Targets and Mechanisms of Geminivirus Silencing Suppressor Protein AC2

Geminiviruses are plant DNA viruses that infect a wide range of plant species and cause significant losses to economically important food and fiber crops. The single-stranded geminiviral genome encodes a small number of proteins which act in an orchestrated manner to infect the host. The fewer proteins encoded by the virus are multifunctional, a mechanism uniquely evolved by the viruses to balance the genome-constraint. The host-mediated resistance against incoming virus includes post-transcriptional gene silencing, transcriptional gene silencing, and expression of defense responsive genes and other cellular regulatory genes. The pathogenicity property of a geminiviral protein is linked to its ability to suppress the host-mediated defense mechanism. This review discusses what is currently known about the targets and mechanism of the viral suppressor AC2/AL2/transcriptional activator protein (TrAP) and explore the biotechnological applications of AC2.

Characterization of a new rice OsMADS1 null mutant generated by homologous recombination-mediated gene targeting

A new, stable, null mutant of OsMADS1 generated by homologous recombination-based gene targeting in an indica rice confirms its regulatory role for floral meristem identity, its determinate development and floral organ differentiation. OsMADS1, an E-class MADS-box gene, is an important regulator of rice flower development. Studies of several partial loss-of-function and knockdown mutants show varied floret organ defects and degrees of meristem indeterminacy. The developmental consequences of a true null mutant on floret meristem identity, its determinate development and differentiation of grass-specific organs such as the lemma and palea remain unclear. In this study, we generated an OsMADS1 null mutant by homologous recombination-mediated gene targeting by inserting a selectable marker gene (hpt) in OsMADS1 and replacing parts of its cis-regulatory and coding sequences. A binary vector was constructed with diphtheria toxin A chain gene (DT-A) as a negative marker to eliminate random integrations and the hpt marker for positive selection of homologous recombination. Precise disruption of the endogenous OsMADS1 locus in the rice genome was confirmed by Southern hybridization. The homozygous osmads1ko null mutant displayed severe defects in all floral organs including the lemma and palea. We also noticed striking instances of floral reversion to inflorescence and vegetative states which has not been reported for other mutant alleles of OsMADS1 and further reinforces the role of OsMADS1 in controlling floral meristem determinacy. Our data suggest, OsMADS1 commits and maintains determinate floret development by regulating floral meristem termination, carpel and ovule differentiation genes (OsMADS58, OsMADS13) while its modulation of genes such as OsMADS15, OsIG1 and OsMADS32 could be relevant in the differentiation and development of palea. Further, our study provides an important perspective on developmental stage-dependent modulation of some OsMADS1 target genes.

Emergence of a Latent Indian Cassava Mosaic Virus from Cassava Which Recovered from Infection by a Non-Persistent Sri Lankan Cassava Mosaic Virus

The major threat for cassava cultivation on the Indian subcontinent is cassava mosaic disease (CMD) caused by cassava mosaic geminiviruses which are bipartite begomoviruses with DNA A and DNA B components. Indian cassava mosaic virus (ICMV) and Sri Lankan cassava mosaic virus (SLCMV) cause CMD in India. Two isolates of SLCMV infected the cassava cultivar Sengutchi in the fields near Malappuram and Thiruvananthapuram cities of Kerala State, India. The Malappuram isolate was persistent when maintained in the Madurai Kamaraj University (MKU, Madurai, Tamil Nadu, India) greenhouse, whereas the Thiruvananthapuram isolate did not persist. The recovered cassava plants with the non-persistent SLCMV, which were maintained vegetative in quarantine in the University of Basel (Basel, Switzerland) greenhouse, displayed re-emergence of CMD after a six-month period. Interestingly, these plants did not carry SLCMV but carried ICMV. It is interpreted that the field-collected, SLCMV-infected cassava plants were co-infected with low levels of ICMV. The loss of SLCMV in recovered cassava plants, under greenhouse conditions, then facilitated the re-emergence of ICMV. The partial dimer clones of the persistent and non-persistent isolates of SLCMV and the re-emerged isolate of ICMV were infective in Nicotiana benthamiana upon agroinoculation. Studies on pseudo-recombination between SLCMV and ICMV in N. benthamiana provided evidence for trans-replication of ICMV DNA B by SLCMV DNA A.

Transgenic tobacco plants expressing siRNA targeted against the Mungbean yellow mosaic virus transcriptional activator protein gene efficiently block the viral DNA accumulation

Mungbean yellow mosaic virus (MYMV) is a bipartite begomovirus that infects many pulse crops such as blackgram, mungbean, mothbean, Frenchbean, and soybean. We tested the efficacy of the transgenically expressed intron-spliced hairpin RNA gene of the transcriptional activator protein (hpTrAP) in reducing MYMV DNA accumulation. Tobacco plants transformed with the MYMV hpTrAP gene accumulated 21-22 nt siRNA. Leaf discs of the transgenic plants, agroinoculated with the partial dimers of MYMV, displayed pronounced reduction in MYMV DNA accumulation. Thus, silencing of the TrAP gene, a suppressor of gene silencing, emerged as an effective strategy to control MYMV.

The Agrobacterium tumefaciens Ti Plasmid Virulence Gene virE2 Reduces Sri Lankan Cassava Mosaic Virus Infection in Transgenic Nicotiana benthamiana Plants

Cassava mosaic disease is a major constraint to cassava cultivation worldwide. In India, the disease is caused by Indian cassava mosaic virus (ICMV) and Sri Lankan cassava mosaic virus (SLCMV). The Agrobacterium Ti plasmid virulence gene virE2, encoding a nuclear-localized, single-stranded DNA binding protein, was introduced into Nicotiana benthamiana to develop tolerance against SLCMV. Leaf discs of transgenic N. benthamiana plants, harboring the virE2 gene, complemented a virE2 mutation in A. tumefaciens and produced tumours. Three tested virE2 transgenic plants displayed reduction in disease symptoms upon agroinoculation with SLCMV DNA A and DNA B partial dimers. A pronounced reduction in viral DNA accumulation was observed in all three virE2 transgenic plants. Thus, virE2 is an effective candidate gene to develop tolerance against the cassava mosaic disease and possibly other DNA virus diseases.

Effect of orientation of transcription of a gene in an inverted transferred DNA repeat on transcriptional gene silencing in rice transgenics-a case study

We studied transgene silencing in two transgenic rice plants, OSM25 and COT-OSM4, which harboured two different types of right border (RB)-centered inverted transferred DNA (T-DNA) repeats (IRs). The T-DNA in OSM25 has three genes gus, OSM and hph, all under the transcriptional control of the Cauliflower mosaic virus 35S promoter (P35S). The gus gene, which is proximal to the RB, is in a convergent orientation of transcription in the IR. OSM25 displayed silencing of all three transgenes. Nuclear run-on transcription analysis revealed that silencing of gus, OSM and hph genes in OSM25 operates at the transcriptional level. P35S showed hypermethylation in OSM25 plants. COT-OSM4 has P35S-driven gus and hph genes in its T-DNA. The hph gene, which is proximal to the RB, is in a divergent orientation of transcription in the IR. Unlike in OSM25, the transgenes in COT-OSM4 showed no silencing. These findings show that convergent orientation of transcription of a gene at the origin of an IR is important for transgene silencing.

