Striga parasitizes transgenic hairy roots of Zea mays and provides a tool for studying plant-plant interactions

Combining Abilities and Heterotic Patterns among Early Maturing Maize Inbred Lines under Optimal and Striga-Infested Environments

Information on the general combining ability of inbred lines and the specific combining ability of hybrid combinations is crucial for successful hybrid development. The objectives of this study were to (i) determine the combining ability of thirty selected early maturing maize inbred lines under Striga-infested and optimal environments, (ii) classify the inbred lines into heterotic groups using the general combining ability effects of multiple traits (HGCAMT) and the single nucleotide polymorphism genetic distance (SNP- GD) methods, and (iii) assess the effectiveness of the heterotic grouping methods. One hundred and fifty single-cross hybrids were generated from the thirty inbred lines using the North Carolina Design II mating method. The hybrids and six local check varieties were tested across optimal and Striga-infested environments in Ghana and Nigeria in 2016 and 2017. The inheritance of grain yield was controlled by the non-additive gene action under both environments and the additive gene action across the two research environments. The non-additive gene action modulated the inheritance of measured traits under Striga-infested environments, except for the Striga damage syndrome rating at 8 weeks after planting. Maternal effects were observed for most traits in each environment and across environments. The inbred lines TZEI 127 and TZEI 40 exhibited significant and positive GCA male and female effects for grain yield under each environment and across the two research environments, indicating the presence of favorable alleles for yield improvements. The SNP-GD heterotic grouping method was identified as the most adequate in grouping the thirty inbred lines.

Genome-Wide Association Studies for Striga asiatica Resistance in Tropical Maize

Striga asiatica L. is a parasitic weed in cereal crops including maize leading to tremendous yield losses up to 100% under severe infestation. The available S. asiatica control methods include cultural control options such as uprooting and burning the Striga plants before they flower, field sanitation, crop rotation, intercropping, organic matter usage, improved fallows, and application of herbicides. Resource limitation among smallholder farmers renders almost all of the control methods impossible. Development and use of Striga resistant genotypes are seen as the most feasible management option. Marker identification formulates tools that are faster, cheaper, and easier to utilise in breeding for S. asiatica resistance which has low heritability. The objective of this study was to identify single nucleotide polymorphism (SNP) markers for Striga resistance using the genome-wide association study (GWAS). Genotyping by sequencing was done on tropical maize inbred lines followed by their evaluation for Striga resistance. Analysis of variance showed significant (p < 0.05) variation among evaluated genotypes for Striga resistance traits such as germination distance, germination percentage, haustoria root attachments, total Striga plants emerged, total biomass, and growth rate. There were also significant differences (p < 0.05) for cobs, leaves, stems, and roots weight. The broad sense heritability was fairly high (up to 61%) for most traits. The means for derived traits on stress tolerance indices were subjected to a t-test, and significant differences (p < 0.05) were found for leaves, stem, roots, shoots, and total biomass. The Manhattan plots from GWAS showed the presence of three SNP markers on chromosome numbers 5, 6, and 7 for total Striga plants emerged. The identified markers for resistance to S. asiatica should be validated and utilised to breed for Striga resistance in tropical maize.

Breeding maize (Zea mays) for Striga resistance: Past, current and prospects in sub-saharan africa

Striga hermonthica, causes up to 100% yield loss in maize production in Sub-Saharan Africa. Developing Striga-resistant maize cultivars could be a major component of integrated Striga management strategies. This paper presents a comprehensive overview of maize breeding activities related to Striga resistance and its management. Scientific surveys have revealed that conventional breeding strategies have been used more than molecular breeding strategies in maize improvement for Striga resistance. Striga resistance genes are still under study in the International Institute for Tropical Agriculture (IITA) maize breeding programme. There is also a need to discover QTL and molecular markers associated with such genes to improve Striga resistance in maize. Marker Assistance Breeding is expected to increase maize breeding efficiency with complex traits such as resistance towards Striga because of the complex nature of the host-parasite relationship and its intersection with other environmental factors. Conventional alongside molecular tools and technical controls are promising methods to effectively assess Striga in Sub-Saharan Africa.

