The nucleotide sequence of pathogenesis-related (PR) 1b protein gene of tobacco

Identification of Lipoxygenase gene repertoire of Cannabis sativa and functional characterization of CsLOX13 gene

Lipoxygenase (LOX) enzymes play a pivotal role in the biosynthesis of oxylipins. The phyto-oxilipins have been implicated in diverse aspects of plant biology, from regulating plant growth and development to providing tolerance against biotic and abiotic stresses. C. sativa is renowned for its bioactive secondary metabolites, namely cannabinoids. LOX route is assumed to be involved in the biosynthesis of hexanoic acid, which is one of the precursors of cannabinoids of C. sativa. For obvious reasons, the LOX gene family deserves thorough investigation in the C. sativa. Genome-wide analysis revealed the presence of 21 LOX genes in C. sativa, which can be further grouped into 13-LOX and 9-LOX depending upon their phylogeny as well as the enzyme activity. The promoter regions of the CsLOX genes were predicted to contain cis-acting elements involved in phytohormones responsiveness and stress response. The qRT-PCR-based expression analysis of 21 LOX genes revealed their differential expression in different plant parts (root, stem, young leaf, mature leaf, sugar leaf, and female flower). The majority of CsLOX genes displayed preferential expression in the female flower, which is the primary site for the biosynthesis of cannabinoids. The highest LOX activity and expression level of a jasmonate marker gene were reported in the female flowers among all the plant parts. Several CsLOX genes were found to be upregulated by MeJA treatment. Based on the transient expression in Nicotiana benthamiana and the development of stable Nicotiana tabacum transgenic lines, we demonstrate that CsLOX13 encodes functional lipoxygenase and play an important role in the biosynthesis of oxylipins.

Tobacco MYC2 regulates jasmonate-inducible nicotine biosynthesis genes directly and by way of the NIC2-locus ERF genes

In Arabidopsis, the MYC2-family basic helix-loop-helix transcription factors mediate transcriptional regulation of jasmonate-responsive genes, and their transcriptional activities are suppressed by physical interactions with jasmonate-ZIM domain (JAZ) proteins. Jasmonate-inducible nicotine formation in Nicotiana plants has been shown to be suppressed by tobacco JAZ proteins, and be regulated by both MYC2-related and NIC2-locus ethylene response factor (ERF) transcription factors. We here show that tobacco MYC2 (NtMYC2) recognizes the G-box sequences, 5'-CAC(G/A)T(G/T)-3', found in the proximal promoter regions of several nicotine biosynthesis genes, including Putrescine N-Methyltransferase 2 (PMT2) and Quinolinate Phosphoribosyltransferase 2 (QPT2). Transient transactivation assays using cultured tobacco cells showed that NtMYC2 and NIC2-locus ERF189 additively activated the PMT2 and QPT2 promoters depending on their cognate binding sites. RNA interference (RNAi) silencing of NtMYC2 in tobacco hairy roots strongly decreased transcript levels of jasmonate-responsive structural genes, including those involved in nicotine biosynthesis, as well as the NIC2-locus ERF genes. Conversely, ERF189 was not required for the expression of NtMYC2. NtMYC2, but not ERF189, interacted with tobacoo JAZs in a yeast two-hybrid assay. These results indicate that NtMYC2 controls nicotine biosynthesis genes in two combinatorial ways, by directly binding the G-box in the target promoters and by up-regulating the NIC2-locus ERF genes.

Disease resistance in plants that carry a feedback-regulated yeast poly(A) binding protein gene

It has been reported that the expression of the yeast poly(A) binding protein gene (PAB1) in plants leads to an induction of disease resistance responses, accompanied by alterations in the growth habit of the plant (Li et al. Plant Mol. Biol. (2000) 42 335). To capitalize on this observation, a feedback-regulated PAB1 gene was assembled and introduced into tobacco and Arabidopsis. The regulation entailed the linking of the expression of the PAB1 gene to control by the lac repressor, and by linking lac repressor expression to the disease resistance state of the plant, such that the induction of systemic defense responses by accumulation of the yeast poly(A) binding protein would turn off the expression of the PAB1 gene. Plants containing this system showed elevated and/or constitutive expression of disease-associated genes and significant resistance to otherwise pathogenic organisms. As well, they displayed a nearly normal growth habit under laboratory and greenhouse settings. These studies indicate that the expression of cytotoxic genes (such as the PAB1 gene) in plants can be controlled so that enhanced disease resistance can be achieved without significantly affecting plant growth and development.