A novel T-DNA integration in rice involving two interchromosomal translocations

A male sterile transgenic rice plant TC-19 harboured a novel T-DNA integration in chromosome 8 with two interchromosomal translocations of 6.55 kb chromosome 3 and 29.8 kb chromosome 9 segments. We report a complex Agrobacterium T-DNA integration in rice (Oryza sativa) associated with two interchromosomal translocations. The T-DNA-tagged rice mutant TC-19, which harboured a single copy of the T-DNA, displayed male sterile phenotype in the homozygous condition. Analysis of the junctions between the T-DNA ends and the rice genome by genome walking showed that the right border is flanked by a chromosome 3 sequence and the left border is flanked by a chromosome 9 sequence. Upon further walking on chromosome 3, a chromosome 3/chromosome 8 fusion was detected. Genome walking from the opposite end of the chromosome 8 break point revealed a chromosome 8/chromosome 9 fusion. Our findings revealed that the T-DNA, together with a 6.55-kb region of chromosome 3 and a 29.8-kb region of chromosome 9, was translocated to chromosome 8. Southern blot analysis of the homozygous TC-19 mutant revealed that the native sequences of chromosome 3 and 9 were restored but the disruption of chromosome 8 in the first intron of the gene Os08g0152500 was not restored. The integration of the complex T-DNA in chromosome 8 caused male sterility.

Mungbean yellow mosaic virus (MYMV) AC4 suppresses post-transcriptional gene silencing and an AC4 hairpin RNA gene reduces MYMV DNA accumulation in transgenic tobacco

Mungbean yellow mosaic virus (MYMV) is a legume-infecting geminivirus that causes yellow mosaic disease in blackgram, mungbean, soybean, Frenchbean and mothbean. AC4/C4, which is nested completely within the Rep gene, is less conserved among geminiviruses. Much less is known about its role in viral pathogenesis other than its known role in the suppression of host-mediated gene silencing. Transient expression of MYMV AC4 by agroinfiltration suppressed post-transcriptional gene silencing in Nicotiana benthamiana 16c expressing green fluorescence protein, at a level comparable to MYMV TrAP expression. AC4 full-length gene and an inverted repeat of AC4 (comprising the full-length AC4 sequence in sense and antisense orientations with an intervening intron) which makes a hairpin RNA (hpRNA) upon transcription were introduced into tobacco by Agrobacterium-mediated leaf disc transformation. Leaf discs of the transgenic plants were agroinoculated with partial dimers of MYMV and used to study the effect of the AC4-sense and AC4 hpRNA genes on MYMV DNA accumulation. Leaf discs of two transgenic plants that express the AC4-sense gene displayed an increase in MYMV DNA accumulation. Leaf discs of six transgenic plants containing the AC4 hpRNA gene accumulated small-interfering RNAs (siRNAs) specific to AC4, and upon agroinoculation with MYMV they exhibited a severe reduction in the accumulation of MYMV DNA. Thus, the MYMV AC4 hpRNA gene has emerged as a good candidate to engineer resistance against MYMV in susceptible plants.

Auxin-responsive OsMGH3, a common downstream target of OsMADS1 and OsMADS6, controls rice floret fertility

GH3 proteins control auxin homeostasis by inactivating excess auxin as conjugates of amino acids and sugars and thereby controlling cellular bioactive auxin. Since auxin regulates many aspects of plant growth and development, regulated expression of these genes offers a mechanism to control various developmental processes. OsMGH3/OsGH3-8 is expressed abundantly in rice florets and is regulated by two related and redundant transcription factors, OsMADS1 and OsMADS6, but its contribution to flower development is not known. We functionally characterize OsMGH3 by overexpression and knock-down analysis and show a partial overlap in these phenotypes with that of mutants in OsMADS1 and OsMADS6. The overexpression of OsMGH3 during the vegetative phase affects the overall plant architecture, whereas its inflorescence-specific overexpression creates short panicles with reduced branching, resembling in part the effects of OsMADS1 overexpression. In contrast, the down-regulation of endogenous OsMGH3 caused phenotypes consistent with auxin overproduction or activated signaling, such as ectopic rooting from aerial nodes. Florets in OsMGH3 knock-down plants were affected in carpel development and pollen viability, both of which reduced fertility. Some of these floret phenotypes are similar to osmads6 mutants. Taken together, we provide evidence for the functional significance of auxin homeostasis and its transcriptional regulation during rice panicle branching and floret organ development.

Antibegomoviral activity of the agrobacterial virulence protein VirE2

Mungbean yellow mosaic geminivirus (MYMV) causes severe yellow mosaic disease in blackgram, mungbean, Frenchbean, pigeonpea, soybean and mothbean. We attempted to induce resistance against this virus using the transcriptional activator protein gene deleted in the C-terminal activation domain (TrAP-∆AD) and Agrobacterium tumefaciens virE2. MYMV is known to replicate in agroinoculated tobacco leaf discs. Three transgenic tobacco plants which harboured a truncated MYMV transcriptional activator protein gene and two tobacco plants transformed with the octopine type A. tumefaciens virE2 gene were agroinoculated with an A. tumefaciens strain which harboured the partial dimers of both DNA A and DNA B of MYMV. The level of viral DNA accumulation in leaf discs of transgenic plants correlated inversely to the level of the MYMV TrAP-∆AD transcript. Two VirE2-transgenic plants, which complemented tumorigenesis of a virE2 mutant A. tumefaciens strain, effectively reduced MYMV DNA accumulation in the leaf disc agroinoculation assay.

Transgene stacking and marker elimination in transgenic rice by sequential Agrobacterium-mediated co-transformation with the same selectable marker gene

Rice chitinase (chi11) and tobacco osmotin (ap24) genes, which cause disruption of fungal cell wall and cell membrane, respectively, were stacked in transgenic rice to develop resistance against the sheath blight disease. The homozygous marker-free transgenic rice line CoT23 which harboured the rice chi11 transgene was sequentially re-transformed with a second transgene ap24 by co-transformation using an Agrobacterium tumefaciens strain harbouring a single-copy cointegrate vector pGV2260::pSSJ1 and a multi-copy binary vector pBin19∆nptII-ap24 in the same cell. pGV2260::pSSJ1 T-DNA carried the hygromycin phosphotransferase (hph) and β-glucuronidase (gus) genes. pBin19∆nptII-ap24 T-DNA harboured the tobacco osmotin (ap24) gene. Co-transformation of the gene of interest (ap24) with the selectable marker gene (SMG, hph) occurred in 12 out of 18 T(0) plants (67%). Segregation of hph from ap24 was accomplished in the T(1) generation in one (line 11) of the four analysed co-transformed plants. The presence of ap24 and chi11 transgenes and the absence of the hph gene in the SMG-eliminated T(1) plants of the line 11 were confirmed by DNA blot analyses. The SMG-free transgenic plants of the line 11 harboured a single copy of the ap24 gene. Homozygous, SMG-free T(2) plants of the transgenic line 11 harboured stacked transgenes, chi11 and ap24. Northern blot analysis of the SMG-free plants revealed constitutive expression of chi11 and ap24. The transgenic plants with stacked transgenes displayed high levels of resistance against Rhizoctonia solani. Thus, we demonstrate the development of transgene-stacked and marker-free transgenic rice by sequential Agrobacterium-mediated co-transformation with the same SMG.

