S cysteine-rich (SCR) binding domain analysis of the Brassica self-incompatibility S-locus receptor kinase

Brassica self-incompatibility, a highly discriminating outbreeding mechanism, has become a paradigm for the study of plant cell-cell communications. When self-pollen lands on a stigma, the male ligand S cysteine-rich (SCR), which is present in the pollen coat, is transmitted to the female receptor, S-locus receptor kinase (SRK). SRK is a membrane-spanning serine/threonine receptor kinase present in the stigmatic papillar cell membrane. Haplotype-specific binding of SCR to SRK brings about pollen rejection. The extracellular receptor domain of SRK (eSRK) is responsible for binding SCR. Based on sequence homology, eSRK can be divided into three subdomains: B lectin-like, hypervariable, and PAN. Biochemical analysis of these subdomains showed that the hypervariable subdomain is responsible for most of the SCR binding capacity of eSRK, whereas the B lectin-like and PAN domains have little, if any, affinity for SCR. Fine mapping of the SCR binding region of SRK using a peptide array revealed a region of the hypervariable subdomain that plays a key role in binding the SCR molecule. We show that residues within the hypervariable subdomain define SRK binding and are likely to be involved in defining haplotype specificity.     

Features of the extracellular domain of the S-locus receptor kinase from Brassica

An S-receptor kinase (SRK) gene associated with self-incompatibility in a Brassica napus subsp. oleifera line has been characterized. The SRK-A14 cDNA shows the highest levels of homology in the 5′ end to the SLG-A14 cDNA present at the same locus. RNA blot analysis shows that the SRK-A14 gene is expressed predominantly in the pistil, and at lower levels in the anthers. The predicted amino acid sequences from the extracellular domain of the SRK-A14 gene and three other SRK genes were compared. The different SRK extracellular domains were for the most part very similar, with the exception of two variable regions containing a high level of amino acid alterations. These extracellular domains also contain a region of similarity to the immunoglobulin domains present in members of the immunoglobulin superfamily. These findings may define regions of the SRK protein that are necessary for interactions between SRK and other proteins.     

Expression of S-locus glycoprotein genes from Brassica oleracea and B. campestris in transgenic plants of self-compatible B. napus cv Westar

Summary Self-compatible Brassica napus var Westar was transformed with SLG, the S-locus-derived gene that encodes S-locus-specific glycoproteins (SLSG). Four allelic variants of SLG isolated from self-incompatible B. oleracea and B. campestris strains homozygous for different S alleles were used. We show that the transgenic plants synthesized SLSG with the same apparent charge, molecular weight, and antigenic properties as that produced by the corresponding self-incompatible strains from which the cloned SLG genes were isolated. In addition, transgene-encoded SLSG was detected specifically in the papillar cells of the stigma, and was correctly targeted to the papillar cell wall. However, SLSG was produced at reduced levels in transgenic plants relative to self-incompatible strains. The introduction of the SLG genes did not confer a self-incompatibility phenotype on the Westar cultivar.     

SRK, the stigma-specific S locus receptor kinase of Brassica, is targeted to the plasma membrane in transgenic tobacco.

The S locus receptor kinase (SRK) gene is one of two S locus genes required for the self-incompatibility response in Brassica. We have identified the product of the SRK6 gene in B. oleracea stigmas and have shown that it has characteristics of an integral membrane protein. When expressed in transgenic tobacco, SRK6 is glycosylated and targeted to the plasma membrane. These results provide definitive biochemical evidence for the existence in plants of a plasma membrane-localized transmembrane protein kinase with a known cell-cell recognition function. The timing of SRK expression in stigmas follows a time course similar to that previously described for another S locus-linked gene, the S locus glycoprotein (SLG) gene, and correlates with the ability of stigmas to mount a self-incompatibility response. Based on SRK6 promoter studies, the site of gene expression overlaps with that of SLG and exhibits predominant expression in the stigmatic papillar cells. Although reporter gene studies indicated that the SRK promoter was active in pollen, SRK protein was not detected in pollen, suggesting that SRK functions as a cell surface receptor exclusively in the papillar cells of the stigma.     

Polymorphism of the kinase domain of the S-locus receptor kinase gene (SRK) in Brassica oleracea L.

