Increase of root induction in Pinus nigra explants using agrobacteria

Agrobacterium rhizogenes: paving the road to research and breeding for woody plants

Woody plants play a vital role in global ecosystems and serve as valuable resources for various industries and human needs. While many woody plant genomes have been fully sequenced, gene function research and biotechnological breeding advances have lagged behind. As a result, only a limited number of genes have been elucidated, making it difficult to use newer tools such as CRISPR-Cas9 for biotechnological breeding purposes. The use of Agrobacterium rhizogenes as a transformative tool in plant biotechnology has received considerable attention in recent years, particularly in the research field on woody plants. Over the past three decades, numerous woody plants have been effectively transformed using A. rhizogenes-mediated techniques. Some of these transformed plants have successfully regenerated. Recent research on A. rhizogenes-mediated transformation of woody plants has demonstrated its potential for various applications, including gene function analysis, gene expression profiling, gene interaction studies, and gene regulation analysis. The introduction of the Ri plasmid has resulted in the emergence of several Ri phenotypes, such as compact plant types, which can be exploited for Ri breeding purposes. This review paper presents recent advances in A. rhizogenes-mediated basic research and Ri breeding in woody plants. This study highlights various aspects of A. rhizogenes-mediated transformation, its multiple applications in gene function analysis, and the potential of Ri lines as valuable breeding materials.

Ex vitro hairy root induction in detached peanut leaves for plant-nematode interaction studies

BACKGROUND

Peanut (Arachis hypogaea) production is largely affected by a variety of abiotic and biotic stresses, including the root-knot nematode (RKN) Meloidogyne arenaria that causes yield losses worldwide. Transcriptome studies of wild Arachis species, which harbor resistance to a number of pests and diseases, disclosed several candidate genes for M. arenaria resistance. Peanut is recalcitrant to genetic transformation, so the use of Agrobacterium rhizogenes-derived hairy roots emerged as an alternative for in-root functional characterization of these candidate genes.

RESULTS

The present report describes an ex vitro methodology for hairy root induction in detached leaves based on the well-known ability of peanut to produce roots spontaneously from its petiole, which can be maintained for extended periods under high-humidity conditions. Thirty days after infection with the A. rhizogenes 'K599' strain, 90% of the detached leaves developed transgenic hairy roots with 5 cm of length in average, which were then inoculated with M. arenaria. For improved results, plant transformation, and nematode inoculation parameters were adjusted, such as bacterial cell density and growth stage; moist chamber conditions and nematode inoculum concentration. Using this methodology, a candidate gene for nematode resistance, AdEXLB8, was successfully overexpressed in hairy roots of the nematode-susceptible peanut cultivar 'Runner', resulting in 98% reduction in the number of galls and egg masses compared to the control, 60 days after M. arenaria infection.

CONCLUSIONS

This methodology proved to be more practical and cost-effective for functional validation of peanut candidate genes than in vitro and composite plant approaches, as it requires less space, reduces analysis costs and displays high transformation efficiency. The reduction in the number of RKN galls and egg masses in peanut hairy roots overexpressing AdEXLB8 corroborated the use of this strategy for functional characterization of root expressing candidate genes. This approach could be applicable not only for peanut-nematode interaction studies but also to other peanut root diseases, such as those caused by fungi and bacteria, being also potentially extended to other crop species displaying similar petiole-rooting competence.

Establishment of pomegranate (Punica granatum) hairy root cultures for genetic interrogation of the hydrolyzable tannin biosynthetic pathway

In contrast to the numerous reports on the human therapeutic applications of hydrolyzable tannins (HTs), genes involved in their biosynthesis have not been identified at the molecular level from any plant species. Although we have previously identified candidate HT biosynthetic genes in pomegranate using transcriptomic and bioinformatic analyses, characterization of in planta enzyme function remains a critical step in biochemical pathway elucidation. We here report the establishment of a pomegranate (Punica granatum) hairy root culture system that produces HTs. Agrobacterium rhizogenes strains transformed with a binary vector harboring a yellow fluorescent protein (YFP) gene were used for hairy root induction, allowing visual, non-destructive, detection of transgene incorporation. It also demonstrated that the pomegranate hairy root culture system is suitable for expressing heterologous genes (YFP in this case). Expression of 26 putative UDP-glycosyltransferase (UGT) genes, obtained from a pomegranate fruit peel (a tissue highly abundant in HTs) RNA-Seq library, were verified in wild type and hairy roots. In addition, two candidate UGTs for HT biosynthesis were identified based on HPLC and differential gene expression analyses of various pomegranate tissues. Together with in vitro enzyme activity assays, the hairy root culture system holds great promise for revealing the undivulged HT biosynthetic pathway using pomegranate as a model system.

