Reversible inhibition of tomato fruit senescence by antisense RNA

Characterization of a family of genes encoding a fruit-specific wound-stimulated protein of bell pepper (Capsicum annuum): identification of a new family of transposable elements

Using a fruit-specific cDNA as a probe we isolated and sequenced the two corresponding homologous genes (Sn-1 and Sn-2) of the bell pepper (Capsicum annuum) genome. Both genes have a single intron and numerous unusual long inverted repeat sequences. The introns share 87% homology and Sn-2 contains one 450 bp additional sequence with structural features of a transposable element, which is highly repetitive in the bell pepper genome. Surprisingly, analysis in data banks showed that genes encoding the potato starch phosphorylase (EC 2.4.1.1) and patatin contain a similar element, named Alien, in their 5'-upstream region. Alien elements are characterized by a conserved 28 bp terminal inverted repeat (TIR), small size, high AT content, potential to form stable DNA secondary structures and they have probably been inserted in TA target sites. Interestingly, the TIR of the Alien elements shares high homology with sequences existing in the TIR of extrachromosomal linear pSKL DNA plasmid of Saccharomyces kluyveri. Northern blot analyses detected Sn-1 transcripts principally in the red fruit whereas no Sn-2 transcripts were detected in neither of the samples monitored. Western blot analyses detected a 16.8 kDa Sn protein principally in the ripe red fruit and wounded areas of green unripe fruit. A comparison of the deduced amino acid sequence of Sn-1 with protein sequences in data banks revealed a significant homology with proteins likely involved in the plant's disease resistance response. Analyses at the subcellular level showed that Sn-1 is localized in the membrane of vacuoles.

Molecular genetic analysis of the ripening-inhibitor and non-ripening loci of tomato: a first step in genetic map-based cloning of fruit ripening genes

Ripening represents a complex developmental process unique to plants. We are using tomato fruit ripening mutants as tools to understand the regulatory components that control and coordinate the physiological and biochemical changes which collectively confer the ripe phenotype. We have genetically characterized two loci which result in significant inhibition of the ripening process in tomato, ripening-inhibitor (rin), and non-ripening (nor), as a first step toward isolating genes likely to encode key regulators of this developmental process. A combination of pooled-sample mapping as well as classical restriction fragment length polymorphism (RFLP) analysis has permitted the construction of high-density genetic maps for the regions of chromosomes 5 and 10 spanning the rin and nor loci, respectively. To assess the feasibility of initiating a chromosome walk, physical mapping of high molecular weight genomic DNA has been employed to estimate the relationship between physical distance (in kb) and genetic distance (in cM) around the targeted loci. Based on this analysis, the relationship in the region spanning the rin locus is estimated to be 200-300 kb/cM, while the nor locus region ratio is approximately 200 kb/1 cM. Using RFLP markers tightly linked to rin and nor, chromosome walks have been initiated to both loci in a yeast artificial chromosome (YAC) library of tomato genomic DNA. We have isolated and characterized several YAC clones linked to each of the targeted ripening loci and present genetic evidence that at least one YAC clone contains the nor locus.

High levels of ripening-specific reporter gene expression directed by tomato fruit polygalacturonase gene-flanking regions

The 1.4 kb 5' polygalacturonase (PG) gene-flanking region has previously been demonstrated to direct ripening-specific chloramphenicol acetyl transferase (CAT) expression in transgenic tomato plants. The steady state level of CAT mRNA in these plants was estimated to be less than 1% of the endogenous PG mRNA. Further constructs containing larger PG gene-flanking regions were generated and tested for their ability to direct higher levels of reporter gene expression. A 4.8 kb 5'-flanking region greatly increased levels of ripening-specific reporter gene activity, while a 1.8 kb 3' region was only shown to have a positive regulatory role in the presence of the extended 5' region. Transgenic plants containing the CAT gene flanked by both of these regions showed the same temporal pattern of accumulation of CAT and PG mRNA, and steady-state levels of the transgene mRNA were equivalent to 60% of the endogenous PG mRNA on a per gene basis. The proximal 150 bp of the PG promoter gave no detectable CAT activity. However, the distal 3.4 kb of the 4.8 kb 5' PG promoter was shown to confer high levels of ripening-specific gene expression when placed in either orientation upstream of the 150 bp minimal promoter. The DNA sequence of the 3.4 kb region revealed a 400 bp imperfect reverse repeat, and sequences which showed similarity to functionally significant sequences from the ripening-related, ethylene-regulated tomato E8 and E4 gene promoters. The possible roles of the flanking regions in regulating PG gene expression are discussed.

