Analysis of a chimeric class-I patatin-GUS gene in transgenic potato plants: High-level expression in tubers and sucrose-inducible expression in cultured leaf and stem explants

Auxin synthesis gene tms1 driven by tuber-specific promoter alters hormonal status of transgenic potato plants and their responses to exogenous phytohormones

Ectopic auxin overproduction in transgenic potato leads to enhanced productivity accompanied with concerted and occasional changes in hormonal status, and causing altered response of transformants to exogenous auxin or cytokinin. Previously, we generated potato transformants expressing Agrobacterium-derived auxin synthesis gene tms1 driven by tuber-specific patatin gene promoter (B33-promoter). Here, we studied the endogenous hormonal status and the response to exogenous phytohormones in tms1 transformants cultured in vitro. Adding indole-3-acetic acid (IAA) or kinetin to culture medium affected differently tuberization of tms1-transformed and control plants, depending also on sucrose content in the medium. Exogenous phytohormones ceased to stimulate the tuber initiation in transformants at high (5-8%) sucrose concentration, while in control plants the stimulation was observed in all experimental settings. Furthermore, exogenous auxin partly inhibited the tuber initiation, and exogenous cytokinin reduced the average tuber weight in most transformants at high sucrose content. The elevated auxin level in tubers of the transformants was accompanied with a decrease in content of cytokinin bases and their ribosides in tubers and most shoots. No concerted changes in contents of abscisic, jasmonic, salicylic acids and gibberellins in tubers were detected. The data on hormonal status indicated that the enhanced productivity of tms1 transformants was due to auxin and not mediated by other phytohormones. In addition, exogenous cytokinin was shown to upregulate the expression of genes encoding orthologs of auxin receptors. Overall, the results showed that tms1 expression and local increase in IAA level in transformants affect both the balance of endogenous cytokinins and the dynamics of tuberization in response to exogenous hormones (auxin, cytokinin), the latter reaction depending also on the carbohydrate supply. We introduce a basic model for the hormonal network controlling tuberization.

Intronic Sequence Regulates Sugar-Dependent Expression of Arabidopsis thaliana Production of Anthocyanin Pigment-1/MYB75

Sucrose-specific regulation of gene expression is recognized as an important signaling response, distinct from glucose, which serves to modulate plant growth, metabolism, and physiology. The Arabidopsis MYB transcription factor Production of Anthocyanin Pigment-1 (PAP1) plays a key role in anthocyanin biosynthesis and expression of PAP1 is known to be regulated by sucrose. Sucrose treatment of Arabidopsis seedlings led to a 20-fold induction of PAP1 transcript, which represented a 6-fold increase over levels in glucose-treated seedlings. The PAP1 promoter was not sufficient for conferring a sucrose response to a reporter gene and did not correctly report expression of PAP1 in plants. Although we identified 3 putative sucrose response elements in the PAP1 gene, none were found to be necessary for this response. Using deletion analysis, we identified a 90 bp sequence within intron 1 of PAP1 that is necessary for the sucrose response. This sequence was sufficient for conferring a sucrose response to a minimal promoter: luciferase reporter when present in multiple copies upstream of the promoter. This work lays the foundation for dissecting the sucrose signaling pathway of PAP1 and contributes to understanding the interplay between sucrose signaling, anthocyanin biosynthesis, and stress responses.

Expression of auxin synthesis gene tms1 under control of tuber-specific promoter enhances potato tuberization in vitro

Phytohormones, auxins in particular, play an important role in plant development and productivity. Earlier data showed positive impact of exogenous auxin on potato (Solanum tuberosum L.) tuberization. The aim of this study was to generate potato plants with increased auxin level predominantly in tubers. To this end, a pBinB33-tms1 vector was constructed harboring the Agrobacterium auxin biosynthesis gene tms1 fused to tuber-specific promoter of the class I patatin gene (B33-promoter) of potato. Among numerous independently generated B33:tms1 lines, those without visible differences from control were selected for detailed studies. In the majority of transgenic lines, tms1 gene transcription was detected, mostly in tubers rather than in shoots. Indoleacetic acid (IAA) content in tubers and the auxin tuber-to-shoot ratio were increased in tms1-expressing transformants. The organ-specific increase in auxin synthesis in B33:tms1-transformants accelerated and intensified the process of tuber formation, reduced the dose of carbohydrate supply required for in vitro tuberization, and decreased the photoperiodic dependence of tuber initiation. Overall, a positive correlation was observed between tms1 expression, IAA content in tubers, and stimulation of tuber formation. The revealed properties of B33:tms1 transformants imply an important role for auxin in potato tuberization and offer prospects to magnify potato productivity by a moderate organ-specific enhancement of auxin content.

Sucrose signaling in plants: a world yet to be explored

The role of sucrose as a signaling molecule in plants was originally proposed several decades ago. However, recognition of sucrose as a true signal has been largely debated and only recently this role has been fully accepted. The best-studied cases of sucrose signaling involve metabolic processes, such as the induction of fructan or anthocyanin synthesis, but a large volume of scattered information suggests that sucrose signals may control a vast array of developmental processes along the whole life cycle of the plant. Also, wide gaps exist in our current understanding of the intracellular steps that mediate sucrose action. Sucrose concentration in plant tissues tends to be directly related to light intensity, and inversely related to temperature, and accordingly, exogenous sucrose supply often mimics the effect of high light and cold. However, many exceptions to this rule seem to occur due to interactions with other signaling pathways. In conclusion, the sucrose role as a signal molecule in plants is starting to be unveiled and much research is still needed to have a complete map of its significance in plant function.

