T-DNA transfer to maize cells: histochemical investigation of beta-glucuronidase activity in maize tissues

Mapping Reaction-Diffusion Networks at the Plant Wound Site With Pathogens

The rich collection of microbes colonizing the plant root making up the rhizosphere function as a multigenomic organ for nutrient distribution. The extent to which its dynamic mutualistic cellular order depends on morphogenic signaling, while likely, remains unknown. We have shown that reaction-diffusion chemical networks constructed with model plant and bacterial metabolites can mimic processes ranging from oxidative burst kinetics to traveling waves and extracellular stationary state reaction-diffusion networks for spatiotemporal ordering of the rhizosphere. Plant parasites and pathogens can be limited by host attachment require dynamic informational networks and continue to provide insight into what controls the rhizosphere. Here we take advantage of Agrobacterium tumefaciens, a plant pathogen with a gated receptor that requires simultaneous perception of two plant metabolites. Genetic manipulations have created receptors allowing each metabolite concentration to be correlated with pathogen behavior. The development of the florescent strains used here provide initial maps of the reaction-diffusion dynamics existing in the rhizosphere, revealing significant differences in the signaling landscape of host and non-host plants before and after wounding, specifically highlighting networks that may inform rhizosphere organization.

T-DNA transfer and T-DNA integration efficiencies upon Arabidopsis thaliana root explant cocultivation and floral dip transformation

T-DNA transfer and integration frequencies during Agrobacterium-mediated root explant cocultivation and floral dip transformations of Arabidopsis thaliana were analyzed with and without selection for transformation-competent cells. Based on the presence or absence of CRE recombinase activity without or with the CRE T-DNA being integrated, transient expression versus stable transformation was differentiated. During root explant cocultivation, continuous light enhanced the number of plant cells competent for interaction with Agrobacterium and thus the number of transient gene expression events. However, in transformation competent plant cells, continuous light did not further enhance cotransfer or cointegration frequencies. Upon selection for root transformants expressing a first T-DNA, 43-69 % of these transformants showed cotransfer of another non-selected T-DNA in two different light regimes. However, integration of the non-selected cotransferred T-DNA occurred only in 19-46 % of these transformants, indicating that T-DNA integration in regenerating root cells limits the transformation frequencies. After floral dip transformation, transient T-DNA expression without integration could not be detected, while stable T-DNA transformation occurred in 0.5-1.3 % of the T1 seedlings. Upon selection for floral dip transformants with a first T-DNA, 8-34 % of the transformants showed cotransfer of the other non-selected T-DNA and in 93-100 % of them, the T-DNA was also integrated. Therefore, a productive interaction between the agrobacteria and the female gametophyte, rather than the T-DNA integration process, restricts the floral dip transformation frequencies.

The efficiency of Arabidopsis thaliana floral dip transformation is determined not only by the Agrobacterium strain used but also by the physiology and the ecotype of the dipped plant

To evaluate the chromosomal background of different Agrobacterium strains on floral dip transformation frequency, eight wild-type Agrobacterium strains, provided by Laboratorium voor Microbiologie Gent (LMG) and classified in different genomic groups, were compared with the commonly used Agrobacterium strains C58C1 Rif(r) (pMP90) and LBA4404 in Arabidopsis thaliana Columbia (Col-0) and C24 ecotypes. The C58C1 Rif(r) chromosomal background in combination with the pMP90 virulence plasmid showed high Col-0 floral dip transformation frequencies (0.76 to 1.57%). LMG201, which is genetically close to the Agrobacterium C58 strain, with the same virulence plasmid showed comparable or even higher transformation frequencies (1.22 to 2.28%), whereas the LBA4404 strain displayed reproducibly lower transformation frequencies (<0.2%). All other tested LMG Agrobacterium chromosomal backgrounds had transformation frequencies between those of the C58C1 Rif(r) (pMP90) and LBA4404 reference strains. None of the strains could transform the C24 ecotype with a frequency higher than 0.1%. Strikingly, all Arabidopsis Col-0 floral dip transformation experiments showed a high transformation variability from plant to plant (even more than 50-fold) within and across the performed biological repeats for all analyzed Agrobacterium strains. Therefore, the physiology of the plant and, probably, the availability of competent flowers to be transformed determine, to a large extent, floral dip transformation frequencies.

