Characterization of an ethylene-regulated flower senescence-related gene from carnation

Building a Flower: The Influence of Cell Wall Composition on Flower Development and Reproduction

Floral patterning is a complex task. Various organs and tissues must be formed to fulfill reproductive functions. Flower development has been studied, mainly looking for master regulators. However, downstream changes such as the cell wall composition are relevant since they allow cells to divide, differentiate, and grow. In this review, we focus on the main components of the primary cell wall-cellulose, hemicellulose, and pectins-to describe how enzymes involved in the biosynthesis, modifications, and degradation of cell wall components are related to the formation of the floral organs. Additionally, internal and external stimuli participate in the genetic regulation that modulates the activity of cell wall remodeling proteins.

Down-Regulation of PpBGAL10 and PpBGAL16 Delays Fruit Softening in Peach by Reducing Polygalacturonase and Pectin Methylesterase Activity

β-galactosidases are cell wall hydrolases that play an important role in fruit softening. However, PpBGALs mechanism impacting on ethylene-dependent peach fruit softening was still unclear. In this study, we found that PpBGAL4, -6, -8, -10, -16, and -17 may be required for ethylene-dependent peach softening and PpBGAL10, -16 may make a main contribution to it among 17 PpBGALs. Utilization of virus-induced gene silencing (VIGS) showed that fruits were firmer than those of the control at 4 and 6 days after harvest (DAH) when PpBGAL10 and PpBGAL16 expression was down-regulated. Suppression of PpBGAL10 and PpBGAL16 expression also reduced PpPG21 and PpPME3 transcription, and polygalacturonase (PG) and pectinmethylesterases (PME) activity. Overall, total cell wall material and protopectin slowly declined, water-soluble pectin slowly increased, and cellulose and hemicellulose was altered significantly at 4 DAH, relative to control fruit. In addition, PpACO1 expression and ethylene production were also suppressed at 4 DAH because of inhibiting PpBGAL10 and PpBGAL16 expression. These results suggested that down-regulation of PpBGAL10 and PpBGAL16 expression delays peach fruit softening by decreasing PG and PME activity, which inhibits cell wall degradation and ethylene production.

Simultaneous knock-down of six β-galactosidase genes in petunia petals prevents loss of pectic galactan but decreases petal strength

Galactose (Gal) is incorporated into cell wall polysaccharides as flowers open, but then is lost because of β-galactosidase activity as flowers mature and wilt. The significance of this for flower physiology resides in the role of galactan-containing polysaccharides in the cell wall, which is still largely unresolved. To investigate this, transcript accumulation of six cell wall-associated β-galactosidases was simultaneously knocked down in 'Mitchell' petunia (Petunia axillaris x (P. axillaris x P. hybrida)) flower petals. The multi-PhBGAL RNAi construct targeted three bud- and three senescence-associated β-galactosidase genes. The petals of the most down-regulated line (GA19) were significantly disrupted in galactose turnover during flower opening, and at the onset of senescence had retained 86% of their galactose compared with 20% in the controls. The Gal content of Na₂CO₃-soluble cell wall extracts and the highly insoluble polysaccharides associated with cellulose were particularly affected. Immunodetection with the antibody LM5 showed that much of the cell wall Gal in GA19 was retained as galactan, presumably the side-chains of rhamnogalacturonan-I. The flowers of GA19, despite having retained substantially more galactan, were no different from controls in their internal cell arrangement, dimensions, weight or timing of opening and senescence. However, the GA19 petals had less petal integrity (as judged by force required to cause petal fracture) after opening and showed a greater decline in this integrity with time than controls, raising the possibility that galactan loss is a mechanism for helping to maintain petal tissue cohesion after flower opening.