Improved Agrobacterium-mediated co-transformation and selectable marker elimination in transgenic rice by using a high copy number pBin19-derived binary vector

A high copy number, selectable marker gene (SMG)-free Agrobacterium binary vector pBin19ΔnptII was constructed by deleting the nptII gene from pBin19. The binary vectors with the RK2 and pVS replication origins exist in 12 and 3 copies, respectively, in Agrobacterium. The tobacco osmotin gene (ap24) was cloned in pBin19ΔnptII and the resultant plasmid pBin19ΔnptII-ap24 was mobilized into the Agrobacterium tumefaciens strain C58C1 Rif(r) harbouring the single-copy cointegrate vector pGV2260::pSSJ1. The T-DNA of the cointegrate vector harboured the hph (SMG) and gus genes. Transformation of Oryza sativa L. var. Pusa Basmati1 with Agrobacterium tumefaciens (pGV2260::pSSJ1, pBin19ΔnptII-ap24) yielded 14 independent hyg+/GUS+ transgenic plants. Southern blot analysis with hph and ap24 probes revealed that 12 out of the 14 transgenic plants were co-transformed and harboured hph, gus and ap24 genes. The new multi-copy binary vector yielded 86% co-transformation efficiency. SMG elimination by genetic separation of the cointegrate T-DNA with the hph/gus genes and binary vector T-DNA with the ap24 gene was accomplished in four out of ten primary co-transformants that were forwarded to the T₁ generation.

Severe stunting in blackgram caused by the Mungbean yellow mosaic virus (MYMV) KA27 DNA B component is ameliorated by co-infection or post-infection with the KA22 DNA B: MYMV nuclear shuttle protein is the symptom determinant

Mungbean yellow mosaic virus-[India:Vigna] (MYMV-[IN:Vig]), a blackgram isolate of MYMV, has five variable and infective DNA B components of which KA22 and KA27 DNA Bs share only 72% nucleotide sequence identity between them. Agroinoculation of blackgram with partial dimers of DNA A and KA27 DNA B caused severe stunting and an inordinate delay in flowering. Interestingly, co-agroinoculation of KA27+KA22 DNA B components along with DNA A ameliorated severe stunting, rescued from the delay in flowering and caused the appearance of yellow mosaic symptom characteristic of KA22 DNA B. Post-agroinoculation of KA27 DNA B-infected blackgram plants with KA22 DNA B also resulted in the amelioration from severe stunting and in the alleviation from the delay in flowering. Alleviation from KA27 DNA B-type of symptom by co-infection or post-infection with KA22 DNA B did not result in a corresponding reduction in KA27 DNA B levels. Swapping of KA27 DNA B with the nuclear shuttle protein gene (NSP) of KA22 DNA B abolished severe stunting and caused the appearance of mild yellow symptom, suggesting that the NSP is the major symptom determinant in MYMV DNA B.

Calcium-promoted protein phosphorylation in plants

Phosphorylation of several polypeptides in corn coleoptiles was promoted by adding calcium. Chlorpromazine, a calmodulin inhibitor, reduced calcium-promoted phosphorylation, suggesting that the phosphorylation was modulated by calmodulin. This is evidence for the role of calcium in protein phosphorylation in plants and could serve as an experimental approach to understanding the molecular mechanism by which calcium modulates various physiological processes in plants.

Virulence genes A, G, and D mediate the double-stranded border cleavage of T-DNA from the Agrobacterium Ti plasmid

Agrobacterium tumefaciens transfers the T-DNA portion of its Ti plasmid to the nuclear genome of plant cells. Upon cocultivation of A. tumefaciens strain A348 with regenerating tobacco leaf protoplasts, restriction endonuclease fragments of the T-DNA were generated that are consistent with double-stranded cleavage of the T-DNA at the border sequences. The T-DNA border cleavage was also induced by acetosyringone, a compound that induces many of the virulence genes. T-DNA cleavage did not occur in Agrobacterium strains harboring Tn3-HoHo1 insertions in the virA, -D, or -G genes. Insertion mutations in virB, -C, or -E did not have any effect on the T-DNA cleavage. Complementation of the mutations in virA, -D, or -G with cosmids containing the respective wild-type genes restored the T-DNA cleavage. Since virA and -G are essential in regulating the expression of other vir genes in response to plant signal molecules, the virD gene product(s) appear to mediate double-stranded T-DNA border cleavage.

Selectable marker elimination in the T0 generation by Agrobacterium-mediated co-transformation involving Mungbean yellow mosaic virus TrAP as a non-conditional negative selectable marker and bar for transient positive selection

Transient selection involving the bar gene and non-conditional negative selection against stable T-DNA integration through the use of the Mungbean yellow mosaic virus (MYMV) transcriptional activator protein gene (TrAP) were used in a novel co-transformation strategy to generate selectable marker gene (SMG)-eliminated transgenic tobacco plants in the T(0) generation itself. Two compatible binary plasmids, pCam-bar-TrAP-gus harbouring bar as an SMG and the MYMV TrAP gene as a non-conditional negative selectable marker, and pGA472 with the nptII gene as an unselected experimental gene of interest (GOI) were placed in the Agrobacterium tumefaciens strain EHA105 and used for co-transformation. Transient selection with 5 mg l(-1) phosphinothricin (PPT) for 2-4 weeks and subsequent establishment in a PPT-minus medium yielded 114 plants from 200 leaf discs. The unselected nptII gene was detected by Southern blot analysis in 13 plants, revealing a co-transformation efficiency of 11.5%. Five of these plants harboured only the nptII gene (GOI) and not the bar gene (SMG). Thus, SMG elimination was achieved in the T(0) generation itself in 4.4% (5/114) of plants, which were transiently selected for 2-4 weeks on PPT. MYMV TrAP, a non-conditional negative selectable marker, effectively reduced the recovery of plants with stable integration of the SMG (bar).