Enhancement of resistance to aphids by introducing the snowdrop lectin genegna into maize plants

In order to enhance the resistance to pests, transgenic maize (Zea mays L.) plants from elite inbred lines containing the gene encoding snowdrop lectin (Galanthus nivalis L. agglutinin; GNA) under control of a phloem-specific promoter were generated through the Agrobacterium tumefaciens-mediated method. The toxicity of GNA-expressing plants to aphids has also been studied. The independently derived plants were subjected to molecular analyses. Polymerase chain reaction (PCR) and Southern blot analyses confirmed that the gna gene was integrated into maize genome and inherited to the following generations. The typical Mendelian patterns of inheritance occurred in most cases. The level of GNA expression at 0.13%-0.28% of total soluble protein was observed in different transgenic plants. The progeny of nine GNA-expressing independent transformants that were derived separately from the elite inbred lines DH4866, DH9942, and 8902, were selected for examination of resistance to aphids. These plants synthesized GNA at levels above 0.22% total soluble protein, and enhanced resistance to aphids was demonstrated by exposing the plants to corn leaf aphid (Rhopalosiphum maidis Fitch) under greenhouse conditions. The nymph production was significantly reduced by 46.9% on GNA-expressing plants. Field evaluation of the transgenic plants supported the results from the inoculation trial. After a series of artificial self-crosses, some homozygous transgenic maize lines expressing GNA were obtained. In the present study, we have obtained new insect-resistant maize material for further breeding work.

Unwounded plants elicit Agrobacterium vir gene induction and T-DNA transfer: transformed plant cells produce opines yet are tumour free

Agrobacterium tumefaciens is well known to cause crown gall tumours at plant wound sites and to benefit from this plant association by obtaining nutrients called opines that are produced by these tumours. Tumourigenesis requires expression of the vir regulon in response to chemical signals that are thought to be released from wound sites. Here, we examine chemical interactions between A. tumefaciens and unwounded plants. To determine whether unwounded plants can release significant amounts of vir gene inducers, we constructed an A. tumefaciens strain carrying a PvirB-gfp fusion. This fusion was strongly induced by co-culture with tobacco seedlings that have been germinated without any intentional wounding. The release of phenolic vir gene inducers was confirmed by GC/MS analysis. We also constructed a strain containing the gfp reporter located on an artificial T-DNA and expressed from a plant promoter. A. tumefaciens efficiently transferred this T-DNA into cells of unwounded plants in the absence of exogenous vir gene inducers. Many cells of seedlings colonized by the bacteria also produced octopine, which was detected using a Pocc-gfp reporter strain. This indicates transfer of the native T-DNA. However, these transformed plant cells did not form tumours. These results suggest that successful colonization of plants by A. tumefaciens, including T-DNA transfer and opine production, does not require wounding and does not necessarily cause cell proliferation. Transformation of plant cells without inciting tumours may represent a colonization strategy for this pathogen that has largely been overlooked.

Agrobacterium-mediated plant transformation: the biology behind the "gene-jockeying" tool

Agrobacterium tumefaciens and related Agrobacterium species have been known as plant pathogens since the beginning of the 20th century. However, only in the past two decades has the ability of Agrobacterium to transfer DNA to plant cells been harnessed for the purposes of plant genetic engineering. Since the initial reports in the early 1980s using Agrobacterium to generate transgenic plants, scientists have attempted to improve this "natural genetic engineer" for biotechnology purposes. Some of these modifications have resulted in extending the host range of the bacterium to economically important crop species. However, in most instances, major improvements involved alterations in plant tissue culture transformation and regeneration conditions rather than manipulation of bacterial or host genes. Agrobacterium-mediated plant transformation is a highly complex and evolved process involving genetic determinants of both the bacterium and the host plant cell. In this article, I review some of the basic biology concerned with Agrobacterium-mediated genetic transformation. Knowledge of fundamental biological principles embracing both the host and the pathogen have been and will continue to be key to extending the utility of Agrobacterium for genetic engineering purposes.