Galactose metabolism in cell walls of opening and senescing petunia petals

Galactose was the major non-cellulosic neutral sugar present in the cell walls of 'Mitchell' petunia (Petunia axillaris x P. axillaris x P. hybrida) flower petals. Over the 24 h period associated with flower opening, there was a doubling of the galactose content of polymers strongly associated with cellulose and insoluble in strong alkali ('residual' fraction). By two days after flower opening, the galactose content of both the residual fraction and a Na(2)CO(3)-soluble pectin-rich cell wall fraction had sharply decreased, and continued to decline as flowers began to wilt. In contrast, amounts of other neutral sugars showed little change over this time, and depolymerisation of pectins and hemicelluloses was barely detectable throughout petal development. Size exclusion chromatography of Na(2)CO(3)-soluble pectins showed that there was a loss of neutral sugar relative to uronic acid content, consistent with a substantial loss of galactose from rhamnogalacturonan-I-type pectin. beta-Galactosidase activity (EC 3.2.1.23) increased at bud opening, and remained high through to petal senescence. Two cDNAs encoding beta-galactosidase were isolated from a mixed stage petal library. Both deduced proteins are beta-galactosidases of Glycosyl Hydrolase Family 35, possessing lectin-like sugar-binding domains at their carboxyl terminus. PhBGAL1 was expressed at relatively high levels only during flower opening, while PhBGAL2 mRNA accumulation occurred at lower levels in mature and senescent petals. The data suggest that metabolism of cell wall-associated polymeric galactose is the major feature of both the opening and senescence of 'Mitchell' petunia flower petals.

Enzymatic activity and substrate specificity of recombinant tomato beta-galactosidases 4 and 5

The open reading frames of tomato beta-galactosidase (TBG) 4 and 5 cDNAs were expressed in yeast, and the enzymes properties and substrate specificities were investigated. The two enzymes had peak activities between pH 4-4.5 and 37-45 degrees C. TBG4 specifically hydrolyzed beta-(1-->4) and 4-linked galactooligosaccharides. TBG5 had a strong preference to hydrolyze beta-(1-->3) and beta-(1-->6)-linked galactooligosaccharides. Exo-beta-galactanase activity of the TBG enzymes was measured by determining the release of galactosyl residues from native tomato cell wall fractions throughout fruit development and ripening. Both TBGs released galactose from all of the fractions and stages tested. TBG4 activity was highest using chelator soluble pectin and alkali soluble pectin at the turning stage of ripening. Using aminopyrene trisulfonate labeled substrates, TBG4 was the only enzyme with strong exo-beta-(1-->4)-galactanase activity on 5 mer or greater galactans. TBG4 and TBG5 were both able to degrade galactosylated rhamnogalacturonan. Neither enzyme was able to degrade galactosylated xyloglucan.

Regulation and execution of molecular disassembly and catabolism during senescence

Senescence is a highly orchestrated developmental stage in the life cycle of plants. The onset of senescence is tightly controlled by signaling cascades that initiate changes in gene expression and the synthesis of new proteins. This complement of new proteins includes hydrolytic enzymes capable of executing catabolism of macromolecules, which in turn sets in motion disassembly of membrane molecular matrices, leading to loss of cell function and, ultimately, complete breakdown of cellular ultrastructure. A distinguishing feature of senescence that sets it apart from other types of programmed cell death is the recovery of carbon and nitrogen from the dying tissue and their translocation to growing parts of the plant such as developing seeds. For this to be accomplished, the initiation of senescence and its execution have to be meticulously regulated. For example, the initiation of membrane disassembly has to be intricately linked with the recruitment of nutrients because their ensuing translocation out of the senescing tissue requires functional membranes. Molecular mechanisms underlying this linkage and its integration with the catabolism of macromolecules in senescing tissues are addressed.

Sucrose prevents up-regulation of senescence-associated genes in carnation petals

cDNA microarrays were used to characterize senescence-associated gene expression in petals of cut carnation (Dianthus caryophyllus) flowers, sampled from anthesis to the first senescence symptoms. The population of PCR fragments spotted on these microarrays was enriched for flower-specific and senescence-specific genes, using subtractive hybridization. About 90% of the transcripts showed a large increase in quantity, approximately 25% transiently, and about 65% throughout the 7 d experiment. Treatment with silver thiosulphate (STS), which blocks the ethylene receptor and prevented the normal senescence symptoms, prevented the up-regulation of almost all of these genes. Sucrose treatment also considerably delayed visible senescence. Its effect on gene expression was very similar to that of STS, suggesting that soluble sugars act as a repressor of ethylene signal transduction. Two fragments that encoded a carnation EIN3-like (EIL) protein were isolated, some of which are key transcription factors that control ethylene response genes. One of these (Dc-EIL3) was up-regulated during senescence. Its up-regulation was delayed by STS and prevented by sucrose. Sucrose, therefore, seems to repress ethylene signalling, in part, by preventing up-regulation of Dc-EIL3. Some other transcription factors displayed an early increase in transcript abundance: a MYB-like DNA binding protein, a MYC protein, a MADS-box factor, and a zinc finger protein. Genes suggesting a role in senescence of hormones other than ethylene encoded an Aux/IAA protein, which regulate transcription of auxin-induced genes, and a cytokinin oxidase/dehydrogenase, which degrades cytokinin. Taken together, the results suggest a master switch during senescence, controlling the co-ordinated up-regulation of numerous ethylene response genes. Dc-EIL3 might be (part of) this master switch.