Generation of selectable marker-free sheath blight resistant transgenic rice plants by efficient co-transformation of a cointegrate vector T-DNA and a binary vector T-DNA in one Agrobacterium tumefaciens strain

Co-transformation of Oryza sativa L. var. Pusa Basmati1 was done using an Agrobacterium tumefaciens strain harbouring a single-copy cointegrate vector and a multi-copy binary vector in the same cell. The T-DNA of the cointegrate vector pGV2260::pSSJ1 carried the hygromycin phosphotransferase (hph) and beta-glucuronidase (gus) genes. The binary vector pCam-chi11, without a plant selectable marker gene, harboured the rice chitinase (chi11) gene under maize ubiquitin promoter. Co-transformation of the gene of interest (chi11) with the selectable marker gene (hph) occurred in 4 out of 20 T(0) plants (20%). Segregation of hph from chi11 was accomplished in two (CoT6 and CoT23) of the four co-transformed plants in the T(1) generation. The selectable marker-free (SMF) lines CoT6 and CoT23 harboured single copies of chi11. Homozygous SMF T(2) plants were established in the lines CoT6 and CoT23. Northern and Western blot analysis of the homozygous SMF lines showed high level of transgene expression. In comparison to untransformed controls, chitinase specific activity was 66- and 22-fold higher in the homozygous SMF T(2) plants of lines CoT6 and CoT23, respectively. The lines CoT6 and CoT23 exhibited 38 and 40% reduction in sheath blight disease, respectively.

The mungbean yellow mosaic begomovirus transcriptional activator protein transactivates the viral promoter-driven transgene and causes toxicity in transgenic tobacco plants

The Begomovirus transcriptional activator protein (TrAP/AC2/C2) is a multifunctional protein which activates the viral late gene promoters, suppresses gene silencing, and determines pathogenicity. To study TrAP-mediated transactivation of a stably integrated gene, we generated transgenic tobacco plants with a Mungbean yellow mosaic virus (MYMV) AV1 late gene promoter-driven reporter gene and supertransformed them with the MYMV TrAP gene driven by a strong 35S promoter. We obtained a single supertransformed plant with an intact 35S-TrAP gene that activated the reporter gene 2.5-fold. However, 10 of the 11 supertransformed plants did not have the TrAP region of the T-DNA, suggesting the likely toxicity of TrAP in plants. Upon transformation of wild-type tobacco plants with the TrAP gene, six of the seven transgenic plants obtained had truncated T-DNAs which lacked TrAP. One plant, which had the intact TrAP gene, did not express TrAP. The apparent toxic effect of the TrAP transgene was abolished by mutations in its nuclear-localization signal or zinc-finger domain and by deletion of its activation domain. Therefore, all three domains of TrAP, which are required for transactivation and suppression of gene silencing, also are needed for its toxic effect.

Expression of full-length and truncated Rep genes from Mungbean yellow mosaic virus-Vigna inhibits viral replication in transgenic tobacco

Mungbean yellow mosaic virus-Vigna (MYMV-Vig) is a bipartite geminivirus that causes a severe yellow mosaic disease in blackgram. An assay was developed to study MYMV-Vig replication by agroinoculation of tobacco leaf discs with partial dimers of the virus. This assay, in a non-host model plant, was used to evaluate pathogen-derived resistance contributed by MYMV-Vig genes in transgenic plants. Viral DNA accumulation was optimum in tobacco leaf discs cultured for 10 days after infection with Agrobacterium tumefaciens strain Ach5 containing partial dimers of both DNA A and DNA B of MYMV-Vig. Transgenic tobacco plants with MYMV-Vig genes for coat protein (CP), replication-associated protein (Rep)-sense, Rep-antisense, truncated Rep (T-Rep), nuclear shuttle protein (NSP) and movement protein (MP) were generated. Leaf discs from transgenic tobacco plants, harbouring MYMV-Vig genes, were agroinoculated with partial dimers of MYMV-Vig and analyzed for viral DNA accumulation. The leaf discs from transgenic tobacco plants harbouring CP and MP genes supported the accumulation of higher levels of MYMV-Vig DNA. However, MYMV-Vig accumulation was inhibited in one transgenic plant harbouring the Rep-sense gene and in two plants harbouring the T-Rep gene. Northern analysis of these plants revealed a good correlation between expression of Rep or T-Rep genes and inhibition of MYMV-Vig accumulation.

Factors contributing to deletion within Mungbean yellow mosaic virus partial dimers in binary vectors used for agroinoculation

Mungbean yellow mosaic virus-Vigna (MYMV) sequences cloned as partial dimers within the T-DNA of a binary vector were deleted at a high frequency upon conjugal mobilization from Escherichia coli into Agrobacterium tumefaciens. This deletion involving the genome-length viral DNA did not occur when the binary plasmid was inside E. coli and when the binary plasmid was introduced into Agrobacterium by electroporation. Deletions occurred in both DNA A and DNA B partial dimers. A minimum of 500-nt continuity on either side of the nonanucleotide in the duplicated common region is required for deletion. A. tumefaciens cells in which deletion was complete, grew as larger colonies reflecting a growth advantage. The small, slow-growing colonies eventually lost the genome-length viral sequences after a few more cycles of growth. Partial dimers in binary plasmids pGA472 and pBin19 with RK2 replicon underwent deletion while those in pPZP with pVS1 replicon did not undergo deletion. Deletion was observed in A. tumefaciens strains C58, A136, A348 and A281 with C58 chromosome background, but not in Ach5 and T37. Interestingly, deletion did not occur in A. tumefaciens strain AGL1 with a recA mutation in C58 chromosome, implying a clear role for recombination in deletion. These observations suggest the choice of Agrobacterium strains and binary vectors for agroinoculation of geminiviruses.

Suppression of RNA silencing by a geminivirus nuclear protein, AC2, correlates with transactivation of host genes

Bipartite geminiviruses encode a small protein, AC2, that functions as a transactivator of viral transcription and a suppressor of RNA silencing. A relationship between these two functions had not been investigated before. We characterized both of these functions for AC2 from Mungbean yellow mosaic virus-Vigna (MYMV). When transiently expressed in plant protoplasts, MYMV AC2 strongly transactivated the viral promoter; AC2 was detected in the nucleus, and a split nuclear localization signal (NLS) was mapped. In a model Nicotiana benthamiana plant, in which silencing can be triggered biolistically, AC2 reduced local silencing and prevented its systemic spread. Mutations in the AC2 NLS or Zn finger or deletion of its activator domain abolished both these effects, suggesting that suppression of silencing by AC2 requires transactivation of host suppressor(s). In line with this, in Arabidopsis protoplasts, MYMV AC2 or its homologue from African cassava mosaic geminivirus coactivated >30 components of the plant transcriptome, as detected with Affymetrix ATH1 GeneChips. Several corresponding promoters cloned from Arabidopsis were strongly induced by both AC2 proteins. These results suggest that silencing suppression and transcription activation by AC2 are functionally connected and that some of the AC2-inducible host genes discovered here may code for components of an endogenous network that controls silencing.