VirD4-independent transformation by CloDF13 evidences an unknown factor required for the genetic colonization of plants via Agrobacterium

Agrobacterium uses a mechanism similar to conjugation for trans-kingdom transfer of its oncogenic T-DNA. A defined VirB/VirD4 Type IV secretion system is responsible for such a genetic transfer. In addition, certain virulence proteins as VirE2 can be mobilized into host cells by the same apparatus. VirE2 is essential to achieve plant but not yeast transformation. We found that the limited host range plasmid CloDF13 can be recruited by the virulence apparatus of Agrobacterium for transfer to eukaryotic hosts. As expected the VirB transport complex was required for such trans-kingdom DNA transfer. However, unexpectedly, the coupling factor VirD4 turned out to be necessary for transfer to plants but not for transport into yeast. The CloDF13 encoded coupling factor (Mob) was essential for transfer to both plants and yeast though. This is interpreted by the different specificities of Mob and VirD4. Hence, Mob being required for the transport of the CloDF13 transferred DNA (to both plants and yeast) and VirD4 being required for transport of virulence proteins such as VirE2. Nevertheless, the presence of the VirE2 protein in the host plant was not sufficient to restore the deficiency for VirD4 in the transforming bacteria. We propose that Mob functions encoded by the plasmid CloDF13 are sufficient for DNA mobilization to eukaryotic cells but that the VirD4-mediated pathway is essential to achieve DNA nuclear establishment specifically in plants. This suggests that other Agrobacterium virulence proteins besides VirE2 are translocated and essential for plant transformation.

Fluorescence microscopy: a powerful technique to detect low GUS activity in vascular tissues

We have previously shown that the Eucalyptus gunnii EgCAD2 promoter was preferentially expressed in vascular tissues in different transgenic plants (poplar, tobacco, Arabidopsis and grapevine). In order to delineate the cis elements governing this vascular expression pattern, promoter deletion analysis was performed allowing us to identify the proximal region [-340/-124] as essential for vascular cambium/xylem-specific expression. In plants transformed with the smallest promoter region [-124/+117], the GUS activity was difficult to detect using conventional bright field microscopy. To overcome this problem, we used fluorescence microscopy, enabling us to show that the [-124/+117] region contained cis-elements driving activity in phloem fibres but not in secondary xylem. The technical improvement of the histochemical detection of GUS activity using fluorescence microscopy enables accurate investigation of low GUS activity in phenol-rich tissues.

The subcellular localization of an unusual rice calmodulin isoform, OsCaM61, depends on its prenylation status

Calmodulin (CaM) is a small Ca2+-binding protein highly conserved in eukaryotes. We have reported previously a novel rice CaM-like protein (OsCaM61) which contains an N-terminal CaM domain and a C-terminal extension with a potential prenylation site. Here we report in vitro activity assays confirm OsCaM61 as a functional CaM. Using the green fluorescent protein (GFP) as a visual marker, we further studied the subcellular localization of OsCaM61 in stably transformed tobacco cells. The GFP-OsCaM61 fusion protein was membrane-associated whereas OsCaM61-GFP was mainly detected in the nucleoplasm. GFP-OsCaM61 was transported into the nucleoplasm upon a block in isoprenoid biosynthesis by mevinolin treatment of cells. These results indicate that the prenylated OsCaM61 molecules are mainly membrane-associated whereas its unprenylated counterparts are transported into the nucleoplasm. Thus, OsCaM61 may play functions in co-ordinating Ca2+ signaling with isoprenoid metabolism.

NtSET1, a member of a newly identified subgroup of plant SET-domain-containing proteins, is chromatin-associated and its ectopic overexpression inhibits tobacco plant growth

The SET- and chromo-domains are recognized as signature motifs for proteins that contribute to epigenetic control of gene expression through effects on the regional organization of chromatin structure. This paper reports the identification of a novel subgroup of SET-domain-containing proteins in tobacco and Arabidopsis, which show highest homologies with the Drosophila position-effect-variegation repressor protein SU(VAR)3-9 and the yeast centromer silencing protein CLR4. The tobacco SET-domain-containing protein (NtSET1) was fused to the green fluorescence protein (GFP) that serves as a visual marker for localization of the recombinant protein in living cells. Whereas control GFP protein alone was uniformly dispersed within the nucleus and cytoplasm, the NtSET1-GFP fusion protein showed a non-uniform localization to multiple nuclear regions in interphase tobacco TBY2 cells. During mitosis, the NtSET1-GFP associated with condensed chromosomes with a non-random distribution. The NtSET1 thus appears to have distinct target regions in the plant chromatin. Overexpression of the NtSET1-GFP in transgenic tobacco inhibited plant growth, implicating the possible involvement of the NtSET1 in transcriptional repression of growth control genes through the formation of higher-order chromatin domains.