Cytomegalovirus-mediated induction of antisense mRNA expression to UL44 inhibits virus replication in an astrocytoma cell line: identification of an essential gene

We have used an antisense RNA approach in the analysis of gene function in human cytomegalovirus (HCMV). An astrocytoma cell line (U373-MG) that is permissive for virus replication was permanently transfected with a construct bearing sequence from HCMV UL44 (coding for the major late DNA-binding protein, ppUL44, also known as pp52 or ICP36) in an antisense orientation and under the control of the immediate-early enhancer-promoter element. Upon HCMV infection at a high multiplicity, we found a marked reduction in UL44 protein products (the ICP36 family of proteins) in established cell transfectants and a strong inhibition of virus yield in infected-cell supernatants at two weeks postinfection, while herpes simplex virus replication was not affected. In infected cells, viral DNA replication was strongly inhibited. While gene products such as pUS22 and pUL32 were also inhibited, pUL123 and pUL82 accumulated in the infected cells over time. Our data suggest an essential role for the UL44 family of proteins in HCMV replication and represent a model of virus inhibition by virus-induced antisense RNA synthesis in genetically modified cells.

Molecular cloning of an ozone-induced 1-aminocyclopropane-1-carboxylate synthase cDNA and its relationship with a loss of rbcS in potato (Solanum tuberosum L.) plants

Acute or chronic exposure of potato plants to ozone (O3) induces ethylene production. We isolated a 1586 bp cDNA (pOIP-1) encoding 1-aminocyclopropane-1-carboxylate (ACC) synthase from a cDNA library constructed with mRNA extracted from O3-treated leaves. The clone has a 1365 bp open reading frame and a 221 bp trailing sequence. The active site found in all ACC synthases and 11 of the 12 amino acid residues conserved in aminotransferases are found in pOIP-1. Northern analysis showed that the mRNA encoding ACC synthase was detectable 1 h after the onset of O3 exposure, and the message increased over time as did ethylene production. Concurrent with the increased ACC synthase mRNA was a decrease in the message for the Rubisco small subunit (rbcS) with no change in the large subunit (rbcL). When the plants were treated with aminooxyacetic acid (AOA), both ethylene production and level of ACC synthase transcript were inhibited. The decline in rbcS was also inhibited by AOA suggesting a correlation between ethylene production and loss of rbcS. Based on nuclear run-on studies it appears that the increase in ACC synthase mRNA may result from O3-induced transcriptional activity.

Factors affecting the inhibition by antisense RNA of granule-bound starch synthase gene expression in potato

Inhibition of expression of specific genes by means of antisense RNA is widely used, although little information is available regarding conditions that affect the efficacy of inhibition. In this study, inhibition of granule-bound starch synthase (GBSS), a key enzyme in starch biosynthesis, is used as a model system. Eleven antisense constructs derived from the full-length GBSS cDNA, the genomic GBSS coding region (gDNA) or fragments of each of these sequences, were analysed with respect to their inhibitory effect. Introduction of full-length gDNA constructs yielded a lower percentage of transgenic clones showing complete inhibition than did introduction of the full-length cDNA constructs. This may be caused by a lower antisense binding capacity of the former due to the relatively low GC content in intron sequences present in the gDNA constructs. The presence of multiple T-DNA insertions was related to a higher degree of inhibition. Putative polyadenylation signals on the antisense strand of the GBSS gene resulted in a premature stop of transcription of some of the antisense genes, as demonstrated by the expression of smaller antisense RNA transcripts. Introduction of antisense constructs driven by the promoter of the (target) GBSS gene resulted in a higher percentage of clones with complete inhibition than introduction of antisense constructs driven by the 35S CaMV promoter. Complete antisense inhibition was achieved in 25% of the clones carrying the antisense construct pKGBA50, which is based on the GBSS promoter and the full-length GBSS cDNA. Thus, it is concluded that the use of pKGBA50 is very suitable for the modification of the composition of potato tuber starch via antisense RNA.