Agrobacterium rhizogenes mediated-transformation of Asimina triloba L. cuttings

The current research compared rooting of pawpaw, with softwood cuttings from mature stands, cuttings from the terminal 3-4 inches of seedlings and cuttings taken from shoots of rooted cuttings from seedlings. Four treatments were used: Agrobacterium rhizogenes strain MSU-1 (A4 wild type), A. rhizogenes strain MT232 (TR105 mutant), indole-3-butyric acid at 20,000 mg L(-1) and control. Only the A. rhizogenes treatments induced rooting and only of the seedling cuttings. No statistical differences in rooting percentage were found among the bacterial strains. While the MSU-1 strain produced more roots, these were shorter and thinner than those produced by MT232 strain. No roots resulted from any other treatment. Roots obtained from infected seedlings were analyzed for transformation using polymerase chain reaction targeting the rolB and rolC vir genes. Roots confirmed as transgenic were 100% for both rolB and rolC.

Enhanced production of asiaticoside from hairy root cultures of Centella asiatica (L.) Urban elicited by methyl jasmonate

Transformed root ("hairy root") cultures have been shown to be a good model for the study of many secondary metabolites. However, economically important compounds such as asiaticoside and madecassoside are produced in insignificant amounts in the root of Centella asiatica (L.) Urban. To overcome this problem, C. asiatica was transformed using Agrobacterium rhizogenes strain R1000 that harbors pCAMBIA1302 encoding the hygromycin phosphotransferase (hpt) and green fluorescence protein (mgfp5) genes and the hairy culture was coupled with elicitation technique. Hairy roots were obtained at a frequency of up to 14.1% from a tissue junction between the leaf and petiole. Abundant hairy roots were observed when co-cultivation of the plant with A. rhizogenes was done for 7 days (36.1%). Transformation was confirmed by PCR and Southern blot analyses. Five weeks after inoculation, no asiaticoside was detected in the hairy root samples. However, when 0.1 mM methyl jasmonate (MJ) was applied as an elicitor to the culture medium for 3 weeks, a large quantity of asiaticoside was generated (7.12 mg/g, dry wt). In the case of gene expression, 12 h after MJ treatment the expression of the CabAS (C. asiatica putative beta-amyrin synthase) gene in the hairy roots is significantly different from that of the control and this level of transcripts was maintained for 14 days. Our results showed that production of C. asiatica hairy roots could be optimized and the resulting cultures could be elicited with MJ treatment for enhanced production of asiaticoside.

Stable transformation of embryogenic tissues of Pinus nigra Arn. using a biolistic method

The stable transformation of embryogenic tissues of Pinus nigra Arn., cell line E104, has been achieved using a biolistic approach. The introduced DNA consisted of the uidA reporter gene under the control of the double CaMV 35S promoter and the nptII selection gene controlled by the single CaMV 35S promoter. Three days after bombardment, putative transformed tissues were selected for continued proliferation on a medium containing 20 mg geneticin l(-1). Resistant embryogenic tissue recovery required 10-12 weeks. The integration of the nptII and uidA genes was confirmed by both histochemical/fluorimetric GUS assays and PCR amplification of the inserts in the five geneticin resistant sub-lines of line E104. The activity of the uidA reporter gene in transgenic, embryogenic tissue lines was stable and could be detected after one year of culture. Somatic embryo maturation was, however, poor and no plantlet regeneration could be obtained.

Novel biotechnological approaches in environmental remediation research

Two novel approaches, the use of Agrobacterium-transformed plant roots and mycelia cultures of fungi, are considered as research tools in the study of the remediation of soil, groundwater, and biowastes. Transformed roots are excellent model systems for screening higher plants that are tolerant of various inorganic and organic pollutants, and for determining the role of the root matrix in the uptake and further metabolism of contaminants. Edible and/or medicinal fungi may also be natural environmental remediators. Liquid cultures of fungal mycelia are appropriate model systems with which to commence screening and biochemical studies in this under-researched area of biotransformation.