The promoter of a basic PR1-like gene, AtPRB1, from Arabidopsis establishes an organ-specific expression pattern and responsiveness to ethylene and methyl jasmonate

Antimicrobial proteins are a key feature underlying the deployment of both pre-formed and inducible defence responses. Probably the most well characterised class are the pathogenesis-related (PR) proteins, which are found in both basic and acidic isoforms. Here we describe the isolation and characterisation of a gene, designated AtPRB1, encoding a basic PR1-like protein from Arabidopsis. This protein showed high amino acid sequence identity with basic and acidic PR1 proteins from other plant species, for example PRB1 from Nicotiana tabacum and PR1 from Brassica napus, at 64% and 78% identity respectively. A genomic DNA fragment containing 2345 bp upstream from the putative transcriptional start site was fused to the gene encoding the luciferase (LUC) gene from Photinus pyralis in order to test for promoter activity. The resulting construct was transformed into Arabidopsis accession Col-0 and analysis of LUC activity, using an ultra-low-light imaging camera system, revealed that the AtPRB1 promoter established an exquisite organ-specific expression pattern. LUC activity was observed in flowers, stems and roots but not in leaf tissue. Superimposed upon this organ-specific expression pattern was responsiveness, in root tissue, to ethylene and methyl jasmonate (MeJA), important cues during the establishment of plant disease resistance. In contrast, AtPRB1::LUC gene expression was repressed in response to salicylic acid treatment. Analysis of a limited series of AtPRB1 5'-promoter deletion mutants, identified a number of promoter regions important for both the establishment of organ-specific expression and responsiveness to ethylene and MeJA. While AtPRB1 gene expression was not induced in response to an avirulent isolate of Peronospora parasitica in leaf tissue, this gene may contribute to horizontal resistance in other tissues and/or to MeJA- and ethylene-dependent defence responses engaged against necrotrophic pathogens in root tissue. It is anticipated that transgenic plants containing AtPRB1-based promoter::reporter constructs will provide useful tools for the future dissection of the cognate signalling networks regulating the expression of this gene.

An as-1-like motif controls the level of expression of the gene for the pathogenesis-related protein 1a from tobacco

Pathogenesis-related proteins of group 1 (PR-1) are strongly induced in plants by pathogen attack, exposure of the plants to (acetyl)salicylic acid (ASA, SA), and by developmental cues. Functional analysis of the PR-1a promoter identified a region of 139 bp (from -691 to -553) mediating expression of the GUS reporter gene in response to ASA. Inspection of this region revealed two TGACG elements reminiscent of activation sequence-1 (as-1). Recently, as-1 has been reported to be responsive to SA in the context of the CaMV 35S RNA promoter. To address the question of whether the as-1-like sequence may be of functional significance for the expression of the PR-1a gene, gel shift assays were performed with TGA1a, a protein been shown to interact with as-1 in vitro. TGA1a was found to bind to the PR-1a as-1-like sequence with similar specificity and affinity as to as-1. Furthermore, mutations were introduced in the as-1-like sequence in the context of the inducible 906 bp PR-1a promoter which are impaired in binding TGA1a in vitro. Significantly reduced levels of GUS reporter gene activity were obtained with the mutant promoter regions as compared to the wild-type PR-1a promoter in response to all stimuli in transgenic tobacco plants. Yet, mutation of the as-1-like sequence did not abolish induction of reporter gene expression. Taken together, these results suggest that the level of expression of the tobacco PR-1a gene is controlled by an as-1-like sequence motif in the PR-1a upstream region, possibly interacting with a factor related to TGA1a.