A critical review on use of Agrobacterium rhizogenes and their associated binary vectors for plant transformation

Agrobacterium rhizogenes, along with A. tumefaciens, has been used to affect genetic transformation in plants for many years. Detailed studies conducted in the past have uncovered the basic mechanism of foreign gene transfer and the implication of Ri/Ti plasmids in this process. A number of reviews exist describing the usage of binary vectors with A. tumefaciens, but no comprehensive account of the numerous binary vectors employed with A. rhizogenes and their successful applications has been published till date. In this review, we recollect a brief history of development of Ri-plasmid/Ri-T-DNA based binary vectors systems and their successful implementation with A. rhizogenes for different applications. The modification of native Ri plasmid to introduce foreign genes followed by development of binary vector using Ri plasmid and how it facilitated rapid and feasible genetic manipulation, earlier impossible with native Ri plasmid, have been discussed. An important milestone was the development of inducible plant expressing promoter systems which made expression of toxic genes in plant systems possible. The successful application of binary vectors in conjunction with A. rhizogenes in gene silencing and genome editing studies which are relatively newer developments, demonstrating the amenability and adaptability of hairy roots systems to make possible studying previously intractable research areas have been summarized in the present review.

Influence of different Agrobacterium rhizogenes strains on hairy root induction and analysis of phenolic and flavonoid compounds in marshmallow (Althaea officinalis L.)

Hairy roots were induced in Althea officinalis using Agrobacterium rhizogenes, strains A4, A13, ATCC15834, and ATCC15834(GUS). The leaf, petiole and shoot explants of marshmallow were used for the hairy roots induction. When hairy roots appeared, cultures were established in MS (Murashige and Skoog) liquid medium without growth regulators. Hairy roots in explants appeared 5-12 days after inoculation. Maximum transformation frequency of 83% was observed on shoot explants with ATCC15834 strain. Among the strains, ATCC15834(GUS) strain showed better potential in the mass production of hairy roots in the hormone-free liquid medium after 50 days of culturing. The highest total phenolic and flavonoids content was found at 1.57 ± 0.1 mg/g dry weight in A13 strain and 3.47 ± 0.3 mg/g in A4 strain, respectively. Secondary metabolite content of hairy roots was found to be strain-specific.

Factors affecting the efficiency of Rhizobium rhizogenes root transformation of the root parasitic plant Triphysaria versicolor and its host Arabidopsis thaliana

BACKGROUND

Rhizobium rhizogenes transformation is commonly used to generate transgenic roots traditionally called hairy roots, for both investigative and commercial applications. While fertile plants can be regenerated from transgenic roots, the transgenic roots are more typically used directly, either to investigate root biology or to produce valuable secondary metabolites. Hairy roots have been particularly useful for genetic studies of rhizosphere interactions; including the recognition of host plant roots by the roots of parasitic angiosperms.

RESULTS

In this manuscript we analyzed various environmental, nutritional and procedural conditions for their effects on transformation of the model hemi-parasitic plant Triphysaria versicolor and Arabidopsis thaliana, one of its hosts. We first examined the effects of media, gelling agents and co-incubation times on Triphysaria root transformation and determined that while all three affected transformation rates, the media were the most significant. Once those primary conditions were fixed, we examined the roles of seedling age, explant type, acetosyringone, temperature and illumination on Triphysaria hairy root transformation rates. Using the optimized procedure approximately 70% of Triphysaria seedlings developed transgenic roots as judged by expression of YFP. These conditions were then used to transform Arabidopsis and similar transformation rates were obtained.

CONCLUSIONS

Analyses of root transformation factors provides a method recovering transgenic roots from both parasitic plants and their hosts at high frequency. In addition to providing an effective in vitro approach for genetic investigations of parasitic plant-host plant interactions, these results are applicable to genetic studies of non-parasitic plants as well.

Genotype-independent Agrobacterium rhizogenes-mediated root transformation of chickpea: a rapid and efficient method for reverse genetics studies

BACKGROUND

Chickpea (Cicer arietinum L.), an important legume crop is one of the major source of dietary protein. Developing an efficient and reproducible transformation method is imperative to expedite functional genomics studies in this crop. Here, we present an optimized and detailed procedure for Agrobacterium rhizogenes-mediated root transformation of chickpea.

RESULTS

Transformation positive roots were obtained on selection medium after two weeks of A. rhizogenes inoculation. Expression of green fluorescent protein further confirmed the success of transformation. We demonstrate that our method adequately transforms chickpea roots at early developmental stage with high efficiency. In addition, root transformation was found to be genotype-independent and the efficacy of our protocol was highest in two (Annigiri and JG-62) of the seven tested chickpea genotypes. Next, we present the functional analysis of chickpea hairy roots by expressing Arabidopsis TRANSPARENT TESTA 2 (AtTT2) gene involved in proanthocyanidins biosynthesis. Overexpression of AtTT2 enhanced the level of proanthocyanidins in hairy roots that led to the decreased colonization of fungal pathogen, Fusarium oxysporum. Furthermore, the induction of transgenic roots does not affect functional studies involving infection of roots by fungal pathogen.