Purification of a beta-galactosidase from cotyledons of Hymenaea courbaril L. (Leguminosae). Enzyme properties and biological function

Beta-galactosidases are enzymes that can be found in most living beings and in the plant kingdom its activity and genes have been detected in several tissues such as ripening fruits, developing leaves and flowers and storage tissues such as cotyledons. In plants, their activities are usually associated with the secondary metabolism or with oligosaccharide or polysaccharide degradation. Polysaccharide specific beta-galactosidases include beta-galactanases, which attack pectic polymers and beta-galactosidases that attack xyloglucans (XG). In the present work we purified an XG-specific beta-galactosidase (named hcbetagal) from cotyledons of developing seedlings of Hymenaea courbaril, a legume tree from the Neotropical region of the world. The enzyme has a molecular weight of 52-62 kDa and was shown to attack specifically xyloglucan oligosaccharides (XGOs) but not the polymer. It has a pH optimum between 3 and 4 and at this pH range the enzyme increases activity linearly up to 50 degrees C. Kinetic studies showed that hcbetagal is inhibited competitively by free galactose (K(i) = 3.7). The biochemical properties of hcbetagal as a whole suggest that it is involved in storage xyloglucan mobilisation during seedling development. Its high specificity towards XGOs, the low pH optimum and the fact that it is inhibited by its product (galactose) suggest that hcbetagal might be one of the biochemical control points in xyloglucan catabolism in vivo. A possible relationship with functional stability of the wall during cell death as cotyledons undergo senescence is discussed.

Isolation of the promoter of a cotton beta-galactosidase gene (GhGal1) and its expression in transgenic tobacco plants

Beta-galactosidases (EC 3.2.1.23) constitute a widespread family of glycosyl hydrolases in plants and are thought to be involved in metabolism of cell wall polysaccharides. A cDNA of the cotton (Gossypium hirsutum) beta-galactosidase gene, designated GhGal1, has previously been identified and its transcripts are highly abundant at the elongation stage of the cotton fiber. To examine the temporal and spatial control of GhGal1 expression, a transcriptional fusion of the GhGal1 promoter region (1770 bp) with the beta-glucuronidase (GUS) reporter gene was introduced into tobacco plants by the Agrobacterium infection method. The resulting transgenic plants showed higher GUS activity of fruit in the transgenic plants than that in the negative and positive controls. Histochemical localization of GUS activity demonstrated that the expression of the GUS gene could be found in the meristem zones of roots, cotyledons, vascular tissues, fruit and trichomes in transgenic tobacco plants. Additionally, sequence analysis of the regulatory region also revealed several conserved motifs among which some were shared with previously reported fruit/seed-specific elements and the others were related with trichome expression. These results indicated the temporal and spatial expression characterization of the GhGal1 promoter in transgenic tobacco plants and provided an important insight into the roles of GhGal1 in cotton fiber development.

A beta-galactosidase gene is expressed during mature fruit abscission of 'Valencia' orange (Citrus sinensis)