Promoters, transcripts, and regulatory proteins of Mungbean yellow mosaic geminivirus

Geminiviruses package circular single-stranded DNA and replicate in the nucleus via a double-stranded intermediate. This intermediate also serves as a template for bidirectional transcription by polymerase II. Here, we map promoters and transcripts and characterize regulatory proteins of Mungbean yellow mosaic virus-Vigna (MYMV), a bipartite geminivirus in the genus Begomovirus. The following new features, which might also apply to other begomoviruses, were revealed in MYMV. The leftward and rightward promoters on DNA-B share the transcription activator AC2-responsive region, which does not overlap the common region that is nearly identical in the two DNA components. The transcription unit for BC1 (movement protein) includes a conserved, leader-based intron. Besides negative-feedback regulation of its own leftward promoter on DNA-A, the replication protein AC1, in cooperation with AC2, synergistically transactivates the rightward promoter, which drives a dicistronic transcription unit for the coat protein AV1. AC2 and the replication enhancer AC3 are expressed from one dicistronic transcript driven by a strong promoter mapped within the upstream AC1 gene. Early and constitutive expression of AC2 is consistent with its essential dual function as an activator of viral transcription and a suppressor of silencing.

Coat proteins of Rice tungro bacilliform virus and Mungbean yellow mosaic virus contain multiple nuclear-localization signals and interact with importin alpha

Transport of the viral genome into the nucleus is an obligatory step in the replication cycle of plant pararetro- and geminiviruses. In both these virus types, the multifunctional coat protein (CP) is thought to be involved in this process. Here, a green fluorescent protein tagging approach was used to demonstrate nuclear import of the CPs of Rice tungro bacilliform virus (RTBV) and Mungbean yellow mosaic virus--Vigna (MYMV) in Nicotiana plumbaginifolia protoplasts. In both cases, at least two nuclear localization signals (NLSs) were identified and characterized. The NLSs of RTBV CP are located within both N- and C-terminal regions (residues 479KRPK/497KRK and 744KRK/758RRK), and those of MYMV CP within the N-terminal part (residues 3KR and 41KRRR). The MYMV and RTBV CP NLSs resemble classic mono- and bipartite NLSs, respectively. However, the N-terminal MYMV CP NLS and both RTBV CP NLSs show peculiarities in the number and position of basic residues. In vitro pull-down assays revealed interaction of RTBV and MYMV CPs with the nuclear import factor importin alpha, suggesting that both CPs are imported into the nucleus via an importin alpha-dependent pathway. The possibility that this pathway could serve for docking of virions to the nucleus is discussed.

Suppression of RNA silencing by a geminivirus nuclear protein, AC2, correlates with transactivation of host genes

Bipartite geminiviruses encode a small protein, AC2, that functions as a transactivator of viral transcription and a suppressor of RNA silencing. A relationship between these two functions had not been investigated before. We characterized both of these functions for AC2 from Mungbean yellow mosaic virus-Vigna (MYMV). When transiently expressed in plant protoplasts, MYMV AC2 strongly transactivated the viral promoter; AC2 was detected in the nucleus, and a split nuclear localization signal (NLS) was mapped. In a model Nicotiana benthamiana plant, in which silencing can be triggered biolistically, AC2 reduced local silencing and prevented its systemic spread. Mutations in the AC2 NLS or Zn finger or deletion of its activator domain abolished both these effects, suggesting that suppression of silencing by AC2 requires transactivation of host suppressor(s). In line with this, in Arabidopsis protoplasts, MYMV AC2 or its homologue from African cassava mosaic geminivirus coactivated >30 components of the plant transcriptome, as detected with Affymetrix ATH1 GeneChips. Several corresponding promoters cloned from Arabidopsis were strongly induced by both AC2 proteins. These results suggest that silencing suppression and transcription activation by AC2 are functionally connected and that some of the AC2-inducible host genes discovered here may code for components of an endogenous network that controls silencing.

Infectivity analysis of two variable DNA B components of Mungbean yellow mosaic virus-Vigna in Vigna mungo and Vigna radiata

Mungbean yellow mosaic virus-Vigna (MYMV-Vig), a Begomovirus that causes yellow mosaic disease, was cloned from field-infected blackgram (Vigna mungo). One DNA A clone (KA30) and five different DNA B clones (KA21, KA22, KA27, KA28 and KA34) were obtained. The sequence identity in the 150-nt common region (CR) between DNA A and DNA B was highest (95%) for KA22 DNA B and lowest (85.6%) for KA27 DNA B. The Rep-binding domain had three complete 11-nt (5'-TGTATCGGTGT-3') iterons in KA22 DNA B (and KA21, KA28 and KA34), while the first iteron in KA27 DNA B (5'-ATCGGTGT-3') had a 3-nt deletion. KA27 DNA B, which exhibited 93.9% CR sequence identity to the mungbean-infecting MYMV, also shared the 3-nt deletion in the first iteron besides having an 18-nt insertion between the third iteron and the conserved nonanucleotide. MYMV was found to be closely related to KA27 DNA B in amino acid sequence identity of BV1 (94.1%) and BC1 (97.6%) proteins and in the organization of nuclear localization signal (NLS), nuclear export signal (NES) and phosphorylation sites. Agroinoculation of blackgram (V. mungo) and mungbean (V. radiata) with partial dimers of KA27 and KA22 DNA Bs along with DNA A caused distinctly different symptoms. KA22 DNA B caused more intense yellow mosaic symptoms with high viral DNA titre in blackgram. In contrast, KA27 DNA B caused more intense yellow mosaic symptoms with high viral DNA titre in mungbean. Thus, DNA B of MYMVVig is an important determinant of host-range between V. mungo and V. radiata.

Analysis of an isolate of Mungbean yellow mosaic virus (MYMV) with a highly variable DNA B component

One DNA A (KA30) and five different DNA B components (KA21, KA22, KA27, KA28 and KA34) of a geminivirus, Mungbean yellow mosaic virus-Vigna (MYMV-Vig) were cloned from a pooled sample of field-infected Vigna mungo plants from Vamban, South India. MYMV-Vig DNA A (KA30) and one of the DNA B components (KA27) exhibited 97% and 95% sequence identities, respectively, to those of MYMV reported from Thailand. However, the DNA B components KA21, KA22, KA28 and KA34 exhibited only 71 to 72% sequence identity to MYMV DNA B. Co-existence of multiple DNA B components in field-infected V. mungo was proved by Southern and PCR analyses. Each of the five DNA B components was infective together with the DNA A upon agroinoculation. Agroinoculation with mixed cultures of Agrobacterium with partial dimers of DNA A and all five DNA Bs proved that all five DNA B components can co-infect a single V. mungo plant.

Analysis of an isolate of Mungbean yellow mosaic virus (MYMV) with a highly variable DNA B component

One DNA A (KA30) and five different DNA B components (KA21, KA22, KA27, KA28 and KA34) of a geminivirus, Mungbean yellow mosaic virus-Vigna (MYMV-Vig) were cloned from a pooled sample of field-infected Vigna mungo plants from Vamban, South India. MYMV-Vig DNA A (KA30) and one of the DNA B components (KA27) exhibited 97% and 95% sequence identities, respectively, to those of MYMV reported from Thailand. However, the DNA B components KA21, KA22, KA28 and KA34 exhibited only 71 to 72% sequence identity to MYMV DNA B. Co-existence of multiple DNA B components in field-infected V. mungo was proved by Southern and PCR analyses. Each of the five DNA B components was infective together with the DNA A upon agroinoculation. Agroinoculation with mixed cultures of Agrobacterium with partial dimers of DNA A and all five DNA Bs proved that all five DNA B components can co-infect a single V. mungo plant.

