Molecular and genetic analysis of nitrite reductase co-suppression in transgenic tobacco plants

Silencing of Nia host genes and transgenes (encoding nitrate reductase) was previously achieved by introducing into tobacco plants the tobacco Nia2 cDNA cloned downstream of the cauliflower mosaic virus (CaMV) 35S promoter. To check whether Nii host genes and transgenes (encoding nitrite reductase, the second enzyme of the nitrate assimilation pathway) were also susceptible to silencing, a transgene consisting of the tobacco Nii1 gene with two copies of the enhancer of the 35S promoter cloned 1 kb upstream of the Nii promoter region was introduced into tobacco plants. Among nine independent transformants analysed, two showed silencing of Nii host genes and transgenes in some descendants after selfing, but never after back-crossing with wild-type plants, suggesting that silencing depends on the number of transgene loci and/or on certain allelic or ectopic combinations of transgene loci. In one transformant carrying a single transgene locus in a homozygous state, silencing was triggered in all progeny plants of each generation, 20 to 50 days after germination. Field trial analysis confirmed that silencing was not triggered when the transgene locus of this latter line was present in a hemizygous state. In addition, it was revealed that silencing can be triggered, albeit at low frequency and later during the development, when this transgene locus is brought into the presence of a non-allelic transgene locus by crossing, suggesting that a homozygous state is not absolutely required.

Identification of sucrose-regulated genes in cultured rice cells using mRNA differential display

In order to get more information about carbon metabolite regulation pathways, cloning and sequence analysis of sucrose-regulated genes from rice-suspension-cultured cells were performed. We used a new method, mRNA differential display, to screen differentially expressed genes under conditions of 3% and no sucrose in the cultured medium. Six candidate clones were identified and sequenced. Clones SI1 and SI2 were repressed by sucrose starvation, while clones SR1, SR2, SR3 and SR4 were induced by sucrose starvation. Nucleotide sequence analysis showed that clone SR2 has 94.8% homology to the salT gene, and clones SI1 and SR3 show 88.3 and 96.9% identity, respectively, to partial cDNA sequences in the GenBank database. The results suggest that mRNA differential display provides an easy and quick way to clone genes involved in the carbon metabolite regulation pathway.

Expression of Aequorea green fluorescent protein in plant cells

The coding region of the green fluorescent protein (GFP) from Aequorea victoria has been fused to the cauliflower mosaic virus 35S promoter and introduced into maize leaf protoplasts. Transient expression of GFP was observed. In addition, the coding region of GFP was fused to an Arabidopsis heat shock promoter and co-transformed with another construct in which GFP has been replaced with chloramphenicol acetyltransferase (CAT). The heat-induced expression of GFP in maize protoplasts parallels that of CAT. While GFP was expressed in both dark-grown and green maize leaf protoplasts, no green fluorescence was observed in similarly transformed Arabidopsis protoplasts.

Sequences required for paramutation of the maize b gene map to a region containing the promoter and upstream sequences

The b gene encodes a transcriptional regulator of the maize anthocyanin biosynthetic pathway. Certain b alleles participate in paramutation, an allele-specific interaction that heritably alters transcription. The moderately transcribed B' allele heritably reduces the transcription of the highly transcribed B-I allele in a B'/B-I heterozygote, such that the B-I allele becomes B'. To identify the cis-acting sequences required for paramutation, we used B' or B-I alleles to isolate intragenic recombinants with B-Peru, an allele that is insensitive to paramutation and has distinct tissue-specific regulation. Physical mapping of the recombinant alleles showed that most of the crossovers were in a small region near the 5' end of the b-transcribed region. Analysis of the recombinant alleles revealed that the ability to cause and respond to paramutation and the control of tissue-specific expression both localize to the 5' region of the gene. The 3' boundary of these functions lies just upstream of the translation initiation codon. The 5' boundary has been estimated to be no more than 0.1 cM further upstream (1-150 kb). Thus, sequences critical for paramutation lie upstream of the b coding sequences and may include transcriptional regulatory sequences.

The Arabidopsis thaliana 4-coumarate:CoA ligase (4CL) gene: stress and developmentally regulated expression and nucleotide sequence of its cDNA

An Arabidopsis cDNA clone encoding 4-coumarate:CoA ligase (4CL), a key enzyme of phenylpropanoid metabolism, was identified and sequenced. The predicted amino acid sequence is similar to those of other cloned 4CL genes. Southern blot analysis indicated that 4CL is single-copy gene in Arabidopsis. Northern blots showed that 4CL expression was activated early during seedling development. The onset of 4CL expression was correlated with the onset of lignin deposition in cotyledons and roots 2-3 days after germination. The timing of the expression of a parsley 4CL1-GUS fusion in transgenic Arabidopsis seedlings was examined in parallel and was very similar to that of endogenous 4CL. In mature plants, highest 4CL expression was observed in bolting stems, where relatively large amounts of lignin accumulate. Both 4CL and 4CL1-GUS mRNA accumulation was strongly and transiently activated by wounding of mature Arabidopsis leaves. 4CL expression was specifically activated within 6 h after infiltration of Arabidopsis ecotype Columbia leaves with a Pseudomonas syringae pv. maculicola strain harboring the bacterial avirulence gene avrB, which causes in incompatible interaction. The timing of 4CL activation was identical to the previously observed activation of PAL gene expression in this interaction. No activation of 4CL expression was observed in a compatible interaction caused by a Pseudomonas syringae pv. maculicola strain without avrB.

Identification of NADPH:protochlorophyllide oxidoreductases A and B: a branched pathway for light-dependent chlorophyll biosynthesis in Arabidopsis thaliana

Illumination releases the arrest in chlorophyll (Chl) biosynthesis in etiolated angiosperm seedlings through the enzymatic photoreduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide), the first light-dependent step in chloroplast biogenesis. NADPH: Pchlide oxidoreductase (POR, EC 1.3.1.33), a nuclear-encoded plastid-localized enzyme, mediates this unique photoreduction. Paradoxically, light also triggers a drastic decrease in the amounts of POR activity and protein before the Chl accumulation rate reaches its maximum during greening. While investigating this seeming contradiction, we identified two distinct Arabidopsis thaliana genes encoding POR, in contrast to previous reports of only one gene in angiosperms. The genes, designated PorA and PorB, by analogy to the principal members of the phytochrome photoreceptor gene family, display dramatically different patterns of light and developmental regulation. PorA mRNA disappears within the first 4 h of greening, whereas PorB mRNA persists even after 16 h of illumination, mirroring the behavior of two distinct POR protein species. Experiments designed to help define the functions of POR A and POR B demonstrate exclusive expression of PorA in young seedlings and of PorB both in seedlings and in adult plants. Accordingly, we propose the existence of a branched light-dependent Chl biosynthesis pathway in which POR A performs a specialized function restricted to the initial stages of greening and POR B maintains Chl levels throughout angiosperm development.