 DNA polymorphism of the S-locus receptor kinase gene (SRK) participating in self-incompatibility in Brassica was analyzed by PCR-RFLP and nucleotide sequencing. In the screening of primers for specific amplification of polymorphic DNA fragments of SRK, the best combination was that of a forward primer (PK1) having the nucleotide sequence of the second exon of S6 SRK and a reverse primer (PK4) having the complementary nucleotide sequence of the fifth exon of S6 SRK. PCR using this primer pair amplified DNA fragments of 0.9–1.0 kb from 36 S haplotypes out of 42 tested. These DNA fragments showed high polymorphism in polyacrylamide-gel electrophoresis after digestion with restriction endonuclease(s): 25 types were found in a double digestion with MboI and AfaI. Nucleotide sequencing of the DNA fragments amplified from five S haplotypes showed that the third, fourth, and fifth exons of SRK are highly conserved, and that there are high variations of the second and third introns of SRK, which produced polymorphism of the band pattern in PCR-RFLPs. Another forward primer (PK5) having the nucleotide sequence of the second exon, which is derived from S2 SRK, amplified DNA fragments of almost the same region of SRK from 27 S haplotypes in combination with PK4. Although SRK alleles of the class-II S haplotypes were not amplified, all of the class-I S-haplotypes were amplified with a primer mixture of PK1, PK4 and PK5. The DNA fragments of both SRK alleles in S heterozygotes, or a 1 : 1 mixture of the genomic DNA of different S homozygotes, were amplified without exception, suggesting the usefulness of these primers for the identification of S heterozygotes. The DNA fragment sizes obtained by digestion with restriction endonucleases served as markers for the identification of S haplotypes. Received: 15 December 1996 / Accepted: 14 February 1997     

A glycoprotein associated with the acquisition of the self-incompatibility system by maturing stigmas of Brassica oleracea

Iso-electric focusing of extracts derived from stigmatic homogenates of Brassica oleracea reveals that the mature stigma possesses large quantities of a glycoprotein not present in earlier stages of development in the bud. Pollen germination experiments carried out in parallel with the biochemical tests suggest that the appearance of this glycoprotein, which has an isoelectric point of pH 5.8, is coincident with the development of the self-incompatibility response. The site of this protein, and the role it may play in pollen-stigma interactions are discussed in terms of current models of the self-incompatibility system in Brassica.Abbreviations PAS periodic acid-schiffs - PEG polyethylene glycol - SI system self-incompatibility system     

Normal growth of transgenic tobacco plants in the absence of cytosolic pyruvate kinase

The coding sequence of the cytosolic isozyme of potato tuber pyruvate kinase (PK) was attached to the transit peptide of the small subunit of pea ribulose-1,5-bisphosphate carboxylase oxygenase and placed under the control of the cauliflower mosaic virus 35S promoter. This construct was transformed into Nicotiana tabacum. Unexpectedly, two primary transformants were recovered in which PK activity in leaves was greatly reduced. The reduction in PK activity appeared to result from the complete absence of the cytosolic form of the enzyme (PK_c). In addition, no PK_c could be detected on western blots of leaf extracts. Metabolite analyses indicated that the levels of phosphoenolpyruvate are substantially higher in PK_c-deficient leaves than in wild-type leaves, consistent with a block in glycolysis at the step catalyzed by PK. PK_c deficiency in the leaves does not appear to adversely affect plant growth. Analysis of progeny indicates that PK_c deficiency is a heritable trait. The leaves of PK_c-deficient transformants have normal rates of photosynthetic O₂ evolution and respiratory O₂ consumption, indicating that these plants are using alternative pathways to bypass PK.     

Expression of bioactive human M-CSF soluble receptor in transgenic tobacco plants

The cDNA encoding N-terminal three immunoglobin-like domains of human M-CSFR was linked to His-tag and endoplasmic reticulum retention sequence (KDEL) before being inserted into the genome of tobacco plant, Nicotiana tabacum cv. NC-89, by Agrobacterium tumefaciens-mediated transformation. The insertion and expression of target gene were confirmed by PCR, ELISA, and Western blot. The recombinant M-CSFsR reached a maximum expression level of 1.92% of total soluble protein in transgenic tobacco plant leaf tissues. The recombinant M-CSFsR could be purified through a one-step IMAC process and its bioactivity was confirmed by the inhibition of colony formation of J6-1 cells. The results suggested that we successfully expressed a high level of bioactive human M-CSFsR in tobacco plants.     