CONCLUSIONS

Transgenic roots expressing genes of interest will be useful in downstream functional characterization using reverse genetics studies. It requires 1 day to perform the root transformation protocol described in this study and the roots expressing transgene can be maintained for 3-4 weeks, providing sufficient time for further functional studies. Overall, the current methodology will greatly facilitate the functional genomics analyses of candidate genes in root-rhizosphere interaction in this recalcitrant but economically important legume crop.

Generation of Soybean (Glycine max) Transient Transgenic Roots

Legumes-because of their nitrogen-fixing capacity-have both ecological and agronomic importance, and are also the major plant protein source for animal consumption. The model legume species are Lotus japonicus, Medicago truncatula, and soybean (Glycine max). These species have sequenced genomes and are amenable to genetic manipulation, as well as to various functional genomic and cell biology approaches. Plant transformation mediated by Agrobacterium is one of the most powerful methods in plant biotechnology. Using the traditional Agrobacterium tumefaciens method, stable transgenic plants take 6 to 12 months to create, depending on species. Besides being time consuming, this approach is often quite laborious. Hence, there is a need for more rapid methods to create transgenic tissues. In the case of roots, this can be done using hairy root transformation mediated by Agrobacterium rhizogenes. This protocol describes a method to generate transgenic soybean roots in as little as 3 weeks. © 2016 by John Wiley & Sons, Inc.

Transcriptomics exposes the uniqueness of parasitic plants

Parasitic plants have the ability to obtain nutrients directly from other plants, and several species are serious biological threats to agriculture by parasitizing crops of high economic importance. The uniqueness of parasitic plants is characterized by the presence of a multicellular organ called a haustorium, which facilitates plant-plant interactions, and shutting down or reducing their own photosynthesis. Current technical advances in next-generation sequencing and bioinformatics have allowed us to dissect the molecular mechanisms behind the uniqueness of parasitic plants at the genome-wide level. In this review, we summarize recent key findings mainly in transcriptomics that will give us insights into the future direction of parasitic plant research.

Parasitic plants of the genus Cuscuta and their interaction with susceptible and resistant host plants

By comparison with plant-microbe interaction, little is known about the interaction of parasitic plants with their hosts. Plants of the genus Cuscuta belong to the family of Cuscutaceae and comprise about 200 species, all of which live as stem holoparasites on other plants. Cuscuta spp. possess no roots nor fully expanded leaves and the vegetative portion appears to be a stem only. The parasite winds around plants and penetrates the host stems via haustoria, forming direct connections to the vascular bundles of their hosts to withdraw water, carbohydrates, and other solutes. Besides susceptible hosts, a few plants exist that exhibit an active resistance against infestation by Cuscuta spp. For example, cultivated tomato (Solanum lycopersicum) fends off Cuscuta reflexa by means of a hypersensitive-type response occurring in the early penetration phase. This report on the plant-plant dialog between Cuscuta spp. and its host plants focuses on the incompatible interaction of C. reflexa with tomato.

A high-throughput RNA interference (RNAi)-based approach using hairy roots for the study of plant-rhizobia interactions

Legumes are major contributors to sustainable agriculture; their key feature is their ability to fix atmospheric nitrogen through symbiotic nitrogen fixation. Legumes are often recalcitrant to regeneration and transformation by Agrobacterium tumefaciens; however, A. rhizogenes-mediated root transformation and composite plant generation are rapid and convenient alternatives to study root biology, including root nodule symbiosis. RNA interference (RNAi), coupled with A. rhizogenes-mediated root transformation, has been very successfully used for analyses of gene function by reverse genetics. Besides being applied to model legumes (Medicago truncatula and Lotus japonicus), this method has been adopted for several other legumes due to the ease and relative speed with which transgenic roots can be generated. Several protocols for hairy root transformation have been published. Here we describe an improved hairy root transformation protocol and the methods to study nodulation in Medicago. We also highlight the major differences between our protocol and others, and key steps that need to be adjusted in order to translate this method to other legumes.