Molecular and physiological properties associated with zebra complex disease in potatoes and its relation with Candidatus Liberibacter contents in psyllid vectors

Zebra complex (ZC) disease on potatoes is associated with Candidatus Liberibacter solanacearum (CLs), an α-proteobacterium that resides in the plant phloem and is transmitted by the potato psyllid Bactericera cockerelli (Šulc). The name ZC originates from the brown striping in fried chips of infected tubers, but the whole plants also exhibit a variety of morphological features and symptoms for which the physiological or molecular basis are not understood. We determined that compared to healthy plants, stems of ZC-plants accumulate starch and more than three-fold total protein, including gene expression regulatory factors (e.g. cyclophilin) and tuber storage proteins (e.g., patatins), indicating that ZC-affected stems are reprogrammed to exhibit tuber-like physiological properties. Furthermore, the total phenolic content in ZC potato stems was elevated two-fold, and amounts of polyphenol oxidase enzyme were also high, both serving to explain the ZC-hallmark rapid brown discoloration of air-exposed damaged tissue. Newly developed quantitative and/or conventional PCR demonstrated that the percentage of psyllids in laboratory colonies containing detectable levels of CLs and its titer could fluctuate over time with effects on colony prolificacy, but presumed reproduction-associated primary endosymbiont levels remained stable. Potato plants exposed in the laboratory to psyllid populations with relatively low-CLs content survived while exposure of plants to high-CLs psyllids rapidly culminated in a lethal collapse. In conclusion, we identified plant physiological biomarkers associated with the presence of ZC and/or CLs in the vegetative potato plant tissue and determined that the titer of CLs in the psyllid population directly affects the rate of disease development in plants.

Characterization of StABF1, a stress-responsive bZIP transcription factor from Solanum tuberosum L. that is phosphorylated by StCDPK2 in vitro

ABF/AREB bZIP transcription factors mediate plant abiotic stress responses by regulating the expression of stress-related genes. These proteins bind to the abscisic acid (ABA)-responsive element (ABRE), which is the major cis-acting regulatory sequence in ABA-dependent gene expression. In an effort to understand the molecular mechanisms of abiotic stress resistance in cultivated potato (Solanum tuberosum L.), we have cloned and characterized an ABF/AREB-like transcription factor from potato, named StABF1. The predicted protein shares 45-57% identity with A. thaliana ABFs proteins and 96% identity with the S. lycopersicum SlAREB1 and presents all of the distinctive features of ABF/AREB transcription factors. Furthermore, StABF1 is able to bind to the ABRE in vitro. StABF1 gene is induced in response to ABA, drought, salt stress and cold, suggesting that it might be a key regulator of ABA-dependent stress signaling pathways in cultivated potato. StABF1 is phosphorylated in response to ABA and salt stress in a calcium-dependent manner, and we have identified a potato CDPK isoform (StCDPK2) that phosphorylates StABF1 in vitro. Interestingly, StABF1 expression is increased during tuber development and by tuber-inducing conditions (high sucrose/nitrogen ratio) in leaves. We also found that StABF1 calcium-dependent phosphorylation is stimulated by tuber-inducing conditions and inhibited by gibberellic acid, which inhibits tuberization.

A transgenic study on affecting potato tuber yield by expressing the rice sucrose transporter genes OsSUT5Z and OsSUT2M

In many plants, sucrose transporters are essential for both sucrose exports from sources and imports into sinks, indicating a function in assimilate partitioning. To investigate whether sucrose transporters can improve the yield of starch plant, potato plants (Solanum tuberosum L. cv. Désirée) were transformed with cDNAs of the rice sucrose transporter genes OsSUT5Z and OsSUT2M under the control of a tuber-specific, class-I patatin promoter. Compared to the controls, the average fructose content of OsSUT5Z transgenic tubers significantly increased. However, the content of the sugars and starch in the OsSUT2M transgenic potato tubers showed no obvious difference. Correspondingly, the average tuber yield, average number of tubers per plant and average weight of single tuber showed no significant difference in OsSUT2M transgenic tubers with controls. In the OsSUT5Z transgenic lines, the average tuber yield per plant was 1.9-fold higher than the controls, and the average number of tubers per plant increased by more than 10 tubers on average, whereas the average weight of a single tuber did not increase significantly. These results suggested that the average number of tubers per plant showed more contribution than the average weight of a single tuber to the tuber yield per plant.

Protein phosphatases type 2A mediate tuberization signaling in Solanum tuberosum L. leaves

Tuber formation in potato (Solanum tuberosum L.) is regulated by hormonal and environmental signals that are thought to be integrated in the leaves. The molecular mechanisms that mediate the responses to tuberization-related signals in leaves remain largely unknown. In this study we analyzed the roles of protein phosphatase type 2A catalytic subunits (PP2Ac) in the leaf responses to conditions that affect tuberization. The responses were monitored by analyzing the expression of the "tuber-specific" genes Patatin and Pin2, which are induced in tubers and leaves during tuber induction. Experiments using PP2A inhibitors, together with PP2Ac expression profiles under conditions that affect tuberization indicate that high sucrose/nitrogen ratio, which promotes tuber formation, increases the transcript levels of Patatin and Pin2, by increasing the activity of PP2As without affecting PP2Ac mRNA or protein levels. Gibberellic acid (GA), a negative regulator of tuberization, down-regulates the transcription of catalytic subunits of PP2As from the subfamily I and decreases their enzyme levels. In addition, GA inhibits the expression of Patatin and Pin2 possibly by a PP2A-independent mechanism. PP2Ac down-regulation by GA may inhibit tuberization signaling downstream of the inductive effects of high sucrose/nitrogen ratio. These results are consistent with the hypothesis that PP2As of the subfamily I may positively modulate the signaling pathways that lead to the transcriptional activation of "tuber-specific" genes in leaves, and act as molecular switches regulated by both positive and negative modulators of tuberization.

Sucrose-mediated translational control

BACKGROUND

Environmental factors greatly impact plant gene expression and concentrations of cellular metabolites such as sugars and amino acids. The changed metabolite concentrations affect the expression of many genes both transcriptionally and post-transcriptionally.

