Promoting genotype-independent plant transformation by manipulating developmental regulatory genes and/or using nanoparticles

KEY MESSAGE

This review summarizes the molecular basis and emerging applications of developmental regulatory genes and nanoparticles in plant transformation and discusses strategies to overcome the obstacles of genotype dependency in plant transformation. Plant transformation is an important tool for plant research and biotechnology-based crop breeding. However, Plant transformation and regeneration are highly dependent on species and genotype. Plant regeneration is a process of generating a complete individual plant from a single somatic cell, which involves somatic embryogenesis, root and shoot organogeneses. Over the past 40 years, significant advances have been made in understanding molecular mechanisms of embryogenesis and organogenesis, revealing many developmental regulatory genes critical for plant regeneration. Recent studies showed that manipulating some developmental regulatory genes promotes the genotype-independent transformation of several plant species. Besides, nanoparticles penetrate plant cell wall without external forces and protect cargoes from degradation, making them promising materials for exogenous biomolecule delivery. In addition, manipulation of developmental regulatory genes or application of nanoparticles could also bypass the tissue culture process, paving the way for efficient plant transformation. Applications of developmental regulatory genes and nanoparticles are emerging in the genetic transformation of different plant species. In this article, we review the molecular basis and applications of developmental regulatory genes and nanoparticles in plant transformation and discuss how to further promote genotype-independent plant transformation.

High frequency in vitro regeneration of adventitious shoots in daylilies (Hemerocallis sp) stem tissue using thidiazuron

BACKGROUND

Daylilies are a lucrative crop used for its floral beauty, medicinal proprieties, landscaping, fire prevention, nutritional value, and research. Despite the importance, daylilies remain extremely challenging for multiplying in vitro. The response difficulty is exacerbated because a few good protocols for daylilies micropropagation are generally difficult to reproduce across genotypes. An efficient strategy, currently applied at Langston University, is to systematically explore individual tissues or organs for their potential to micropropagation. This article is a partial report of the investigation carried out under room environmental conditions and focuses on developing an efficient daylilies in vitro propagation protocol that uses the stem tissue as the principal explant.

RESULTS

In less than three months, using thidiazuron, the use of the stem tissue as the in vitro experimental explant was successful in inducing multiple shoots several folds greater than current daylilies shoot organogenesis protocols. The study showed that tissue culture can be conducted successfully under unrestricted room environmental conditions as well as under the controlled environment of a growth chamber. It also showed that splitting lengthwise stem explants formed multiple shoots several folds greater than cross-sectioned and inverted explants. Shoot conversion rate was mostly independent of the number of shoots formed per explants. The overall response was explant and genotype-dependent. Efficient responses were observed in all thidiazuron treatments.

CONCLUSION

An efficient protocol, which can be applied for mass multiple shoots formation using the daylilies stem tissue as the main explant, was successfully developed. This could lead to a broad and rapid propagation of the crop under an array of environmental conditions to meet the market demand and hasten exogenous gene transfer and breeding selection processes.

Callus mediated shoot organogenesis and regeneration of cytologically stable plants of Ledebouria revoluta: An ethnomedicinal plant with promising antimicrobial potency

Ledebouria revoluta are important ethnomedicinal plant found in India and South Africa. Micropropagation via indirect shoot organogenesis had been established from three types of explant (i.e. scale leaf, leaf lamina and root) of L. revoluta. Scale leaf was found superior as compared to leaf lamina and root explant with respect to their organogenic callus induction potentiality. Murashige and Skoog (1962) [MS] media supplemented with 3.0 mg L⁻¹ 2,4-dichlorophenoxyacetic acid, 0.75 mg L⁻¹ β-naphthoxyacetic acid were best effective for inducing organogenic callus. Maximum 17.0 ± 0.52 bulblets were induced from about 500 mg of callus within 42–46 days sub-culturing on a medium containing 0.75 mg L⁻¹ kinetin. The bulblets were matured (86.7% success) after one month culture on the same medium composition. The best result of in vitro root induction with 100% response and 8.4 ± 0.31 roots per bulb was achieved after 18 days of implantation on MS medium containing 2.0 mg L⁻¹ indole-3-butyric acid. Plantlets were acclimatized with a 96.0% survival rate. Chromosomal studies revealed cytological stability of callus cells and all regenerants containing 2n = 30 chromosomes, same as parental plants. Antimicrobial activity of L. revoluta was tested against two Gram-positive bacteria, three Gram-negative bacteria and two fungi. The methanol and ethanol extract proved more effective against bacteria, whereas acetone and chloroform extract shows potential anti-fungal activities. Present protocol can be applied reliably to produce uniform planting materials in large scale. In addition, this efficient indirect regeneration pathway via callus culture opens a way for improvement through genetic transformation.

Physical state of the culture medium triggers shift in morphogenetic pattern from shoot bud formation to somatic embryo in Solanum khasianum

Solanum khasianum is a rich source of steroidal alkaloids that are important secondary metabolites with enormous pharmaceutical uses. Development of plantlets from somatic tissues, under in vitro conditions, takes place both through adventitious shoot bud differentiation or somatic embryogenesis (SE) pathway. We observed that the physical state of medium, solid or liquid, determined the regenerant differentiation patterns from root segment explants in S. khasianum. In the solidified medium, the root segments developed adventitious shoot buds whereas somatic embryos were regenerated in the liquid medium. Varying gradients from liquid to solid medium were further used to confirm the effect of solidified condition on regeneration pathway. Histological analysis of developing shoot buds and somatic embryos was also performed to confirm their development and differentiation patterns. In order to further confirm the developmental pathways, qRT-PCR analysis of the marker genes of SE and shoot regeneration was also performed. While SOMATIC EMBRYOGENESIS RECEPTOR KINASE1 (SkSERK1) expression was significantly up-regulated during the early embryogenic stage, the LATE EMBRYOGENESIS ABUNDANT (SkLEA) protein was found to be highly expressed in the mature embryos. Expression of the HISTONE DEACETYLASE (HDA6), a repressor of SE related genes, was highly decreased during embryogenesis in the liquid culture. Furthermore, expression of the ENHANCER OF SHOOT REGENERATION  (ESR) gene was comparatively increased during shoot regeneration in the culture using solid medium. Our results point out that the physical state of the medium in S. khasianum plays a decisive role in differentiation pattern which was independent of hormonal supplements.