At the maize/Agrobacterium interface: natural factors limiting host transformation

BACKGROUND

Agrobacterium tumefaciens has been successfully harnessed as the only natural vector for the incorporation of foreign genes into higher plants, but its use in the grain crops is often limited. Low transformation efficiency has been partly attributed to a failure in the initial events in the transformation process, specifically in the capacity of the VirA/VirG two-component system to induce expression of the virulence genes.

RESULTS

Here we show that the root exudate of Zea mays seedlings specifically inhibits virulence gene expression, determine that 2-hydroxy-4,7-dimethoxybenzoxazin-3-one (MDIBOA), which constitutes > 98% of the organic exudate of the roots of these seedlings, is the most potent and specific inhibitor of signal perception in A. tumefaciens-mediated gene transfer yet discovered, and develop a model that is able to predict the MDIBOA concentration at any distance from the root surface. Finally, variants of A. tumefaciens resistant to MDIBOA-mediated inhibition of vir gene expression have been selected and partially characterized.

CONCLUSIONS

These results suggest a strategy in which a plant may resist pathogen invasion by specifically blocking virulence gene activation and yet ensure that the 'resistance factor' does not accumulate to levels sufficient to impose toxicity and selection pressure on the pathogen. The data further establish that naturally occurring inhibitors directed against signal perception by the VirA/VirG two-component regulatory system can play an important role in host defense. Finally, selected variants resistant to specific MDIBOA inhibition may now be used to extend the transformation efficiency of maize and possibly other cereals.

AGROBACTERIUM AND PLANT GENES INVOLVED IN T-DNA TRANSFER AND INTEGRATION

The phytopathogenic bacterium Agrobacterium tumefaciens genetically transforms plants by transferring a portion of the resident Ti-plasmid, the T-DNA, to the plant. Accompanying the T-DNA into the plant cell is a number of virulence (Vir) proteins. These proteins may aid in T-DNA transfer, nuclear targeting, and integration into the plant genome. Other virulence proteins on the bacterial surface form a pilus through which the T-DNA and the transferred proteins may translocate. Although the roles of these virulence proteins within the bacterium are relatively well understood, less is known about their roles in the plant cell. In addition, the role of plant-encoded proteins in the transformation process is virtually unknown. In this article, I review what is currently known about the functions of virulence and plant proteins in several aspects of the Agrobacterium transformation process.

T-DNA transfer to maize plants

Agrobacterium-mediated transformation is the method of choice to engineer desirable genes into plants. Here we describe a protocol for demonstrating T-DNA transfer from Agrobacterium into the economically important graminaceous plant maize. Expression of the T-DNA-located GUS gene was observed with high efficiency on shoots of young maize seedlings after cocultivation with Agrobacterium.

Transfer and Integration of T-DNA without Cell Injury in the Host Plant

Agrobacterium colonizes plant cells via a gene transfer mechanism that results in plant tumorigenesis. Virulence (vir) genes are transcriptionally activated in the bacteria by plant metabolites released from the wound site. Hence, it is believed that agrobacteria use injuries to facilitate their entrance into the host plant and that the wounded state is required for plant cell competence for Agrobacterium-mediated gene delivery. However, our experiments using vir gene-activated bacteria sprayed onto tobacco plantlets demonstrated that cells in unwounded plants could also be efficiently transformed. The condition of the plant cells was monitored using [beta]-glucuronidase under the control of a wound-inducible promoter. Infection of leaf tissue is light dependent, and it is drastically reduced when abscisic acid is exogenously applied to the plant. Under these experimental conditions, stomatal opening seems to be used by Agrobacterium to circumvent the physical barrier of the cuticle. These results thus show that the proposed cellular responses evoked by wounding in higher plants are not essential for Agrobacterium-mediated transformation.