Arabinogalactan protein-rich cell walls, paramural deposits and ergastic globules define the hyaline bodies of rhinanthoid Orobanchaceae haustoria

BACKGROUND AND AIMS

Parasitic plants obtain nutrients from their hosts through organs called haustoria. The hyaline body is a specialized parenchymatous tissue occupying the central parts of haustoria in many Orobanchaceae species. The structure and functions of hyaline bodies are poorly understood despite their apparent necessity for the proper functioning of haustoria. Reported here is a cell wall-focused immunohistochemical study of the hyaline bodies of three species from the ecologically important clade of rhinanthoid Orobanchaceae.

METHODS

Haustoria collected from laboratory-grown and field-collected plants of Rhinanthus minor, Odontites vernus and Melampyrum pratense attached to various hosts were immunolabelled for cell wall matrix glycans and glycoproteins using specific monoclonal antibodies (mAbs).

KEY RESULTS

Hyaline body cell wall architecture differed from that of the surrounding parenchyma in all species investigated. Enrichment in arabinogalactan protein (AGP) epitopes labelled with mAbs LM2, JIM8, JIM13, JIM14 and CCRC-M7 was prominent and coincided with reduced labelling of de-esterified homogalacturonan with mAbs JIM5, LM18 and LM19. Furthermore, paramural bodies, intercellular deposits and globular ergastic bodies composed of pectins, xyloglucans, extensins and AGPs were common. In Rhinanthus they were particularly abundant in pairings with legume hosts. Hyaline body cells were not in direct contact with haustorial xylem, which was surrounded by a single layer of paratracheal parenchyma with thickened cell walls abutting the xylem.

CONCLUSIONS

The distinctive anatomy and cell wall architecture indicate hyaline body specialization. Altered proportions of AGPs and pectins may affect the mechanical properties of hyaline body cell walls. This and the association with a transfer-like type of paratracheal parenchyma suggest a role in nutrient translocation. Organelle-rich protoplasts and the presence of exceptionally profuse intra- and intercellular wall materials when attached to a nitrogen-fixing host suggest subsequent processing and transient storage of nutrients. AGPs might therefore be implicated in nutrient transfer and metabolism in haustoria.

A comparison of strategies for multiple-gene co-transformation via hairy root induction

Hairy root is a transformed root tissue in which transfer DNA (T-DNA) is inserted in the genome by Agrobacterium rhizogenes. To establish a system for multiple-gene co-transformation in hairy roots, we evaluated four different strategies using A. rhizogenes. The genes gusA and mgfp5 were located in separate plasmids, which were transformed into two different batches of A. rhizogenes (strategy 2AR) or a single batch (strategy 2BV). The two reporter genes were also inserted in one T-DNA (strategy 1TD) or two different T-DNAs (strategy 2TD) in a binary vector. Over 90 % of infected Nicotiana tabacum leaf discs formed hairy roots in all four groups, which was not significantly different from the infection efficiency of wild-type A. rhizogenes. Proportions of co-transformed hairy roots with strategies 2AR, 2BV, 1TD, and 2TD were 65.4, 40.0, 78.6, and 82.1 %, respectively, which indicated that all of the strategies were suitable for co-transformation of multiple genes. High variation in growth rate and heterologous protein expression indicated that further screening is required to identify the clone with the highest productivity. Our results indicated that strategies 1TD and 2TD achieved the highest co-transformation efficiency. Combination with strategy 2AR or 2BV provides additional options for co-transformation of multiple transgenes.

Induction of hairy roots in Arnica montana L. by Agrobacterium rhizogenes

The induction of hairy roots in Arnica montana L. by Agrobacterium rhizogenes mediated system was established. The frequency of genetic transformation varied from 4.8 to 12% depended on method of infection. The cefotaxime at concentration of 200 mg/l proved to suppress effectively the growth of A. rhizogenes after co-cultivation. Among the three tested nutrient media: Murashige and Skoog (MS), Gamborg’s (B5) and Schenk and Hildebrandt (SH), MS medium was superior for growth and high biomass production of transformed roots compared to other culture media. After culturing for 40 days the fresh weight of clone T4 increased 7.6 fold over the non-transformed roots. The transfer of rol A, rol B and rol C genes into Arnica genome was confirmed by PCR analysis. Established genetic transformation techniques in A. montana efficiently provided and generated a large number of transformed roots — an excellent system for studying gene function and could be used for the production of secondary metabolites synthesized in roots.