Mungbean yellow mosaic virus-Vi Agroinfection by Codelivery of DNA A and DNA B From One Agrobacterium Strain

Agroinfection of bipartite geminiviruses is routinely done by mixing two Agrobacterium strains that independently harbor partial tandem repeats of DNA A and DNA B. We report here an improved agroinfection method for bipartite geminiviruses that utilizes one strain of Agrobacterium that harbors DNA A and DNA B partial tandem repeats on two compatible replicons. A cointegrate vector, pGV2260∷pGV1.3A, with the partial tandem repeat of Mungbean yellow mosaic virus-Vi (MYMV-Vi) DNA A and a binary vector, pGA1.9B, with the partial tandem repeat of MYMV-Vi DNA B gave an agroinfection efficiency of 24% when harbored in two Agrobacterium strains and an efficiency of 61% when harbored in one Agrobacterium strain. A combination of binary vectors, pGA1.9A with MYMV-Vi DNA A partial tandem repeat and pGA1.9B with DNA B partial tandem repeat, gave an agroinfection efficiency of 74% when harbored in two strains. But pGA1.9A and pPZP1.9B (a partial tandem repeat of DNA B), when present in the same Agrobacterium strain, gave 100% agroinfection. Accumulation of viral DNA was shown by Southern blotting. The single-strain method using two compatible replicons consistently gave 100% agroinfection efficiency.

Efficient vir gene induction in Agrobacterium tumefaciens requires virA, virG, and vir box from the same Ti plasmid

The vir genes of octopine, nopaline, and L,L-succinamopine Ti plasmids exhibit structural and functional similarities. However, we observed differences in the interactions between octopine and nopaline vir components. The induction of an octopine virE(A6)::lacZ fusion (pSM358cd) was 2.3-fold higher in an octopine strain (A348) than in a nopaline strain (C58). Supplementation of the octopine virG(A6) in a nopaline strain with pSM358 did not completely restore virE(A6) induction. However, addition of the octopine virA(A6) to the above strain increased virE(A6) induction to a level almost comparable to that in octopine strains. In a reciprocal analysis, the induction of a nopaline virE(C58)::cat fusion (pUCD1553) was two- to threefold higher in nopaline (C58 and T37) strains than in octopine (A348 and Ach5) and L,L-succinamopine (A281) strains. Supplementation of nopaline virA(C58) and virG(C58) in an octopine strain (A348) harboring pUCD1553 increased induction levels of virE(C58)::cat fusion to a level comparable to that in a nopaline strain (C58). Our results suggest that octopine and L,L-succinamopine VirG proteins induce the octopine virE(A6) more efficiently than they do the nopaline virE(C58). Conversely, the nopaline VirG protein induces the nopaline virE(C58) more efficiently than it does the octopine virE(A6). The ability of Bo542 virG to bring about supervirulence in tobacco is observed for an octopine vir helper (LBA4404) but not for a nopaline vir helper (PMP90). Our analyses reveal that quantitative differences exist in the interactions between VirG and vir boxes of different Ti plasmids. Efficient vir gene induction in octopine and nopaline strains requires virA, virG, and vir boxes from the respective Ti plasmids.

The distinct disease phenotypes of the common and yellow vein strains of Tomato golden mosaic virus are determined by nucleotide differences in the 3'-terminal region of the gene encoding the movement protein

In Nicotiana benthamiana, the common strain of the bipartite geminivirus Tomato golden mosaic virus (csTGMV) induces extensive chlorosis whereas the yellow vein strain (yvTGMV) produces veinal chlorosis on systemically infected leaves. In Datura stramonium, csTGMV produces leaf distortion and a severe chlorotic mosaic whereas yvTGMV produces only small chlorotic lesions on systemically infected leaves. Genetic recombination and site-directed mutagenesis studies using infectious clones of csTGMV and yvTGMV have identified a role in symptom production for the gene encoding the movement protein (MP). The MP amino acid at position 272, either valine (csTGMV) or isoleucine (yvTGMV), influenced symptoms in both hosts by inducing an intermediate phenotype when exchanged between the two strains. Exchange of an additional strain-specific MP amino acid at position 288, either glutamine (csTGMV) or lysine (yvTGMV), resulted in the change of symptom phenotype to that of the other strain. In situ hybridization analysis in N. benthamiana demonstrated that there was no qualitative difference in the tissue distribution of the two strains although csTGMV accumulated in higher amounts, suggesting that the efficiency of virus movement rather than distinct differences in tissue specificity of the strains is responsible for the symptom phenotypes.

Genetic transformation of cotyledon explants of cowpea (Vigna unguiculata L. Walp) using Agrobacterium tumefaciens

Mature de-embryonated cotyledons with intact proximal end of Vigna unguiculata were cultured on B5 basal medium containing varying concentrations of BAP. Thirty-six percent of the explants produced shoots on B5 medium supplemented with 8× 10(-6) M BAP. Cotyledon explants were pre-incubated for 24 h, inoculated with A. tumefaciens pUCD2614 carrying pUCD2340, co-cultivated for 48 h and transferred to hygromycin-B (25 mg/l) containing shoot induction medium. Approximately 15-19% of the explants produced shoots on the selection medium. The elongated shoots were subsequently rooted on B5 basal medium containing hygromycin. The transgenic plants were later established in pots. The presence of hpt gene in the transgenic plants was confirmed by Southern blot hybridization.

Agrobacterium tumefaciens-mediated transformation of Vigna mungo (L.) Hepper

Transformed Vigna mungo (blackgram) calli were obtained by cocultivating segments of primary leaves with Agrobacterium tumefaciens vir helper strains harbouring the binary vector pGA472 having kanamycin resistance gene as plant transformation marker. Transformed calli were selected on Murashige and Skoog medium supplemented with 50 mg/l kanamycin and 500 mg/l carbenicillin. Transformed calli were found to be resistant to kanamycin up to 900 mg/l concentration. Expression of kanamycin resistance gene in transformed calli was demonstrated by neomycin phosphotransferase assay. Stable integration of transferred DNA into V. mungo genome was confirmed by Southern blot analysis.

Rice scutellum induces Agrobacterium tumefaciens vir genes and T-strand generation

For successful transformation of a plant by Agrobacterium tumefaciens it is essential that the explant used in cocultivation has the ability to induce Agrobacterium tumour-inducing (Ti) plasmid virulence (vir) genes. Here we report a significant variation in different tissues of Indica rice (Oryza sativa L. cv. Co43) in their ability to induce Agrobacterium tumefaciens vir genes and T-strand generation, using explants preincubated in liquid Murashige and Skoog (MS) medium. An analysis of rice leaf segments revealed that they neither induced vir genes nor inhibited vir gene induction. Of different parts of rice plants of different ages analysed only scutellum from four-day old rice seedlings induced vir genes and generation of T-strands. We observed that the physical presence of preincubated scutella is required for vir gene induction. Conditioned medium from which preincubated scutella were removed did not induce the vir genes. Scutellum-derived calli, cultured for 25 days on medium containing 2,4-D, also induced virE to an appreciable level. These results suggest that scutellum and scutellum-derived calli may be the most susceptible tissues of rice for Agrobacterium-mediated transformation.

