Ludmila Mlynárová

Reduced Position Effect in Mature Transgenic Plants Conferred by the Chicken Lysozyme Matrix-Associated Region

Matrix-associated regions may be useful for studying the role of chromatin architecture in transgene activity of transformed plants. The chicken lysozyme A element was shown to have specific affinity for tobacco nuclear matrices, and its influence on the variability of transgene expression in tobacco plants was studied. T-DNA constructs in which this element flanked either the [beta]-glucuronidase (GUS) reporter gene or both reporter and selection gene were introduced in tobacco. The variation in GUS gene activity was reduced significantly among mature first-generation transgenic plants carrying the A element. Average GUS activity became somewhat higher, but the maximum attainable level of gene expression was similar for all three constructs. Transient gene expression assays showed that the A element did not contain general enhancer functions; therefore, its presence seemed to prevent the lower levels of transgene expression. The strongest reduction in variability was found in plants transformed with the construct carrying the A elements at the borders of the T-DNA. In this population, expression levels became copy number dependent. The presence of two A elements in the T-DNA did not interfere with meiosis.

The MAR-Mediated Reduction in Position Effect Can Be Uncoupled from Copy Number-Dependent Expression in Transgenic Plants

To study the role of matrix-associated regions (MARs) in establishing independent chromatin domains in plants, two transgenes were cloned between chicken lysozyme A elements. These transgenes were the neomycin phosphotransferase (NPTII) gene under control of the nopaline synthase (nos) promoter and the [beta]-glucuronidase (GUS) gene controlled by the double cauliflower mosaic virus (dCaMV) 35S promoter. The A elements are supposed to establish an artificial chromatin domain upon integration into the plant DNA, resulting in an independent unit of transcriptional regulation. Such a domain is thought to be characterized by a correlated and position-independent, hence copy number-dependent, expression of the genes within the domain. The presence of MARs resulted in a higher relative transformation efficiency, demonstrating MAR influence on NPTII gene expression. However, variation in NPTII gene expression was not significantly reduced. The selection bias for NPTII gene expression during transformation could not fully account for the lack of reduction in variation of NPTII gene expression. Topological interactions between the promoter and A element may interfere with the A element as a domain boundary. In contrast, the GUS gene on the same putative chromatin domain showed a highly significant reduction in variation of gene expression, as expected from previous results. Surprisingly, no copy number-dependent GUS gene expression was observed: all plants showed approximately the same GUS activity. We concluded, therefore, that dCaMV 35S-GUS gene expression in mature tobacco plants is regulated by some form of dosage compensation.

Approaching the Lower Limits of Transgene Variability.

The inclusion of chicken lysozyme matrix-associated regions (MARs) in T-DNA has been demonstrated to reduce the variation in [beta]-glucuronidase (GUS) gene expression among first-generation transformed plants. The residual variation observed between transgenic plant lines with MARs at the T-DNA borders was investigated. By definition, any phenotypic variance between or within genetically identical plants is caused by random or environmental variation. This variation therefore sets a lower limit to the variation in GUS activities. The variance of GUS activity in offspring plant populations of genetically identical individuals was used as an estimate of environmental variation. For transgenic plants with MARs at the T-DNA borders, the variation between independent transformants could not be distinguished from the environmental variation. The variation could be attributed mainly to the variation in the GUS activity measurement. Therefore, the MAR element approached the maximal possible reduction of transgene variability given current technology and sample sizes. The role of MARs in offspring plants was evaluated by comparing such populations of transgenic plants for the magnitude of and variation in GUS activity. Pairwise comparisons showed that the presence of MARs reduced variation in offspring generations in the same manner as demonstrated for primary transformants. The populations carrying a doubled cauliflower mosaic virus 35S promoter-GUS gene tended to be more variable than the Lhca3.St.1 promoter-GUS gene-carrying populations. This tendency indicated an intrinsic susceptibility of the doubled cauliflower mosaic virus 35S promoter to variation. Homozygous plants were approximately twice as active as the corresponding hemizygous plants and tended to be more variable than the hemizygous plants. We hypothesized that the magnitude of environmental variations is related to a higher susceptibility to transgene silencing.

