Horticultural characterization of Angelonia salicariifolia plants transformed with wild-type strains of Agrobacterium rhizogenes

Kalanchoë blossfeldiana naturally transformed with Rhizobium rhizogenes exhibits superior root phenotype

Plant transformation with root oncogenic loci (rol) genes and open reading frames (ORFs) from Rhizobium rhizogenes have not yet targeted the underground root phenotype of these transformants. Hence, there is a need to develop plants with more efficient root system architecture (RSA). Here, RSA was assessed in naturally transformed (NT) and single rol/ORF Kalanchoë blossfeldiana 'Molly' lines in an aeroponic growth system combined with gene expression analysis. Three NT lines; 306, 324 and 331; exhibited better-developed RSA with longer roots and increased root biomass. In line 306, longest root was 6.3 ± 0.3 cm while WT had 4.8 ± 0.1 cm. However, root length of all overexpressing lines was ca. 30% shorter than WT. Root fresh weight of NT lines was 4.5-fold higher than WT. The expression of rolB, ∆ORF13a and ORF14 in the leaves of overexpressing lines was many folds higher than in NT lines. Increased expression of ∆ORF13a and ORF14 in leaves and roots may contribute more to a stronger compact phenotype than previously assumed. The moderate compact phenotype of NT lines combined with improved RSA compared to the overexpressing lines and WT strongly indicate that the use of R. rhizogenes has great potential to produce Kalanchoë phenotypes with enhanced RSA.

Induction of dwarf and early flowering phenotypes in Tricyrtis sp. by ectopic expression of LEAFY from Arabidopsis thaliana

LEAFY (LFY), which encodes a plant-specific transcription factor, plays an important role in the transition from vegetative to reproductive development. Ectopic expression of LFY has been reported to induce dwarfism and early flowering in some model plants. In order to examine the possibility of using LFY for molecular breeding of ornamental plants, we produced and characterized transgenic plants ectopically expressing LFY from Arabidopsis thaliana (AtLFY) in the liliaceous ornamental plant Tricyrtis sp. Nine independent transgenic plants have been obtained, and all of them exhibited dwarf phenotypes compared with the vector control. These transgenic plants could be classified into three types according to the degree of dwarfism: one showed an extreamly dwarf phenotype with smaller leaves (Type I); two showed moderately dwarf phenotypes (Type II); and six showed slightly dwarf phenotypes (Type III). All of Type I, Type II and Type III transgenic plants produced flower buds 1-3 weeks earlier than the vector control. Vector control and Type III transgenic plants produced 1-4 apical flower buds, whereas Type I and Type II transgenic plants produced only a single apical flower bud. Type I and Type II transgenic plants often produced non-fully-opened flowers. Quantitative real-time reverse transcription-polymerase chain reaction analysis showed that the AtLFY expression level generally correlated with the degree of dwarfism. These results indicate that morphological alterations observed in the transgenic plants was induced by ectopic expression of AtLFY. Lower levels of ectopic expression of LFY may be valuable for producing dwarf and early flowering ornamental plants.

Rhizogenic agrobacteria as an innovative tool for plant breeding: current achievements and limitations

Compact plant growth is an economically important trait for many crops. In practice, compactness is frequently obtained by applying chemical plant growth regulators. In view of sustainable and environmental-friendly plant production, the search for viable alternatives is a priority for breeders. Co-cultivation and natural transformation using rhizogenic agrobacteria result in morphological alterations which together compose the Ri phenotype. This phenotype is known to exhibit a more compact plant habit, besides other features. In this review, we highlight the use of rhizogenic agrobacteria and the Ri phenotype with regard to sustainable plant production and plant breeding. An overview of described Ri lines and current breeding applications is presented. The potential of Ri lines as pre-breeding material is discussed from both a practical and legal point of view.

Increasing genetic variability in oilseed rape (Brassica napus) - Genotypes and phenotypes of oilseed rape transformed by wild type Agrobacterium rhizogenes

Brassica napus (oilseed rape) is a major oil crop worldwide. Due to the short domestication period of oilseed rape the genetic variability is limited compared to other crops. Transfer of rol and aux genes from Agrobacterium rhizogenes is used in horticulture to increase genetic variability. In the current study, we explore transformation by A. rhizogenes as a biotechnological approach in breeding for more branched and shorter oilseed rape. In the 2nd generation of transformed oilseed rape, branch numbers increased significantly by 49% from 7.7 ± 0.4 to 11.5 ± 1.9 when comparing rol+/aux+ plants with WT. Simultaneously, the apical height of plants was reduced by 25% from 81.3 ± 1.9 cm to 62.4 ± 6.7 cm in rol+/aux+ plants at the onset of flowering. Reproductive parameters affecting yield as seed size and number were negatively affected in rol+/aux+ plants. Interestingly, oil composition was changed in rol+/aux+ seeds. Oleic acid (ω9) contents were reduced by more than 3% whereas α-linolenic acid (ω6) increased by more than 25% in mature seeds. To obtain shorter and more branched breeding material of oilseed rape we suggest crossing plants with the rol+/aux+ genotype back into the parental breeding line. This could reduce the negative impact of rol+/aux+ on yield.