beta-galactosidases have been detected in a wide range of plants and are characterized by their ability to hydrolyse terminal non-reducing beta-D-galactosyl residues from beta-D-galactosides. These enzymes have been detected in a wide range of plant organs and tissues. In a search for differentially expressed genes during the abscission process in citrus, sequences encoding beta-galactosidase were identified. Three cDNA fragments of a beta-galactosidase gene were isolated from a cDNA subtraction library constructed from mature fruit abscission zones 48 h after the application of a mature fruit-specific abscission agent, 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMN-pyrazole). Based on sequence information derived from these fragments, a full-length cDNA of 2847 nucleotides (GenBank accession number AY029198) encoding beta-galactosidase was isolated from mature fruit abscission zones by 5'- and 3'-RACE approaches. The beta-galactosidase cDNA encoded a protein of 737 amino acid residues with a calculated molecular weight of 82 kDa. The deduced protein was highly homologous to plant beta-galactosidases expressed in fruit ripening. Southern blot analysis demonstrated that at least two closely related beta-galactosidase genes were present in 'Valencia' orange. Temporal expression patterns in mature fruit abscission zones indicated beta-galactosidase mRNA was detected 48 h after treatment of CMN-pyrazole and ethephon in mature fruit abscission zones. beta-galactosidase transcripts were detected in leaf abscission zones only after ethephon application. The citrus beta-galactosidase was expressed in stamens and petals of fully opened flowers and young fruitlets. The results suggest that this beta-galactosidase may play a role during abscission as well as early growth and development processes in flowers and fruitlets.

Barley beta-galactosidase: structure, function, heterogeneity, and gene origin

Barley (Hordeum vulgare) beta-galactosidase is composed of a large (45 kDa) and a small (33 kDa) polypeptide. N-terminal sequencing of the polypeptides and antibody reactivity data place the barley enzyme and heterodimeric plant beta-galactosidases from jack bean, maize, and wheat in family 35 of the glycosyl hydrolases. Sequence analysis indicates the existence of a subfamily of genes coding for polypeptide precursors that are cleaved to produce the two subunits in heterodimeric beta-galactosidases. The heterogeneity of the barley holoenzyme is related, but not restricted, to the N-glycosylation of the small polypeptide. Both polypeptides are essential for the catalytic activity of the enzyme.

A beta-galactosidase-like gene is expressed during tobacco pollen development

cDNA clone (TP5) with significant homology to ss-galactosidases has been isolated from a mature tobacco pollen cDNA library by differential screening. The predicted protein of 715 aa shows high levels of homology to plant beta-galactosidases expressed during fruit ripening and senescence. Northern analysis shows that the TP5 transcript is expressed exclusively in developing anthers and mature pollen. The transcript is present at very low levels at meiosis and increases dramatically, late in microspore development after mitosis suggesting that the primary role for the protein is during pollen tube growth. beta-galactosidase activity, measured by scanning densitometry of histochemically stained tobacco microspores, is first detectable in the early to mid-vacuolate stage, and reaches a peak at microspore mitosis, thereafter decreasing as the microspores reach maturity. Southern analysis indicates that the TP5 gene is present in two copies, probably corresponding to the two ancestral genomes of N. tabacum.

A family of at least seven beta-galactosidase genes is expressed during tomato fruit development

During our search for a cDNA encoding beta-galactosidase II, a beta-galactosidase/exogalactanase (EC 3.2.1.23) present during tomato (Lycopersicon esculentum Mill.) fruit ripening, a family of seven tomato beta-galactosidase (TBG) cDNAs was identified. The shared amino acid sequence identity among the seven TBG clones ranged from 33% to 79%. All contained the putative active site-containing consensus sequence pattern G-G-P-[LIVM]-x-Q-x-E-N-E-[FY] belonging to glycosyl hydrolase family 35. Six of the seven single-copy genes were mapped using restriction fragment length polymorphisms of recombinant inbred lines. RNA gel-blot analysis was used to evaluate TBG mRNA levels throughout fruit development, in different fruit tissues, and in various plant tissues. RNA gel-blot analysis was also used to reveal TBG mRNA levels in fruit of the rin, nor, and Nr tomato mutants. The TBG4-encoded protein, known to correspond to beta-galactosidase II, was expressed in yeast and exo-galactanase activity was confirmed via a quantified release of galactosyl residues from cell wall fractions containing beta(1-->4)-D-galactan purified from tomato fruit.