The Presence of a Chromatin Boundary Appears to Shield a Transgene in Tobacco from RNA Silencing

We present isogenic transgenic tobacco lines that carry at a given chromosomal position a β-glucuronidase (GUS) reporter gene either with or without the presence of the matrix-associated region known as the chicken lysozyme A element. Plants were generated with the Cre-lox site–specific recombination system using heterospecific lox sites. Analysis of GUS gene expression in plant populations demonstrates that the presence of the A element can shield against RNA silencing of the GUS gene. Protection was observed in two of three independent tobacco transformants. Plants carrying an A element 5′ of the GUS gene always had stable GUS activity, but upon removal of this A element, the GUS gene became silenced over time in two lines, notably when homozygous.

A self-excising Cre recombinase allows efficient recombination of multiple ectopic heterospecific lox sites in transgenic tobacco

To study the impact of different DNA configurations on the stability of transgene expression, a variant of the cre gene was developed. This variant allows for the highly efficient in planta removal of its own loxP-flanked coding sequence as well as other DNAs flanked by ectopic heterospecific lox sites, either lox511 or lox2272 or both, in trans. The plant intron-containing cre gene, creINT, was configured in such a way that self-excision generated an intact hygromycin resistance selectable marker gene. In this combination, all selected transformants showed highly efficient excision. Plants obtained showed no indication of any chimerism, indicating a cell autonomous nature of the hygromycin selection during transformation and regeneration. The highly efficient concomitant removal of wildtype and heterospecific lox site-flanked DNA demonstrated that upon retransformation with the self-excising creINT, sufficient amounts of Cre enzyme were produced prior to its removal. Plants obtained with creINT showed much less frequently the Cre-associated phenomenon of reduced fertility than plants obtained with a continuous presence of Cre recombinase. The creINT system has therefore advantages over systems with a continuously present Cre. The creINT system was successfully used for removal of two chromatin boundary elements from transgene cassettes in tobacco. Analysis of plants with and without boundary elements on the same chromosomal location will contribute to a better evaluation of the role of such elements in the regulation of transgene expression in plants.

Tentacles of in vitro-grown round-leaf sundew (Drosera rotundifolia L.) show induction of chitinase activity upon mimicking the presence of prey

Induction of plant-derived chitinases in the leaves of a carnivorous plant was demonstrated using aseptically grown round-leaf sundew (Drosera rotundifolia L.). The presence of insect prey was mimicked by placing the chemical inducers gelatine, salicylic acid and crustacean chitin on leaves. In addition, mechanical stirring of tentacles was performed. Chitinase activity was markedly increased in leaf exudates upon application of notably chitin. Application of gelatine increased the proteolytic activity of leaf exudates, indicating that the reaction of sundew leaves depends on the molecular nature of the inducer applied. In situ hybridization of sundew leaves with a Drosera chitinase probe showed chitinase gene expression in different cell types of non-treated leaves, but not in the secretory cells of the glandular heads. Upon induction, chitinase mRNA was also present in the secretory cells of the sundew leaf. The combined results indicate that chitinase is likely to be involved in the decomposition of insect prey by carnivorous plants. This adds a novel role to the already broad function of chitinases in the plant kingdom and may contribute to our understanding of the molecular mechanisms behind the ecological success of carnivorous plants in nutritionally poor environments.