A novel cDNA from Drosophila encoding a protein with similarity to mammalian cysteine-rich secretory proteins, wasp venom antigen 5, and plant group 1 pathogenesis-related proteins

The CAP protein family is made up of a group of secreted proteins that share sequence similarity. Members of this family are found in animals, plants, and fungi, and their shared sequence similarity suggests that members share a common, but as yet unknown, molecular function. As a first step in defining the function of CAP family proteins, an 878 bp partial cDNA encoding a novel member of the CAP family was cloned by the polymerase chain reaction (PCR) from total RNA of adult Drosophila. The cDNA contained the complete coding sequence for a protein 256 amino acids in length, as well as the complete 3' untranslated region (UTR) and a portion of the 5' UTR. The protein, named Antigen 5-related (Agr), was most similar in sequence to antigen 5 (Ag5), a CAP family member found in social wasps and ants. The corresponding Agr RNA is about 1 kb in length and is present at all stages of development, with highest levels observed in adults. Agr RNA is transcribed from a single gene that is located within region 12F of the X chromosome. The identification of Agr in Drosophila expands the number of known CAP family members to well over four dozen. Further studies of Agr and the gene which encodes this protein using the Drosophila model system may help provide important insight into the molecular functioning of this little known, but increasingly significant protein family.

The PR-1a promoter contains a number of elements that bind GT-1-like nuclear factors with different affinity

The 900 bp promoter region of the tobacco PR-1a gene was divided into eight fragments using PCR. The fragments were tested for their ability to bind to nuclear factors isolated from tobacco leaf. Band shift assays demonstrated that all but one of the fragments specifically interacted with nuclear proteins. From competition experiments it was determined that the same nuclear factors bind various promoter fragments with different affinity. Moreover, efficient competition with a synthetic tetramer of box II of the rbcS promoter indicated that GT-1-like nuclear factors are involved in these interactions. Furthermore, in comparison to extracts from untreated plants, nuclear protein preparations from tobacco mosaic virus-infected tobacco showed a reduced GT-1 binding activity. These results will be discussed in relation to induced PR-1a gene expression.

The upstream region of the gene for the pathogenesis-related protein 1a from tobacco responds to environmental as well as to developmental signals in transgenic plants

Pathogenesis-related proteins (PR proteins) are a heterogeneous group of proteins which are induced in plants by diverse stimuli, e.g. PR proteins are elicited by pathogen attack in the course of the hypersensitive defense reaction of the plant. To examine the regulation of these genes, the 5'-flanking region of the tobacco (Nicotiana tabacum cv. Wisconsin 38) PR-1a gene up to position -1533 was isolated from genomic DNA by the polymerase chain reaction. Two chimeric gene constructs containing 1533 bp and 906 bp, respectively, of the PR-1a upstream region fused to the GUS reporter gene were stably integrated into the tobacco genome. All primary transformants exhibited induced expression of the reporter gene after infection of the plants with tobacco mosaic virus or treatment with acetylsalicylic acid. In addition, similar expression of the reporter gene was observed in leaves of adult transgenic plants without any prior inductive treatments. To study this phenomenon in more detail, the F1 progeny of independent transgenic lines were monitored during the ontogeny of the plants. In normally developing tobacco plants, strong GUS activities were typically detected approximately 12 weeks after germination in the lowest leaves of vegetative plants. When successive leaves of individual plants were tested during the following weeks, a clear gradient of reporter gene activity had developed in the green leaves including the sepals from the bottom to the top of the plants. In all cases analyzed, this gradient of reporter gene expression was strictly parallelled by the expression of the endogenous PR-1 proteins. These results suggest that the acidic PR-1 proteins from tobacco fulfill a role during the later stages of plant development and that the PR-1a upstream region -906 to -335 contains positive regulatory elements for both environmental and developmental signals.

