Regeneration of transgenic plants from two indica rice (Oryza sativa L.) cultivars using shoot apex explants

A comprehensive review of in planta stable transformation strategies

Plant transformation remains a major bottleneck to the improvement of plant science, both on fundamental and practical levels. The recalcitrant nature of most commercial and minor crops to genetic transformation slows scientific progress for a large range of crops that are essential for food security on a global scale. Over the years, novel stable transformation strategies loosely grouped under the term "in planta" have been proposed and validated in a large number of model (e.g. Arabidopsis and rice), major (e.g. wheat and soybean) and minor (e.g. chickpea and lablab bean) species. The in planta approach is revolutionary as it is considered genotype-independent, technically simple (i.e. devoid of or with minimal tissue culture steps), affordable, and easy to implement in a broad range of experimental settings. In this article, we reviewed and categorized over 300 research articles, patents, theses, and videos demonstrating the applicability of different in planta transformation strategies in 105 different genera across 139 plant species. To support this review process, we propose a classification system for the in planta techniques based on five categories and a new nomenclature for more than 30 different in planta techniques. In complement to this, we clarified some grey areas regarding the in planta conceptual framework and provided insights regarding the past, current, and future scientific impacts of these techniques. To support the diffusion of this concept across the community, this review article will serve as an introductory point for an online compendium about in planta transformation strategies that will be available to all scientists. By expanding our knowledge about in planta transformation, we can find innovative approaches to unlock the full potential of plants, support the growth of scientific knowledge, and stimulate an equitable development of plant research in all countries and institutions.

One-Step Regeneration of Hairy Roots to Induce High Tanshinone Plants in Salvia miltiorrhiza

Salvia miltiorrhiza is a traditional Chinese medicinal plant of Labiatae, which has been widely utilized to treat a variety of cardiovascular and cerebrovascular diseases. However, due to the long growth cycle, low content of active ingredients, and serious quality deterioration of S. miltiorrhiza, the use of biotechnology to improve S. miltiorrhiza to meet the growing demand for clinical applications has become a research hotspot. In this study, a novel one-step hairy root regeneration method was developed, which could rapidly obtain hairy roots and regenerated plants with high tanshinone content. By optimizing the parameters of Agrobacterium rhizogenes transformation in S. miltiorrhiza, it was finally established that the explants were infected in Ar.qual (OD₆₀₀ = 0.6) for 10 min, co-cultured for 3 days, and then screened on the screening medium containing 7.5 mg/l hygromycin, the maximum transformation frequency can reach 73.85%. GFP and PCR detection yielded a total of 9 positive transgenic hairy root lines and 11 positive transgenic regenerated plants. SmGGPPS1 was successfully overexpressed in positive transgenic regenerated plants, according to the results of qRT-PCR. The content of tanshinone IIA and cryptotanshinone were dramatically enhanced in transgenic regenerated plants and hairy roots by Ultra Performance Liquid Chromatography analysis. Based on the Agrobacterium-mediated transformation of S. miltiorrhiza, this study developed a new method for regenerating plants with transgenic hairy roots. This method provides a foundation for the breeding of S. miltiorrhiza and the sustainable development of medicinal plant resources, as well as provides a useful reference for the application of other species.

Role of activated charcoal and amino acids in developing an efficient regeneration system for foxtail millet (Setaria italica (L.) Beauv.) using leaf base segments

An efficacious, reproducible direct in vitro regeneration system has been developed from leaf base segments (LBs) of six high yielding genotypes of foxtail millet (Setaria italica (L.) Beauv.). LBs excised from 4-day-old seedling were inoculated on Murashige and Skoog (MS) medium supplemented with different types and concentrations of cytokinins. The shoots induced per explant significantly increased with the supplementation of BAP to auxin containing medium. The results showed that a maximum shoot induction, 58.8% was obtained on MS medium incorporated with 8.9 µM BAP and 2.7 µM NAA in 'CO5' genotype. Further, the highest frequency of multiple shoots was produced on MS(I) medium containing 8.9 µM BAP, 2.7 µM NAA, 700 mg L^-1 proline, 0.5 mg L^-1 cysteine, 2.0 mg L^-1 glycine and 150 mg L^-1 arginine. MS(I) medium additionally fortified with 5.0 g L^-1 activated charcoal (AC) was found to achieve the best precocious plant regeneration. Elongated shoots were rooted on half-strength MS medium amended with 2.9 µM IAA and achieved maximum root number (8.7) within 10 days. Rooted plantlets were acclimated in soil with 92% survival rate. Molecular marker analysis of in vitro regenerated and field grown plants revealed no somaclonal variations. Briefly, amino acids and activated charcoal could significantly enhance the foxtail millet direct multiple shoot proliferation and plant regeneration. Here we report, a short-term, genotype independent, direct plant regeneration protocol for future genetic transformation studies in foxtail millet genotypes.