Induction of tomato stress protein mRNAs by ethephon, 2,6-dichloroisonicotinic acid and salicylate

To study the possible involvement of plant hormones in the synthesis of stress proteins in tomato upon inoculation with Cladosporium fulvum, we investigated the induction of mRNAs encoding PR proteins and ethylene biosynthesis enzymes by ethephon, 2,6-dichloroisonicotinic acid (INA) and salicylic acid (SA) by northern blot analysis. Ethephon slightly induced some but not all mRNAs encoding intra- and extracellular PR proteins. INA induced all PR protein mRNAs analysed, except for intracellular chitinase and extracellular PR-4. SA induced all PR protein mRNAs analyzed, except for intracellular chitinase and osmotin. None of the inducers affected the expression of ACC synthase mRNA, whereas all three induced ethylene-forming enzyme (EFE) mRNA.

Expression of apple 1-aminocyclopropane-1-carboxylate synthase in Escherichia coli: kinetic characterization of wild-type and active-site mutant forms

The pyridoxal phosphate-dependent enzyme 1-aminocyclopropane-1-carboxylate synthase (ACC synthase; S-adenosyl-L-methionine methylthioadenosine-lyase, EC 4.4.1.14) catalyzes the conversion of S-adenosylmethionine (AdoMet) to ACC and 5'-methylthioadenosine, the committed step in ethylene biosynthesis in plants. Apple ACC synthase was overexpressed in Escherichia coli (3 mg/liter) and purified to near homogeneity. A continuous assay was developed by coupling the ACC synthase reaction to the deamination of 5'-methylthioadenosine by adenosine deaminase (adenosine aminohydrolase, EC 3.5.4.4) from Aspergillus oryzae. The enzyme is dimeric, with kcat = 9s-1 per monomer and Km = 12 microM for AdoMet. The pyridoxal phosphate-binding site of ACC synthase appears to be highly homologous to that of aspartate aminotransferase, suggesting similar roles for corresponding residues. Site-directed mutagenesis of Lys-273, Arg-407, and Tyr-233 (corresponding to residues 258, 386, and 225 in aspartate aminotransferase) and kinetic analyses of the mutants confirms their importance in the ACC synthase mechanism. The Lys-273 to Ala mutant has no detectable activity, supporting the identification of this residue as the base catalyzing C alpha proton abstraction. Mutation of Arg-407 to Lys results in a precipitous drop in kcat/Km and an increase in Km for AdoMet of at least 20-fold, in accordance with its proposed role as principal ligand for the substrate alpha-carboxylate group. Replacement of Tyr-233 with Phe causes a 24-fold increase in the Km for AdoMet and no change in kcat, suggesting that this residue plays a role in orienting the pyridoxal phosphate cofactor in the active site.

Field evaluation of transgenic potato plants expressing an antisense granule-bound starch synthase gene: increase of the antisense effect during tuber growth

Transgenic plants of a tetraploid potato cultivar were obtained in which the amylose content of tuber starch was reduced via antisense RNA-mediated inhibition of the expression of the gene encoding granule-bound starch synthase (GBSS). GBSS is one of the key enzymes in the biosynthesis of starch and catalyses the formation of amylose. The antisense GBSS genes, based on the full-length GBSS cDNA driven by the 35S CaMV promoter or the potato GBSS promoter, were introduced into the potato genome by Agrobacterium tumefaciens-mediated transformation. Expression of each of these genes resulted in the complete inhibition of GBSS gene expression, and thus in the production of amylose-free tuber starch, in mature field-grown plants originating from rooted in vitro plantlets of 4 out of 66 transgenic clones. Clones in which the GBSS gene expression was incompletely inhibited showed an increase of the extent of inhibition during tuber growth. This is likely to be due to the increase of starch granule size during tuber growth and the specific distribution pattern of starch components in granules of clones with reduced GBSS activity. Expression of the antisense GBSS gene from the GBSS promoter resulted in a higher stability of inhibition in tubers of field-grown plants as compared to expression from the 35S CaMV promoter. Field analysis of the transgenic clones indicated that inhibition of GBSS gene expression could be achieved without significantly affecting the starch and sugar content of transgenic tubers, the expression level of other genes involved in starch and tuber metabolism and agronomic characteristics such as yield and dry matter content.