RECENT PROGRESS

Sucrose acts as a signalling molecule in the control of translation of the S1 class basic leucine zipper transcription factor (bZIP) genes. In these genes the main bZIP open reading frames (ORFs) are preceded by upstream open reading frames (uORFs). The presence of uORFs generally inhibits translation of the following ORF but can also be instrumental in specific translational control. bZIP11, a member of the S1 class bZIP genes, harbours four uORFs of which uORF2 is required for translational control in response to sucrose concentrations. This uORF encodes the Sucrose Control peptide (SC-peptide), which is evolutionarily conserved among all S1 class bZIP genes in different plant species. Arabidopsis thaliana bZIP11 and related bZIP genes seem to be important regulators of metabolism. These proteins are targets of the Snf1-related protein kinase 1 (SnRK1) KIN10 and KIN11, which are responsive to energy deprivation as well as to various stresses. In response to energy deprivation, ribosomal biogenesis is repressed to preserve cellular function and maintenance. Other key regulators of ribosomal biogenesis such as the protein kinase Target of Rapamycin (TOR) are tightly regulated in response to stress.

CONCLUSIONS

Plants use translational control of gene expression to optimize growth and development in response to stress as well as to energy deprivation. This Botanical Briefing discusses the role of sucrose signalling in the translational control of bZIP11 and the regulation of ribosomal biogenesis in response to metabolic changes and stress conditions.

The sweet potato sporamin promoter confers high-level phytase expression and improves organic phosphorus acquisition and tuber yield of transgenic potato

The sweet potato sporamin promoter was used to control the expression in transgenic potato of the E. coli appA gene, which encodes a bifunctional enzyme exhibiting both acid phosphatase and phytase activities. The sporamin promoter was highly active in leaves, stems and different size tubers of transgenic potato, with levels of phytase expression ranging from 3.8 to 7.4% of total soluble proteins. Phytase expression levels in transgenic potato tubers were stable over several cycles of propagation. Field tests showed that tuber size, number and yield increased in transgenic potato. Improved phosphorus (P) acquisition when phytate was provided as a sole P source and enhanced microtuber formation in cultured transgenic potato seedlings when phytate was provided as an additional P source were observed, which may account for the increase in leaf chloroplast accumulation (important for photosynthesis) and tuber yield of field-grown transgenic potato supplemented with organic fertilizers. Animal feeding tests indicated that the potato-produced phytase supplement was as effective as a commercially available microbial phytase in increasing the availability of phytate-P to weanling pigs. This study demonstrates that the sporamin promoter can effectively direct high-level recombinant protein expression in potato tubers. Moreover, overexpression of phytase in transgenic potato not only offers an ideal feed additive for improving phytate-P digestibility in monogastric animals but also improves tuber yield, enhances P acquisition from organic fertilizers, and has a potential for phytoremediation.

Expression of alternansucrase in potato plants

Alternan, which consists of alternating α-(1→3)/α-(1→6)-linked glucosyl residues, was produced in potato tubers by expressing a mature alternansucrase (Asr) gene from Leuconostoc mesenteroides NRRL B-1355 in potato. Detection of alternan was performed by enzyme-linked immunosorbent assay in tuber juices, revealing a concentration between 0.3 and 1.2 mg g^-1 fresh wt. The Asr transcript levels correlated well with alternan accumulation in tuber juices. It appeared that the expression of sucrose-regulated starch-synthesizing genes (ADP-glucose pyrophosphorylase subunit S and granule-bound starch synthase I) was down-regulated. Despite this, the physico-chemical properties of the transgenic starches were unaltered. These results are compared to those obtained with other transgenic potato plants producing mutan [α-(1→3)-linked glucosyl residues] and dextran [α-(1→6)-linked glucosyl residues].

Fusion proteins comprising the catalytic domain of mutansucrase and a starch-binding domain can alter the morphology of amylose-free potato starch granules during biosynthesis

It has been shown previously that mutan can be co-synthesized with starch when a truncated mutansucrase (GtfICAT) is directed to potato tuber amyloplasts. The mutan seemed to adhere to the isolated starch granules, but it was not incorporated in the starch granules. In this study, GtfICAT was fused to the N- or C-terminus of a starch-binding domain (SBD). These constructs were introduced into two genetically different potato backgrounds (cv. Kardal and amf), in order to bring GtfICAT in more intimate contact with growing starch granules, and to facilitate the incorporation of mutan polymers in starch. Fusion proteins of the appropriate size were evidenced in starch granules, particularly in the amf background. The starches from the various GtfICAT/SBD transformants seemed to contain less mutan than those from transformants with GtfICAT alone, suggesting that the appended SBD might inhibit the activity of GtfICAT in the engineered fusion proteins. Scanning electron microscopy showed that expression of SBD-GtfICAT resulted in alterations of granule morphology in both genetic backgrounds. Surprisingly, the amf starches containing SBD-GtfICAT had a spongeous appearance, i.e., the granule surface contained many small holes and grooves, suggesting that this fusion protein can interfere with the lateral interactions of amylopectin sidechains. No differences in physico-chemical properties of the transgenic starches were observed. Our results show that expression of granule-bound and "soluble" GtfICAT can affect starch biosynthesis differently.

Seasonal control of tuberization in potato: conserved elements with the flowering response

Fluctuations in day length determine the time to flower in many plants and in potato are critical to promote differentiation of tubers. Day length is perceived in the leaves and under inductive conditions these synthesize a systemic signal that is transported to the underground stolons to induce tuber development. Flowering tobacco shoots grafted into potato stocks promote tuberization in the stocks, indicating that the floral and tuber-inducing signals might be similar. We describe recent progress in the identification of the molecular mechanisms underlying day-length recognition in potato. Evidence has been obtained for a conserved function of the potato orthologs of the CONSTANS (CO) and FLOWERING LOCUS T (FT) proteins in tuberization control under short days (SDs). These observations indicate that common regulatory pathways are involved in both flowering and tuberization photoperiodic responses in plants.

Production of dextran in transgenic potato plants

The production of dextran in potato tubers and its effect on starch biosynthesis were investigated. The mature dextransucrase (DsrS) gene from Leuconostoc mesenteroides was fused to the chloroplastic ferredoxin signal peptide (FD) enabling amyloplast entry, which was driven by the highly tuber-expressed patatin promoter. After transformation of two potato genotypes (cv. Kardal and the amylose-free (amf) mutant), dextrans were detected by enzyme-linked immunosorbent assay (ELISA) in tuber juices of Kardal and amf transformants. The dextran concentration appeared two times higher in the Kardal (about 1.7 mg/g FW) than in the amf transformants. No dextran was detected by ELISA inside the starch granule. Interestingly, starch granule morphology was affected, which might be explained by the accumulation of dextran in tuber juices. In spite of that, no significant changes of the physicochemical properties of the starches were detected. Furthermore, we have observed no clear changes in chain length distributions, despite the known high acceptor efficiency of DSRS.

