Phenylalanine ammonia‐lyase (PAL) catalyses the first step in the biosynthesis of phenylpropanoids, which form a wide variety of plant secondary products. The transcription of PAL is regulated in response to various factors that induce the accumulation of flavonoids, lignin and compounds thought to be involved in plant defence reactions. The 5′ upstream sequence of a PAL gene from Phaseolus vulgaris was fused to the coding region of the reporter gene encoding beta‐glucuronidase (GUS), and transformed into potato and tobacco plants. Histochemical analysis of GUS expression showed that the PAL promoter was active in specific cell types that accumulated phenylpropanoid derivatives in response to mechanical wounding, and also during normal development of the xylem and flower. In xylem that had undergone secondary thickening, GUS activity occurred in rays of cells thought to be the xylem parenchyma. It was postulated that PAL activity in these cells could provide intermediates for lignin synthesis in xylem vessels that had terminally differentiated.

Phenylalanine ammonia-lyase (PAL) catalyses the first committed step in the biosynthesis of phenylpropanoids, which perform a variety of functions in plant development and in their interactions with the environment. French bean contains a small family of genes encoding PAL and two of these genes, PAL2 and PAL3, have been shown to be differentially expressed at the mRNA level in bean tissues. The transcriptional activities of the PAL2 and PAL3 genes have been investigated by fusing their promoters to the reporter gene beta-glucuronidase (GUS) and transforming these constructs into Arabidopsis, potato and tobacco. The PAL2- and PAL3-GUS constructs exhibited different spatial and temporal patterns of expression during development and in response to environmental stimuli. The consistency of these data with previous mRNA analysis in bean suggests that the differential expression of these two PAL genes is, at least in part, a function of their promoter activities. New patterns of PAL2 and PAL3 promoter activities were also characterized. Some species-specific differences in GUS expression were observed and these may reflect differences in phenylpropanoid metabolism or the signals that modulate PAL gene transcription.

SummaryPhenylalanine ammonia‐lyase (PAL) catalyses the first committed step in the biosynthesis of phenyl‐propanoids, which perform a variety of functions in plant development and in their interactions with the environment. French bean contains a small family of genes encoding PAL and two of these genes, PAL2 and PAL3, have been shown to be differentially expressed at the mRNA level in bean tissues. The transcriptional activities of the PAL2 and PAL3 genes have been investigated by fusing their promoters to the reporter gene β‐glucuronidase (GUS) and transforming these constructs into Arabidopsis, potato and tobacco. The PAL2‐ and PAL3‐GUS constructs exhibited different spatial and temporal patterns of expression during development and in response to environmental stimuli. The consistency of these data with previous mRNA analysis in bean suggests that the differential expression of these two PAL genes is, at least in part, a function of their promoter activities. New patterns of PAL2 and PAL3 promoter activities were also characterized. Some species‐specific differences in GUS expression were observed and these may reflect differences in phenylpropanoid metabolism or the signals that modulate PAL gene transcription.

We have investigated the efficacy of using the Escherichia coli lac operator‐repressor system to control plant gene expression. The lacI gene was modified to allow optimal expression in plant cells and then placed downstream of the cauliflower mosaic virus (CaMV) 35S RNA promoter. This construct was introduced into tobacco plants by leaf disc transformation. Transgenic tobacco plants synthesized significant quantities of LacI protein (up to 0.06% of total soluble protein). We have used the E.coli beta‐glucuronidase gene (gus) as the reporter gene by placing it downstream of the maize chlorophyll a/b binding protein (CAB) gene promoter. Lac operators were introduced into several positions within the CAB promoter and operator‐free plasmid was used as control. Repression was assessed by comparing the transient expression from CAB‐operator‐gus reporter constructs in protoplasts expressing lac protein, with that in control cells not expressing the repressor. Repression varied between 10 and 90% with different operator positions. Transient assays were also performed in the presence of the inducer, isopropyl‐beta‐D‐thiogalactoside (IPTG). In lacI protoplasts the presence of IPTG manifested itself in a 4.2‐fold relief of repression. The study was extended to show regulation of expression in stable transformants. Tobacco transformants harbouring a CAB‐operator‐gus reporter construct and the lacI gene were shown to have repressed GUS levels, but in the presence of IPTG, repression was relieved 15‐fold. We conclude that the lac repressor can enter the plant cell nucleus, find its cognate operator sequence in the chromatin to form a repressor‐‐operator complex and effectively block transcription of a downstream gene.

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