Reduced ethylene synthesis by transgenic tomatoes expressing S-adenosylmethionine hydrolase

We have utilized a gene from bacteriophage T3 that encodes the enzyme S-adenosylmethionine hydrolase (SAMase) to generate transgenic tomato plants that produce fruit with a reduced capacity to synthesize ethylene. S-adenosylmethionine (SAM) is the metabolic precursor of 1-aminocyclopropane-1-carboxylic acid, the proximal precursor to ethylene. SAMase catalyzes the conversion of SAM to methylthioadenosine and homoserine. To restrict the presence of SAMase to ripening fruit, the promoter from the tomato E8 gene was used to regulate SAMase gene expression. Transgenic tomato plants containing the 1.1 kb E8 promoter bore fruit that expressed SAMase during the breaker and orange stage of fruit ripening and stopped expression after the fruit fully ripened. Plants containing the 2.3 kb E8 promoter expressed SAMase at higher levels during the post-breaker phases of fruit ripening and had a substantially reduced capacity to synthesize ethylene.

Effects of an antisense napin gene on seed storage compounds in transgenic Brassica napus seeds

To manipulate the quantity and quality of storage components in Brassica napus seeds, we have constructed an antisense gene for the storage protein napin. The antisense gene was driven by the 5'-flanking region of the B. napus napin gene to express antisense RNA in a seed-specific manner. Seeds of transgenic plants with antisense genes often contained reduced amounts of napin. In some transgenic plants, no accumulation of napin was observed. However, the total protein content of transgenic and wild-type seeds did not differ significantly. Seeds lacking napin accumulated 1.4 to 1.5 times more cruciferin than untransformed seeds, although the oleosin content was not affected. Fatty acid content and composition in the seeds of transgenic plants were also analyzed by gas chromatography. Though the total fatty acid content of the transformants was the same as that of non-transformants, there was a reduction in 18:1 contents and a concomitant increase of 18:2 in seeds with reduced napin levels. This observed change in fatty acid composition was inherited in the next generation.

Timing of fungal invasion using host's ripening hormone as a signal

In many postharvest fruit diseases, fungi remain latent until the fruit ripens. How the fungus times its infection at ripening of the host is not known. We have found that the volatiles produced by the climacteric tomato, avocado, and banana fruits induce germination and appressorium formation in Colletotrichum gloeosporioides and Colletotrichum musae. Exposure of the spores of these fungi to ethylene, the host's ripening hormone, at </=1 microl/liter, caused germination, branching of the germ tube, and formation of up to six appressoria from a single spore. Propylene, an ethylene analog, but not the hydrocarbon gas methane was able to induce spore germination and multiple appressorium formation. The ethylene effect on the fungi appears to be a plant-like response as it was inhibited by silver ion and 2,5-norbornadiene; the inhibition by the latter could be reversed by higher ethylene concentrations. Ethylene induced germination and appressorium formation in the Colletotrichum sp. penetrating climacteric fruit but not in other Colletotrichum strains. That the ethylene induction of multiple appressorium formation could be relevant to postharvest infection was indicated by the observation that C. gloeosporioides spores formed multiple appressoria on normally ripening tomato that produces ethylene, whereas on transgenic tomato and orange, fruits incapable of producing ethylene, exogenous ethylene was required to induce multiple appressorium formation and lesion formation. These results strongly suggest that these fungi must have coevolved to develop a mechanism to use the host's ripening hormone as a signal to differentiate into multiple infection structure and thus time the infection process.