Structural diversity and differential transcription of the patatin multicopy gene family during potato tuber development

The patatin multicopy gene family encodes the major storage protein in potato tubers and is organized as a single cluster in the potato genome. We sequenced a 154-kb bacterial artificial chromosome (BAC) clone containing a portion of the patatin gene cluster. Two putatively functional patatin genes were found in this BAC. These two genes are embedded within arrays of patatin pseudogenes. Using a chromatin immunoprecipitation method we demonstrate that the dramatic increase of patatin gene expression during the transition from stolons to tubers coincides with an increase of histone H4 lysine acetylation. We used 3' rapid amplification of cDNA ends to profile expression of different patatin genes during tuber development. The profiling results revealed differential expression patterns of specific patatin gene groups throughout six different stages of tuber development. One group of patatin gene transcripts, designated patatin gene group A, was found to be the most abundant group during all stages of tuber development. Other patatin gene groups, with a 48-bp insertion in the 3'-untranslated region, are not expressed in stolons but display a gradual increase in expression level following the onset of tuberization. These results demonstrate that the patatin genes exhibit alterations in chromatin state and differential transcriptional regulation during the developmental transition from stolons into tubers, in which there is an increased demand for protein storage.

Mutan produced in potato amyloplasts adheres to starch granules

Production of water-insoluble mutan polymers in Kardal potato tubers was investigated after expression of a full-length (GtfI) and a truncated mutansucrase gene referred to as GtfICAT (GtfI without glucan-binding domain) from Streptococcus downei. Subsequent effects on starch biosynthesis at the molecular and biochemical levels were studied. Expression of the GtfICAT gene resulted in the adhesion of mutan material on starch granules, which stained red with erythrosine, and which was hydrolysed by exo-mutanase. In addition, GtfICAT-expressing plants exhibited a severely altered tuber phenotype and starch granule morphology in comparison to those expressing the full-length GtfI gene. In spite of that, no structural changes at the starch level were observed. Expression levels of the sucrose-regulated, AGPase and GBSSI genes were down-regulated in only the GTFICAT transformants, showing that GtfICAT expression interfered with the starch biosynthetic pathway. In accordance with the down-regulated AGPase gene, a lower starch content was observed in the GTFICAT transformants. Finally, the rheological properties of the GTFICAT starches were modified; they showed a higher retrogradation during cooling of the starch paste.

Differential regulation of glucose-6-phosphate dehydrogenase isoenzyme activities in potato

In plants, Glc-6-phosphate dehydrogenase (G6PDH) isoenzymes are present in the cytosol and in plastids. The plastidic enzymes (P1 and P2) are subject to redox regulation, but mechanisms that adjust cytosolic G6PDH activity are largely unknown. We adopted a leaf disc system for monitoring the effects of various conditions on G6PD isoform expression and enzyme activities in potato (Solanum tuberosum). Cytosolic G6PDH activity remained constant during water incubation in the dark. In continuous light or in the presence of metabolizable sugars in the dark, cytosolic G6PDH activity increased 6-fold within 24 h. Cycloheximide incubation demonstrated that enhanced cytosolic G6PDH activity depends on de novo protein synthesis. Osmotic change, phosphate sequestration, or oxidative stress did not affect cytosolic G6PDH activity. Furthermore, enzyme activity and protein contents closely followed the corresponding mRNA levels. Together with the fact that multiple SURE elements are present in the promoter region of the gene, these results suggest that cytosolic G6PDH activity is regulated by sugar availability at the transcriptional level. Plastidic G6PDH activity stayed constant during water incubation in the light and dropped to minimal levels within 6 h in the dark. Conversely, plastidic G6PDH activity of leaf discs incubated on Paraquat rose to 10-fold higher levels, which was not prevented by cycloheximide. Similar increases were found with nitrite, nitrate, or sulfate. No major changes in protein or mRNA contents of the plastidic P1 and P2 isoforms were registered. K(m) (Glc-6-phosphate) values of plastidic G6PDH activity differed between samples incubated on water or Paraquat, suggesting posttranslational modification of the plastidic enzyme(s). Immunoprecipitation of (32)P-labeled samples with P1 isoform-specific antibodies showed that the chloroplast enzyme is subject to protein phosphorylation. Obviously, in extended dark periods, G6PDH activity in the stroma is restricted but can be stimulated in response to high demands for NADPH.

Isolation and functional characterization of a stolon specific promoter from potato (Solanum tuberosum L.)

In the search for time- and tissue-specific promoters an RNA fingerprinting technique called cDNA-AFLP was used. A transcript derived fragment (TDF511) was isolated which showed high similarity to alcohol dehydrogenases. The gene corresponding to this TDF, named Stgan, is likely to be involved in biosynthesis or breakdown of compounds affecting gibberellic acid (GA) levels in the plant [Plant J. 25(6) (2001) 595]. In this article the isolation and characterization of a Stgan promoter region is reported. The promoter region of this gene was fused to a reporter gene encoding beta-glucuronidase (GUS) and introduced in potato plants. GUS staining was detected uniquely in stolon tips and nodes. RNA in situ hybridization experiments revealed that this gene was specifically expressed in parenchyma cells, in the stolon cortex. Comparison of this promoter sequence with several promoter databases resulted in the identification of several potential binding sites for transcription factors. From the in vitro-culture experiments Stgan transcription appears to be induced by long days, sucrose and different hormones such as gibberellic acid, ancymidol, ethylene and cytokinins.