Signal transduction and regulation of IbpreproHypSys in sweet potato

Hydroxyproline-rich glycopeptides (HypSys) are small signalling peptides containing 18-20 amino acids. The expression of IbpreproHypSys, encoding the precursor of IbHypSys, was induced in sweet potato (Ipomoea batatas cv. Tainung 57) through wounding and IbHypSys treatments by using jasmonate and H2 O2 . Transgenic sweet potatoes overexpressing (OE) and silencing [RNA interference (RNAi)] IbpreproHypSys were created. The expression of the wound-inducible gene for ipomoelin (IPO) in the local and systemic leaves of OE plants was stronger than the expression in wild-type (WT) and RNAi plants after wounding. Furthermore, grafting experiments indicated that IPO expression was considerably higher in WT stocks receiving wounding signals from OE than from RNAi scions. However, wounding WT scions highly induced IPO expression in OE stocks. These results indicated that IbpreproHypSys expression contributed towards sending and receiving the systemic signals that induced IPO expression. Analysing the genes involved in the phenylpropanoid pathway demonstrated that lignin biosynthesis was activated after synthetic IbHypSys treatment. IbpreproHypSys expression in sweet potato suppressed Spodoptera litura growth. In conclusion, wounding induced the expression of IbpreproHypSys, whose protein product was processed into IbHypSys. IbHypSys stimulated IbpreproHypSys and IPO expression and enhanced lignin biosynthesis, thus protecting plants from insects.

Suppression of B function strongly supports the modified ABCE model in Tricyrtis sp. (Liliaceae)

B class MADS-box genes play important roles in petal and stamen development. Some monocotyledonous species, including liliaceous ones, produce flowers with petaloid tepals in whorls 1 and 2. A modified ABCE model has been proposed to explain the molecular mechanism of development of two-layered petaloid tepals. However, direct evidence for this modified ABCE model has not been reported to date. To clarify the molecular mechanism determining the organ identity of two-layered petaloid tepals, we used chimeric repressor gene-silencing technology (CRES-T) to examine the suppression of B function in the liliaceous ornamental Tricyrtis sp. Transgenic plants with suppressed B class genes produced sepaloid tepals in whorls 1 and 2 instead of the petaloid tepals as expected. In addition, the stamens of transgenic plants converted into pistil-like organs with ovule- and stigma-like structures. This report is the first to describe the successful suppression of B function in monocotyledonous species with two-layered petaloid tepals, and the results strongly support the modified ABCE model.

Overexpression of the gibberellin 2-oxidase gene from Torenia fournieri induces dwarf phenotypes in the liliaceous monocotyledon Tricyrtis sp

Gibberellins (GAs) are the plant hormones that control many aspects of plant growth and development, including stem elongation. Genes encoding enzymes related to the GA biosynthetic and metabolic pathway have been isolated and characterized in many plant species. Gibberellin 2-oxidase (GA2ox) catalyzes bioactive GAs or their immediate precursors to inactive forms; therefore, playing a direct role in determining the levels of bioactive GAs. In the present study, we produced transgenic plants of the liliaceous monocotyledon Tricyrtis sp. overexpressing the GA2ox gene from the linderniaceous dicotyledon Torenia fournieri (TfGA2ox2). All six transgenic plants exhibited dwarf phenotypes, and they could be classified into two classes according to the degree of dwarfism: three plants were moderately dwarf and three were severely dwarf. All of the transgenic plants had small or no flowers, and smaller, rounder and darker green leaves. Quantitative real-time reverse transcription-polymerase chain reaction (PCR) analysis showed that the TfGA2ox2 expression level generally correlated with the degree of dwarfism. The endogenous levels of bioactive GAs, GA1 and GA4, largely decreased in transgenic plants as shown by liquid chromatography-mass spectrometry (LC-MS) analysis, and the level also correlated with the degree of dwarfism. Exogenous treatment of transgenic plants with gibberellic acid (GA3) resulted in an increased shoot length, indicating that the GA signaling pathway might normally function in transgenic plants. Thus, morphological changes in transgenic plants may result from a decrease in the endogenous levels of bioactive GAs. Finally, a possibility of molecular breeding for plant form alteration in liliaceous ornamental plants by genetically engineering the GA metabolic pathway is discussed.