S proteins control rejection of incompatible pollen in Petunia inflata

Flowering plants have evolved various stratagems to prevent inbreeding and promote outcrosses. One such mechanism, gametophytic self-incompatibility, provides a genetic barrier to self-fertilization, and in the simplest cases is controlled by the highly polymorphic S locus. Growth of a pollen tube in the style is arrested when the S allele carried by the pollen matches one of the two S alleles carried by the pistil. Putative S allele proteins of the pistil have been identified in several solanaceous species based on their co-segregation with S alleles, and they have been shown to be ribonucleases. So far, there has been only correlative or indirect evidence for the claim that these S allele-associated proteins (S proteins) are involved in recognition and rejection of self pollen. Here we show that inhibition of synthesis of S3 and S2 proteins in Petunia inflata plants of S2S3 genotype by the antisense S3 gene resulted in failure of the transgenic plants to reject S3 and S2 pollen. We further show that expression of the transgene encoding S3 protein in P. inflata plants of S1S2 genotype confers on the transgenic plants the ability to reject S3 pollen. The self-incompatibility behaviour of the pollen was not affected by the transgene in either set of experiments. Taken together, these findings provide direct in vivo evidence that S proteins control the self-incompatibility behaviour of the pistil.

Fruit developmental regulation of the kiwifruit actinidin promoter is conserved in transgenic petunia plants

We have examined the expression of actinidin, a cysteine protease found in kiwifruit, over the course of fruit development. Protease activity was first seen in fruit that had reached about half their final weight, and rose to high levels at harvest. The 5'-flanking region (nucleotides -1301 to +58) of a kiwifruit actinidin gene was fused to the beta-glucuronidase (GUS)-coding region, and the chimaeric gene was introduced into transgenic petunia plants. Induction of the GUS gene was observed during the later stages of seed pod development, closely resembling the pattern of actinidin induction in fruit tissues of kiwifruit. Some GUS expression was also detected in the vascular system of the receptacle, leaves, stems and roots. A shorter promoter fragment consisting of nucleotides -115 to +58 conferred similar spatial and temporal regulation in some of the transgenic plants.

Ethylene gas: it's not just for ripening any more!

Hormones play a central role in regulating plant growth and development. There are five well-characterized plant hormones, the chemically simplest of which is the gaseous olefin ethylene. Molecular genetic studies of Arabidopsis are beginning to reveal the mechanisms controlling ethylene biosynthesis, perception and signal transduction.

Differential expression of the partially duplicated chloroplast S10 ribosomal protein operon

The chloroplast S10 ribosomal protein operon is partially duplicated in many plants because it initiates within the inverted repeat of the circular chloroplast genome. In spinach, the complete S10 operon (S10B) spans the junction between inverted repeat B (IRB) and the large single-copy (LSC) region. The S10 operon is partially duplicated in the inverted repeat A (IRA), but the sequence of S10A completely diverges from S10B at the junction of S10A and the LSC region. The DNA sequence shared by S10A and S10B includes trnI1, the rpl23 pseudogene (rpl23 psi), the intron-containing rpl2 and rps19, which is truncated in S10A at the S10A/LSC junction (rps19'). Transcription of rps19' from the promoter region of S10A could result in the synthesis of a mutant S19 protein. Analysis of RNA accumulation and run-on transcription from S10A and S10B using unique probes from the S10A/LSC and S10B/LSC junctions reveals that expression of S10A is reduced. The difference in S10A and S10B expression appears to be the result of reduced transcription from S10A, rather than differences in RNA stability. Transcription of S10B can initiate at three distinct promoter regions, P1, P2 and P3, which map closely to transcripts detected by S1 nuclease analysis. P1 is located upstream of trnI1 and has the highest transcription initiation frequency in vitro of the three promoter regions. The DNA sequence of P1 is most similar to the chloroplast promoter consensus DNA sequence. Interference by the highly and convergently transcribed psbA-trnH1 operon is considered as a mechanism to explain the reduced activity of the S10A promoters.