Targeted expression of human serum albumin to potato tubers

Complementary DNA expression of mature human serum albumin was engineered into potato plants under the transcriptional control of patatin B33 promoter and potato proteinase inhibitor II terminator. Protein secretion was achieved by using the signal sequence from potato proteinase inhibitor II. Recombinant albumin accumulated up to 0.2% of total soluble tuber protein in single transformant lines, regardless of the potato cultivar used. Electrophoretic mobility and N-terminal amino acid sequence analysis of partially purified recombinant albumin confirmed proper processing of an immune responsive recombinant albumin, and revealed that the proteinase inhibitor II signal sequence was correctly removed. No further optimisation of these yields was obtained by HSA expression in patatin antisense plants (line Pas58). Subcellular localisation showed that recombinant protein was successfully targeted to the apoplast. Potato tubers may be used, by applying this technology, to produce other heterologous proteins of interest in the biopharmaceutical industry.

Storekeeper defines a new class of plant-specific DNA-binding proteins and is a putative regulator of patatin expression

The expression of class I patatin genes is restricted to potato tubers but can be induced in other tissues by exogenous sucrose. Here we show that tuber-specific and sucrose-inducible gene expression is reduced in transgenic potato plants by mutations in a conserved 10 base pair motif within the B-box of the patatin promoter. In a southwestern screen, we have isolated a novel DNA-binding protein designated Storekeeper (STK) that specifically recognises the B-box motif in vitro. Gel shift experiments with an STK-specific antibody suggest that STK is the B-box binding protein found in tuber nuclei. We propose that STK, the defining member of a new class of DNA binding proteins, regulates patatin expression in potato tubers via the B-box motif.

Sucrose metabolism in plastids

The question whether sucrose (Suc) is present inside plastids has been long debated. Low Suc levels were reported to be present inside isolated chloroplasts, but these were argued to be artifacts of the isolation procedures used. We have introduced Suc-metabolizing enzymes in plastids and our experiments suggest substantial Suc entry into plastids. The enzyme levansucrase from Bacillus subtilis efficiently synthesizes fructan from Suc. Targeting of this enzyme to the plastids of tobacco (Nicotiana tabacum) and potato (Solanum tuberosum) plants leads to high-level fructan accumulation in chloroplasts and amyloplasts, respectively. Moreover, introduction of this enzyme in amyloplasts leads to an altered starch structure. Expression of the yeast invertase in potato tuber amyloplasts results in an 80% reduction of total Suc content, showing efficient hydrolysis of Suc by the plastidic invertase. These observations suggest that Suc can enter plastids efficiently and they raise questions as to its function and metabolism in this organelle.

Production of hepatitis B surface antigen in transgenic plants for oral immunization

Here we present data showing oral immunogenicity of recombinant hepatitis B surface antigen (HBsAg) in preclinical animal trials. Mice fed transgenic HBsAg potato tubers showed a primary immune response (increases in HBsAg-specific serum antibody) that could be greatly boosted by intraperitoneal delivery of a single subimmunogenic dose of commercial HBsAg vaccine, indicating that plants expressing HBsAg in edible tissues may be a new means for oral hepatitis B immunization. However, attainment of such a goal will require higher HBsAg expression than was observed for the potatoes used in this study. We conducted a systematic analysis of factors influencing the accumulation of HBsAg in transgenic potato, including 5' and 3' flanking elements and protein targeting within plant cells. The most striking improvements resulted from (1) alternative polyadenylation signals, and (2) fusion proteins containing targeting signals designed to enhance integration or retention of HBsAg in the endoplasmic reticulum (ER) of plant cells.

Photosynthesis, sugars and the regulation of gene expression

Sugar-mediated regulation of gene expression is a mechanism controlling the expression of many different plant genes. In this review, a compilation of the genes encoding photosynthetic proteins, subject to this mode of regulation, is presented. Several groups have devised different screening strategies to obtain Arabidopsis mutants in sugar sensing and signalling. An overview of these strategies has been included. Sugar-mediated regulation of gene expression is thought to require the hexokinase (HXK) protein. It has previously been shown that one such sugar, mannose, is capable of blocking germination in Arabidopsis. This inhibition is also mediated by HXK and occurs in the low millimolar concentration range. Here, the use of germination on mannose as an effective screening strategy for putative sugar sensing and signalling mutants is reported. T-DNA- and EMS-mutagenized collections were used to isolate 31 mannose-insensitive germination (mig) mutants. With the use of these mutants, a comparison between this screen and other existing sugar-sensing screens is presented.

A/T-rich sequences act as quantitative enhancers of gene expression in transgenic tobacco and potato plants

The role of an A/T-rich positive regulatory region (P268, -444 to -177 from the translation start site) of the pea plastocyanin gene (PetE) promoter has been investigated in transgenic plants containing chimeric promoters fused to the beta-glucuronidase (GUS) reporter gene. This region enhanced GUS expression in leaves of transgenic tobacco plants when fused in either orientation to a minimal pea PetE promoter (-176 to +4) and in roots when fused in either orientation upstream or downstream of a minimal cauliflower mosaic virus 35S promoter (-90 to +5). The region was also able to enhance GUS expression in microtubers of transgenic potato plants when placed in either orientation upstream of a minimal class I patatin promoter (-332 to +14). Dissection of P268 revealed that cis elements responsible for enhancing GUS expression from the minimal PetE promoter were distributed throughout P268. Multiple copies of a 31 bp A/T-rich sequence from within P268 and of a 26 bp random A/T sequence were able to enhance GUS expression from the minimal PetE promoter, indicating that A/T-rich sequences are able to act as quantitative, non-tissue-specific enhancer elements in higher plants.

Sucrose-specific signalling represses translation of the Arabidopsis ATB2 bZIP transcription factor gene

The Arabidopsis bZIP transcription factor gene ATB2 has been shown previously to be expressed in a light-regulated and tissue-specific way. Here we describe the precise localization of ATB2 expression, using transgenic lines containing an ATB2 promoter-GUS reporter gene construct. The observed expression pattern suggests a role for ATB2 in the control of processes associated with the transport or utilization of metabolites. Remarkably, expression of the ATB2-GUS reporter gene construct was specifically repressed by sucrose. Other sugars, such as glucose and fructose, alone or in combination, were ineffective. Repression was observed at external sucrose concentrations exceeding 25 mM. Transcript levels of both the endogenous ATB2 gene and the ATB2-GUS reporter gene were not repressed by sucrose, suggesting that sucrose affects mRNA translation. This translational regulation involves the ATB2 leader sequence because deletion of the leader resulted in loss of sucrose repression. Our results provide evidence for a sucrose-specific sugar sensing and signalling system in plants.

