by Ahmad and Cashmore (1993). Mutants lacking one or several of these receptors have provided useful information concerning the role of individual photoreceptors and their interaction in transducing light signals (reviewed in Barnes et al., 1997).Much less is known concerning molecules that link light perception to gene activation. The CONSTITUTIVE PHOTO-MORPHOGENIC/DEETIOLATED/FUSCA (COP/DET/FUS) class of proteins has been suggested to play an important role in regulating the repression of light-inducible genes and the maintenance of skotomorphogenesis (Chory et al., 1989). For some of these proteins, the genes have been cloned and include COP1 (Deng et al., 1992), DET1 (Pepper et al., ...