Sequence complexity of the S receptor kinase gene family in Brassica

A genomic library from an S ₂₉/S ₂₉ self-incompatible genotype of Brassica oleracea was screened with a probe carrying part of the catalytic domain of a Brassica S-receptor kinase (SRK)-like gene. Six positive phage clones with varying hybridisation intensities (K1 to K6) were purified and characterised. A 650–700 by region corresponding to the probe was excised from each clone and sequenced. DNA and predicted protein sequence comparisons based on a multiple alignment identified K5 as a pseudogene, whereas the others could encode functional proteins. K3 was found to have lost an intron from its genomic sequence. The six genes display different degrees of sequence similarity and form two distinct clusters in a dendrogram. The 98% similarity between K4 and K6, which extends across intron sequences, suggests that these might be very recently diverged alleles or daughters of a duplication. In addition, K2 showed a comparably high similarity to the probe. Clones K1, K3 and K5 cross-hybridised with an SLG ₂₉ cDNA probe, indicating the presence of upstream receptor domains homologous to the Brassica SLG gene. This suggests that the previously reported S sequence complexity may be ascribed to a large receptor kinase gene family.     

A plant receptor-like gene, the S-locus receptor kinase of Brassica oleracea L., encodes a functional serine/threonine kinase.

To investigate the catalytic properties of the Brassica oleracea S-locus receptor kinase (SRK), we have expressed the domain that is homologous to protein kinases as a fusion protein in Escherichia coli. Following in vivo labeling of cultures with 32P-labeled inorganic phosphate, we observed phosphorylation of the fusion protein on serine and threonine, but not on tyrosine. In contrast, labeling was not observed when lysine-524, a residue conserved among all protein kinases, was mutated to arginine, thus confirming that SRK phosphorylation was the result of intrinsic serine/threonine kinase activity.     

A Brassica S-locus related gene promoter directs expression in both pollen and pistil of tobacco

A portion (1.5 kb) of the promoter region of an S63 S-locus related (SLR) glycoprotein gene from the sporophytically self-incompatible species Brassica oleracea was inserted upstream of the β-glucuronidase (GUS) gene in binary vector pBI101.1. The resulting construct was then introduced into Nicotiana tabacum through Agrobacterium-mediated transformation. The expression pattern of GUS under the control of the S63 SLR promoter fragment was found to be similar to that already reported for expression of the GUS gene directed by an S-locus specific gene promoter in transgenic N. tabacum. Furthermore, this pattern of expression resembled more closely that reported for S-genes of the self-incompatible species Nicotiana alata, which has a gametophytic self-incompatibility system, than the predicted pattern of expression of S-genes in B. oleracea.     

Increased yield of heterologous viral glycoprotein in the seeds of homozygous transgenic tobacco plants cultivated underground.

The use of transgenic plants in the production of recombinant proteins for human therapy, including subunit vaccines, is being investigated to evaluate the efficacy and safety of these emerging biopharmaceutical products. We have previously shown that synthesis of recombinant glycoprotein B (gB) of human cytomegalovirus can be targeted to seeds of transgenic tobacco when directed by the rice glutelin 3 promoter, with gB retaining critical features of immunological reactivity (E.S. Tackaberry et al. 1999. Vaccine, 17: 3020-3029). Here, we report development of second generation transgenic plant lines (T1) homozygous for the transgene. Twenty progeny plants from two lines (A23T(1)-2 and A24T(1)-3) were grown underground in an environmentally contained mine shaft. Based on yields of gB in their seeds, the A23T(1)-2 line was then selected for scale-up in the same facility. Analyses of mature seeds by ELISA showedthat gB specific activity in A23T(1)-2 seeds was over 30-fold greater than the best T0 plants from the same transformation series, representing 1.07% total seed protein. These data demonstrate stable inheritance, an absence of transgene inactivation, and enhanced levels of gB expression in a homozygous second generation plant line. They also provide evidence for the suitability of using this environmentally secure facility to grow transgenic plants producing therapeutic biopharmaceuticals.