Tomato fructokinases exhibit differential expression and substrate regulation

Two divergent genes encoding fructokinase, Frk1 and Frk2, have been previously shown to be expressed in tomato (Lycopersicon esculentum L.) and have now been further characterized with regard to their spatial expression and the enzymic properties of the encoded proteins. Frk1 and Frk2 mRNA levels were coordinately induced by exogenous sugar, indicating that both belong to the growing class of sugar-regulated genes. However, in situ hybridization indicated that Frk1 and Frk2 were expressed in a spatially distinct manner, with Frk2 mRNA primarily localized in cells of the fruit pericarp, which store starch, and Frk1 mRNA distributed ubiquitously in pericarp tissue. To evaluate the biochemical characteristics of the products of the Frk1 and Frk2 genes, each cDNA was expressed in a mutant yeast (Saccharomyces cerevisiae) line defective in hexose phosphorylation and unable to grow on glucose or fructose (Fru). Both Frk1 and Frk2 proteins expressed in yeast conferred the ability to grow on Fru and exhibited fructokinase activity in vitro. Although both Frk1 and Frk2 both utilized Fru as a substrate, only Frk2 activity was inhibited at high Fru concentrations. These results indicate that Frk2 can be distinguished from Frk1 by its sensitivity to substrate inhibition and by its temporal and spatial pattern of expression, which suggests that it plays a primary role in plant cells specialized for starch storage.

CARBOHYDRATE-MODULATED GENE EXPRESSION IN PLANTS

Plant gene responses to changing carbohydrate status can vary markedly. Some genes are induced, some are repressed, and others are minimally affected. As in microorganisms, sugar-sensitive plant genes are part of an ancient system of cellular adjustment to critical nutrient availability. However, in multicellular plants, sugar-regulated expression also provides a mechanism for control of resource distribution among tissues and organs. Carbohydrate depletion upregulates genes for photosynthesis, remobilization, and export, while decreasing mRNAs for storage and utilization. Abundant sugar levels exert opposite effects through a combination of gene repression and induction. Long-term changes in metabolic activity, resource partitioning, and plant form result. Sensitivity of carbohydrate-responsive gene expression to environmental and developmental signals further enhances its potential to aid acclimation. The review addresses the above from molecular to whole-plant levels and considers emerging models for sensing and transducing carbohydrate signals to responsive genes.

Isolation and characterisation of cDNA clones representing the genes encoding the major tuber storage protein (dioscorin) of yam (Dioscorea cayenensis Lam.)

cDNA clones encoding dioscorins, the major tuber storage proteins (M(r) 32,000) of yam (Dioscorea cayenesis) have been isolated. Two classes of clone (A and B, based on hybrid release translation product sizes and nucleotide sequence differences) which are 84.1% similar in their protein coding regions, were identified. The protein encoded by the open reading frame of the class A cDNA insert is of M(r) 30,015. The difference in observed and calculated molecular mass might be attributed to glycosylation. Nucleotide sequencing and in vitro transcription/translation suggest that the class A dioscorin proteins are synthesised with signal peptides of 18 amino acid residues which are cleaved from the mature peptide. The class A and class B proteins are 69.6% similar with respect to each other, but show no sequence identity with other plant proteins or with the major tuber storage proteins of potato (patatin) or sweet potato (sporamin). Storage protein gene expression was restricted to developing tubers and was not induced by growth conditions known to induce expression of tuber storage protein genes in other plant species. The codon usage of the dioscorin genes suggests that the Dioscoreaceae are more closely related to dicotyledonous than to monocotyledonous plants.

Expression patterns of three genes in the stem of lucerne (Medicago sativa)

We have identified three stem abundantly expressed genes in lucerne (alfalfa, Medicago sativa). A cDNA library, constructed from lucerne stem polyadenylated RNA, was screened by differential hybridization. From this screening, cDNA clones that correspond to genes which are preferentially, or specifically, expressed in the stem were isolated. MsaS1 encodes an unidentified protein, MsaS2 encodes an S-adenosyl-homocysteine hydrolase and MsaS3 encodes an extensin-like protein. Northern blot analysis of RNA isolated from individual stem internodes indicated that the three corresponding genes show differing developmental patterns of expression. The expression of MsaS1 was confined to the youngest stem tissue and may be regulated by sucrose. In stem tissue the level of RNA for the three genes decreased in response to wounding. Tissue print hybridization analysis was used to localize the expression of the genes to the xylem side of vascular bundles in lucerne stems.

Solanum brevidens possesses a non-sucrose-inducible patatin gene

The patatin gene is the best known "tuber-specific" gene of potato (Solanum tuberosum). Patatin is encoded by a multigene family that can be divided into two classes. Class I genes are highly expressed in tubers and are sucrose inducible, while class II genes are under developmental control and are expressed mainly in root tips. Here we report the isolation and characterization of cDNA clones corresponding to a patatin gene of the non-tuberizing Solanum species S. brevidens. We show that the gene is 94-100% homologous to the class I type patatin genes of S. tuberosum; the homology includes the sequences in the 5' and the 3' untranslated regions. However, the patatin gene of S. brevidens is regulated like class II type patatin genes and cannot be transcriptionally activated by elevated levels of sucrose. This result further supports the idea that the components required for tuberization may be present in non-tuberizing solanaceous plants, but are regulated differently.

Nuclear protein factors binding to a class I patatin promoter region are tuber-specific and sucrose-inducible

Genes encoding patatin, the major storage protein of the potato tuber, are generally divided into two classes, class I and class II. The expression of the class I patatin genes is normally tuber-specific, but can be induced in leaves by high concentrations of sucrose. By employing electrophoretic mobility shift assays (EMSA), we have identified nuclear protein factors that interact specifically with the proximal portion of the class I patatin promoter that is required for tuber-specific and sucrose-inducible expression. The factors were detected in nuclear extracts prepared from potato tubers and sucrose-induced leaves, but not in extracts from leaves of normal potato plants. Four putative transcription factor-binding sites were localized using DNase I footprinting. Competitive EMSA was employed to show that the same protein factor binds to at least two of the sites (boxes D and M). Interestingly, these two binding sites are highly homologous to light-responsive elements present in genes for the ribulose-1,5-bisphosphate carboxylase small subunit.