Sequence-specific binding of protein factors to two independent promoter regions of the acidic tobacco pathogenesis-related-1 protein gene (PR-1)

Gel shift mobility analysis, using the proximal 0.3 kb fragment of the tobacco pathogenesis-related protein 1a gene (PR-1a) and nuclear extracts from healthy Samsun NN tobacco leaves, which do not produce PR-1 proteins, showed a broad shifted signal with low mobility. This signal was not detected with nuclear proteins from the interspecific hybrid of Nicotiana glutinosa x Nicotiana debneyi, which constitutively produces the PR-1a protein. Similar shifted signals were detected with both proximal and distal regions of the 0.3 kb fragment using nuclear proteins from healthy Samsun NN tobacco, but not with proteins from the interspecific hybrid. Further experiments, performed using 5' or 3' truncated fragments of the 0.3 kb fragment, identified two independent binding sites: a distal site between -179 and -168 bp from the transcription start site, and a proximal site between -61 and -37 bp. Footprint analysis revealed two protected sequences, a distal region between -184 and -172 bp, and a proximal region between -68 and -51 bp. These results indicate the presence of regulatory factor(s) for expression of the acidic PR-1a gene. The possibility of negative regulation of the gene is discussed.

Activation of a truncated PR-1 promoter by endogenous enhancers in transgenic plants

PR-1 genes are induced by various environmental stimuli such as pathogen attack or exposure of the plants to certain chemicals. To examine the regulation of these genes, the 5' flanking regions of the PR-la gene and of two PR-1 pseudogenes were joined by a transcriptional fusion to the Escherichia coli beta-glucuronidase (GUS) gene. These constructs were stably integrated into the tobacco genome and independent primary transformants were monitored for the expression of the reporter gene. Unexpectedly, out of 55 transformants analysed, four plants exhibited considerable GUS activities without any inductive treatment of the plants. Expression of the endogenous PR-1 genes, however, could not be detected in these plants. Primer extension analyses revealed correct initiation of the PR1/GUS hybrid transcripts from the PR-1a TATA box. When the plants were analysed at the cellular level, clear differences regarding the tissue specificity of expression of the reporter gene were observed. These results strongly suggest that the PR1/GUS hybrid promoter expression cassettes may be activated when integrated in the vicinity of heterologous enhancer elements dispersed in the tobacco genome. In order to support this hypothesis, domain B of the enhancer of the 35S RNA promoter from cauliflower mosaic virus (CaMV) was fused to various PR1/GUS hybrid genes upstream as well as downstream from the RNA start site. These constructs were stably introduced into the tobacco genome. In any primary transformant analysed, strong GUS activities were observed with the PR1/GUS hybrid RNAs originating from the normal transcription start site of the PR-1a gene. The tissue specificity of gene expression was identical to that described previously for the CaMV 35S domain B enhancer element. Thus, modulations of the transcriptional activity of the PR-1 promoter can be achieved by heterologous enhancers in transgenic plants and may be encountered upon random integration of PR-1 promoter constructs into the tobacco genome.

Molecular analysis of two PR-1 pseudogenes from tobacco

Two independent PR-1 lambda genomic clones (W38/1 and W38/3) were isolated and characterized from a tobacco (Nicotiana tabacum cv. Wisconsin 38) library. Neither clone is identical to the previously described PR-1 cDNA clones, and both clones carry mutations within the highly conserved PR-1 protein coding region. For example, clone W38/1 has a GAA Glu codon instead of the translation stop codon thus harbouring an open reading frame extended by 16 additional amino acids. Furthermore, both clones display considerable variations in the genomic flanking sequences when compared to the PR-1a gene. In order to test whether the encoded genes are active, their upstream sequences were fused to the E. coli beta-glucuronidase (GUS) reporter gene. While significant GUS activities as compared to the 35S RNA promoter from cauliflower mosaic virus (CaMV) were obtained with the W38/1 and W38/3 sequences in transient gene expression assays, no transcriptional activities could be observed upon stable transformation of the same constructs. In addition, the protein coding region of W38/1 was joined to the CaMV 35S RNA promoter and transgenic tobacco plants were generated. However, neither transcripts nor a protein could be detected deriving from the W38/1 structural gene with this chimaeric construct in the transformants. Taken together, these data indicate that the genes contained in lambda clones W38/1 and W38/3 are not active in planta.