Finger Millet [Eleusine coracana (L.) Gaertn.] Improvement: Current Status and Future Interventions of Whole Genome Sequence

The whole genome sequence (WGS) of the much awaited, nutrient rich and climate resilient crop, finger millet (Eleusine coracana (L.) Gaertn.) has been released recently. While possessing superior mineral nutrients and excellent shelf life as compared to other major cereals, multiploidy nature of the genome and relatively small plantation acreage in less developed countries hampered the genome sequencing of finger millet, disposing it as one of the lastly sequenced genomes in cereals. The genomic information available for this crop is very little when compared to other major cereals like rice, maize and barley. As a result, only a limited number of genetic and genomic studies has been undertaken for the improvement of this crop. Finger millet is known especially for its superior calcium content, but the high-throughput studies are yet to be performed to understand the mechanisms behind calcium transport and grain filling. The WGS of finger millet is expected to help to understand this and other important molecular mechanisms in finger millet, which may be harnessed for the nutrient fortification of other cereals. In this review, we discuss various efforts made so far on the improvement of finger millet including genetic improvement, transcriptome analysis, mapping of quantitative trait loci (QTLs) for traits, etc. We also discuss the pitfalls of modern genetic studies and provide insights for accelerating the finger millet improvement with the interventions of WGS in near future. Advanced genetic and genomic studies aided by WGS may help to improve the finger millet, which will be helpful to strengthen the nutritional security in addition to food security in the developing countries of Asia and Africa.

Development of a genotype independent and transformation amenable regeneration system from shoot apex in rice (Oryza sativa spp. indica) using TDZ

Agrobacterium-mediated transformation of indica rice has been established in only a limited number of cultivars because the regeneration of plants from transformed embryogenic calli is highly cultivar-specific. Establishment of a highly efficient plant regeneration system from shoot apex explants applicable to many cultivars of indica rice will accelerate the application of transformation technology in breeding programs and functional genomics study. We established an efficient shoot multiplication and plant regeneration system from shoot apices of indica rice using thidiazuron (TDZ) as a plant growth regulator. Shoot apices cultured on MS basal medium devoid of plant growth regulators formed solitary shoots in 90% of cultures. Addition of TDZ or benzylaminopurine to regeneration medium significantly influenced formation of multiple shoots directly from shoot apex explants without an intervening callus stage. Best shoot proliferation response (10.3 shoots per explant) was recorded when shoot apices were cultured on media supplemented with 4 mg/l TDZ. No synergistic effect on shoot proliferation was observed when indole-3-acetic acid and indole-3-butyric acid were supplemented to media containing 4 mg/l TDZ. The regeneration system was efficient in evoking multiple shoot proliferation in eight different cultivars of indica rice. Shoots were rooted in MS basal medium and plantlets were acclimatized with 100% survival rate. The shoot apex explants of all the eight cultivars of indica rice were found competent to Agrobacterium-mediated transformation while explants from IR-64 showed highest transient GUS expression. This variety-independent transformation amenable regeneration system from shoot apices may widely be applicable for genetic transformation of indica varieties.

An improved protocol for efficient transformation and regeneration of diverse indica rice cultivars

BACKGROUND

Rice genome sequencing projects have generated remarkable amount of information about genes and genome architecture having tremendous potential to be utilized in both basic and applied research. Success in transgenics is paving the way for preparing a road map of functional genomics which is expected to correlate action of a gene to a trait in cellular and organismal context. However, the lack of a simple and efficient method for transformation and regeneration is a major constraint for such studies in this important cereal crop.