Genetic Transformation of Wheat Mediated by Agrobacterium tumefaciens

A rapid Agrobacterium tumefaciens-mediated transformation system for wheat was developed using freshly isolated immature embryos, precultured immature embryos, and embryogenic calli as explants. The explants were inoculated with a disarmed A. tumefaciens strain C58 (ABI) harboring the binary vector pMON18365 containing the [beta]-glucuronidase gene with an intron, and a selectable marker, the neomycin phosphotransferase II gene. Various factors were found to influence the transfer-DNA delivery efficiency, such as explant tissue and surfactants present in the inoculation medium. The inoculated immature embryos or embryogenic calli were selected on G418-containing media. Transgenic plants were regenerated from all three types of explants. The total time required from inoculation to the establishment of plants in soil was 2.5 to 3 months. So far, more than 100 transgenic events have been produced. Almost all transformants were morphologically normal. Stable integration, expression, and inheritance of the transgenes were confirmed by molecular and genetic analysis. One to five copies of the transgene were integrated into the wheat genome without rearrangement. Approximately 35% of the transgenic plants received a single copy of the transgenes based on Southern analysis of 26 events. Transgenes in T1 progeny segregated in a Mendelian fashion in most of the transgenic plants.

High efficiency transformation of maize (Zea mays L.) mediated by Agrobacterium tumefaciens

Transformants of maize inbred A188 were efficiently produced from immature embryos cocultivated with Agrobacterium tumefaciens that carried "super-binary" vectors. Frequencies of transformation (independent transgenic plants/embryos) were between 5% and 30%. Almost all transformants were normal in morphology, and more than 70% were fertile. Stable integration, expression, and inheritance of the transgenes were confirmed by molecular and genetic analysis. Between one and three copies of the transgenes were integrated with little rearrangement, and the boundaries of T-DNA were similar to those in transgenic dicotyledons and rice. F1 hybrids between A188 and five other inbreds were transformed at low frequencies.

Early transcription of Agrobacterium T-DNA genes in tobacco and maize

We developed a sensitive procedure to investigate the kinetics of transcription of an Agrobacterium tumefaciens transferred (T)-DNA-encoded beta-glucuronidase gusA (uidA) gene soon after infection of plant suspension culture cells. The procedure uses a reverse transcriptase-polymerase chain reaction and enables detection of gusA transcripts within 18 to 24 hr after cocultivation of the bacteria with either tobacco or maize cells. Detection of gusA transcripts depended absolutely on the intact virulence (vir) genes virB, virD1/virD2, and virD4 within the bacterium. Mutations in virC and virE resulted in delayed and highly attenuated expression of the gusA gene. A nonpolar transposon insertion into the C-terminal coding region of virD2 resulted in only slightly decreased production of gusA mRNA, although this insertion resulted in the loss of the nuclear localization sequence and the important omega region from VirD2 protein and rendered the bacterium avirulent. However, expression of gusA transcripts in tobacco infected by this virD2 mutant was more transient than in cells infected by a wild-type strain. Infection of tobacco cells with an Agrobacterium strain harboring a mutant virD2 allele from which the omega region had been deleted resulted in similar transient expression of gusA mRNA. These data indicate that the C-terminal nuclear localization signal of the VirD2 protein is not essential for nuclear uptake of T-DNA and further suggest that the omega domain of VirD2 may be required for efficient integration of T-DNA into the plant genome. The finding that the initial kinetics of gusA gene expression in maize cells are similar to those shown in infected tobacco cells but that the presence of gusA mRNA in maize is highly transient suggests that the block to maize transformation involves T-DNA integration and not T-DNA entry into the cell or nuclear targeting.