The rolC promoter of Agrobacterium rhizogenes Ri plasmid is activated by sucrose in transgenic tobacco plants

The 5'-upstream region of the rolC gene of the Ri plasmid is expressed specifically in phloem cells of transgenic higher plants. In this study, we demonstrated that the rolC promoter is activated by sucrose in phloem cells of transgenic tobacco seedlings bearing rolC promoter-uidA chimeric fusion gene. Since the rolC promoter is not activated by sorbitol, sucrose metabolism rather than osmotic pressure exerted by the disaccharide may be responsible for induction. Thus, experiments using 5'-upstream deletion mutants, internal deletion mutants, and chimeric constructs with a heterologous promoter (-90 region of the cauliflower mosaic virus 35S promoter) were conducted to define the region of the rolC promoter involved in sucrose activation. The results indicated that a cis-acting sucrose responsive region of the rolC promoter is located between -135 and -94 bp with respect to the transcription initiation site. In phloem cells, high concentrations of sucrose are encountered owing to ongoing translocation of photosynthates from source to sink tissues. Therefore, sucrose as a signal molecule may regulate the phloem-specific expression of the rolC promoter.

Feedback control of gene expression

Although feedback regulation of photosynthesis by carbon metabolites has long been recognized and investigated, its underlying molecular mechanisms remain unclear. The recent discovery that glucose and acetate trigger global repression of maize photosynthetic gene transcription provides the first direct evidence that a fundamental mechanism is used for feedback regulation of photosynthesis in higher plants. The metabolic repression of photosynthetic genes has now been found in many higher plants and is likely universal. It overrides other regulation by light, tissue type and developmental stage, and serves potentially as the molecular basis of interactions between sink and source tissues. Using simplified and convenient cellular systems and transgenic plants, the study of metabolic regulation of gene expression offers an excellent opportunity for the understanding of global and coordinate gene control and metabolite-mediated signal transduction in higher plants.

Metabolic regulation of alpha-amylase gene expression in transgenic cell cultures of rice (Oryza sativa L.)

Expression of two genes in the alpha-amylase gene family is controlled by metabolic regulation in rice cultured cells. The levels of RAmy3D and RAmy3E mRNAs in rice cultured cells are inversely related to the concentration of sugar in the culture medium. Other genes in the rice alpha-amylase gene family have little or no expression in cultured cells; these expression levels are not controlled by metabolic regulation. A RAmy3D promoter/GUS gene fusion was metabolically regulated in the transgenic rice cell line 3DG, just as the endogenous RAmy3D gene is regulated. An assay of GUS enzyme activity in 3DG cells demonstrated that RAmy3D/GUS expression is repressed when sugar is present in the culture medium and induced when sugar is removed from the medium. The 942 bp fragment of the RAmy3D promoter that was linked to the coding region of the GUS reporter gene thus contains all of the regulatory sequences necessary for metabolic regulation of the gene.

The nuclear factor SP8BF binds to the 5'-upstream regions of three different genes coding for major proteins of sweet potato tuberous roots

Sporamin and beta-amylase are two major proteins of tuberous roots of sweet potato, and expression of genes coding for sporamin and beta-amylase is induced concomitantly in leaves with the petioles attached by exogenous supply of sucrose or polygalacturonic acid. We have used a DNase I footprinting assay to characterize nuclear factors that bind to the 5'-upstream regions of gSPO-A1, gSPO-B1 and g beta-Amy genes that encode A-type sporamin, B-type sporamin and the subunit of beta-amylase, respectively. Nuclear extracts from sucrose-treated petioles protected a region around -155 relative to the transcription start site of gSPO-A1 and a region around -880 of g beta-Amy from DNase I digestion on both strands. These two protected regions both contained the sequence ACTGTGTA, designated SP8a, in opposite orientation with respect to the direction of transcription. A gel mobility shift assay with SP8a oligonucleotide and competition experiments indicated that a common factor SP8BF binds to the SP8a sequence in gSPO-A1 and g beta-Amy. Binding of SP8BF to the SP8a oligonucleotide was abolished by mutation within the SP8a sequence. Fragments of the 5'-upstream region of gSPO-B1 also competed for the binding of SP8BF to the SP8a oligonucleotide, and the DNase I footprinting assay revealed three binding sites for SP8BF in the 5'-upstream region of gSPO-B1. These three sites in gSPO-B1 all contained the sequence TACTATT, designated SP8b, which shared 4 nucleotides at identical positions with the SP8a sequence. An inverted repeat of the SP8b sequence was also present at one protected site in the 5'-upstream region of g beta-Amy. In addition to sucrose-treated petioles, SP8BF activity was also present in tuberous roots and untreated fresh petioles of sweet potato. Furthermore, the activity was also detected in stems of tobacco plantlets, suggesting that SP8BF is an ubiquitous factor.

Production of cyclodextrins, a novel carbohydrate, in the tubers of transgenic potato plants

Cyclodextrins (CDs) are cyclic oligosaccharides containing six (alpha), seven (beta), or eight (gamma) glucose molecules, respectively. The cyclodextrin glycosyltransferases (CGT), which produce CDs from starch, are found only in bacteria and are used in batch fermentors with hydrolyzed starch to produce CDs commercially. Using a CGT gene from Klebsiella, we attempted to engineer the tubers of developing potatoes to produce these novel, high-value carbohydrates. A chimeric gene, consisting of (1) the patatin promoter for tuber-specific expression, (2) the small subunit of ribulose bisphosphate carboxylase (SSU) transit peptide for plastid targeting, (3) the CGT structural gene from Klebsiella and (4) the nopaline synthase 3' region, was introduced into potatoes. Both alpha and beta CDs were produced in tubers of transgenic potatoes at levels corresponding to 0.001-0.01% of the starch being converted to CDs.