Isolation of two soybean G-box binding factors which interact with a G-box sequence of an auxin-responsive gene

G-box binding factors (GBFs) constitute a family of plant DNA-binding proteins that bind to the G-box motif, a regulatory cis element present in many plant genes with a palindromic DNA motif of CACGTG. Previously TCCACGTGTC, a G-box motif, from an auxin responsive gene GmAux28 has been identified as a sequence-specific protein-binding site. Here the isolation of two soybean cDNA clones, referred to as SGBF-1 and SGBF-2, encoding proteins which bind to the G-box motif is reported. The primary structure of SGBF-1 and SGBF-2 predicts that these proteins contain a basic leucine zipper (bZIP) DNA-binding domain and an N-terminal proline-rich domain. A dramatic difference in the pattern of protein-DNA complex formation was observed when recombinant SGBF-1 and SGBF-2 proteins were analyzed by electrophoretic mobility shift assays (EMSAs). The SGBF-1 binding pattern obtained with the G-box probe resulted in three major retarded bands while the SGBF-2 formed a single complex. This shows that the characteristically diffuse banding pattern of plant nuclear proteins interacting with the G-box is also observed in a binding assay using only one recombinant GBF. EMSAs were performed with a few selected binding sequences to study the effect of flanking nucleotides to the hexanucleotide G-box core motif. The binding specificity of the SGBF proteins resembles that described for type A cauliflower nuclear G-box binding proteins which bind class I G-box elements [(G/T)(C/A)CACGTG(G/T)(A/C)]. Phylogenetic analysis of 13 GBF-like proteins from various plant species reveals that the SGBF-1 and SGBF-2 proteins belong to different lineages, suggesting that they may have distinct functions in activating transcription.

GA3-regulated cDNAs from Hordeum vulgare leaves

The barley mutant dbg 576 shows an extreme vegetative dwarf phenotype. This is reversed after the application of GA3 which also induces the mutant florets to become partly fertile. cDNA clones ES1A and ES2A were isolated by differential screening with subtracted probes from a DNA library prepared from mutant leaf blades after GA3 treatment. Both the ES1A and the ES2A mRNA level increases as early as 30 min after GA3 treatment and decreases later. Accumulation of ES1A and ES2A mRNAs is leaf blade-specific and both are ca. 750 nucleotides long. ES1A encodes a protein of approximately 6 kD which shows a significant homology with mammalian epidermal growth factors (EGFs). ES2A encodes a protein of 22 kDa with homology, in regions with potential amphiphilic helices, with the D7 family of late embryogenesis-abundant proteins (LEA).

Activity of constitutive promoters in various species from the Liliaceae

In this paper we first review literature on the performance of various promoters in monocotyledonous species. In general, promoters isolated from monocots show a higher activity in monocot species. Moreover, the presence of an intron between the promoter and reporter gene increases transcription levels. We used the same approach to study gene expression in Liliaceae. The activities of the CaMV 35S, maize Adh1-based pEmu, rice Act1 and maize Ubi promoters, coupled to the beta-glucuronidase (gus) reporter gene, were evaluated for transient gene expression upon particle bombardment of tissues of tobacco, rice, tulip, lily and leek. Although monocot promoters performed very well in rice tissues, the results of this study show that this cannot be generalized for other monocot species. The transcription inducing effects of monocot promoters were less pronounced or even absent in tissues of Liliaceae, while the presence of an intron between promoter and gus gene reduced promoter activity.

An anther-specific gene encoded by an S locus haplotype of Brassica produces complementary and differentially regulated transcripts

The self-incompatibility locus of Brassica consists of a coadapted gene complex that contains at least two genes required for the recognition and inhibition of pollen by the stigma when self-pollinated. Here, we report the identification of a third S locus-linked gene from the S2 haplotype of Brassica oleracea. This gene, which we designated SLA (for S Locus Anther), is a novel gene with an unusual structure. SLA is transcribed from two promoters to produce two complementary anther-specific transcripts, one spliced and the other unspliced, that accumulate in an antiparallel manner in developing microspores and anthers. The sequence of the spliced transcript showed the presence of two open reading frames that predict proteins of 10 and 7.5 kD. Neither transcript was produced in a self-compatible B. napus strain carrying an S2-like haplotype, indicating that the SLA gene in this strain is nonfunctional. Interestingly, sequences related to SLA were not detected in DNA or RNA from plants carrying S haplotypes other than S2. The haplotype specificity of SLA, its anther-specific expression, and its physical linkage to the S locus are properties expected for a gene that encodes a determinant of S2 specificity in pollen.

Gerbera hybrida (Asteraceae) imposes regulation at several anatomical levels during inflorescence development on the gene for dihydroflavonol-4-reductase

In the ornamental cut flower plant Gerbera hybrida the spatial distribution of regulatory molecules characteristic of differentiation of the composite inflorescence is visualized as the various patterns of anthocyanin pigmentation of different varieties. In order to identify genes that the plant can regulate according to these anatomical patterns, we have analysed gene expression affecting two enzymatic steps, chalcone synthase (CHS) and dihydroflavonol-4-reductase (DFR), in five gerbera varieties with spatially restricted anthocyanin pigmentation patterns. The dfr expression profiles vary at the levels of floral organ, flower type and region within corolla during inflorescence development according to the anthocyanin pigmentation of the cultivars. In contrast, chs expression, although regulated in a tissue-specific manner during inflorescence development, varies only occasionally. The variation in the dfr expression profiles between the varieties reveals spatially specific gene regulation that senses the differentiation events characteristic of the composite inflorescence.

Separation of AG function in floral meristem determinacy from that in reproductive organ identity by expressing antisense AG RNA

The Arabidopsis floral homeotic gene AGAMOUS (AG) is a regulator of early flower development. The ag mutant phenotypes suggest that AG has two functions in flower development: (1) specifying the identity of stamens and carpels, and (2) controlling floral meristem determinacy. To dissect these two AG functions, we have generated transgenic Arabidopsis plants carrying an antisense AG construct. We found that all of the transgenic plants produced abnormal flowers, which can be classified into three types. Type I transgenic flowers are phenocopies of the ag-1 mutant flowers, with both floral meristem indeterminacy and floral organ conversion; type II flowers are indeterminate and have partial conversion of the reproductive organs; and type III flowers have normal stamens and carpels, but still have an indeterminate floral meristem inside the fourth whorl of fused carpels. The existence of type III flowers indicates that AG function can be perturbed to affect only floral meristem determinacy, but not floral organ identity. Furthermore, the fact that floral meristem determinacy is affected in all transformants, but floral organ identity only in a subset of them, suggests that the former may required a higher level of AG activity than the latter. This hypothesis is supported by the levels of AG mRNA detected in different transformants with different frequencies of distinct types of abnormal antisense AG transgenic flowers. Finally, since AG inhibits the expression of another floral regulatory gene AP1, we examined AP1 expression in antisense AG flowers, and found that AP1 is expressed at a relatively high level in the center of type II flowers, but very little or below detectable levels in the inner whorls of type III flowers. These results provide further insights into the interaction of AG and AP1 and how such an interaction may control both organ identity and floral meristem determinacy.