Xylem-specific expression of wound-inducible rice peroxidase genes in transgenic plants

A peroxidase gene, poxA, was isolated from a rice (Oryza sativa L.) genomic library. The gene consists of four exons whose combined sequences were identical to that of the prxRPA mRNA whose levels were dramatically stimulated by wounding as well as by treatment of rice shoots with ethephon or UV irradiation [H. Ito, F. Kimizuka, A. Ohbayashi, H. Matsui, M. Honma, A. Shinmyo, Y. Ohashi, A.B. Caplan, R.L. Rodriguez, Molecular cloning and characterization of two complementary DNAs encoding putative peroxidases from rice (Oryza sativa L.) shoots, Plant Cell Rep. 13 (1994) 361-366]. The temporal and spatial expression properties of the poxA gene promoter as well as that from a second related peroxidase gene, poxN, were analyzed in transgenic tobacco and rice plants using the uidA gene as a reporter. In transgenic tobacco, UV- and wound-responsive cis-elements were located within 144 bp from the translational start codon of the poxA gene. The poxN promoter, however, was inactive in the heterologous host as no significant GUS activity was evident. On the other hand, chimeric uidA genes containing 2.2 kb of the poxA promoter or 1.4 kb of poxN promoter were active in transgenic rice plants. Both peroxidase promoters directed GUS activities in a spatial and tissue specific manner coincident with the expression patterns exhibited by their mRNAs. Histochemical analysis of transgenic rice plants showed that both peroxidase genes are expressed in the vascular bundles of the shoot apex and lamina joint, and in xylem-parenchyma cells of the leaf blade and sheath.

Programmed cell death during pollination-induced petal senescence in petunia

Petal senescence, one type of programmed cell death (PCD) in plants, is a genetically controlled sequence of events comprising its final developmental stage. We characterized the pollination-induced petal senescence process in Petunia inflata using a number of cell performance markers, including fresh/dry weight, protein amount, RNA amount, RNase activity, and cellular membrane leakage. Membrane disruption and DNA fragmentation with preferential oligonucleosomal cleavage, events characteristic of PCD, were found to be present in the advanced stage of petal senescence, indicating that plant and animal cell death phenomena share one of the molecular events in the execution phase. As in apoptosis in animals, both single-stranded DNase and double-stranded DNase activities are induced during petal cell death and are enhanced by Ca(2+). In contrast, the release of cytochrome c from mitochondria, one commitment step in signaling of apoptosis in animal cells, was found to be dispensable in petal cell death. Some components of the signal transduction pathway for PCD in plants are likely to differ from those in animal cells.

A nitrilase-like protein interacts with GCC box DNA-binding proteins involved in ethylene and defense responses

Ethylene-responsive element-binding proteins (EREBPs) of tobacco (Nicotiana tabacum L.) bind to the GCC box of many pathogenesis-related (PR) gene promoters, including osmotin (PR-5). The two GCC boxes on the osmotin promoter are known to be required, but not sufficient, for maximal ethylene responsiveness. EREBPs participate in the signal transduction pathway leading from exogenous ethylene application and pathogen infection to PR gene induction. In this study EREBP3 was used as bait in a yeast two-hybrid interaction trap with a tobacco cDNA library as prey to isolate signal transduction pathway intermediates that interact with EREBPs. One of the strongest interactors was found to encode a nitrilase-like protein (NLP). Nitrilase is an enzyme involved in auxin biosynthesis. NLP interacted with other EREBP family members, namely tobacco EREBP2 and tomato (Lycopersicon esculentum L.) Pti4/5/6. The EREBP2-EREBP3 interaction with NLP required part of the DNA-binding domain. The specificity of interaction was further confirmed by protein-binding studies in solution. We propose that the EREBP-NLP interaction serves to regulate PR gene expression by sequestration of EREBPs in the cytoplasm.

Cloning of a DNA-binding protein that interacts with the ethylene-responsive enhancer element of the carnation GST1 gene

Ethylene transcriptionally activates a glutathione S-transferase gene (GST1) at the onset of the senescence program in carnation (Dianthus caryophyllus L.) flower petals. A 126 bp region of the GST1 promoter sequence has been identified as an ethylene-responsive enhancer element (ERE). In this paper, we demonstrate the ability of nuclear proteins from senescing petals to recognize a 22 bp sequence within the ERE (ERE oligonucleotide). Mutation of the ERE oligonucleotide sequence significantly alters the strength of this nuclear protein-DNA association. The wild-type ERE oligonucleotide sequence was used to isolate a cDNA clone encoding a sequence-specific DNA binding protein. Nucleotide sequencing and deduced amino acid sequence analysis of this cDNA predicted a 32 kDa protein which we have designated carnation ethylene-responsive element-binding protein-1 (CEBP-1). The mRNA expression pattern of CEBP-1 suggests that it is not transcriptionally regulated by ethylene. The amino acid sequence homology of CEBP-1 with other plant nucleic acid binding proteins indicates a conserved nucleic acid binding domain. Within this domain are two highly conserved RNA-binding motifs, RNP-1 and RNP-2. An acidic region and a putative nuclear localization signal are also identified.

