Antisense oligodeoxynucleotide inhibition as a potent diagnostic tool for gene function in plant biology

Oligo-Not Only for Silencing: Overlooked Potential for Multidirectional Action in Plants

Oligo technology is a low-cost and easy-to-implement method for direct manipulation of gene activity. The major advantage of this method is that gene expression can be changed without requiring stable transformation. Oligo technology is mainly used for animal cells. However, the use of oligos in plants seems to be even easier. The oligo effect could be similar to that induced by endogenous miRNAs. In general, the action of exogenously introduced nucleic acids (Oligo) can be divided into a direct interaction with nucleic acids (genomic DNA, hnRNA, transcript) and an indirect interaction via the induction of processes regulating gene expression (at the transcriptional and translational levels) involving regulatory proteins using endogenous cellular mechanisms. Presumed mechanisms of oligonucleotides' action in plant cells (including differences from animal cells) are described in this review. Basic principles of oligo action in plants that allow bidirectional changes in gene activity and even those that lead to heritable epigenetic changes in gene expression are presented. The effect of oligos is related to the target sequence at which they are directed. This paper also compares different delivery methods and provides a quick guide to using IT tools to help design oligonucleotides.

Antisense oligonucleotide technology as a research tool in plant biology

An antisense oligonucleotide (ASO) is a short single-stranded deoxyribonucleotide complementary to the sense strand of a selected nucleic acid. As a result, an ASO can modulate gene expression through several mechanisms. The technology based on ASO has already been applied in studies on gene function in mammalian cells and selective therapeutic strategies for many diseases. The conceptual simplicity and low cost of this method, and the developments in the field of plant genome sequencing observed in the last decades, have paved the way for the ASO method also in plant biology. It is applied in gene function analysis as well as the development of non-invasive plant production technology involving gene modifications without transgenesis. Therefore, the first part of this review provides a comprehensive overview of the structure, mechanism of action and delivery methods of ASOs in plants and shows the most important features essential for the proper design of individual experiments. We also discuss potential issues and difficulties that may arise during practical ASO implementation. The second part of this article contains an analysis of ASO applications in various studies in the field of plant biology. We presented for the first time that ASOs were also successfully applied in cucumber.

A Dual-Promoter Gene Orchestrates the Sucrose-Coordinated Synthesis of Starch and Fructan in Barley

Sequential carbohydrate synthesis is important for plant survival because it guarantees energy supplies for growth and development during plant ontogeny and reproduction. Starch and fructan are two important carbohydrates in many flowering plants and in human diets. Understanding this coordinated starch and fructan synthesis and unraveling how plants allocate photosynthates and prioritize different carbohydrate synthesis for survival could lead to improvements to cereals in agriculture for the purposes of greater food security and production quality. Here, we report a system from a single gene in barley employing two alternative promoters, one intronic/exonic, to generate two sequence-overlapping but functionally opposing transcription factors, in sensing sucrose, potentially via sucrose/glucose/fructose/trehalose 6-phosphate signaling. The system employs an autoregulatory mechanism in perceiving a sucrose-controlled trans activity on one promoter and orchestrating the coordinated starch and fructan synthesis by competitive transcription factor binding on the other promoter. As a case in point for the physiological roles of the system, we have demonstrated that this multitasking system can be exploited in breeding barley with tailored amounts of fructan to produce healthy food ingredients. The identification of an intron/exon-spanning promoter in a hosting gene, resulting in proteins with distinct functions, adds to the complexity of plant genomes.

Oligodeoxynucleotides Can Transiently Up- and Downregulate CHS Gene Expression in Flax by Changing DNA Methylation in a Sequence-Specific Manner

Chalcone synthase (CHS) has been recognized as an essential enzyme in the phenylpropanoid biosynthesis pathway. Apart from the leading role in the production of phenolic compounds with many valuable biological activities beneficial to biomedicine, CHS is well appreciated in science. Genetic engineering greatly facilitates expanding knowledge on the function and genetics of CHS in plants. The CHS gene is one of the most intensively studied genes in flax. In our study, we investigated engineering of the CHS gene through genetic and epigenetic approaches. Considering the numerous restrictions concerning the application of genetically modified (GM) crops, the main purpose of this research was optimization of the plant's modulation via epigenetics. In our study, plants modified through two methods were compared: a widely popular agrotransformation and a relatively recent oligodeoxynucleotide (ODN) strategy. It was recently highlighted that the ODN technique can be a rapid and time-serving antecedent in quick analysis of gene function before taking vector-mediated transformation. In order to understand the molecular background of epigenetic variation in more detail and evaluate the use of ODNs as a tool for predictable and stable gene engineering, we concentrated on the integration of gene expression and gene-body methylation. The treatment of flax with a series of short oligonucleotides homologous to a different part of CHS gene isoforms revealed that those directed to regulatory gene regions (5'- and 3'-UTR) activated gene expression, directed to non-coding region (introns) caused gen activity reduction, while those homologous to a coding region may have a variable influence on its activity. Gene expression changes were accompanied by changes in its methylation status. However, only certain (CCGG) motifs along the gene sequence were affected. The analyzed DNA motifs of the CHS flax gene are more accessible for methylation when located within a CpG island. The methylation motifs also led to rearrangement of the nucleosome location. The obtained results suggest high specificity of ODN action and establish a potential valuable alternative for improvement of crops.