Isolation of cDNA clones of genes with altered expression levels in phosphate-starved Brassica nigra suspension cells

Differential gene expression at the transcriptional level was examined as an initial step in the investigation of the P(i) starvation response of Brassica nigra suspension cells. Total RNA was extracted from 7-day old cells grown in media containing either no P(i), 1.25 mM or 10 mM Pi. In vitro translation was carried out using their respective poly(A)+ RNA isolates and the resultant polypeptides were separated on a high-resolution SDS-PAGE gel. Scanning densitometry identified four polypeptides (ca. 31.7, 32.3, 52.5 and 64.8 kDa) present only in the P(i)-starved samples. Screening by differential hybridization was performed on a cDNA library constructed from mRNA isolated from P(i)-starved cells. Probes prepared from mRNA from P(i)-deficient and P(i)-sufficient cells identified a number of clones representing mRNA species that were preferentially transcribed under P(i) deficiency. These phosphate starvation-responsive (psr) clones were placed into eleven groups as determined by cross-hybridization. Northern blots showed that the corresponding genes are inducible in both mild and severe P(i) starvation conditions. Preliminary sequencing identified one of the clones as being homologous to beta-glucosidases from several plant species. The possible role of beta-glucosidase during Pi starvation and the identities of the other psr genes are discussed.

An S-RNase promoter from Nicotiana alata functions in transgenic N. alata plants but not Nicotiana tabacum

Nicotiana tabacum and Nicotiana alata plants were transformed with genomic clones of two S-RNase alleles from N. alata. Neither the S2 clone, with 1.6 kb of 5' sequence, nor the S6 clone, with 2.8 kb of 5' sequence, were expressed at detectable levels in transgenic N. tabacum plants. In N. alata, expression of the S2 clone was not detected, however the S6 clone was expressed (at low levels) in three out of four transgenic plants. An S6-promoter-GUS fusion gene was also expressed in transgenic N. alata but not N. tabacum. Although endogenous S-RNase genes are expressed exclusively in floral pistils, the GUS fusion was expressed in both styles and leaves.

Sink limitation induces the expression of multiple soybean vegetative lipoxygenase mRNAs while the endogenous jasmonic acid level remains low

The response of individual members of the lipoxygenase multigene family in soybeans to sink deprivation was analyzed. RNase protection assays indicated that a novel vegetative lipoxygenase gene, vlxC, and three other vegetative lipoxygenase mRNAs accumulated in mature leaves in response to a variety of sink limitations. These data suggest that several members of the lipoxygenase multigene family are involved in assimilate partitioning. The possible involvement of jasmonic acid as a signaling molecule regulating assimilate partitioning into the vegetative storage proteins and lipoxygenases was directly assessed by determining the endogenous level of jasmonic acid in leaves from plants with their pods removed. There was no rise in the level of endogenous jasmonic acid coincident with the strong increase in both vlxC and vegetative storage protein VspB transcripts in response to sink limitation. Thus, expression of the vegetative lipoxygenases and vegetative storage proteins is not regulated by jasmonic acid in sink-limited leaves.

Early iron deficiency stress response in leaves of sugar beet

Iron nutrient deficiency was investigated in leaves of hydroponically grown sugar beets (Beta vulgaris) to determine how ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) gene expression is affected when thylakoid components of photosynthesis are diminished. Rubisco polypeptide content was reduced by 60% in severely iron-stressed leaves, and the reduction was directly correlated to chlorophyll content. The concentration of Rubisco protein in iron-stressed leaves was found to be regulated by availability of mRNAs, and CO2 fixation by Rubisco was reduced from 45 mumol CO2 m-2 s-1 in extracts from iron-sufficient leaves to 20 mumol CO2 m-2 s-1 in extracts from severely stressed leaves. The rate of CO2 fixation was directly correlated to leaf chlorophyll content. Rubisco in iron-sufficient control leaves was 59% activated, whereas in severely stressed leaves grown under the same light, Rubisco was 43% activated. RNA synthesis was reduced by about 50% in iron-deficient leaves, but 16S and 25S rRNA and ctDNA were essentially unaffected by iron stress.

Characterization of a low-temperature-induced cDNA from winter Brassica napus encoding the 70 kDa subunit of tonoplast ATPase

A cDNA clone, pBN59, was isolated by differential screening of a cDNA library of winter Brassica napus during cold acclimation. Nucleotide sequence of BN59 was found to be homologous to that encoding the 70 kDa subunit of the vacuolar H(+)-ATPase in plants. Transcripts hybridizing to BN59 accumulated during exposure to low temperatures and to the exogenous application of abscisic acid (ABA). Western blot analyses also indicated an increase in the 70 kDa subunit during cold acclimation. The accumulation of an endomembrane H(+)-ATPase is consistent with the observation of osmotic adjustment, increases in endogenous ABA and the proliferation of endomembranes during cold acclimation.

Stress responses in alfalfa (Medicago sativa L.) XIX. Transcriptional activation of oxidative pentose phosphate pathway genes at the onset of the isoflavonoid phytoalexin response

We have isolated cDNA clones encoding the pentose phosphate pathway enzymes 6-phosphogluconate dehydrogenase (6PGDH, EC 1.1.1.44) and glucose 6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) from alfalfa (Medicago sativa L.). These exhibit extensive nucleotide and amino acid sequence similarity to the corresponding genes from bacteria, Drosophila and mammals. Transcripts encoding both enzymes are expressed at high levels in roots and nodules. Exposure of alfalfa suspension cells to an elicitor from yeast cell walls results in co-ordinated increases in transcription rates for both genes, followed by increased steady state transcript levels but only slightly increased extractable enzyme activities, at the onset of accumulation of isoflavonoid phytoalexins. Levels of NADPH and NADP remain relatively constant in alfalfa cells following elicitation. The rapid transcriptional activation of 6PGDH and G6PDH does not therefore appear to be a response to altered pyridine nucleotide redox state. These genes appear to respond to early events in elicitor-mediated signalling rather than to subsequent elicitor-induced changes in secondary metabolism. Hydrogen peroxide, a potential signal for elicitation of anti-oxidative genes in biologically stressed plant cells, did not induce 6PGDH or G6PDH transcripts or enzymatic activity.

Transgenic analysis of the 5'- and 3'-flanking regions of the NADH-dependent hydroxypyruvate reductase gene from Cucumis sativus L

The 5'- and 3'-flanking regions of HPRA, a cucumber gene that encodes hydroxypyruvate reductase, were evaluated for regulatory activity with respect to light responsiveness and organ specificity. To define the functional regions of the 5'-flanking region of HPRA, a series of deletions was generated and the remaining portions fused to the beta-glucuronidase (GUS) reporter gene (uidA) containing a minimal 35S promoter truncated at -90. The region from -66 to +39 was found to be necessary for light-regulated expression of the uidA reporter gene, while the region from -382 to -67 was found to be necessary for its leaf-specific expression. Further deletion of the HPRA 5' flanking region to -590 resulted in high levels of root expression, suggesting the presence of a negative regulatory element responsible for silencing root expression of the HPRA gene between -590 and -383. The 3'-flanking region of the HPRA gene downstream of the polyadenylation site contains several sequence motifs resembling regulatory elements present in the promoters of several light-responsive genes. An 823 bp portion of the HPRA 3'-flanking region containing these putative regulatory elements enhanced GUS expression in leaves when placed downstream of the uidA reporter gene in the forward orientation, but not in the reverse orientation. When placed 5' of the -90 35S promoter, the 823 bp fragment enhanced slightly, independently of orientation, the root tip-specific expression pattern intrinsic to the -90 35S promoter, indicating that in some cases this region can act as a transcriptional enhancer.

