Is the nectar redox cycle a floral defense against microbial attack?

Many angiosperms use a remarkable reproductive strategy that relies on attracting animals (insect, avian or mammalian pollinators) to transfer pollen between plants. Relying on other organisms for sexual reproduction seems evolutionarily untenable, but the great diversity of angiosperms illustrates how highly successful this strategy is. To attract pollinators, plants offer a variety of rewards. Perhaps the primary floral reward is floral nectar. Plant nectar has long been considered a simple sugar solution but recent work has demonstrated that nectar is a complex biological fluid containing significant and important biochemistry with the potential function of inhibiting microbial growth. These results lead the way to novel insights into the mechanisms of floral defense and the co-evolution of angiosperms and their pollinators.

Tobacco nectarin I. Purification and characterization as a germin-like, manganese superoxide dismutase implicated in the defensE of floral reproductive tissues

Nectarin I, a protein that accumulates in the nectar of Nicotiana sp. , was determined to contain superoxide dismutase activity by colorimetric and in-gel assays. This activity was found to be remarkably thermostable. Extended incubations at temperatures up to 90 degrees C did not diminish the superoxide dismutase activity of nectarin I. This attribute allowed nectarin I to be purified to homogeneity by heat denaturation of the other nectar proteins. By SDS-polyacrylamide gel electrophoresis, nectarin I appeared as a 29-kDa monomer. If the protein sample was not boiled prior to loading the gel, then nectarin I migrated as 165-kDa oligomeric protein. By matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, the protomer subunit was found to be a 22.5-kDa protein. Purified nectarin I contained 0.5 atoms of manganese/monomer, and the superoxide dismutase activity of nectarin I was not inhibited by either H(2)O(2) or NaCN. Following denaturation, the superoxide dismutase activity was restored after Mn(2+) addition. Addition of Fe(2+), Cu(2+), Zn(2+), and Cu(2+)/Zn(2+) did not restore superoxide dismutase activity. The quaternary structure of the reconstituted enzyme was examined, and only tetrameric and pentameric aggregates were enzymatically active. The reconstituted enzyme was also shown to generate H(2)O(2). Putative nectarin I homologues were found in the nectars of several other plant species.

Wound-inducible expression of a potato inhibitor II-chloramphenicol acetyltransferase gene fusion in transgenic tobacco plants

A potato inhibitor II gene (IIK) was isolated from a library of potato genes in lambda bacteriophage. An 8-kilobase-pair (kbp) insert was identified using a tomato inhibitor II cDNA as a hybridization probe, and a 2.6-kbp fragment containing the gene was subcloned into the plasmid pUC13 and characterized. The nucleotide sequence of the isolated gene exhibited 87% identity with the wound-inducible tomato inhibitor II cDNA sequence. The amino acid sequence of inhibitor IIK, deduced from the potato gene, exhibited 84% identity with the tomato inhibitor II protein. A 1000-bp restriction fragment from the 5' flanking region of the gene was fused to the open reading frame of the chloramphenicol acetyltransferase (CAT) gene. This fusion was terminated in two ways: (i) with a terminator sequence from the potato inhibitor II gene and (ii) with a terminator from the 6b gene of Ti plasmid pTiA6. These chimeric genes were transferred into tobacco cells via a binary Ti vector system, and transgenic plants were regenerated. The CAT gene was expressed in leaves of transformed plants in response to wounding when fused with the inhibitor IIK promoter and terminator regions. The chimeric gene containing the 6b terminator did not express CAT in response to wounding. The wound-inducible expression of CAT activity was systemic and was induced in tissues distal to the wounded tissues. The time course of wound induction of CAT activity in transgenic tobacco leaves is similar to that found for wound-inducible inhibitor I and II mRNAs in tomato leaves. These results demonstrate that sequences necessary and sufficient for wound inducibility are present within approximately 1000 bp of the control regions of the inhibitor IIK genes and that wound-inducible components of tobacco leaf cells can regulate these sequences.