RESULTS

In the present study, we have developed an easy, rapid and highly efficient transformation and regeneration protocol using mature seeds as explants and found its successful applicability to a choice of elite indica rice genotypes. We have optimized various steps of transformation and standardized different components of the regeneration medium including growth hormones and the gelling agent. The modified regeneration medium triggers production of large number of shoots from smaller number of calli and promotes their faster growth, hence significantly advantageous over the existing protocols where the regeneration step requires maximum time. Using this protocol, significantly higher transformation efficiency (up to 46%) and regeneration frequency (up to 92% for the untransformed calli and 59% for the transformed calli) were achieved for the four tested cultivars. We have used this protocol to produce hundreds of independent transgenic lines of different indica rice genotypes. Upon maturity, these transgenic lines were fertile thereby indicating that faster regeneration during tissue culture did not affect their reproductive potential.

CONCLUSIONS

This speedy, yet less labor-intensive, protocol overcomes major limitations associated with genetic manipulation in rice. Moreover, our protocol uses mature seeds as the explant, which can easily be obtained in quantity throughout the year and kept viable for a long time. Such an easy, efficient and generalized protocol has the potential to be a major tool for crop improvement and gene-function studies on the model monocot plant rice.

Agrobacterium-mediated transformation of finger millet (Eleusine coracana (L.) Gaertn.) using shoot apex explants

A new Agrobacterium-mediated transformation system was developed for finger millet using shoot apex explants. The Agrobacterium strain LBA4404 harboring binary vector pCAMBIA1301, which contained hygromycin phosphotransferase (hptII) as selectable marker gene and β-glucuronidase (GUS) as reporter gene, was used for optimization of transformation conditions. Two finger millet genotypes, GPU 45 and CO 14, were used in this study. The optimal conditions for the Agrobacterium-mediated transformation of finger millet were found to be the co-cultivation of explants obtained on the 16th day after callus induction (DACI), exposure of explants for 30 min to agrobacterial inoculum and 3 days of co-cultivation on filter paper placed on medium supplemented with 100 μM acetosyringone (AS). Addition of 100 μM L: -cysteine in the selection medium enhanced the frequency of transformation and transgenic plant recovery. Both finger millet genotypes were transformed by Agrobacterium. A frequency of 19% transient expression with 3.8% stable transformation was achieved in genotype GPU 45 using optimal conditions. Five stably transformed plants were fully characterized by Southern blot analysis. A segregation analysis was also performed in four R(1) progenies, which showed normal Mendelian pattern of transgene segregation. The inheritance of transgenes in R(1) progenies was also confirmed by Southern blot analysis. This is the first report on Agrobacterium-mediated transformation of finger millet. This study underpins the introduction of numerous agronomically important genes into the genome of finger millet in the future.

Genetic engineering of millets: current status and future prospects

This review summarizes progress on the genetic transformation of millets and discusses the future prospects for the development of improved varieties. Only a limited number of studies have been carried out on genetic improvement of millets despite their nutritional importance in supplying minerals, calories and protein. Most genetic transformation studies of millets have been restricted to pearl millet and bahiagrass and most studies have been limited to the assessment of reporter and marker gene expression. Biolistic-mediated gene delivery has been frequently used for the transformation of millets but Agrobacterium-mediated transformation is still lagging. Improved transformation of millets, allied to relevant gene targets which may offer, for example, improved nutritional quality, resistance to abiotic and biotic stresses, and resistance to fungal infection will play important roles in millet improvement.

Tissue specific response of Agrobacterium tumefaciens attachment to Sorghum bicolor (L) Moench

Agrobacterium mediated genetic transformation of plants have advantages over other methods, especially for making single copy transgenic plants with reduced chances of gene silencing and instability. However, monocotyledonous plant species could not utilize the full potential of this system because of possible limitations in Agrobacterium interaction with monocot plant cells. Agrobacterium attachment as a factor in genetic transformation was studied in the leaf, shoot apex, and leaf derived callus of sorghum (Sorghum bicolor (L) Moench). Pre-induction of Agrobacterium with acetosyringone was found necessary for Agrobacterium attachment to sorghum tissues. All the explants responded positively, with preferential Agrobacterium attachment and colonization around the tissues having actively dividing cells. Callus proved to be the best explant for Agrobacterium attachment as observed in scanning electron microscopy and transient GUS expression. Loss of Agrobacterium attachment was observed with an increase in the degree of tissue differentiation.