Peroxisomal thiolase mRNA is induced during mango fruit ripening

Fruit ripening is a complex, developmentally regulated process. A series of genes have been isolated from various ripening fruits encoding enzymes mainly involved in ethylene and cell wall metabolism. In order to aid our understanding of the molecular basis of this process in a tropical fruit, a cDNA library was prepared from ripe mango (Mangifera indica L. cv. Manila). By differential screening with RNA poly(A)+ from unripe and ripe mesocarp a number of cDNAs expressing only in ripe fruit have been isolated. This paper reports the characterization of one such cDNA (pTHMF 1) from M. indica which codes for a protein highly homologous to cucumber, rat and human peroxisomal thiolase (EC 2.3.1.16), the catalyst for the last step in the beta-oxidation pathway. The cDNA for the peroxisomal mango thiolase is 1305 bp in length and codes for a protein of 432 amino acids with a predicted molecular mass of 45,532 Da. Mango thiolase is highly homologous to cucumber thiolase (80%), the only other plant thiolase whose cloning has been reported, and to rat and human thiolases (55% and 55% respectively). It is shown by northern analysis that during fruit ripening THMF 1 is up-regulated. A similar pattern of expression was detected in tomato fruit. Wounding and pathogen infection do not appear to affect THMF 1 expression. The possible involvement of thiolase in fatty acid metabolism during fruit ripening will be discussed. To our knowledge this is the first report cloning of a plant gene involved in fatty acid metabolism showing an induction during fruit ripening.

Hydrogen peroxide does not function downstream of salicylic acid in the induction of PR protein expression

The roles of salicylic acid (SA) and H2O2 in the induction of PR proteins in tobacco have been examined. Studies were conducted on wild-type tobacco and plants engineered to express a bacterial salicylate hydroxylase capable of metabolizing SA to catechol (SH-L plants). Wild-type and PR-1a-GUS-transformed plants express PR-1a following challenge with Pseudomonas syringae pathovar syringae, SA or 2,6-dichloro-isonicotinic acid (INA). In contrast, SH-L plants failed to respond to SA but did express PR-1a following INA treatment. H2O2 and the irreversible catalase inhibitor 3-amino-1,2,4-triazole (3-AT) were found to be weak inducers of PR-1a expression (relative to SA) in wild-type tobacco but were unable to induce PR-1a in SH-L plants, suggesting that the action of these compounds depends upon the accumulation of SA. A model has been proposed suggesting that SA binds to and inhibits a catalase inducing an increase in H2O2 leading to PR protein expression. Catalase activity has been measured in tobacco and no significant changes in activity following infection with P. syringae pv. syringae were detected. Furthermore, inhibition of catalase activity in vitro in plant extracts requires pre-incubation and only occurs at SA concentrations above 250 microM. Leaf disks preincubated with 1 mM SA do accumulate SA to these levels and PR-1a is efficiently induced but there is no apparent inhibition of catalase activity. It is also shown that a SA-responsive gene, PR-1a, and a H2O2-sensitive gene, AoPR-1, are both relatively insensitive to 3-AT suggesting that induction of these genes is unlikely to be due entirely to inhibition of an endogenous catalase.

Expression of two related vacuolar H(+)-ATPase 16-kilodalton proteolipid genes is differentially regulated in a tissue-specific manner

The 16-kD proteolipid subunit is the principal integral membrane protein of the vacuolar H(+)-ATPase (V-ATPase) complex that forms the proton channel responsible for translocating protons across lipid bilayers. Two degenerate synthetic oligonucleotides, COT11 and COT12, corresponding to highly conserved transmembrane domains in all 16-kD subunits sequenced so far, were used to amplify a partial cDNA of the V-ATPase proteolipid subunit from cotton (Gossypium hirsutum L.) by polymerase chain reaction (PCR). These PCR products were used to isolate two full-length cDNAs from a -3 d postanthesis cotton ovule library. Both clones, CVA16.2 and CVA16.4, consisting of 816 and 895 bp, respectively, encode the 16-kD proteolipid subunit of the V-ATPase. At the nucleotide level, the complete sequences of the two clones show 73.5% identity, but share about 95% identity within the coding region, although the two polypeptides differ by only one amino acid. Comparison of deduced amino acid sequences of the proteolipid subunits revealed that the four transmembrane domains and the two cytosolic extramembrane domains are highly conserved in all eukaryotes. Southern blot analysis of cotton genomic DNA showed that these clones belong to small gene families in related diploid and allotetraploid species. Northern blot analysis suggested that the three major V-ATPase subunits (69, 60, and 16 kD) are coordinately regulated, in part, at the transcriptional level. RNA analysis and reverse-transcription PCR established that 16-kD proteolipid transcripts differentially accumulate in different tissues and increase dramatically in tissues undergoing rapid expansion, particularly in anthers, ovules, and petals. The CVA16.4 proteolipid transcript is the most prevalent of the two proteolipid messages in expanding ovules harvested 10 d post-anthesis. In contrast, the two proteolipid mRNAs accumulate to similar levels in developing petals.

Distinct classes of mitotic cyclins are differentially expressed in the soybean shoot apex during the cell cycle

Protein phosphorylation by the complexes of cyclin and cyclin-dependent kinase plays a key role in cell cycle progression in all eukaryotes. The amplification by polymerase chain reaction of a cyclin box from developing root nodules and root apices of soybean showed the expression of a number of different molecular species of mitotic cyclins in plant meristems, and they were classified into five distinct groups based on their sequence similarities. The complete soybean cyclin cDNAs, cyc1Gm to cyc5Gm, corresponding to each group were isolated, and their predicted amino acid sequences showed clear similarities to mitotic cyclins identified from various organisms. These genes are expressed predominantly in such meristematic tissues as root and shoot apices and young developing nodules. Double-target in situ hybridization involving histone H4 as an S-phase marker allowed us to estimate the phases during which these cyclin genes are abundantly expressed. The results indicated that cyc5Gm is expressed in G2-to-M phases and cyc3Gm is expressed from late S-to-G2 phases. These expression patterns, together with the sequence criteria, strongly suggest that cyc3Gm and cyc5Gm encode the plant cognates for A- and B-type cyclins, respectively. In addition, the expression of cyc1Gm was restricted during a short period in S phase, suggesting that it belongs to a novel class of plant cyclins. Sequence comparison of 18 plant mitotic cyclins cloned thus far showed that they can be divided into four distinct structural groups with different functions in cell cycle progression.

Heat-inducible expression of FLP gene in maize cells

The soybean heat-shock gene promoter (Gmhsp 17.5-E) has been used to direct expression of gusA and FLP genes in maize cells. At inducible temperatures, in transient expression assays, gusA gene expression controlled by the heat-shock promoter is about 10-fold higher than the expression directed by the CaMV 35S promoter. The Gmhsp 17.5-E promoter preserves its regulatory functions in heterologous maize cells after random integration into genomic DNA. Heat-shock inducible expression of the FLP gene was investigated by co-transformation of the FLP expression vector (pHsFLP) and a recombination test vector (pUFNeoFmG) into maize protoplasts. Co-transformed protoplasts were incubated at 42 degrees C for 2 h. This treatment induced recombination of 20-25% of the available FRT sites in transient assays. As a result of heat-shock treatment of stably co-transformed maize cells, activation of gusA gene expression and an associated decrease or elimination of NPT-II activity in transgenic maize lines was observed. Molecular evidence was obtained of the expected DNA excision process catalyzed by the FLP protein in maize transgenic cells. Thus, the experiments presented in this paper indicate that the FLP protein can recognize and subsequently recombine the FRT target sites that had integrated into plant genomic DNA, and that regulated expression of the FLP gene is possible in maize cells using the soybean heat-shock promoter.