Patatin and four serine proteinase inhibitor genes are differentially expressed during potato tuber development

A highly efficient and synchronous in vitro tuberization system is described. One-node stem pieces from potato (Solanum tuberosum cv. Bintje) plants grown under short day-light conditions containing an axillary bud were cultured in the dark on a tuber-inducing medium. After 5 or 6 days all axillary buds started to develop tubers. To study gene expression during tuber development, RNA isolated from tuberizing axillary buds was used for both in vitro translation and northern blot hybridizations. The genes encoding the proteinase inhibitors I and II (PI-I and PI-II), a Kunitz- and a Bowman-Birk-type proteinase inhibitor were already expressed in uninduced axillary buds. The length of the day-light conditions differently influenced the expression level of the individual genes. In addition, the expression of each of these genes changed specifically during the development of the axillary bud to tuber. In contrast to the expression of these proteinase inhibitor genes, patatin gene expression was only detectable from the day tuberization was manifested as a radial expansion of the axillary bud. These results are discussed with respect to the regulation of the expression of the genes studied in relation to the regulation of tuber development.

Genetic and physical mapping of the patatin genes in potato and tomato

Genes for the major storage protein of potato, patatin, have been mapped genetically and physically in both the potato and tomato genomes. In potato, all patatin genes detected by the cDNA clone pGM01 map to a single locus at the end of the long arm of chromosome 8. By means of pulsed field gel electrophoresis (PFGE) it was possible further to delimit this locus, containing 10-15 copies of the gene, to a maximum size of 1.4 million base pairs. Hybridizations with class-specific clones suggest that the locus is at least partially divided into domains containing the two major types of patatin genes, class I and II. In tomato, patatin-homologous sequences were found to reside at the orthologous locus at the end of chromosome 8. The approximately three copies in tomato were localized by PFGE to a single fragment of 300 kilobases. Whereas the class II-specific 5' promoter sequences reside in tomato at the same locus as the coding sequences, the single class I-specific copy of the 5' promoter sequences was localized on chromosome 3 with no coding sequence attached to it. A clone from this chromosome 3 locus of tomato was isolated and by restriction fragment length polymorphism mapping it could be further shown that a similar class I-specific sequence also exists on chromosome 3 of potato. As in tomato, this copy on chromosome 3 is not linked to a coding sequence for patatin. The results are discussed with respect to genome evolution and PFGE analysis of complex gene families.

High-level expression of a sweet potato sporamin gene promoter: beta-glucuronidase (GUS) fusion gene in the stems of transgenic tobacco plants is conferred by multiple cell type-specific regulatory elements

Genes coding for sporamin, the most abundant protein of the tuberous root of the sweet potato, are expressed at a high levels in the stems of plantlets cultured axenically on sucrose-containing medium. Their expression is also induced in leaf-petiole explants by high concentrations of sucrose. A fusion gene comprising of the 1 kb 5' upstream region of the gSPO-A1 gene coding for the A-type sporamin and the coding sequence of bacterial beta-glucuronidase (GUS) was introduced into the tobacco genome by Agrobacterium-mediated transformation. Transgenic tobacco plants cultured axenically on sucrose-containing medium expressed GUS activity predominantly in their stems. Histochemical examination of GUS activity using a chromogenic substrate showed a distinct spatial pattern of GUS staining in the stem. Strong GUS activity was detected in the internal phloem of the vascular system and at the node, especially at the base of the axillary bud. Relatively weaker GUS activity was also detected in pith parenchyma. A 5' deletion of the promoter to nucleotide -305, relative to the transcription start site, did not alter significantly the level of GUS activity or the spatial pattern of GUS staining in the stem. However, further deletions to -237 and -192 resulted in a decrease in the level of GUS activity in the stem that occurred simultaneously with the loss of GUS staining in both the internal phloem and at the base of the axillary bud. However, plants with these deletion constructs still exhibited the predominant expression pattern of GUS activity in the stem and GUS staining in the pith parenchyma cells. Deletion to -94 completely abolished the expression of GUS activity. These results indicate that a sequence between -305 and -237 contains a cis-regulatory element(s) that is required for expression of the GUS reporter gene in both the internal phloem and at the base of the axillary bud, while a sequence between -192 and -94 contains a cis-acting element(s) that is required for expression in pith parenchyma cells.

cis regulatory elements directing tuber-specific and sucrose-inducible expression of a chimeric class I patatin promoter/GUS-gene fusion

The 5'-upstream region of the class I patatin gene B33 directs strong expression of the beta-glucuronidase (GUS) reporter gene in potato tubers and in leaves treated with sucrose. Cis-acting elements affecting specificity and level of expression were identified by deletion analysis in transgenic potato plants. A putative tuber-specific element is located downstream from position -195. Nuclear proteins present in leaf and tuber extracts bind specifically to a conserved AT rich motif within this region. A DNA fragment between -183 and -143, including the binding site is, however, not able to enhance the expression of a truncated 35S promoter from cauliflower mosaic virus. Independent positive elements contributing to a 100-fold increase relative to the basic tuber-specific element are located between -228 and -195; -736 and -509, -930 and -736 and -1512 and -951. Sucrose inducibility is controlled by sequences downstream of position -228, indicating that the tuber-specific and sucrose-inducible elements are in close proximity.

Gene expression during tuber development in potato plants

Potato tubers are modified stems that have differentiated into storage organs. Factors such as day-length, nitrogen supply, and levels of the phytohormones cytokinin and gibberellic acid, are known to control tuberization. Morphological changes during tuber initiation are accompanied by the accumulation of a characteristic set of proteins, thought to be involved in N-storage (i.e. patatin) or defense against microbial or insect attack (i.e. proteinase inhibitor II). Additionally, deposition of large amounts of starch occurs during tuber formation, which is paralleled by an increase in sucrose synthase and other enzymes involved in starch biosynthesis (i.e. ADP-glucose pyrophosphorylase, starch synthases, and branching enzyme). Potential controlling mechanisms for genes expressed during tuberization are discussed.