We demonstrate that phenylalanine ammonia‐lyase (PAL) in parsley (Petroselinum crispum) is encoded by a small family of at least four genes. The levels of mRNA from three identified PAL genes increase considerably upon treatment of cultured parsley cells with UV light or fungal elicitor and upon wounding of parsley leaves or roots. In cultured cells these changes were shown to involve transcriptional activation. We present the first primary structure of a plant PAL gene (parsley PAL‐1) and the deduced amino acid sequence of the enzyme. Inducible in vivo footprints in the PAL‐1 promoter define two nucleotide sequences, within the motifs CTCCAACAAACCCCTTC and ATTCTCACCTACCA, involved in the responses to both UV irradiation and elicitor application. These motifs are conserved at similar positions in several elicitor or light‐responsive genes from different species. In two cases they are found within short regions known to confer elicitor or UV‐light inducibility. The conserved motifs in the parsley 4‐coumarate:CoA ligase gene, which is coordinately regulated with PAL, also display UV‐light inducible in vivo footprints. Taken together, our findings suggest a general role of these putative cis‐acting elements in the responses of plants to such stresses.

The isolation and characterization of mutants hypersensitive to ultraviolet (UV) radiation has been a powerful tool to learn about the mechanisms that protect plants against UV-induced damage. To increase our understanding of the various mechanisms of defense against UVB radiation, we searched for mutations that would increase the level of tolerance of Arabidopsis plants to UV radiation. We describe a single gene dominant mutation (uvt1) that leads to a remarkable tolerance to UVB radiation conditions that would kill wild-type plants. Pigment analyses show a constitutive increase in accumulation of UV-absorbing compounds in uvt1 that increases the capacity of the leaves to block UVB radiation and therefore is likely to be responsible for the elevated resistance of this mutant to UVB radiation. These increases in absorption in the UV region are due, at least in part, to increases in flavonoid and sinapate accumulation. Expression of chalcone synthase (CHS) mRNA was shown to be constitutively elevated in uvt1 plants, suggesting that the increases in absorption may be a consequence of changes in gene expression. Expression of CHS in uvt1 was shown to be still inducible by UV, indicating that the uvt1 lesion may not affect the UV-mediated regulation of CHS gene expression. Our data support an important role for UV screens in the overall protection of plants to UVB radiation. The uvt1 mutant could prove to be an important tool to elucidate further the exact role of UV-absorbing pigments in UV protection as well as the relative contribution of other mechanisms to the overall tolerance of plants to UV radiation.

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We analyzed the developmental regulation and the activation by wounding of several stressrelated genes in various parsley organs. The genes encode phenylalanine ammonia-lyase (PAL) and 4-coumarate: CoA ligase (4 CL), two enzymes of general phenylpropanoid metabolism; a flavonoid specific enzyme, chalcone synthase (CHS); a furanocoumarin specific enzyme, bergaptol O-methyltransferase (BMT); and a pathogenesis-related protein (PR 1). All genes or gene families exhibited high levels of expression in roots and during certain stages of leaf development. PAL, 4 CL and CHS were preferentially expressed in young leaves, BMT and PR 1 in old leaves. An appreciable increase in CHS mRNA levels was observed in wounded leaves. By contrast, root wounding led to a decrease in the existing CHS m RNA levels. A biphasic response (a decrease followed by an increase) to wounding was seen for BMT and PR 1 m RNA s in roots and for BMT mRNA in attached leaves. Using gene-specific oligonucleotide probes to measure the expression rates of three of the four PAL genes and of the two 4 CL genes separately we observed a differential behavior of the individual family members under many of the conditions tested. While PAL-3 was preferentially activated in wounded leaves and 4CL-1 in wounded roots, PAL-2 and 4C L-2 were primarily responsible for the high constitutive expression levels in roots and flowering stems respectively. Despite the differential expression of their individual members, the PAL and 4 CL gene families displayed very similar changes in the overall patterns of expression, reflecting their closely related functions in phenylpropanoid metabolism.

Summary Cultured parsley (Petroselinum crispum) cells respond differentially to UV‐containing white light and fungal elicitor. Both stimuli activate the transcription of genes encoding enzymes of partly overlapping phenylpropanoid biosynthetic pathways. Irradiation induces vacuolar accumulation of flavonoids, whereas elicitor treatment stimulates the secretion of furanocoumarins. Simultaneous treatment of parsley cells with UV light and elicitor results in quantitative changes in both responses. Irradiation reduces elicitor‐induced furanocoumarin synthesis, possibly by post‐transcriptional mechanisms, whereas elicitor treatment completely blocks the light‐induced accumulation of flavonoids by repressing the transcription of the chalcone synthase gene. We have identified elicitor‐sensitive regions in the chalcone synthase promoter by transient expression analysis of selected promoter constructs linked to the β‐D‐glucuronidase reporter gene in parsley protoplasts. These regions are identical to those that were found to be sufficient for light inducibility of the chalcone synthase promoter.

The nature and function of the transcriptionally competent state of chromatin remains an enigma despite much work. Two properties are generally characteristic of competent chromatin: an increased sensitivity to digestion by DNase I and a reduced frequency of cytosine methylation at CpG positions (63). However, the notion of "competent" chromatin is poorly defined. Generally it is taken to mean a region of chromatin that is committed, by virtue of a special conformation, to the present or future transcription of one or more of its resident genes.Little is known about the nature of the DNase I-sensitive conformation of competent chromatin. The DNase I-sensitive state encompasses both the active genes themselves and the nontranscribed regions of chromatin flanking them. This contrasts with the situation for transcriptionally induced perturbations of chromatin, for which several structural features have been described (3,14,18,22,40,58,67,79). These perturbations are limited both temporally and spatially to the regions actually undergoing the transcription and are therefore distinct from the all-inclusive DNase I sensitivity of competent chromatin. The large DNase I-sensitive regions characteristic of competent chromatin may include several genes and, in the most completely mapped instances, cover discrete domains that extend uniformly through 10 to 100 kilobase pairs (kb) of chromatin (2, 39, 49). These domains can even include more nontranscribed flanking chromatin than actively transcribed gene chromatin. Moreover, the nontranscribed regions are just as sensitive to DNase I digestion as are the transcribed regions, provided the last-cut approach to the measurement of DNase I sensitivity is used (2,39,49,67,72).