Genes encoding small GTP-binding proteins analogous to mammalian rac are preferentially expressed in developing cotton fibers

In animals, the small GTP-binding proteins, Rac and Rho, of the ras superfamily participate in the signal transduction pathway that regulates the organization of the actin cytoskeleton. We report here on the characterization of two distinct cDNA clones isolated from a cotton fiber cDNA library that code for homologs of animal Rac proteins. Using gene-specific probes, we have determined that amphidiploid cotton contains two genes that code for each of the two Rac proteins, designated Rac13 and Rac9, respectively. The gene for Rac13 shows highly enhanced expression in developing cotton fibers, with maximal expression occurring at the time of transition between primary and secondary wall synthesis. This is also the time at which reorganization of the cytoskeleton occurs, and thus the pattern of expression of Rac13 is consistent with its possible role, analogous to animal Rac, in the signal transduction pathway that controls cytoskeletal organization.

Expression of a flower-specific Myb protein in leaf cells using a viral vector causes ectopic activation of a target promoter

The promoter of the bean PAL2 gene (encoding phenylalanine ammonia-lyase; EC 4.3.1.5) is a model for studies of tissue-restricted gene expression in plants. Petal epidermis is one of the tissues in which this promoter is activated in tobacco. Previous work suggested that a major factor establishing the pattern of PAL2 expression in tobacco petals is the tissue distribution of a protein closely related to Myb305, which is a Myb-like transcriptional activator from snapdragon. In the present work, we show that Myb305 expression in tobacco leaves causes ectopic activation of the PAL2 promoter. To achieve Myb305 expression in planta, a viral expression vector was used. This approach combines the utility of transient assays with the possibility of direct biochemical detection of the introduced factor and may have wider application for studying the function of plant transcription factors.

Expression of a flower-specific Myb protein in leaf cells using a viral vector causes ectopic activation of a target promoter

The promoter of the bean PAL2 gene (encoding phenylalanine ammonia-lyase; EC 4.3.1.5) is a model for studies of tissue-restricted gene expression in plants. Petal epidermis is one of the tissues in which this promoter is activated in tobacco. Previous work suggested that a major factor establishing the pattern of PAL2 expression in tobacco petals is the tissue distribution of a protein closely related to Myb305, which is a Myb-like transcriptional activator from snapdragon. In the present work, we show that Myb305 expression in tobacco leaves causes ectopic activation of the PAL2 promoter. To achieve Myb305 expression in planta, a viral expression vector was used. This approach combines the utility of transient assays with the possibility of direct biochemical detection of the introduced factor and may have wider application for studying the function of plant transcription factors.

The GA5 locus of Arabidopsis thaliana encodes a multifunctional gibberellin 20-oxidase: molecular cloning and functional expression

The biosynthesis of gibberellins (GAs) after GA12-aldehyde involves a series of oxidative steps that lead to the formation of bioactive GAs. Previously, a cDNA clone encoding a GA 20-oxidase [gibberellin, 2-oxoglutarate:oxygen oxidoreductase (20-hydroxylating, oxidizing), EC 1.14.11.-] was isolated by immunoscreening a cDNA library from liquid endosperm of pumpkin (Cucurbita maxima L.) with antibodies against partially purified GA 20-oxidase. Here, we report isolation of a genomic clone for GA 20-oxidase from a genomic library of the long-day species Arabidopsis thaliana Heynh., strain Columbia, by using the pumpkin cDNA clone as a heterologous probe. This genomic clone contains a GA 20-oxidase gene that consists of three exons and two introns. The three exons are 1131-bp long and encode 377 amino acid residues. A cDNA clone corresponding to the putative GA 20-oxidase genomic sequence was constructed with the reverse transcription-PCR method, and the identity of the cDNA clone was confirmed by analyzing the capability of the fusion protein expressed in Escherichia coli to convert GA53 to GA44 and GA19 to GA20. The Arabidopsis GA 20-oxidase shares 55% identity and > 80% similarity with the pumpkin GA 20-oxidase at the derived amino acid level. Both GA 20-oxidases share high homology with other 2-oxoglutarate-dependent dioxygenases (2-ODDs), but the highest homology was found between the two GA 20-oxidases. Mapping results indicated tight linkage between the cloned GA 20-oxidase and the GA5 locus of Arabidopsis. The ga5 semidwarf mutant contains a G-->A point mutation that inserts a translational stop codon in the protein-coding sequence, thus confirming that the GA5 locus encodes GA 20-oxidase. Expression of the GA5 gene in Ara-bidopsis leaves was enhanced after plants were transferred from short to long days; it was reduced by GA4 treatment, suggesting end-product repression in the GA biosynthetic pathway.

Identification of auxin-responsive elements of parB and their expression in apices of shoot and root

Detailed analysis of transgenic tobaccos containing a series of chimeric parB promoter/beta-glucuronidase (GUS) gene constructs allowed us to define two auxin-responsive elements (AREs) of 48 bp and 95 bp (positions -210 to -163 and -374 to -280) in the parB promoter. The two AREs responded independently to physiological concentrations of auxin. Gel retardation assays revealed binding of nuclear protein(s) to the sequence conserved between ARE I and ARE II. The auxin responsiveness of the parB promoter did not mediate the pathway through the as-1 element and transcription factor ASF-1. AREs I and II were responsive to auxin at physiological concentrations, whereas as-1 responded only to higher concentrations of auxin which may be interpreted as stress, though as-1 had been reported to be a minimal ARE [Liu, X. & Lam, E. (1994) J. Biol. Chem. 269, 668-675]. Histochemical staining of transgenic tobacco that contained a parB promoter/GUS construct demonstrated the expression of GUS activity in the shoot apex as well as in the root tips, suggesting the involvement of parB expression in meristematic activity or differentiation. The drastic change in auxin responsiveness in the transgenic plants between the 6th and 10th day after imbibition of seeds implies the development or the activation of auxin signal transduction systems during plant development.

Early nodulin gene expression during Nod factor-induced processes in Vicia sativa

Rhizobium leguminosarum bv. viciae-secreted Nod factors are able to induce root hair deformation, the formation of nodule primordia and the expression of early nodulin genes in Vicia sativa (vetch). To obtain more insight into the mode of action of Nod factors the expression of early nodulin genes was followed during Nod factor-induced root hair deformation and nodule primordium formation. The results of these studies suggested that the expression of VsENOD5 and VsENOD12 is not required for root hair deformation. In the Nod factor-induced primordia both VsENOD12 and VsENOD40 are expressed in a spatially controlled manner similar to that found in Rhizobium-induced nodule primordia. In contrast, VsENOD5 expression has never been observed in Nod factor-induced primordia, showing that the induction of VsENOD5 and VsENOD12 expression are not coupled. VsENOD5 expression is induced in the root epidermis by Nod factors and in Rhizobium-induced nodule primordia only in cells infected by the bacteria, suggesting that the Nod factor does not reach the inner cortical cells.