Transfer of non-T-DNA portions of the Agrobacterium tumefaciens Ti plasmid pTiA6 from the left terminus of TL-DNA

We introduced a plant selection marker, nptII, to the left of border A in the Agrobacterium Ti plasmid pTiA6. Infection of tobacco leaf discs with the modified Agrobacterium strain gave rise to kanamycin-resistant calli which grew in a hormone-dependent manner. Southern hybridization analysis of DNA isolated from four transformants indicated initiation of DNA transfer at or near border A and absence of T-DNA sequences. These results demonstrate that DNA transfer events starting at a left border on a native Ti plasmid and moving away from the T-DNA region occur and that they can be detected by designing a suitable selection strategy.

Opines stimulate induction of the vir genes of the Agrobacterium tumefaciens Ti plasmid

Upon incubation of Agrobacterium tumefaciens A348 with acetosyringone, the vir genes encoded by the Ti (tumor-inducing) plasmid are induced. The addition of certain opines, including octopine, nopaline, leucinopine, and succinamopine, enhanced this induction 2- to 10-fold. The compounds mannopine, acetopine, arginine, pyruvate, and leucine did not stimulate the induction of the vir genes to such an extent. The enhancement of vir gene induction by opines depended on acetosyringone and the genes virA and virG. Opines stimulated the activity of the vir genes, the double-stranded cleavage of the T (transferred)-DNA at the border repeat sequences, and the production of T-strands by the bacterium. The transformation efficiency of cotton shoot tips was markedly increased by the addition of acetosyringone and nopaline at the time of infection.

Virulence genes, borders, and overdrive generate single-stranded T-DNA molecules from the A6 Ti plasmid of Agrobacterium tumefaciens

Agrobacterium tumefaciens transfers the T-DNA portion of its Ti plasmid to the nuclear genome of plant cells. Upon cocultivation of A. tumefaciens A348 with regenerating tobacco leaf protoplasts, six distinct single-stranded T-DNA molecules (T strands) were generated in addition to double-stranded T-DNA border cleavages which we have previously reported (K. Veluthambi, R.K. Jayaswal, and S.B. Gelvin, Proc. Natl. Acad. Sci. USA 84:1881-1885, 1987). The T region of an octopine-type Ti plasmid has four border repeats delimiting three T-DNA regions defined as T left (TL), T center (TC), and T right (TR). The six T strands generated upon induction corresponded to the TL, TC, TR, TL + TC, TC + TR, and TL + TC + TR regions, suggesting that the initiation and termination of T-strand synthesis can occur at each of the four borders. Most TL + TC + TR T-strand molecules corresponded to the top T-DNA strand, whereas the other five T strands corresponded to the bottom T-DNA strand. Generation of T strands required the virA, virG, and virD operons. Extra copies of vir genes, harbored on cosmids within derivatives of A. tumefaciens A348, enhanced production of T strands. The presence of right and left border repeats in their native orientation is important for the generation of full-length T strands. When a right border repeat was placed in the opposite orientation, single-stranded T-DNA molecules that corresponded to the top strand were generated. Deletion of overdrive, a sequence that flanks right border repeats and functions as a T-DNA transmission enhancer, reduced the level of T-strand generation. Induction of A. tumefaciens cells by regenerating tobacco protoplasts increased the copy number of the Ti plasmid relative to the bacterial chromosome.

Double-stranded cleavage of T-DNA and generation of single-stranded T-DNA molecules in Escherichia coli by a virD-encoded border-specific endonuclease from Agrobacterium tumefaciens

The virD locus of Agrobacterium tumefaciens Ti plasmid pTiA6 was sequenced. Computer analysis of the sequence indicated five possible open reading frames (ORFs) within this locus. Two additional ORFs were identified distal to this locus. However, only two polypeptides of apparent molecular masses 16 and 56 kilodaltons, the products of ORFs 1 and 2, were detected in Escherichia coli, both in vivo and in an in vitro coupled transcription-translation system. The virD locus was cloned in expression vector pKK223.3 under control of a tac promoter and introduced into an E. coli strain harboring mini-Ti plasmid pAL1050. When induced with isopropyl-beta-D-thiogalactopyranoside, the virD gene products exhibited double-stranded T-DNA border-specific endonuclease activity. Deletion analysis demonstrated that this activity is encoded within the 5'-proximal 1.7-kilobase-pair portion of the virD locus that carries ORF 1 and most of ORF 2. Neither ORF 1 nor ORF 2 independently showed endonuclease activity; complementation studies indicated that the products of ORFs 1 and 2 together have this activity. The expression of this 1.7-kilobase-pair region of the virD locus caused double-stranded cleavage of the T-DNA at or near the borders and generated single-stranded T-DNA molecules with approximately equal frequencies in E. coli.

In vitro and in vivo protein phosphorylation in Avena sativa L. coleoptiles: effects of Ca2+, calmodulin antagonists, and auxin

In vitro and in vivo protein phosphorylations in oat (Avena sativa L.) coleoptile segments were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis and by two-dimensional gel electrophoresis. In vitro phosphorylation of several polypeptides was distinctly promoted at 1 to 15 micromolar free Ca2+ concentrations. Ca2(+)-stimulated phosphorylation was markedly reduced by trifluoperazine, chlorpromazine, and naphthalene sulfonamide (W7). Two polypeptides were phosphorylated both under in vitro and in vivo conditions, but the patterns of phosphorylation of several other polypeptides were different under the two conditions indicating that the in vivo phosphorylation pattern of proteins is not truly reflected by in vitro phosphorylation studies. Trifluoperazine, W7, or ethylene glycol-bis-(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) + calcium ionophore A23187 treatments resulted in reduced levels of in vivo protein phosphorylation of both control and auxin-treated coleoptile segments. Analysis by two-dimensional electrophoresis following in vivo phosphorylation revealed auxin-dependent changes of certain polypeptides. A general inhibition of phosphorylation by calmodulin antagonists suggested that both control and auxin-treated coleoptiles exhibited Ca2+, and calmodulin-dependent protein phosphorylation in vivo.

Effect of Calcium on Tuberization in Potato (Solanum tuberosum L.)

Tuberization response of single-node leaf cuttings from induced potato plants (Solanum tuberosum L.) was reversed when pretreated with 5 millimolar ethyleneglycol-bis-(beta-aminoethyl ether)N,N'-tetraacetic acid (EGTA) + 50 micromolar calcium ionophore (A23187) and resumed when transferred to a CaCl(2)-containing medium. Tuberization was inhibited by LaCl(3), chlorpromazine, and trifluoperazine at 5 to 10 micromolar. These results suggest a role for calcium in the tuberization process.