Functional analysis of the 3' control region of the potato wound-inducible proteinase inhibitor II gene

Proteinase inhibitor genes are expressed strongly in specific plant tissues under both developmental and environmental regulation. We have studied the role of the 3' control region of the potato proteinase inhibitor II gene (PI-II) that is inducible in leaves in response to herbivore attacks or other severe wounding. Comparison of the terminator from the PI-II gene with two different terminators from the 6b and 7 genes, driven by a common PI-II promoter-cat fusion molecule, indicated that the PI-II terminator provided the most efficient expression of cat. The PI-II terminator also caused a significantly elevated cat gene expression driven by the cauliflower mosaic virus 35S promoter. The increase in the level of expression is probably not due to the presence of an enhancer element in the PI-II terminator region, but to cis-acting elements involved in mRNA processing or stability. Both transient and stable transformation analyses of the deletion mutants in the 3'-flanking sequence indicated that about a 100-base pair DNA fragment surrounding the polyadenylation site is essential for the efficient gene expression. This region seems to consist of several regulatory elements, including the conserved sequence, CGTGTCTT, which is located 9 bases downstream from the polyadenylation site. The elements appear to contribute to the increased stability of mRNAs containing the PI-II terminator.

Tobacco nectarin V is a flavin-containing berberine bridge enzyme-like protein with glucose oxidase activity

Ornamental tobacco (Nicotiana langsdorffii X N. sanderae) secretes a limited array of proteins (nectarins) into its floral nectar. Careful sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of tobacco nectar revealed that a broad protein band from 61 to 65 kD actually consists of five discrete protein bands. N-terminal sequencing and tryptic peptide mass spectrometry fingerprint analysis demonstrated that the upper three bands are isoforms of the same protein, NEC5 (Nectarin V), whereas the lower two bands, NEC4 (Nectarin IV), are related to each other but not to NEC5. Reverse transcription-polymerase chain reaction (RT-PCR) based upon N-terminal sequence of NEC5 generated a short cDNA that encoded the N terminus of the NEC5 protein. Two rounds of inverse-PCR using genomic DNA permitted the isolation of approximately one-half of the coding region of the nec5 gene along with 787 nucleotides of the 5'-flanking region. This DNA fragment was used as a probe to isolate a near full-length nec5 clone from a nectary-derived cDNA library. BLAST analysis identified the nec5 cDNA as a berberine bridge enzyme-like protein. Approximately 40% of the cDNA sequence corresponded to peptides that were identified by tryptic peptide mass spectrometry fingerprint analysis of the NEC5 protein, thereby confirming that this cDNA encoded the NEC5 protein. In-gel assays also demonstrated that NEC5 contains a covalently linked flavin, and it possesses glucose oxidase activity. RT-PCR-based expression analyses showed that nec5 expression is limited exclusively to the nectary gland during late stages of floral development.

Comparison of major protein antigens and protein profiles of Treponema pallidum and Treponema pertenue

The protein profiles of Treponema pallidum and Treponema pertenue, the causative agents of syphilis and yaws, respectively, were compared by one- and two-dimensional gel electrophoresis. One-dimensional gels showed essentially no differences in the protein patterns of these treponemes. On two-dimensional gels most radiolabeled protein species were shared; however, variations were noticed in several minor protein species. Antigenic comparison by radioimmunoprecipitation and Western blotting also demonstrated similarities between these spirochetes. However, lactoperoxidase-catalyzed iodination of T. pallidum and T. pertenue suggested differences in their surface proteins.

Nectarin IV, a potent endoglucanase inhibitor secreted into the nectar of ornamental tobacco plants. Isolation, cloning, and characterization

We have isolated and characterized the Nectarin IV (NEC4) protein that accumulates in the nectar of ornamental tobacco plants (Nicotiana langsdorffii x Nicotiana sanderae var LxS8). This 60-kD protein has a blocked N terminus. Three tryptic peptides of the protein were isolated and sequenced using tandem mass spectroscopy. These unique peptides were found to be similar to the xyloglucan-specific fungal endoglucanase inhibitor protein (XEGIP) precursor in tomato (Lycopersicon esculentum) and its homolog in potato (Solanum tuberosum). A pair of oligonucleotide primers was designed based on the potato and tomato sequences that were used to clone a 1,018-bp internal piece of nec4 cDNA from a stage 6 nectary cDNA library. The remaining portions of the cDNA were subsequently captured by 5' and 3' rapid amplification of cDNA ends. Complete sequencing of the nec4 cDNA demonstrated that it belonged to a large family of homologous proteins from a wide variety of angiosperms. Related proteins include foliage proteins and seed storage proteins. Based upon conserved identity with the wheat (Triticum aestivum) xylanase inhibitor TAXI-1, we were able to develop a protein model that showed that NEC4 contains additional amino acid loops that are not found in TAXI-1 and that glycosylation sites are surface exposed. Both these loops and sites of glycosylation are on the opposite face of the NEC4 molecule from the site that interacts with fungal hemicellulases, as indicated by homology to TAXI-I. NEC4 also contains a region homologous to the TAXI-1 knottin domain; however, a deletion in this domain restructures the disulfide bridges of this domain, resulting in a pseudoknottin domain. Inhibition assays were performed to determine whether purified NEC4 was able to inhibit fungal endoglucanases and xylanases. These studies showed that NEC4 was a very effective inhibitor of a family GH12 xyloglucan-specific endoglucanase with a K(i) of 0.35 nm. However, no inhibitory activity was observed against other family GH10 or GH11 xylanases. The patterns of expression of the NEC4 protein indicate that, while expressed in nectar at anthesis, it is most strongly expressed in the nectary gland after fertilization, indicating that inhibition of fungal cell wall-degrading enzymes may be more important after fertilization than before.