Activation of two osmotin-like protein genes by abiotic stimuli and fungal pathogen in transgenic potato plants

Osmotin-like proteins are encoded by at least six members of a multigene family in Solanum commersonii. A genomic clone (lambda pGEM2a-7) that contains two osmotin-like protein genes (OSML13 and OSML81) arranged in the same transcriptional orientation has been isolated. Restriction mapping and sequence analysis indicated that the two intronless genes correspond to the previously characterized pA13 and pA81 cDNAs. To study the transcriptional activation of OSML13 and OSML81 promoters, the 5' flanking DNA sequence (-1078 to +35 of OSML13 and -1054 to +41 of OSML81) was fused to the beta-glucoronidase (GUS) coding region, and the chimeric gene fusions were introduced into wild potato (S. commersonii) plants via Agrobacterium-mediated transformation. Analysis of the chimeric gene expression in transgenic potato plants showed that both 5' flanking DNA sequences are sufficient to impart GUS inducibility by abscisic acid, NaCl, salicylic acid, wounding, and fungal infection. Low temperature activated both chimeric genes only slightly. Infection with Phytophthora infestans resulted in strong GUS expression from both chimeric genes primarily in the sites of pathogen invasion, suggesting a limited diffusion of fungal infection-mediated signals. The expression patterns of both osmotin-like protein genes implicate their dual functions in osmotic stress and plant pathogen defense.

Promoters from kin1 and cor6.6, two Arabidopsis thaliana low-temperature- and ABA-inducible genes, direct strong beta-glucuronidase expression in guard cells, pollen and young developing seeds

The ability of most higher plants to withstand freezing can be enhanced by cold acclimation, although the freezing tolerance of plant tissues is also affected by their developmental stage. In addition, low temperature has pleiotropic effects on many plant developmental processes such as vernalization. The interaction between plant development and low temperature implies that some genes are regulated by both environmental factors and developmental cues. Although a number of cold-inducible genes from plants have been identified, information concerning their regulation during plant development is limited. In order to understand their developmental regulation and obtain possible clues as to function, the promoters of kin1 and cor6.6, two cold- and abscisic acid (ABA)-regulated genes from Arabidopsis thaliana, were fused to the beta-glucuronidase (GUS)-coding sequence and the resulting constructs were used to transform tobacco and A. thaliana. Transgenic plants with either the kin1 or cor6.6 promoter showed strong GUS expression in pollen, developing seeds, trichomes and, most interestingly, in guard cells. During pollen development, maximum GUS activity was found in mature pollen. In contrast, the maximum GUS activity during seed development was during early embryogenesis. These patterns of expression distinguish kin1 and cor6.6 from related lea genes which are strongly expressed during late embryogenesis. There was no major qualitative difference in patterns of GUS expression between kin1 and cor6.6 promoters and the results were similar for transgenic tobacco and Arabidopsis. Considering the results described, as well as those in an accompanying paper (Wang et al., 1995, Plant Mol Biol 28: 605-617 (this issue), we suggest that osmotic potential might be a major factor in regulating the expression of kin1 and cor6.6 during several developmental processes. The implication of the results for possible function of the gene products is discussed.

A region of the Arabidopis Lhcb1*3 promoter that binds to CA-1 activity is essential for high expression and phytochrome regulation

We have previously characterized a protein from Arabidopsis thaliana, called CA-1, that bound to a specific region of the Lhcb1*3 promoter. This binding activity was of interest because the sequence to which it bound is included in a portion of the promoter that is sufficient for phytochrome regulation and because the activity was absent in photomorphogenic mutant det1 seedlings (L. Sun, R.A. Doxsee, E. Harel, E.M. Tobin [1993] Plant Cell 5: 109-121). We have now directly tested whether the nucleotide sequence to which CA-1 binds is required for regulation of the transcription of this gene by phytochrome. A mutation that abolished CA-1 binding in vitro was introduced into a 1.15-kb segment of the Lhcb1*3 promoter, and both the wild-type and mutant promoter fragments were fused to a uidA reporter gene and used to stably transform A. thaliana. Ten different homozygous lines were examined for phytochrome responsiveness for each of the two constructs by assaying beta-glucuronidase activity. The wild-type construct showed normal phytochrome responsiveness. The mutant construct showed no phytochrome response, and the overall level of beta-glucuronidase activity in etiolated seedlings was decreased by about 2 orders of magnitude. We did not detect a response to a B photoreceptor other than phytochrome itself for either the wild-type or mutant construct. We conclude that information essential for both a high level of expression and phytochrome responsiveness is contained in a 27-bp region to which the CA-1 activity binds.

Mechanism of the activation of proteinase inhibitor synthesis by systemin involves beta-sheet structure, a specific DNA-binding protein domain

We analyzed a tertiary structure of systemin, the first identified polypeptide plant hormone, using two-dimensional NMR spectroscopy. From these data and molecular dynamics calculations we concluded that the peptide can adopt a Z-like-beta-sheet structure, which has previously been found in many specific DNA-binding proteins. Using DNA-cellulose affinity chromatography, we showed that systemin binds strongly to DNA. We suggest that the specific systemin-DNA interaction, particularly in a promoter region of the proteinase inhibitors, could effect gene expression and thus explain the biological activity of systemin.

Promoters from kin1 and cor6.6, two homologous Arabidopsis thaliana genes: transcriptional regulation and gene expression induced by low temperature, ABA, osmoticum and dehydration

The Arabidopsis thaliana genes kin1 and cor6.6 belong to the same family and were expressed at higher levels following low temperature and ABA treatments. In an attempt to elucidate the mechanism of gene regulation by low temperature, the relationship between low-temperature- and abscisic acid (ABA)-induced gene expression and possible differential expression of the two genes, we have cloned a 5.3 kb genomic fragment harboring kin1 and cor6.6 and their respective 5' sequences. The putative promoters of both genes were fused to the beta-glucuronidase (GUS) coding sequence and GUS expression was analysed in transgenic tobacco and Arabidopsis plants. The cor6.6 promoter produced a higher basal level of expression than the kin1 promoter in transgenic tobacco. Enzyme assays of inducible GUS activity in transgenic Arabidopsis and tobacco plants showed that GUS activity directed by both kin1 and cor6.6 promoters was significantly induced by ABA, dehydration and osmoticum, but not by low temperature. Northern analysis revealed, in contrast, that GUS mRNA was significantly induced in these transgenic plants by low temperature. Further analysis showed that, at low temperature, GUS protein synthesis from the induced GUS mRNA was inhibited. Together these results reveal induction of kin1 and cor6.6 transcription by low temperature, exogenous ABA and dehydration. However, low-temperature expression is dramatically reduced at the translational level.