Calcium- and calmodulin-regulated phosphorylation of soluble and membrane proteins from corn coleoptiles

In vitro phosphorylation of several membrane polypeptides and soluble polypeptides from corn (Zea mays var. Patriot) coleoptiles was promoted by adding Ca(2+). Ca(2+)-promoted phosphorylation of the membrane polypeptides was further increased in the presence of calmodulin. Both Ca(2+)-stimulated and Ca(2+)- and calmodulin-stimulated phosphorylations of membrane polypeptides were inhibited by chlorpromazine, a calmodulin antagonist. Ca(2+)-stimulated phosphorylation of soluble polypeptides increased with increasing Ca(2+) concentration. The calmodulin antagonists chlorpromazine and trifluoperazine inhibited the Ca(2+)-promoted phosphorylation of soluble polypeptides. Added calmodulin promoted the Ca(2+)-dependent phosphorylation of a 98 kilodaltons polypeptide. Both Ca(2+)-dependent and Ca(2+)-independent phosphorylations required Mg(2+) at an optimal concentration of 5 to 10 millimolar. Cyclic AMP was found to have no stimulatory effect on protein phosphorylation. Sodium molybdate, an inhibitor of protein phosphatase, increased the net phosphorylation of several polypeptides. Rapid loss of radioactivity from the phosphorylated polypeptides following incubation in unlabeled ATP indicated the presence of phosphoprotein phosphatase activity.

Threonine Synthase of Lemna paucicostata Hegelm. 6746

Threonine synthase (TS) was purified approximately 40-fold from Lemna paucicostata, and some of its properties determined by use of a sensitive and specific assay. During the course of its purification, TS was separated from cystathionine gamma-synthase, establishing the separate identity of these enzymes. Compared to cystathionine gamma-synthase, TS is relatively insensitive to irreversible inhibition by propargylglycine (both in vitro and in vivo) and to gabaculine, vinylglycine, or cysteine in vitro. TS is highly specific for O-phospho-l-homoserine (OPH) and water (hydroxyl ion). Nucleophilic attack by hydroxyl ion is restricted to carbon-3 of OPH and proceeds sterospecifically to form threonine rather than allo-threonine. The K(m) for OPH, determined at saturating S-adenosylmethionine (AdoMet), is 2.2 to 6.9 micromolar, two orders of magnitude less than values reported for TS from other plant tissues. AdoMet markedly stimulates the enzyme in a reversible and cooperative manner, consistent with its proposed role in regulation of methionine biosynthesis. Cysteine (1 millimolar) caused a slight (26%) reversible inhibition of the enzyme. Activities of TS isolated from Lemna were inversely related to the methionine nutrition of the plants. Down-regulation of TS by methionine may help to limit the overproduction of threonine that could result from allosteric stimulation of the enzyme by AdoMet.No evidence was obtained for feedback inhibition, repression, or covalent modification of TS by threonine and/or isoleucine.

Polyamine-stimulated phosphorylation of proteins from corn (Zea mays L.) coleoptiles

The effect of polyamines (spermine, spermidine and putrescine) on in vitro phosphorylation of proteins from corn coleoptiles was investigated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. Spermine promoted the phosphorylation of several membrane and soluble proteins and most of the proteins phosphorylated were different from those phosphorylated in the presence of calcium. Spermidine promoted the phosphorylation to a lesser extent and putrescine had very little stimulatory effect. Spermine-promoted phosphorylation of soluble proteins was dependent upon the presence of Mg2+ and was discernible at 100 microM spermine concentration.

Auxin-regulated polypeptide changes at different stages of strawberry fruit development

The pattern of polypeptides at different stages of strawberry (Fragaria ananassa Duch. cv Ozark Beauty) fruit development was studied by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. An 81,000-dalton polypeptide appeared between 5 and 10 days after pollination. Polypeptides with molecular weights of 76,000 and 37,000 daltons were formed after 10 days. The control exerted by auxin in the stage-specific formation of polypeptides was investigated by stopping fruit growth after removing the achenes and reinitiating fruit growth by the application of a synthetic auxin, alpha-naphthaleneacetic acid (NAA). When the achenes were removed from the 5- and 10-day-old fruits, the fruits failed to grow, the 81,000 dalton polypeptide was not formed between 5 and 10 days, and the 76,000- and 37,000-dalton polypeptides were not formed between 10 and 20 days. Application of NAA to fruits deprived of auxin by removal of achenes resulted in the resumption of growth and also in the appearance of these polypeptides. Removal of achenes of the 5- or 10-day-old fruits and growing them without auxin resulted in the formation of 52,000- and 57,000-dalton polypeptides. These two polypeptides were not formed when NAA was applied to fruits after removal of achenes. Supply of NAA to auxin-deprived fruits 5 days after removal of achenes resulted in resumption of growth and also in the disappearance of these two polypeptides, pointing out their possible relation to the inhibition of fruit growth.

Trehalose Toxicity in Cuscuta reflexa: Cell Wall Synthesis Is Inhibited upon Trehalose Feeding

alpha,alpha-Trehalose induced a rapid blackening of the terminal 2.5-centimeter region of excised Cuscuta reflexa Roxb. vine. The incorporation of radioactivity from [(14)C]glucose into alkali-insoluble fraction of shoot tip was markedly inhibited by 12 hours of trehalose feeding to an excised vine. This inhibition was confined to the apical segment of the vine in which cell elongation occurred. The rate of blackening of shoot tip explants was hastened by the addition of gibberellic acid A(3), which promoted elongation growth of isolated Cuscuta shoot tips. The symptom of trehalose toxicity was duplicated by 2-deoxyglucose, which has been shown to be a potent inhibitor of cell wall synthesis in yeast. The observations suggest that trehalose interferes with the synthesis of cell wall polysaccharides, the chief component of which was presumed to be cellulose.

Trehalose Toxicity in Cuscuta reflexa: SUCROSE CONTENT DECREASES IN SHOOT TIPS UPON TREHALOSE FEEDING

Trehalose, an alpha,alpha-diglucoside, induced a rapid blackening and death of shoot tips of Cuscuta reflexa (dodder) cultured in vitro. The onset of toxic symptom was delayed if any of the several sugars which support the in vitro growth of Cuscuta was supplied with trehalose. The rate of trehalose uptake or its accumulation in the tissue was not affected by sugar cofeeding. The levels of total and reducing sugars declined appreciably in the trehalose-fed shoot tip explants compared to control tissue cultured in absence of a carbon source. This was not due to an increased rate of respiration of the trehalose-treated tissue. In shoot tips cultured in presence of both trehalose and sucrose, the decline in total and reducing sugars was curtailed. There was a marked fall in the level of sucrose; and invertase activity was higher in trehalose-fed shoot tips. The incorporation of label from [(14)C]glucose into sucrose in the shoot tip explant was reduced as early as 12 h of trehalose feeding. The results suggest that increased utilization of sucrose as well as an inhibition of its synthesis contribute to the drastic fall in the sucrose content upon trehalose feeding.