Identification of an ethylene-responsive region in the promoter of a fruit ripening gene

Transcription of the E4 gene is controlled by an increase in ethylene concentration during tomato fruit ripening. To investigate the molecular basis for ethylene regulation, we have examined the E4 promoter to identify cis elements and trans-acting factors that are involved in E4 gene expression. In transgenic tomato plants a chimeric gene construct containing a 1.4-kilobase E4 promoter fused to a beta-glucuronidase reporter gene is rapidly induced by ethylene in ripening fruit. Deletion of E4 promoter sequences to 193 base pairs reduces the level of GUS activity but does not affect ethylene induction. Transient expression of E4 promoter-luciferase chimeric gene constructs containing various deletions, introduced into tomato fruit pericarp by particle bombardment, indicates that a positive ethylene-responsive region is localized between nucleotides -161 and -85 relative to the transcription start site. DNase I footprint analysis shows that a nuclear factor in unripe fruit interacts specifically with sequences in this element, from -142 to -110, which are required for the ethylene response. The DNase I footprint of this factor is reduced in ethylene-treated unripe fruit and undetectable in ripe fruit. Based on the correlation of a nuclear factor binding site with promoter sequences required for ethylene induction, we propose that this in vitro DNA-binding activity may represent a factor that is involved in ethylene-regulated E4 gene expression.

Isolation of a ripening and wound-induced cDNA from Cucumis melo L. encoding a protein with homology to the ethylene-forming enzyme

A cDNA clone (pMEL1) was isolated from a climacteric melon fruit cDNA library using the tomato ethylene-forming-enzyme (EFE) cDNA, pTOM13, as a probe. Northern analysis of RNA isolated from wounded leaf and fruit tissue and from preclimacteric and climacteric pericarp tissue was used to determine the pattern of pMEL1 RNA expression. pMEL1 hybridized to a 1.3-kb transcript in climacteric fruit and wounded leaf tissue but was undetectable in preclimacteric fruit and unwounded leaves. Maximal expression of pMEL1 RNA occurred in wounded ripe fruit. pMEL1 contained a 1230-bp insert containing a predicted open reading frame of 318 amino acids and molecular mass of 35.3 kDa. The predicted amino acid sequence of pMEL1 was 73-81% identical to the deduced amino acid sequences of tomato (pTOM13) EFE and EFE-related genes isolated from tomato and avocado fruit and senescent carnation petals. Genomic Southern analysis indicated that pMEL1 hybridized to a number of genomic fragments consistent with the presence of more than one pMEL1-related gene in melon. On Western analysis of total protein extracts from ripe tomato and melon fruit, using an antibody raised against tomato EFE (pTOM13) expressed in Escherichia coli, a single 35.5-kDa protein was detected. A 35-kDa product translated from in-vitro transcribed pMEL1 and immunoadsorbed by anti-EFE serum was very similar in size to the predicted 35.3-kDa pMEL1 cDNA protein product. These results indicate that pMEL1 may encode an EFE gene involved in ethylene biosynthesis during fruit ripening and may be identical to or share extensive sequence similarity with an EFE expressed in response to tissue wounding.

CTR1, a negative regulator of the ethylene response pathway in Arabidopsis, encodes a member of the raf family of protein kinases

We isolated a recessive Arabidopsis mutant, ctr1, that constitutively exhibits seedling and adult phenotypes observed in plants treated with the plant hormone ethylene. The ctr1 adult morphology can be phenocopied by treatment of wild-type plants with exogenous ethylene and is due, at least in part, to inhibition of cell elongation. Seedlings and adult ctr1 plants show constitutive expression of ethylene-regulated genes. The epistasis of ctr1 and other ethylene response mutants has defined the position of CTR1 in the ethylene signal transduction pathway. The CTR1 gene has been cloned, and the DNA sequences of four mutant alleles were determined. The gene encodes a putative serine/threonine protein kinase that is most closely related to the Raf protein kinase family.