Early transcription of Agrobacterium T-DNA genes in tobacco and maize

We developed a sensitive procedure to investigate the kinetics of transcription of an Agrobacterium tumefaciens transferred (T)-DNA-encoded beta-glucuronidase gusA (uidA) gene soon after infection of plant suspension culture cells. The procedure uses a reverse transcriptase-polymerase chain reaction and enables detection of gusA transcripts within 18 to 24 hr after cocultivation of the bacteria with either tobacco or maize cells. Detection of gusA transcripts depended absolutely on the intact virulence (vir) genes virB, virD1/virD2, and virD4 within the bacterium. Mutations in virC and virE resulted in delayed and highly attenuated expression of the gusA gene. A nonpolar transposon insertion into the C-terminal coding region of virD2 resulted in only slightly decreased production of gusA mRNA, although this insertion resulted in the loss of the nuclear localization sequence and the important omega region from VirD2 protein and rendered the bacterium avirulent. However, expression of gusA transcripts in tobacco infected by this virD2 mutant was more transient than in cells infected by a wild-type strain. Infection of tobacco cells with an Agrobacterium strain harboring a mutant virD2 allele from which the omega region had been deleted resulted in similar transient expression of gusA mRNA. These data indicate that the C-terminal nuclear localization signal of the VirD2 protein is not essential for nuclear uptake of T-DNA and further suggest that the omega domain of VirD2 may be required for efficient integration of T-DNA into the plant genome. The finding that the initial kinetics of gusA gene expression in maize cells are similar to those shown in infected tobacco cells but that the presence of gusA mRNA in maize is highly transient suggests that the block to maize transformation involves T-DNA integration and not T-DNA entry into the cell or nuclear targeting.

Intracellular Agrobacterium can transfer DNA to the cell nucleus of the host plant

Agrobacterium tumefaciens is a Gram-negative, soil-borne bacterium responsible for the crown gall disease of plants. The galls result from genetic transformation of plant cells by the bacteria. Genes located on the transferred DNA (T-DNA), which is part of the large tumor-inducing (Ti) plasmid of Agrobacterium, are integrated into host plant chromosomes and expressed. This transfer requires virulence (vir) genes that map outside the T-DNA on the Ti plasmid and that encode a series of elaborate functions that appear similar to those of interbacterial plasmid transfer. It remains a major challenge to understand how T-DNA moves from Agrobacterium into the plant cell nucleus, in view of the complexity of obstacles presented by the eukaryotic host cell. Specific anchoring of bacteria to the outer surface of the plant cell seems to be an important prelude to the mobilization of the T-DNA/protein complex from the bacterial cell to the plant cell. However, the precise mode of infection is not clear, although a requirement of wounded cells has been documented. By using a microinjection approach, we show here that the process of T-DNA transfer from the bacteria to the eukaryotic nucleus can occur entirely inside the plant cell. Such transfer is absolutely dependent on induction of vir genes and a functional virB operon. Thus, A. tumefaciens can function as an intracellular infectious agent in plants.

Constitutive expression of the virulence genes improves the efficiency of plant transformation by Agrobacterium

Inducible virulence (vir) genes of the Agrobacterium tumefaciens tumor-inducing (Ti) plasmid are under control of a two-component regulatory system. In response to environmental factors (phenolic compounds, sugars, pH) VirA protein phosphorylates VirG, which in turn interacts with the promoters of other vir genes, causing induction. A mutation of virG, virGN54D (which codes for a Asn-54-->Asp amino acid change in the product), causes constitutive expression of other vir genes independent of virA. We have investigated whether providing Agrobacterium with a plasmid containing virGN54D augments the efficiency of transfer of the T-DNA (transferred DNA). For both tobacco and cotton, we observed an enhancement of transformation efficiency when the inciting Agrobacterium strain carries the virGN54D mutation. We also tested whether supplying Agrobacterium with a similar plasmid containing wild-type virG affects the efficiency of T-DNA transfer. An intermediate efficiency was observed when this plasmid was employed. Using a beta-glucuronidase (GUS) reporter gene to assess transient expression of T-DNA after transfer to tobacco and maize tissues, we observed a higher frequency of GUS-expressing foci after inoculation with Agrobacterium strains carrying virGN54D than with Agrobacterium carrying the wild-type virG. Gene-transfer efficiency to maize by an octopine strain was greatly improved upon introduction of virGN54D. Multiple copies of wild-type virG were equally effective in promoting transient expression efficiency in tobacco but were virtually ineffective in maize. We propose the use of virGN54D to improve the efficiency of Agrobacterium-mediated transformation, especially for recalcitrant plant species.