The MYB305 transcription factor regulates expression of nectarin genes in the ornamental tobacco floral nectary

We have isolated and characterized the cDNA encoding the ornamental tobacco (Nicotiana langsdorffii X N. sanderae) homolog of the antirrhinum (Antirrhinum majus) MYB305. This transcription factor was robustly expressed at Stage 12 of nectary development but was only weakly expressed in the earlier Stage 6 nectaries. The ornamental tobacco MYB305 contains a conserved R2R3 MYB DNA binding domain with 76 amino acids in the activation domain. A green fluorescent protein-MYB305 fusion localized to nucleus of tobacco protoplasts and yeast one-hybrid assays demonstrated that it functions as a transcription activator. A conserved 23-amino acid C-terminal domain is required to activate gene expression. The coding region of the myb305 cDNA was expressed in Escherichia coli as a glutathione S-transferase fusion protein and was purified to homogeneity. This protein shows binding to two consensus MYB binding sites on the ornamental tobacco Nectarin I (nec1) promoter as well as to the single site located on the Nectarin V (nec5) promoter. Deletions of either of the binding sites from the nec1 promoter significantly reduced expression in nectary tissues. Temporally, MYB305 expression precedes that of nec1 and nec5, as would be expected if the MYB305 factor regulates expression of the nec1 and nec5 genes. Ectopic expression of MYB305 in foliage was able to induce expression of both nec1 and nec5, as well as two flavonoid biosynthetic genes in the foliage. Finally, RNA interference knockdown of MYB305 resulted in reduced expression of both nectarins and flavonoid biosynthetic genes. We conclude that expression of MYB305 regulates expression of the major nectarin genes in the floral nectary.

Molecular characterization of a wound-inducible inhibitor I gene from potato and the processing of its mRNA and protein

Genomic blotting of restriction fragments of Russet Burbank DNA indicated that at least 6 copies of Inhibitor I are present in the tetraploid potato genome. A library of potato genes in bacteriophage λ was screened for the presence of Inhibitor I genes using a wound-inducible tomato Inhibitor I cDNA as a hybridization probe. One phage with an insert of 13.1 kb was isolated that hybridized most strongly with the probe. A 4.2 kb Eco RI fragment containing the gene was isolated from the clone and 2.2 kb region was sequenced that included about 800 bp of both the 5' and 3' regions. The gene contained two introns of 479 and 417 bp respectively, and the splice junctions were typical of other eukaryotic genes. Putative TATAA and CAAT boxes were identified. The nucleotide sequence, when compared with a wound-inducible tomato Inhibitor I cDNA, exhibited over 90% identity. The gene codes for a prepro-Inhibitor I protein of 96 amino acids. The putative pre-sequence of 19 amino acids, differs in only one residue from that of tomato Inhibitor I. The potato pro-sequence, however, is lacking a tetrapeptide that is found in the tomato pro-sequence in the region of pro-peptide processing. This deletion, together with a substitution of a Gln for a Leu (4 residues toward the N terminus) provides an explanation for the differences at the N-termini between tomato and potato Inhibitor I natural proteins by providing different processing sites in the two pro-inhibitors. Thus, amino acid sequence differences between the N termini of tomato and potato Inhibitor I are easily explained by the mutational events. The different proposed pro-processing sites of the tomato and potato inhibitors suggest that a processing protease may be present in the vacuole with a specificity for Asn-X and Gln-X bonds.