Errors in genomics and proteomics

To the editor: Genetic flow between transgenic and native maize has apparently occurred in Mexico1, resulting in wild strains containing one or more transgenic sequences (most likely encoding Bt toxin). These “transgenic” native maizes not only have every single trait that has been selected and preserved for thousands of years (making them perfectly adapted to specific geographic regions), but now also possess an additional and desirable characteristic— insect resistance, a trait likely to be consciously preferred by Mexican peasant farmers. Diversity will not be affected. On the contrary, we can predict that this useful transgene will be found in increasing numbers and types of native maizes. We believe it is important to stress this is not genetic contamination! Contamination means unexpected, undesirable, and uncontrollable spread; that is not happening. The spread will be induced because of the advantage of having a native corn with resistance to insects. Maize is so dependent on human intervention that it cannot survive in the wild. Maize seeds are attached to a cob and cannot free themselves: it absolutely requires human intervention. As maize was first domesticated more than 6,000 years ago, only genes and alleles that are important for humans have been selected and preserved. Still, if someone wants to remove the transgene from these plants, the procedure would be simple: select and multiply those susceptible maizes and do not harvest and multiply the insect-resistant ones. That is something no Mexican farmer will do. Teosintes, ancestors and close relatives of corn, do not seem to be affected by genetic flow from (any) maize. Teosintes growing naturally in cornfields yield a very poor hybrid progeny. They do not release their seeds, and therefore the probability is very low for natural genetic introgression (incorporation of a gene or allele in a population) into teosintes. We also have found that teosintes are highly susceptible to insects and pathogens when growing under more intensive experimental field conditions, but they appear to be resistant to them when growing naturally in the wild. Thus we conclude that even if the Bt transgene could be introgressed into teosintes, it will provide no biological advantage and thus would be lost by natural evolution. To reiterate2, there is no need for concern. Juan Pablo Ricardo Martínez-Soriano and Ana María Bailey, Unidad de Biotecnología e Ingeniería Genética de Plantas, Centro de Investigación y Estudios Avanzados del Instituto Politécnico Nacional, Apartado Postal 629, 36500 Irapuato, Gto, México (jpms@ira.cinvestav.mx) Joel Lara-Reyna,Instituto de Fitosanidad,Colegio de Postgraduados,Montecillo, Edo de México, México, and Diana Sara Leal-Klevezas Secretaría de Salud de Guanajuato, Tamazuca 4 Centro, Guanajuato, Gto., México

The promiscuity of heterospecific lox sites increases dramatically in the presence of palindromic DNA

Heterospecific lox sites are mutated lox sites that in the presence of Cre recombinase recombine with themselves but not or much less with wildtype loxP. We here show that in Escherichia coli both lox511 and lox2272 sites become highly promiscuous with respect to loxP when in the presence of Cre one of the recombination partners is present in a larger stretch of an inverted repeat of non-lox DNA. In such a palindromic DNA configuration, also the occurrence of other DNA repeat-mediated recombination events is somewhat increased in the presence of Cre. The results indicate that in recombinase mediated cassette exchange or other double lox applications based on the exclusivity of heterospecific lox sites, or in research combining Cre-lox approaches with hairpin RNA for gene silencing, the presence of duplicated DNA around lox sites has to be taken into account. It is proposed that the presence of palindromic non-lox DNA interferes with the homology search of the Cre enzyme prior to the actual recombination event.

Activity of the promoter of the Lhca3.St.1 gene, encoding the potato apoprotein 2 of the light-harvesting complex of photosystem I, in transgenic potato and tobacco plants

We have isolated cDNA and genomic clones for the potato (Solanum tuberosum) apoprotein 2 of the light harvesting complex of Photosystem I, designated Lhca3.St.1. The protein shows all characteristics of the family of chlorophyll a/b-binding proteins. Potato Lhca3.1 gene expression occurs predominantly in leaves, and is transcriptionally regulated by light. One gene copy is present per haploid genome. The sequence of the 5′ upstream region was determined. Most boxes identified in the promoter sequences of genes whose expression is light-regulated recur in the Lhca3.St.1 sequence. Functional analyses of the Lhca3.St.1 promoter and two deletion derivatives in transgenic potato transformed with a promoter-GUS fusion show high promoter activity in leaves and other green parts of the plant, which depends on light. Activity is absent in roots and potato tubers. The 500 bp promoter fragment is as active as the full 2.0 kb sequence, showing that all regulatory elements are present on the smallest deletion derivative. In transgenic tobacco (Nicotiana tabacum) plants carrying the largest promoter derivative a similar distribution of activity is found. Promoter activity is not restricted to the phloem, but also prominent in the xylem of the young stem, which contrasts with promoters of other photosynthesis-associated genes.

Expression of a cucumber class III chitinase and Nicotiana plumbaginifolia class I glucanase genes in transgenic potato plants

The genes encoding for a cucumber class III chitinase and Nicotianaplumbaginifolia class I glucanase were co-introduced into Slovak potato (Solanum tuberosum L.) breeding line 116/86 using Agrobacterium tumefaciens. For both transgenes the number of integrated copies and level of RNA expression were determined. These analyses demonstrated low variation and significant correlation in expression of the introduced transgenes. The effect of transgene expression on fungal susceptibility of transformants was evaluated in vitro. Hyphal extension assays revealed no obvious differences in the ability of extracts from transformants to inhibit growth of Rhizoctonia solani comparing to non-transformed potato.