Efficient and specific inhibition of plant microRNA function by anti-microRNA oligonucleotides (AMOs) in vitro and in vivo

Anti-microRNA oligonucleotides (AMOs) are efficient and sequence-specific inhibitors of plant miRNA function both in vitro and in vivo. MicroRNAs (miRNAs) are small non-coding RNAs that play critical roles in developmental and physiological processes in plants and animals. Although miRNA knockdown by chemically modified antisense oligonucleotides prevails in animal and therapeutic studies, no such application has ever been reported in plants. Here, we show that sucrose-mediated delivery of 2'-O-methyl (2'-O-Me) anti-miRNA oligonucleotides (AMOs) is an efficient and sequence-specific way of inhibiting plant miRNA activity both in vitro and in vivo. Administration of AMOs to rice protoplasts and intact leaves resulted in efficient inhibition of miRNAs with concurrent de-repression of their target genes. AMOs caused simultaneous inhibition of miRNAs from the same family but exerted negligible effects on miRNAs from different families. In rice seedlings, a single-dose AMO treatment conferred long-lasting miRNA inhibition for at least 7 days. Although simultaneous dysregulation of multiple miRNAs by an AMO-and-miRNA-mimic mixture resulted in severe root defects, the phenotypic effects of individual AMOs and miRNA mimics were negligible, suggesting that those miRNAs function together in regulatory networks to ensure homeostasis. Our results validate the utility of AMOs as an efficient tool for plant miRNA loss-of-function studies in vivo, and this approach may prove to be a highly promising general method for unraveling miRNA-mediated gene-regulatory networks.

A selection strategy in plant transformation based on antisense oligodeoxynucleotide inhibition

Antisense oligodeoxynucleotide (asODN) inhibition was developed in the 1970s, and since then has been widely used in animal research. However, in plant biology, the method has had limited application because plant cell walls significantly block efficient uptake of asODN to plant cells. Recently, we have found that asODN uptake is enhanced in a sugar solution. The method has promise for many applications, such as a rapid alternative to time-consuming transgenic studies, and high potential for studying gene functionality in intact plants and multiple plant species, with particular advantages in evaluating the roles of multiple gene family members. Generation of transgenic plants relies on the ability to select transformed cells. This screening process is based on co-introduction of marker genes into the plant cell together with a gene of interest. Currently, the most common marker genes are those that confer antibiotic or herbicide resistance. The possibility that traits introduced by selectable marker genes in transgenic field crops may be transferred horizontally is of major public concern. Marker genes that increase use of antibiotics and herbicides may increase development of antibiotic-resistant bacterial strains or contribute to weed resistance. Here, we describe a method for selection of transformed plant cells based on asODN inhibition. The method enables selective and high-throughput screening for transformed cells without conferring new traits or functions to the transgenic plants. Due to their high binding specificity, asODNs may also find applications as plant-specific DNA herbicides.

Comprehensive analysis of tobacco pollen transcriptome unveils common pathways in polar cell expansion and underlying heterochronic shift during spermatogenesis

BACKGROUND

Many flowering plants produce bicellular pollen. The two cells of the pollen grain are destined for separate fates in the male gametophyte, which provides a unique opportunity to study genetic interactions that govern guided single-cell polar expansion of the growing pollen tube and the coordinated control of germ cell division and sperm cell fate specification. We applied the Agilent 44 K tobacco gene chip to conduct the first transcriptomic analysis of the tobacco male gametophyte. In addition, we performed a comparative study of the Arabidopsis root-hair trichoblast transcriptome to evaluate genetic factors and common pathways involved in polarized cell-tip expansion.

RESULTS

Progression of pollen grains from freshly dehisced anthers to pollen tubes 4 h after germination is accompanied with > 5,161 (14.9%) gametophyte-specific expressed probes active in at least one of the developmental stages. In contrast, > 18,821 (54.4%) probes were preferentially expressed in the sporophyte. Our comparative approach identified a subset of 104 pollen tube-expressed genes that overlap with root-hair trichoblasts. Reverse genetic analysis of selected candidates demonstrated that Cu/Zn superoxide dismutase 1 (CSD1), a WD-40 containing protein (BP130384), and Replication factor C1 (NtRFC1) are among the central regulators of pollen-tube tip growth. Extension of our analysis beyond the second haploid mitosis enabled identification of an opposing-dynamic accumulation of core regulators of cell proliferation and cell fate determinants in accordance with the progression of the germ cell cycle.

CONCLUSIONS

The current study provides a foundation to isolate conserved regulators of cell tip expansion and those that are unique for pollen tube growth to the female gametophyte. A transcriptomic data set is presented as a benchmark for future functional studies using developing pollen as a model. Our results demonstrated previously unknown functions of certain genes in pollen-tube tip growth. In addition, we highlighted the molecular dynamics of core cell-cycle regulators in the male gametophyte and postulated the first genetic model to account for the differential timing of spermatogenesis among angiosperms and its coordination with female gametogenesis.