Arabidopsis thaliana contains a large family of germin-like proteins: characterization of cDNA and genomic sequences encoding 12 unique family members

We have identified 39 Arabidopsis thaliana ESTs encoding germin-like proteins (GLPs) and have completely sequenced 25 of these cDNAs. Our analysis demonstrates that the Arabidopsis genome contains a gene family with at least 12 GLP genes. Comparisons with other known germins and germin-like proteins indicate that these Arabidopsis GLP subfamilies are unique from wheat germin. All other known GLPs fall into one of these subfamilies. The translated GLPs show approximately 35% amino acid identity with other GLPs outside of their subfamily and significantly higher levels of identity within their respective subfamily. The 3' ends of many of the GLP cDNAs are heterogeneous and several sites of polyadenylation are used. Ten of the GLPs have N-terminal signal sequences and most appear to be exported from the cell. Structurally, the GLPs are predicted to have a high content of beta-pleated sheet. Seven conserved regions of beta-sheet were found in each of the GLP proteins along with alpha-helices located at both N- and C-termini. These same structural elements are also conserved in wheat germin. With one exception, all GLP family members contain at least one N-glycosylation site. All of these sites are conserved in an unstructured loop between beta-1 and beta-2. Genes for two of these GLPs were identified in genomic sequences previously deposited in the GenBank. The GLP3b gene is physically linked to the polyubiquitin 4 gene. The 3' end of the GLP3b mRNA is only 0.5 kb from the ubq4 start of transcription. Analysis of the GLP3b promoter shows the presence of a single putative auxin-response sequence located at -124 to -111 upstream from the 5' end of the GLP3b mRNA. The GLP9 gene was identified in an Arabidopsis contig from Chromosome 4.

Structural characterization of the active site of Brucella abortus Cu-Zn superoxide dismutase: a 15N and 1H NMR investigation

Prokaryotic Cu-Zn superoxide dismutases (SODs) are rare and poorly characterized compared to their eukaryotic counterparts. To better characterize the structure of the prokaryotic enzyme, an NMR investigation of Brucella abortus Cu-Zn SOD in the reduced form was undertaken. The enzyme studied was a recombinant form, expressed in Escherichia coli. The enzyme initially lacked a full complement of Cu and Zn ion. After demetallation and remetallation with a stoichiometric amount of Cu and Zn ion, the specific activity of the recombinant B. abortus Cu-Zn SOD was comparable to the specific activity of the bovine enzyme. The 15N and 1H resonances of seven active site histidine imidazole rings were assigned using two-dimensional NMR methods. A self-consistent set of nuclear Overhauser effects between imidazole ring protons was observed, which was in agreement with the predictions of a model based on the X-ray crystallographic structure of the oxidized bovine enzyme (Tainer, J.A., Getzoff, E. D., Beem, K. M., Richardson, J.S., & Richardson, D.C. (1982) J. Mol. Biol. 160, 181-217). These observations strongly suggest that the structure of the active site of the prokaryotic enzyme is similar to that of the eukaryotic enzyme. Differences in the observed and predicted nuclear Overhauser effects could be ascribed to differences in the oxidation state of the Cu ion (Cu(I) in the reduced B. abortus enzyme and Cu(II) in the oxidized bovine enzyme), as much as they could to the different origins of the enzymes. The NMR data were also compared to a similar 1H NMR study of the human enzyme (Bertini, I., Capozzi, F., Luchinat, C., Piccioli, M., & Viezzoli, M. S. (1991) Eur. J. Biochem. 197, 691-697). The pattern of nuclear Overhauser effects and the chemical shifts of corresponding resonances were very similar in 1H NMR spectra of the human and B. abortus enzymes. Significant differences in the chemical shifts or exchange behavior of a few resonances indicated differences in the environments of several histidines in the active sites of reduced B. abortus and human Cu-Zn SODs. This is consistent with the presence of a number of insertions and deletions in the loop regions that make up the active site as indicated by amino acid sequence alignment studies. The tautomeric and protonation states of the active site histidines were also determined in this study, and the results were in agreement with previous studies. The resonances of nitrogen atoms coordinated to metal ions were found to fall between those of protonated and unprotonated nitrogens on histidine imidazoles.