Isolation and analysis of cDNAs encoding tomato cysteine proteases expressed during leaf senescence

Several cDNAs for mRNAs that change in abundance during tomato leaf senescence were isolated. In this paper we report molecular cloning and expression analysis of two cysteine proteases. SENU2 is identical to the cDNA C14 which encodes a cysteine protease previously shown to be expressed in response to extremes of temperature in tomato fruit [43]. SENU3 cDNA clone was 1.2 kb in length and hybridized to a transcript of 1.4 kb which suggested that the clone was not full-length. The missing 5' end was isolated using rapid amplification of cDNA ends (RACE). Southern blot analysis of tomato genomic DNA indicates that SENU3 is encoded by a single or low copy gene. SENU3 was also shown to have significant homology with known cysteine proteases. These two senescence-associated cysteine proteases are also expressed during other developmental processes, including seed germination, consistent with a role in protein turnover. SENU2 and SENU3 mRNAs were detectable in young fully expanded leaves and increased in abundance with leaf age, reaching a maximum during the later stages of visible leaf senescence. Such a pattern of expression suggests that the onset of leaf senescence is a gradual event. Analysis of senescence in transgenic plants deficient in ethylene biosynthesis, in which leaf senescence is delayed, indicated that enhanced accumulation of SENU2 and SENU3 mRNA was similarly delayed but not prevented.

Senescence-induced expression of a homologue of delta 9 desaturase in rose petals

cDNAs for senescence-inducible genes were isolated by differential hybridization from a cDNA library derived from mRNAs from the petals of rose flowers. The amino acid sequence deduced from these cDNAs exhibited significant homology to those of delta 9 acyl-lipid desaturases of cyanobacteria and of delta 9 acyl-CoA desaturases of a yeast and mammals. There was no amino-terminal sequence indicative of a leader peptide for targeting to the chloroplasts or to mitochondria. Northern blot analysis indicated that the transcripts of the cDNAs were expressed specifically in petals at late developmental stages and during senescence. It is proposed that a delta 9 desaturase in the senescing petals play an important role in the degradation of saturated fatty acids of membrane lipids.

Identification of an ethylene-responsive region in the promoter of a tobacco class I chitinase gene

The Chn48 gene is a representative of a family of tobacco class I basic chitinase genes, and the expression is induced by the stress hormone ethylene. To investigate the molecular basis for transcriptional regulation by ethylene we have examined the Chn48 promoter to identify cis-elements and trans-acting factors that are involved in the chitinase gene expression. In transgenic tobacco plants, a chimeric gene construct containing a 2 kb Chn48 promoter fused to a beta-glucuronidase reporter gene was induced by ethylene in leaf tissues. Deletion analysis indicated that a positive ethylene-responsive region is located between nucleotides -503 and -358 relative to the transcription initiation site. This 146 bp sequence was found to confer ethylene-responsive reporter gene expression when inserted in either orientation upstream of the heterologous promoter, indicating that the sequence functions as a regulatory enhancer. The ethylene-responsive region contains two copies of a GCC-box (TAAGAGCCGCC), which is conserved in a number of ethylene-responsive defense genes. The sequences within this ethylene-responsive region that are necessary for ethylene-responsive transcription were further localized to the 71 bp sequence between positions -480 and -410 containing two copies of the GCC-box by loss-of-function analysis. Gel mobility-shift experiments showed the presence of leaf nuclear factors that interact with the DNA sequences included in the ethylene-responsive region.