Pollen viability and transgene expression following storage in honey

Transgenic plants of tobacco and Arabidopsis that produce genetically marked pollen, expressing the reporter gene uidA (gusA), were generated to determine whether pollen proteins can be expressed and stable in honey, a potential route by which foreign proteins might enter the wider environment. Hydrated tobacco pollen was found to lose viability rapidly in honey, while pollen in the natural dehydrated form remained viable for at least several days and in some cases several weeks, as determined by FDA staining activity and germinability. Dehydrated pollen was found to be capable of transient foreign gene expression, following microprojectile bombardment, after incubation in honey for at least 120 h. PCR amplification of transgene sequences in pollen of transgenic plants revealed that pollen DNA can remain relatively intact after 7 weeks in honey. GUS enzyme activity analysis and SDS-PAGE of pollen proteins revealed that foreign and native pollen proteins are stable in pollen incubated in honey for at least 6 weeks. We conclude that pollen may represent an ecologically important vector for transgenic protein products.

Selective DNA amplification regulates transcript levels in plant mitochondria

Most plant mitochondrial genomes exist as subgenomic-size fragments apparently due to recombination between repetitive sequences. This leads to the possibility that independently replicating subgenomic domains could result in mitochondrial gene copy number variation. We show, through Southern-blot analysis of both restricted and intact mtDNA, that there are gene-specific copy number differences in the monocot Zea mays. Comparison of two different maize genotypes, B37(N) and B37(T), a cytoplasmic male-sterile strain, reveal fewer gene copy number differences for B37(T) than for B37(N). In contrast to maize, significant gene copy number differences are not detected in the dicot Brassica hirta. We also demonstrate that mitochondrial transcriptional rates in both species are apparently dependent on gene copy number since relative rates determined by run-on analysis are proportional to relative gene copy numbers. Thus a direct relationship exists between plant mitochondrial gene copy number and transcriptional rate.

Nature and regulation of pistil-expressed genes in tomato

The specialized reproductive functions of angiosperm pistils are dependent in part upon the regulated activation of numerous genes expressed predominantly in this organ system. To better understand the nature of these pistil-predominant gene products we have analyzed seven cDNA clones isolated from tomato pistils through differential hybridization screening. Six of the seven cDNAs represent sequences previously undescribed in tomato, each having a unique pistil- and/or floral-predominant expression pattern. The putative protein products encoded by six of the cDNAs have been identified by their similarity to sequences in the database of previously sequenced genes, with a seventh sequence having no significant similarity with any previously reported sequence. Three of the putative proteins appear to be targeted to the endomembrane system and include an endo-beta-1,4-glucanase which is expressed exclusively in pistils at early stages of development, and proteins similar in sequence to gamma-thionin and miraculin which are expressed in immature pistils and stamens, and in either sepals or petals, respectively. Two other clones, similar in sequence to each other, were expressed primarily in immature pistils and stamens and encode distinct proteins with similarity to leucine aminopeptidases. An additional clone, which encodes a protein similar in sequence to the enzyme hyoscyamine 6-beta-hydroxylase and to other members of the family of Fe2+/ascorbate-dependent oxidases, was expressed at high levels in pistils, stamens and sepals, and at detectable levels in some vegetative organs. Together, these observations provide new insight into the nature and possible functional roles of genes expressed during reproductive development.

An Arabidopsis thaliana root-specific kinase homolog is induced by dehydration, ABA, and NaCl

Genomic and cDNA clones have been isolated for an Arabidopsis thaliana gene, ARSK1, that encodes a protein with structural similarities to serine/threonine kinases. Expression of ARSK1 is root specific and is induced by exposing roots to air during growth or by treatment of roots with ABA or NaCl. ARSK1 gene expression in transgenic plants is confined to cells in the tissues of the root as measured by beta-glucuronidase (GUS) expression from an ARSK1 gene promoter-GUS gene construct. Transverse sections of the stained roots further defined the tissue-specificity; high levels of expression in the epidermal, endoepidermal and cortex regions, but no or very little expression in the vascular system. Another feature of the expression pattern of the ARSK1 gene was a gradual increase in the expression expression level along the root with the highest level of expression in the region closest to the root meristem. These studies suggest that ARSK1 may have a role in the signal transduction pathway of osmotic stress.

Aerobic fermentation in tobacco pollen

We characterized the genes coding for the two dedicated enzymes of ethanolic fermentation, alcohol dehydrogenase (ADH) and pyruvate decarboxylase (PDC), and show that they are functional in pollen. Two PDC-encoding genes were isolated, which displayed reciprocal regulation: PDC1 was anaerobically induced in leaves, whereas PDC2 mRNA was absent in leaves, but constitutively present in pollen. A flux through the ethanolic fermentation pathway could be measured in pollen under all tested environmental and developmental conditions. Surprisingly, the major factor influencing the rate of ethanol production was not oxygen availability, but the composition of the incubation medium. Under optimal conditions for pollen tube growth, approximately two-thirds of the carbon consumed was fermented, and ethanol accumulated into the surrounding medium to a concentration exceeding 100 mM.

The homeobox gene ATK1 of Arabidopsis thaliana is expressed in the shoot apex of the seedling and in flowers and inflorescence stems of mature plants

The homeodomain is a DNA-binding domain present in a large family of eukaryotic regulatory proteins. Homeodomain proteins have been shown to play key roles in controlling developmental programs in various organisms. Here we report the isolation and characterisation of a homeobox gene from Arabidopsis thaliana designated ATK1. The gene was isolated using as a probe the homeobox domain of the KN1 gene from maize. The homeodomain of ATK1 is highly homologous to the homeodomain of the KN1 gene of maize (81%) but shows only poor homology outside the homeodomain. Therefore ATK1 is probably not the Arabidopsis homologue of the KN1 gene from maize. It contains the four invariant amino acid residues present in the recognition helix 3 of all other homeodomain proteins. Outside the homeodomain a region rich in aspartate and glutamate residues is found suggesting that ATK1 is a transcriptional activator. The gene contains four introns which is similar in the KN1 gene of maize and the Osh1 gene of rice. Primer extension reveals the presence of two transcription initiation sites. The leader sequence of the genuine transcript is 342 nucleotides long and contains two upstream open reading frames. ATK1 is strongly expressed in the shoot apex of seedlings, while in mature plants the gene is primarily expressed in flowers and inflorescence stems. Such an expression pattern is reminiscent of that of the KN1 gene of maize and therefore ATK1 could similarly be involved in determining cell fate.

Translational arrest in hypoxic potato tubers is correlated with the aberrant association of elongation factor EF-1 alpha with polysomes

Translation elongation factor EF-1 alpha became stably associated with potato tuber polysomes at the onset of hypoxia, coincident with a sharp rise in lactate and decrease in tissue pH. This aberrant association of EF-1 alpha with polysomes also occurred when aerobic tuber extracts were acidified in vitro. Upon resumption of protein synthesis, an increase in the steady-state levels of EF-1 alpha, and expression of an EF-1 alpha/GUS transgene was observed. These results indicate that translational arrest results from to the failure of EF-1 alpha to dissociate from ribosomes during the elongation cycle, and that restoration of protein synthesis is coordinated with expression of EF-1 alpha.