Nectarin I is a novel, soluble germin-like protein expressed in the nectar of Nicotiana sp

We have identified a limited number of proteins secreted into the nectar of tobacco plants. Nectarin I is the most highly expressed nectar protein and has a monomer molecular mass of 29 kDa. The other major nectar proteins are expressed at lower levels and have monomer molecular masses of 41, 54, and 65 kDa respectively. Nectarin I was purified and antiserum was raised against the protein. Under nondenaturing conditions, Nectarin I has an apparent molecular mass of > 120 kDa. The expression of Nectarin I was restricted to nectary tissues and to a much lower level in the ovary. No Nectarin I was found in petals, stems, leaves, or roots or other floral tissues. The expression of Nectarin I was also developmentally regulated. It is expressed in nectary tissues only while nectar is being actively secreted. Subsequently, the N-terminus of purified Nectarin I was sequenced. Sequence identity showed Nectarin I is related to wheat germin. Although hydrogen peroxide is readily detectable in tobacco floral nectar, we were unable to demonstrate any oxalate oxidase activity for Nectarin I. A partial cDNA encoding the mature Nectarin I N-terminus was isolated and used to probe a Nicotiana plumbaginifolia genomic library. The Nectarin I gene was isolated and the translated sequence was consistent with both N-terminal and internal cyanogen bromide-derived amino acid sequence. The gene contains a single 386 nt intron and encodes a mature protein of 197 amino acids.

Tobacco nectaries express a novel NADPH oxidase implicated in the defense of floral reproductive tissues against microorganisms

Hydrogen peroxide produced from the nectar redox cycle was shown to be a major factor contributing to inhibition of most microbial growth in floral nectar; however, this obstacle can be overcome by the floral pathogen Erwinia amylovora. To identify the source of superoxide that leads to hydrogen peroxide accumulation in nectary tissues, nectaries were stained with nitroblue tetrazolium. Superoxide production was localized near nectary pores and inhibited by diphenylene iodonium but not by cyanide or azide, suggesting that NAD(P)H oxidase is the source of superoxide. Native PAGE assays demonstrated that NADPH (not NADH) was capable of driving the production of superoxide, diphenyleneiodonium chloride was an efficient inhibitor of this activity, but cyanide and azide did not inhibit. These results confirm that the production of superoxide was due to an NADPH oxidase. The nectary enzyme complex was distinct by migration on gels from the leaf enzyme complex. Temporal expression patterns demonstrated that the superoxide production (NADPH oxidase activity) was coordinated with nectar secretion, the expression of Nectarin I (a superoxide dismutase in nectar), and the expression of NOX1, a putative gene for a nectary NADPH oxidase that was cloned from nectaries and identified as an rbohD-like NADPH oxidase. Further, in situ hybridization studies indicated that the NADPH oxidase was expressed in the early stages of flower development although superoxide was generated at later stages (after Stage 10), implicating posttranslational regulation of the NADPH oxidase in the nectary.

A yeast mitochondrial leader peptide functions in vivo as a dual targeting signal for both chloroplasts and mitochondria

A fusion protein was expressed in transgenic tobacco and yeast cells to examine the functional conservation of mechanisms for importing precursor proteins from the cytosol into mitochondria and chloroplasts. The test protein consisted of the mitochondrial leader peptide from the yeast precursor to cytochrome oxidase subunit Va (prC5) fused to the reporter protein chloramphenicol acetyltransferase. This protein, denoted prC5/CAT, was transported into the mitochondrial interior in yeast and tobacco cells. In both organisms, the mitochondrial form of prC5/CAT was smaller than the primary translation product, suggesting that proteolytic processing occurred during the transport process. prC5/CAT also was translocated into chloroplasts in vivo, accumulating to approximately the same levels as in plant mitochondria. However, accumulation of prC5/CAT in chloroplasts relative to mitochondria varied with the conditions under which plants were grown. The chloroplast form of prC5/CAT also appeared to have been proteolytically processed, yielding a mature protein of the same apparent size as that seen in mitochondria of either tobacco or yeast. Chloramphenicol acetyltransferase lacking a mitochondrial targeting peptide did not associate with either chloroplasts or mitochondria. The results demonstrated that in plant cells a single leader peptide can interact functionally with the protein translocation systems of both chloroplasts and mitochondria, and raised the possibility that certain native proteins might be shared between these two organelles.