Characterization of three members of the ACC synthase gene family in Solanum tuberosum L

Two genomic clones corresponding to three members of the 1-aminocyclopropane-1-carboxylic acid (ACC) synthase gene family in potato (Solanum tuberosum L.) have been isolated and sequenced. Two highly homologous genes, ST-ACS1A and ST-ACS1B, transcribed in opposite directions were found in an 8.9 kb region. Their coding sequences are interrupted by two introns at identical positions. Their closest relative in tomato is the LE-ACS3 gene. The third gene in potato, ST-ACS2, was found in a 4 kb region and shows a gene structure similar to that of the tomato LE-ACS4 gene and to the mung bean VR-ACS4 and VR-ACS5 genes. Based on its lack of significant homology to the tomato gene family and its closeness to the VR-ACS4 and VR-ACS5 genes, we propose that LE-ACS7 represents an additional isoform in the tomato genome. Moreover, in a phylogenetic comparison of known ACC synthases, the ST-ACS2 isoform was grouped in a separate lineage together with the mung bean VR-ACS4 and VR-ACS5, and the moth orchid DS-ACS1A and DS-ACS1B gene products. Expression of the three potato genes was studied by reverse transcription-polymerase chain reaction on total RNA. The twin genes are positively regulated by indole-3-acetic acid in hypocotyls and expression is modulated by wounding in the leaves. The third gene is responsive to ethylene and wounding mainly in tubers. The roles of these three genes and of other members of the ACC synthase gene family in vegetative processes of potato such as tuberization, dormancy, and sprouting have yet to be determined.

Biphasic and differential expression of cytosolic glutamine synthetase genes of radish during seed germination and senescence of cotyledons

Three structurally distinct cDNA clones for cytosolic glutamine synthetase (GS1) were isolated from libraries prepared from senescing radish cotyledons. Northern blot analysis showed that transcripts from two of the three genes encoding GS1, Gln1;1 and Gln1;3, accumulated in the cotyledons during both dark-induced and natural senescence. Transcripts from the last gene, Gln1;2, remained at a low level during both processes. Transcripts from all three Gln1 genes accumulated in cotyledons of germinating seeds. We infer from these findings that GS1 enzymes function in both germination and senescence to convert ammonium to glutamine to remobilize nitrogen from source to sink organs. We have also examined the pattern of expression of these genes in different tissues. All three genes are expressed in roots. A large amount of transcripts from Gln1;1 accumulated in hypocotyls. Whereas none were transcribed in flowers. During dark-induced senescence of cotyledons, application of inorganic nitrogen delayed chlorophyll degradation. Inorganic nitrogen enhanced the accumulation of Gln1;1 transcripts, but decreased those of Gln1;3. In contrast, application of glutamine promoted yellowing of cotyledons during the dark treatment, and slightly increased the amounts of transcripts from Gln1;3 but decreased those of Gln1;1. Transcription of the three Gln1 genes appears, therefore, to be differentially regulated in radish cotyledons during senescence and germination.

An ethylene-responsive enhancer element is involved in the senescence-related expression of the carnation glutathione-S-transferase (GST1) gene

The increased production of ethylene during carnation petal senescence regulates the transcription of the GST1 gene encoding a subunit of glutathione-S-transferase. We have investigated the molecular basis for this ethylene-responsive transcription by examining the cis elements and trans-acting factors involved in the expression of the GST1 gene. Transient expression assays following delivery of GST1 5' flanking DNA fused to a beta-glucuronidase receptor gene were used to functionally define sequences responsible for ethylene-responsive expression. Deletion analysis of the 5' flanking sequences of GST1 identified a single positive regulatory element of 197 bp between -667 and -470 necessary for ethylene-responsive expression. The sequences within this ethylene-responsive region were further localized to 126 bp between -596 and -470. The ethylene-responsive element (ERE) within this region conferred ethylene-regulated expression upon a minimal cauliflower mosaic virus-35S TATA-box promoter in an orientation-independent manner. Gel electrophoresis mobility-shift assays and DNase I footprinting were used to identify proteins that bind to sequences within the ERE. Nuclear proteins from carnation petals were shown to specifically interact with the 126-bp ERE and the presence and binding of these proteins were independent of ethylene or petal senescence. DNase I footprinting defined DNA sequences between -510 and -488 within the ERE specifically protected by bound protein. An 8-bp sequence (ATTTCAAA) within the protected region shares significant homology with promoter sequences required for ethylene responsiveness from the tomato fruit-ripening E4 gene.