Effects of light and chloroplast functional state on expression of nuclear genes encoding chloroplast glyceraldehyde-3-phosphate dehydrogenase in long hypocotyl (hy) mutants and wild-type Arabidopsis thaliana

In a previous study of Arabidopsis thaliana (J. Dewdney, T.R. Conley, M.-C. Shih, H.M. Goodman [1993] Plant Physiol 103: 1115-1121), it was postulated that both blue light receptor- and phytochrome-mediated pathways contribute to regulation of the nuclear genes encoding A and B subunits of glyceraldehyde-3-phosphate dehydrogenase (GAPA and GAPB). Here were report on the involvement of a nuclear gene encoding a putative blue-light receptor (HY4) and of a nuclear gene encoding phytochrome A apoprotein (PHYA) in regulation of the GAPA and GAPB genes in response to blue and far-red light. Continuous light irradiation experiments with the hy4 mutant demonstrate that the HY4 gene product is required for full expression of GAPA, GAPB, and one or more of the nuclear genes encoding small subunits of of ribulose-1,5-bisphosphate carboxylase/oxygenase. Continuous light irradiation and fluence-response studies with the phyA-101 mutant show that phytochrome A functions in far-red light regulation of GAPA, GAPB, nuclear genes encoding small subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase, and CAB genes. Phytochromes A and B alone either do not participate in red light-mediated gene regulation or have redundant functions, as shown by analysis of phyA-101 and phyB-1 single mutants. In addition, the hypothesis that chloroplast-nucleus interactions affect GAPA and GAPB gene regulation was tested. Herbicide-mediated photooxidative damage to chloroplasts in A thaliana seedlings strongly decreased the maximum amount of GAPA and GAPB steady-state mRNA detected in continuous-light irradiation experiments. Full expression of the GAPB genes is dependent on the presence of functional chloroplasts.

Developmental regulation of ribosomal protein L16 genes in Arabidopsis thaliana

Lateral roots can be synchronously induced in Arabidopsis by a brief auxin treatment. An early event in the development of a lateral root primordium is the accumulation of mRNAs encoding ribosomal proteins. In situ hybridizations show that mRNA encoding one ribosomal protein, L16, accumulates in all rapidly proliferating tissues including the shoot and root apical meristems and lateral root primordia. To understand further the mechanisms by which ribosomal proteins are coordinately synthesized, two genes encoding the ribosomal protein L16 were isolated from Arabidopsis thaliana. Promoter sequences from each RPL16A and RPL16B were fused to the beta-glucuronidase reporter gene GUS. The promoter of RPL16B(from -848 to -19) conferred X-Gluc staining in proliferating tissues including the shoot and root apical meristems. When GUS was expressed from the RPL16A promoter (from -875 to -22), X-Gluc staining was observed in cells in the root stele and in anthers. When seedlings transformed with either promoter construct were treated with auxin to induce lateral roots, X-Gluc staining accumulated in the lateral root primordia by 16 h after induction. Transcription of the RPL16B promoter appears to be correlated with cell division, while transcription of the RPL16A promoter is very cell specific. Expression of two genes encoding L16 during the early phase of lateral root initiation and in developing pollen may serve to increase levels of ribosomal proteins during the rapid growth of these tissues.

Posttranscriptional and posttranslational control of enolase expression in the facultative Crassulacean acid metabolism plant Mesembryanthemum Crystallinum L

During the induction of Crassulacean acid and metabolism by environmental stresses in the common ice plant (Mesembryanthemum crystallinum L.), enzyme activities involved in glycolysis and gluconeogenesis, including enolase (2-phospho-D-glycerate hydrolase), increase significantly. In this study, we describe two nearly identical cDNA clones (Pgh1a and Pgh1b) encoding enolase from the common ice plant. This cytoplasmically localized enzyme is encoded by a gene family of at least two members. The polypeptides encoded by these cDNAs share a high degree of amino acid sequence identity (86.7-88.3%) with other higher plant enolases. Enolase activity increased more than 4-fold in leaves during salt stress. This increase was accompanied by a dramatic increase in Pgh1 transcription rate and the accumulation of enolase transcripts in leaves. Pgh1 transcript levels also increased in leaves in response to low temperature, drought, and anaerobic stress conditions and upon treatment of unstressed plants with the plant growth regulators abscisic acid and 6-benzylaminopurine. In roots, enolase transcripts increased in abundance in response to salt, low and high temperature, and anaerobic stresses. Surprisingly, we observed no increase in enolase protein levels, despite the increased levels of mRNA and enzyme activity during salt stress. The stress-induced increase in enolase activity is therefore due to posttranslational regulation of steady-state enzyme pools. Our results demonstrate that the stress-induced shift to Crassulacean acid metabolism in the ice plant involves complex regulatory control mechanisms that operate at the transcriptional, posttranscriptional, and postranslational levels.

Isolation of a flax (Linum usitatissimum) gene induced during susceptible infection by flax rust (Melampsora lini)

In a susceptible infection of flax (Linum usitatissimum), the obligate rust pathogen (Melampsora lini) can grow in the leaf without triggering the hypersensitive resistance response. The rust establishes specialized structures (haustoria) in plant mesophyll cells and induces changes in plant subcellular organization. Subtraction hybridization methods were used to isolate cDNA clones of mRNAs that have altered expression in infected leaves. Most of the cDNAs recovered were of fungal origin, but one clone, pFIS 1 (flax inducible sequence No. 1), recovered from several independent experiments, was a plant-specified mRNA that showed a 10-fold increase in steady-state levels during susceptible growth. The increase in fis 1 mRNA levels was not seen in the resistant reaction (hypersensitive reaction) and the predicted protein sequence (551 amino acids with a predicted molecular weight of 61 kDa) has no similarity to known pathogenesis-related proteins. Searches of sequence data bases showed that fis 1 encodes a protein which contains amino acid sequence motifs that are conserved in all previously characterized aldehyde dehydrogenases.

Cis-acting elements and expression pattern of the spinach rps22 gene coding for a plastid-specific ribosomal protein

In order to study the regulation of nuclear genes coding for plastid ribosomal proteins, we have analysed the promoter region of spinach rps22 using both in vitro and in vivo approaches. By footprinting analyses, we have identified eight DNA elements interacting with spinach leaf nuclear factors in the 300 bp promoter region upstream of the transcription start site. Among these elements, four are short AT-rich sequences and one is identical to the Hex motif characterized initially in wheat histone genes. In transgenic tobacco plants, the reporter gene coding for the beta-glucuronidase (GUS) directed by a 1.2 kb upstream region of rps22 was expressed in several plant organs, with high levels in leaf mesophyll, embryo cotyledons and root meristematic cells and very low levels in other cell types. Interestingly, when deleted to -295, the promoter, which contained all the foot-printed elements, was still able to confer the same expression pattern, although the activity was relatively lower than with the 1.2 kb promoter. When deleted further to -154, the promoter, from which the AT-rich elements were eliminated, loses its activity almost completely, suggesting that these AT-rich elements are important for the rps22 promoter activity. Altogether, our results show that rps22 gene expression is controlled by specific cis elements not present in other nuclear-encoded plastid ribosomal protein genes studied so far.

Shoot development in plants: time for a change

The shoot system in plants progresses through several discrete phases during its development. Changes in the timing of these phases have important consequences for the morphogenesis of the shoot and are likely to be important in plant evolution. Genetic analysis of phase change in herbaceous plants, such as maize and Arabidopsis, has defined some of the genes involved in this phenomenon and has suggested a model for the regulation of this key feature of plant development.