Petunia nectar proteins have ribonuclease activity

Plants requiring an insect pollinator often produce nectar as a reward for the pollinator's visitations. This rich secretion needs mechanisms to inhibit microbial growth. In Nicotiana spp. nectar, anti-microbial activity is due to the production of hydrogen peroxide. In a close relative, Petunia hybrida, limited production of hydrogen peroxide was found; yet petunia nectar still has anti-bacterial properties, suggesting that a different mechanism may exist for this inhibition. The nectar proteins of petunia plants were compared with those of ornamental tobacco and significant differences were found in protein profiles and function between these two closely related species. Among those proteins, RNase activities unique to petunia nectar were identified. The genes corresponding to four RNase T2 proteins from Petunia hybrida that show unique expression patterns in different plant tissues were cloned. Two of these enzymes, RNase Phy3 and RNase Phy4 are unique among the T2 family and contain characteristics similar to both S- and S-like RNases. Analysis of amino acid patterns suggest that these proteins are an intermediate between S- and S-like RNases, and support the hypothesis that S-RNases evolved from defence RNases expressed in floral parts. This is the first report of RNase activities in nectar.

Systemic induction of a potato pin2 promoter by wounding, methyl jasmonate, and abscisic acid in transgenic rice plants

To address the question whether common signal(s) and transduction pathways are used to mediate a systemic wound response in monocot and dicot plants, a fusion of the potato proteinase inhibitor II gene (pin2) promoter and the bacterial beta-glucuronidase gene (Gus)-coding region was introduced into rice. In transgenic rice plants, the expression of the pin2-Gus fusion gene displays a systemic wound response, although the expression level is relatively low. Incorporation of the first intron from the rice actin 1 gene (Act1) into the 5'-untranslated region of the pin2-Gus construct results in high-level, systemically wound-inducible expression of the modified construct in transgenic rice plants. Histochemical analysis shows that this high-level, wound-inducible expression is associated with the vascular tissue in both leaves and roots. Furthermore, the expression of the pin2-Act1 intron-Gus fusion gene in transgenic rice plants can be systemically induced by both methyl jasmonate (MJ) and the phytohormone abscisic acid (ABA). These results suggest that the signal(s) mediating the observed systemic wound response and certain steps of the transduction pathways are conserved between dicot and monocot plants. Transient expression assays show that the pin2-Act1 intron-Gus construct is also actively expressed in transformed cells and tissues of several other monocot plants. Thus, the wound-inducible pin2 promoter in combination with the rice Act1 intron 1 might be used as an efficient regulator for foreign gene expression in transgenic monocot plants.

Auxin Levels Regulate the Expression of a Wound-Inducible Proteinase Inhibitor II-Chloramphenicol Acetyl Transferase Gene Fusion in Vitro and in Vivo

Proteinase inhibitor genes are expressed in solanaceous and leguminous plants following wounding of the foliage by mechanical methods. Previous studies have shown that a cloned proteinase inhibitor II-chloramphenicol acetyl transferase (pin2-CAT) chimeric gene is regulated in a wound-inducible manner in transgenic plants. In this study, we analyzed transgenic plant tissues for expression of the pin2-CAT gene in response to various plant hormones. We found that CAT activity was induced in tobacco (Nicotiana tabacum) callus incubated in the absence of any plant growth regulators. Addition of growth regulators to the medium thus permitted us to measure the effects of these substances on the activity of the pin2-CAT gene construction. Cytokinin (BAP) and ethylene (ethophon) even at low concentrations stimulated the expression of CAT activity by 25 to 50%. Abscisic acid at concentrations up to 4.4 x 10(-5) molar had no effect upon CAT activity, but increasing auxin (naphthalene acetic acid) levels completely inhibited the synthesis of CAT protein. Gibberellic acid had little effect except at very high concentration (2.9 x 10(5) molar). The kinetics of activation of the pin2-CAT gene were quite long (5 to 7 days) when unwounded calli were plated on media lacking auxin. This effect was documented for calli derived from several transformed plants, containing the full, chimeric pin2-CAT (pRT45) gene. In addition, calli from tissues transformed with wild-type vectors or from several plants transformed with pRT50 (a noninducible derivative of pRT45) were not induced by plating on media lacking auxin. Other naturally occurring and synthetic auxins had similar effects to naphthalene acetic acid in inhibiting the induction of the chimeric gene fusion. Finally, leaf discs from transformed plants were induced by incubation in MS liquid medium in the presence and absence of naphthalene acetic acid. NAA was also effective in down regulating the chimeric gene in whole plant tissues.