Analysis of an osmotically regulated pathogenesis-related osmotin gene promoter

Osmotin is a small (24 kDa), basic, pathogenesis-related protein, that accumulates during adaptation of tobacco (Nicotiana tabacum) cells to osmotic stress. There are more than 10 inducers that activate the osmotin gene in various plant tissues. The osmotin promoter contains several sequences bearing a high degree of similarity to ABRE, as-1 and E-8 cis element sequences. Gel retardation studies indicated the presence of at least two regions in the osmotin promoter that show specific interactions with nuclear factors isolated from cultured cells or leaves. The abundance of these binding factors increased in response to salt, ABA and ethylene. Nuclear factors protected a 35 bp sequence of the promoter from DNase I digestion. Different 5' deletions of the osmotin promoter cloned into a promoter-less GUSNOS plasmid (pBI 201) were used in transient expression studies with a Biolistic gun. The transient expression studies revealed the presence of three distinct regions in the osmotin promoter. The promoter sequence from -108 to -248 bp is absolutely required for reporter gene activity, followed by a long stretch (up to -1052) of enhancer-like sequence and then a sequence upstream of -1052, which appears to contain negative elements. The responses to ABA, ethylene, salt, desiccation and wounding appear to be associated with the -248 bp sequence of the promoter. This region also contains a putative ABRE (CACTGTG) core element. Activation of the osmotin gene by various inducers is discussed in view of antifungal activity of the osmotin protein.

DNA-protein interactions on a cis-DNA element essential for ethylene regulation

The PRB-1b gene encodes for a basic-type component of the pathogenesis-related PR-1 protein family. In leaves of tobacco plants, PRB-1b mRNA accumulation is rapidly induced by the application of exogenous ethylene. Promoter deletion analysis was performed in transgenic tobacco plants to delineate cis-acting elements necessary for ethylene responsiveness of the PRB-1b gene. The promoter sequence from position -213 was sufficient to enhance a 20 fold increase of beta-glucuronidase reporter gene expression in transgenic tobacco leaves exposed to 20 microliters/l of ethylene, however -141 bp were not. The functional study was correlated with in vitro analysis of the nuclear protein-DNA complexes formed on the promoter element identified as necessary for ethylene induction. Gel-shift analysis using restriction fragments spanning the sequence between position -237 and -143 revealed two distinct nuclear protein-DNA interactions. The protein-binding sequences were mapped to the contiguous regions G (-200 to -178) and Y (-179 to -154) by gel-shift analysis using oligonucleotides. Fractionation of crude nuclear extract by heparin-agarose chromatography resulted in the differential elution of the two binding activities. The DNA-nuclear protein interactions characterized in vitro can be part of the molecular events which mediate the transcriptional regulation of the PRB-1b gene by ethylene.

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.

Characterization of an ethylene-responsive glutathione S-transferase gene cluster in carnation

In this paper we present the structural analysis of two tightly linked genes from the glutathione S-transferase (GST) gene family in carnation (Dianthus caryophyllus). Southern blot analysis and restriction endonuclease mapping revealed a single cloned region of the carnation genome was highly homologous to the previously characterized ethylene-responsive GST mRNA expressed in flower petals during senescence. Nucleotide sequencing of this region revealed the presence of two tandemly arranged genes designated GST1 and GST2. Comparison of the nucleotide sequences of the cloned genomic region with the previously characterized GST cDNA clone pSR8 revealed that GST1 contains the entire transcription unit in 10 exons interrupted by 9 introns. The transcription unit of GST2 was found to be very similar to GST1 with complete conservation of intron position. In addition, the length and nucleotide sequences of the two genes' introns were highly conserved. GST2 was not completely represented by the cloned genomic region, missing the 3' portion of the transcription unit. Primer extension analysis indicated a single transcriptional start site for transcripts which accumulate in senescing carnation petals. The 5'-flanking sequences of GST1 and GST2 were compared and regions of homology and divergence identified. These upstream sequences were compared with other plant ethylene-responsive genes and GST genes and several sequence motifs of potential importance in the regulation of GST expression were identified. A chimeric gene constructed between -1457 bp of the 5'-flanking DNA of GST1 and the coding region of beta-glucuronidase was found to confer ethylene-inducible expression in flower petals following delivery of the construct into tissue by particle bombardment.