Horizontal gene transfer from genus agrobacterium to the plant linaria in nature

Genes can be transferred horizontally between prokaryotes and eukaryotes in nature. The best-studied examples occur between Agrobacterium rhizogenes and certain Nicotiana spp. To investigate possible additional cases of horizontal gene transfer in nature between Agrobacterium and plants, a real-time polymerase chain reaction-based approach was employed to screen 127 plant species, belonging to 38 families of Dicotyledones, for the presence of oncogenes homologous to the transfer DNA fragments (T-DNA) from both A. tumefaciens and A. rhizogenes. Among all of the analyzed plant species, we found that only Linaria vulgaris contained sequences homologous to the T-DNA of A. rhizogenes. All screened L. vulgaris plants from various parts of Russia contained the same homologous sequences, including rolB, rolC, ORF13, ORF14, and mis genes. The same opine gene is found in the species of Nicotiana which contain genes of A. rhizogenes. In L. vulgaris, there are two copies of T-DNA organized as a single tandem imperfect direct repeat. The plant DNA sequence of the site of integration shows similarity to a retrotransposon. This site is most likely silent, suggesting that the T-DNA is not expressed. Attempts to demonstrate expression of the T-DNA genes were negative. Our study indicates that the frequency of gene transfer and fixation in the germline from Agrobacterium to plant hosts is rare in the natural environment.

Genetic transformation of Bacopa monnieri by wild type strains of Agrobacterium rhizogenes stimulates production of bacopa saponins in transformed calli and plants

We have developed an efficient transformation system for Bacopa monnieri, an important Indian medicinal plant, using Agrobacterium rhizogenes strains LBA 9402 and A4. Transformed roots induced by strain LBA 9402 spontaneously dedifferentiated to callus while excised roots induced by strain A4 spontaneously showed induction of shoot buds within 10 days. PCR and RT-PCR analysis revealed the presence and expression of the rolAB and rolC genes at the transcription level in pRi A4 transformed cultures indicating that the TL-DNA was integrated retained and expressed in the A4-Ri transformed shoots. Transformed calli showed the presence of rolAB or rol A, TR and ags genes. Transformed plants showed morphological features typically seen in transgenic plants produced by A. rhizogenes. Growth and biomass accumulation was significantly higher in the transformed shoots (twofold) and roots (fourfold) than in the non-transformed (WT) plants. In pRi A4-transformed plants, the content of bacopasaponin D, bacopasaponin F, bacopaside II and bacopaside V was enhanced significantly as compared to WT plants of similar age while bacoside A3 and bacopasaponin C content was comparable with that of WT plants. Significant increase in content of five bacopa saponins could be detected in pRi 9402-transformed callus cultures. There is an overall stimulatory effect on accumulation of bacopa saponins in transformed plants and cells of B. monnieri establishing the role of endogenous elicitation by Ri T-DNA of A. rhizogenes.

Transformation of Kalanchoe blossfeldiana with rol-genes is useful in molecular breeding towards compact growth

Dwarf genotypes of the economically important flowering potted plant Kalanchoe blossfeldiana were developed by molecular breeding. Root inducing (Ri)-lines were regenerated by applying CPPU to the hairy roots, which were produced by inoculating leaf explants with a wild-type Agrobacterium rhizogenes strain ATCC15834. Amplification by polymerase chain reaction (PCR) and Southern blot analysis confirmed the presence of T-DNA in the Ri-lines. Six Ri-lines were characterised in a greenhouse trial revealing that several morphological traits changed with respect to ornamental value such as plant height, number of lateral shoots, leaf size, leaf number, flower size and number of flowers. The Ri-lines differed in their degree of Ri-phenotype, and the internodes of the Ri-lines were clearly shorter, giving a compact growth habit compared to control plants. Time to anthesis was the same in Ri-line 331 as in control plants and delayed by only 3 days in Ri-line 306 as compared to control plants. A compact plant without delayed flowering can be assumed to be valuable for further breeding.

Influence of rol genes in floriculture

Traditionally, new traits have been introduced into ornamental plants through classical breeding. However, genetic engineering now enables specific alterations of single traits in already successful varieties. New or improved varieties of floricultural crops can be obtained by acting on floral traits, such as color, shape or fragrance, on vase life in cut-flower species, and on rooting potential or overall plant morphology. Overexpression of the rol genes of the Ri plasmid of Agrobacterium rhizogenes in plants alters several of the plant's developmental processes and affects their architecture. Both A. rhizogenes- and rol-transgenic plants display the "hairy-root phenotype", although specific differences are found between species and between transgenic lines. In general, these plants show a dwarfed phenotype, reduced apical dominance, smaller, wrinkled leaves, increased rooting, altered flowering and reduced fertility. Among the rol genes, termed rolA, B, C and D, rolC has been the most widely studied because its effects are the most advantageous in terms of improving ornamental and horticultural traits. In addition to the dwarfness and the increase in lateral shoots that lead to a bushy phenotype, rolC-plants display more, smaller flowers, and advanced flowering; surprisingly, these plants may have better rooting capacity and they show almost no undesirable traits. rolD, the least studied among the rol genes, offers promising applications due to its promotion of flowering. Although the biochemical functions of rol genes remain poorly understood, they are useful tools for improving ornamental flowers, as their expression in transgenic plants yields many beneficial traits.