Amyloplast to chromoplast conversion in developing ornamental tobacco floral nectaries provides sugar for nectar and antioxidants for protection

Tobacco floral nectaries undergo changes in form and function. As nectaries change from green to orange, a new pigment is expressed. Analysis demonstrated that it is β-carotene. Plastids undergo dramatic changes. Early in nectary development, they divide and by stage 9 (S9) they are engorged with starch. About S9, nectaries shift from quiescent anabolism to active catabolism resulting in starch breakdown and production of nectar sugars. Starch is replaced by osmiophilic bodies, which contain needle-like carotenoid crystals. Between S9 and S12, amyloplasts are converted to chromoplasts. Changes in carotenoids and ascorbate were assayed and are expressed at low levels early in development; however, following S9 metabolic shift, syntheses of β-carotene and ascorbate greatly increase in advance of expression of nectar redox cycle. Transcript analysis for carotenoid and ascorbate biosynthetic pathways showed that these genes are significantly expressed at S6, prior to the S9 metabolic shift. Thus, formation of antioxidants β-carotene and ascorbate after the metabolic shift is independent of transcriptional regulation. We propose that biosynthesis of these antioxidants is governed by availability of substrate molecules that arise from starch breakdown. These processes and events may be amenable to molecular manipulation to provide a better system for insect attraction, cross pollination, and hybridization.

Tobacco Nectarin III is a bifunctional enzyme with monodehydroascorbate reductase and carbonic anhydrase activities

Tobacco plants secrete a limited array of proteins (nectarins) into their floral nectar. N-terminal sequencing of the Nectarin II ( NEC2; 35kD) and the Nectarin III ( NEC3; 40kD) proteins revealed that they both share identity with dioscorin, the major soluble protein of yam tubers. These sequences also revealed that NEC2 is a breakdown product of NEC3. Using these N-terminal peptide sequences, degenerate oligonucleotides were designed that permitted the isolation of a partial NEC3 cDNA. This cDNA was then used to probe a nectary specific cDNA library and a full-length NEC3 cDNA clone was isolated. Complete sequence analysis confirmed the identity of NEC3 as a dioscorin-like protein. MALDI-TOF mass spectrometric fingerprinting of tryptic peptides derived from the purified NEC3 confirmed that this protein was encoded by the isolated cDNA. NEC3 was shown to possess both carbonic anhydrase and monodehydroascorbate reductase activities. RT-PCR based expression analyses demonstrated that NEC3 transcript is expressed throughout nectary development as well as in other floral organs. A proposed function in the maintenance of pH and oxidative balance in nectar is discussed.

Wounding Nicotiana tabacum Leaves Causes a Decline in Endogenous Indole-3-Acetic Acid

We have previously observed that auxin can act as a repressor of the wound-inducible activation of a chimeric potato proteinase inhibitor II-CAT chimeric gene (pin2-CAT) in transgenic tobacco (Nicotiana tobacum) callus and in whole plants. Therefore, this study was designed to examine endogenous levels of indole-3-acetic acid (IAA) in plant tissues both before and after wounding. Endogenous IAA was measured in whole plant tissues by gas chromatography-mass spectrometry using an isotope dilution technique. (13)C-Labeled IAA was used as an internal standard. The endogenous levels of IAA declined two- to threefold within 6 hours after a wound. The kinetics of auxin decline are consistent with the kinetics of activation of the pin2-CAT construction in the foliage of transgenic tobacco.

Identification of S-RNase and peroxidase in petunia nectar

Previous SDS PAGE gel analysis of the floral nectars from petunia and tobacco plants revealed significant differences in the protein patterns. Petunia floral nectar was shown to contain a number of RNase activities by in gel RNase activity assay. To identify these proteins in more detail, the bands with RNase activity were excised from gel and subjected to trypsin digestion followed by LC-MS/MS analysis. This analysis revealed that S-RNases accumulate in nectar from Petunia hybrida, where they should carry out a biological function different from self-pollen rejection. In addition, other proteins were identified by the LC-MS/MS analysis. These proteins include a peroxidase, an endochitinase, and a putative fructokinase. Each of these proteins contained a secretory signal sequence that marked them as potential nectar proteins. We developed RT-PCR assays for each of these five proteins and demonstrated that each of these proteins was expressed in the petunia floral nectary. A discussion of the role of these proteins in antimicrobial activity in nectar is presented.