Use of a tomato mutant constructed with reverse genetics to study fruit ripening, a complex developmental process

Fruit ripening is one of the most dramatic developmental transitions associated with extensive alteration in gene expression. The plant hormone ethylene is considered to be the causative ripening agent. Transgenic tomato plants were constructed expressing antisense or sense RNA to the key enzyme in the ethylene (C2H4) biosynthetic pathway, 1-aminocyclopropane-1-carboxylate (ACC) synthase using the constitutive CaMV 35S and fruit specific E8 promoters. Fruits expressing antisense LE-ACS2 RNA produce less ethylene and fail to ripen only when ethylene production is suppressed by more than 99% (> 0.1 nl/g fresh weight). Ethylene production is considerably inhibited (50%) in fruits expressing sense LE-ACS2 RNA. Antisense fruits accumulate normal levels of polygalacturonase (PG), ACC oxidase (pTOM13), E8, E17, J49, and phytoene desaturase (D2) mRNAs which were previously thought to be ethylene-inducible. E4 gene expression is inhibited in antisense fruits and its expression is not restored by treatment with exogenous propylene (C3H6). Antisense fruits accumulate PG mRNA, but it is not translated. Immunoblotting experiments indicate that the PG protein is not expressed in antisense fruits but its accumulation is restored by propylene (C3H6) treatment. The results suggest that at least two signal-transduction pathways are operating during tomato fruit ripening. The independent (developmental) pathway is responsible for the transcriptional activation of genes such as PG, ACC oxidase, E8, E17, D2, and J49. The ethylene-dependent pathway is responsible for the transcriptional and posttranscriptional regulation of genes involved in lycopene, aroma biosynthesis, and the translatability of developmentally regulated genes such as PG.

Modulation of glutamine synthetase gene expression in tobacco by the introduction of an alfalfa glutamine synthetase gene in sense and antisense orientation: molecular and biochemical analysis

A glutamine synthetase (GS) cDNA isolated from an alfalfa cell culture cDNA library was found to represent a cytoplasmic GS. The full-length alfalfa GS1 coding sequence, in both sense and antisense orientation and under the transcriptional control of the cauliflower mosaic virus 35S promoter, was introduced into tobacco. Leaves of tobacco plants transformed with the sense construct contained greatly elevated levels of GS transcript and GS polypeptide which assembled into active enzyme. Leaves of the plants transformed with the antisense GS1 construct showed a significant decrease in the level of both GS1 and GS2 polypeptides and GS activity, but did not show any significant decrease in the level of endogenous GS mRNA. We have proposed that antisense inhibition using a heterologous antisense GS RNA occurs at the level of translation. Our results also suggest that the post-translational assembly of GS subunits into a holoenzyme requires an additional factor(s) and is under regulatory control.

Cellulase and polygalacturonase involvement in the abscission of leaf and fruit explants of peach

Ethylene-induced abscission in leaf and fruit explants of peach involves different enzymes. In leaves abscission is accompanied by increased occurrence of cellulase forms differing in isoelectric point (pI 6.5 and 9.5). A polypeptide with a molecular mass of 51 kDa gives in a western blot a strong cross-reaction with an antibody raised against a maturation cellulase from avocado fruit. Cellulase activity is also found in abscising fruit explants but the amount is very low compared to that of the leaf explants. A northern analysis with a cellulase clone from avocado reveals the presence of two hybridizing mRNAs with a size of 2.2 kb and 1.8 kb, respectively. The steady-state level of the 2.2 kb mRNA is significantly increased by treatment with ethylene. Polygalacturonases are not detected in abscising leaves, but are strongly induced by ethylene in fruit explants. Of the three forms found, two are exopolygalacturonases while the third is an endoenzyme. Ethylene activates preferentially the endoenzyme and the basic exoenzyme but depresses the acid exopolygalacturonases. A northern analysis carried out with a cDNA coding for tomato endopolygalacturonase shows hybridization only with one endopolygalacturonase mRNA form in the fruit abscission zone. Treatment with ethylene causes an increase in the steady-state level of this mRNA. The differences in the enzyme patterns observed in fruit and leaf abscission zones and a differential enzyme induction suggest the feasibility to regulate fruit abscission in peach with the aid of antisense RNA genes.