The nectary-specific pattern of expression of the tobacco Nectarin I promoter is regulated by multiple promoter elements

The major protein secreted into the nectar of tobacco plants (Nectarin I) is a germin-like protein that has superoxide dismutase activity. We have isolated the gene encoding Nectarin I (NECI) and analyzed the expression of a chloramphenicol acetyl transferase (CAT) marker gene driven by its promoter in transgenic plants. Transgenic plant lines that expressed the CAT gene under control of the full-length NECI promoter showed high levels of CAT expression in mature floral nectaries. Tissue specificity of NECI-CAT expression demonstrated that the construct was expressed uniquely in nectaries with a small level of expression in ovary. Further, analysis of its temporal expression showed that the construct is expressed uniquely during those times when nectar is actively being secreted from flowers. An examination of the transcription start site verified that the initiation site of the NECI-CAT mRNA in transgenic plants is identical with that of the native gene in vivo. Two promoter deletion constructs were also prepared and analyzed. Analysis in transgenic plants revealed that the nectary-specific expression is the result of multiple promoter elements and suggests that nectar secretion and flower opening may be coordinately regulated.

Isolation and Characterization of UMP Synthase Mutants from Haploid Cell Suspensions of Nicotiana tabacum

Uridine 5'-monophosphate (UMP) synthase mutants of tobacco have been produced from haploid cell-suspension cultures of a transgenic Nicotiana tabacum line, Tr25. The mutants were induced by incubating the suspension-cultured cells with 1 mmN-nitroso-N-methylurea for either 5 or 12 hours. Twenty mutant calli were isolated on selection medium containing 20 milligrams per liter of 5-fluoroorotic acid. Of those tested, most had reduced regeneration capacity. Characterization of UMP synthase activities in the isolated calli showed that UMP synthase activity varied from 8 to nearly 100% of the wild-type activity. The growth of the calli on the media containing different levels of 5-fluoroorotic acid correlated with decreasing UMP synthase activity. Because the UMP synthase enzyme has two separate enzymic activities (orotate phosphoribosyl transferase and orotidine-5'-monophosphate decarboxylase), several mutants were further characterized to determine how the mutations affected each of the two enzymic activities. In each case, the enzymic activity affected was the orotate phosphoribosyl transferase and not the orotidine-5'-monophosphate decarboxylase. The wound-inducible phenotype of the Tr25 plants as measured by the activation of the pin2-CAT gene remained unchanged by introduction of the UMP synthase mutations.

Time-course proteome analysis of developing extrafloral nectaries of Ricinus communis

Floral and extrafloral nectaries are unique organs that secrete energy rich chemical components, but their contribution for nectar production is largely unknown. Here, we present the first comparative proteome dataset of four developmental stages of the extrafloral nectaries from castor plant (Ricinus communis), an important biofuel crop. Respectively, from stage I-IV, we identified 626, 613, 449 and 356 proteins, respectively, summing up 882 nonredundant proteins. Surprisingly, we identified two isoforms of the potent toxin ricin, indicating that ricin expression is not limited to seeds, but it may serve a general defense purpose for the castor plant. To date, this is the most complete dataset of proteins either from floral or extrafloral nectaries, thus contributing to lay the foundations for investigations on their ecological and evolutionary importance.

Knockdown of MYB305 disrupts nectary starch metabolism and floral nectar production

MYB transcription factors have important roles during floral organ development. In this study, we generated myb305 RNAi knockdown tobacco plants and studied the role of MYB305 in the growth of the floral nectary. We have previously shown the MYB305 regulates the expression of flavonoid metabolic genes as well as of nectar proteins (nectarins); however, the myb305 plants showed other floral phenotypes that we investigate in these studies. The nectaries of myb305 plants show juvenile character at late stages of development and secrete reduced levels of nectar. Because starch metabolism is intimately involved in nectar secretion and is strongly regulated during normal nectary development, we examined the accumulation of starch in the nectaries of the myb305 plants. The myb305 plants accumulated lower levels of starch in their nectaries than did wild-type plants. The reduced starch correlated with the reduced expression of the ATP-glucose pyrophosphorylase (small subunit) gene in nectaries of the myb305 plants during the starch biosynthetic phase. Expression of genes encoding several starch-degrading enzymes including β-amylase, isoamylase 3, and α-amylase was also reduced in the myb305 plants. In addition to regulating nectarin and flavonoid metabolic gene expression, these results suggest that MYB305 may also function in the tobacco nectary maturation program by controlling the expression of starch metabolic genes.