Efficient, one step and cultivar independent shoot organogenesis of potato

An efficient, one step and genotype independent protocol of shoot organogenesis was developed from leaf and internodal explants taken from microshoots of different cultivars of potato (Solanum tuberosum L.). Initially, microshoots were cultured on basal Murashige and Skoog medium additionally supplemented with 10 µM AgNO₃ (MS1 medium) to achieve healthy shoot growth required to get the quality explants. Shoot organogenesis was induced from both types of explants (leaf and internodal) on MS1 medium variously supplemented with 6-benzyladenine (BA) and gibberellic acid (GA₃). Maximum explants were induced shoot organogenesis on MS1 medium supplemented with 10 µM BA and 15.0 µM GA₃ from both the cultivars namely 'Kufri Chipsona 1' and 'Kufri Pukhraj'. Among the types of explants used, better response was observed from internodal segments as compared to leafs. This optimized medium combination was found to be equally effective for all the eight cultivars tested namely 'Kufri Pukhraj', 'Kufri Chipsona 1', 'Kufri Chipsona 2', 'Kufri Jyoti', 'Kufri Surya', 'Kufri Chandramukhi', 'Kufri Khyati' and 'Desiree'. The clonal uniformity of the regenerated shoots was confirmed using random amplified polymorphic DNA and inter-simple sequence repeats markers.

Temporary Immersion System for Date Palm Micropropagation

The temporary immersion system (TIS) is being used with tremendous success for automation of micropropagation of many plant species. TIS usually consists of a culture vessel comprising two compartments, an upper one with the plant material and a lower one with the liquid culture medium and an automated air pump. The latter enables contact between all parts of the explants and the liquid medium by setting overpressure to the lower part of the container. These systems are providing the most satisfactory conditions for date palm regeneration via shoot organogenesis and allow a significant increase of multiplication rate (5.5-fold in comparison with that regenerated on agar-solidified medium) and plant material quality, thereby reducing production cost.

Low Agrobacterium tumefaciens inoculum levels and a long co-culture period lead to reduced plant defense responses and increase transgenic shoot production of sunflower (Helianthus annuus L.)

Agrobacterium-mediated plant transformation is typically conducted by inoculating plant tissues with an Agrobacterium suspension containing approximately 108-109 bacteria mL-1, followed by a 2-3-d co-culture period. Use of longer co-culture periods could potentially increase transformation efficiencies by allowing more time for Agrobacterium to interact with plant cells, but bacterial overgrowth is likely to occur, leading to severe tissue browning and reduced transformation and regeneration. Low bacterial inoculum levels were therefore evaluated as a means to reduce the negative outcomes associated with long co-culture. The use of low inoculum bacterial suspensions (approximately 6 × 102 bacteria mL-1) followed by long co-culture (15 d) led to the production of an average of three transformed sunflower shoots per explant while the use of high inoculum (approximately 6 × 108 bacteria mL-1) followed by short co-culture (3 d) led to no transformed shoots. Low inoculum and long co-culture acted synergistically, and both were required for the improvement of sunflower transformation. Gene expression analysis via qRT-PCR showed that genes related to plant defense response were generally expressed at lower levels in the explants treated with low inoculum than those treated with high inoculum during 15 d of co-culture, suggesting that low inoculum reduced the induction of plant defense responses. The use of low inoculum with long co-culture (LI/LC) led to large increases in sunflower transformation efficiency. This method has great potential for improving transformation efficiencies and expanding the types of target tissues amenable for transformation of different plant species.

Efficient in vitro direct shoot organogenesis from seedling derived split node explants of maize (Zea mays L.)

Maize is one of the important cereal crops around the world. An efficient and reproducible regeneration protocol via direct organogenesis has been established using split nodes as ex-plants derived from 7 to 10 day old in vitro grown seedlings. Surface sterilized maize seeds were germinated on MS medium lacking plant growth regulators. Nodal sections of 7–10 day old seedlings were isolated, split longitudinally into two halves and cultured on regeneration medium containing different concentrations of 6-benzyladenine (2.20, 4.40, 6.60, 8.80, 11.0 and 13.2 μM) or kinetin (2.32, 4.65, 6.97, 9.29, 11.6 and 13.9 μM). Inclusion of 8.80 μM BA into MS supplemented medium triggered a high frequency of regeneration response from split node explants with a maximum number of shoots (12.0 ± 1.15) and the highest shoot length (3.0 ± 0.73) was obtained directly (without an intervening callus phase) within 4 weeks of culture. Further shoot elongation was achieved on medium containing 4.40 μM BA. The elongated micro shoots were rooted on MS medium fortified with 1.97 μM indole-3-butyric acid. The regenerated plantlets with roots were successfully hardened on earthen pots after proper acclimatization under greenhouse conditions. This new efficient regeneration method provides a solid foundation for genetic manipulation of maize for biotic and abiotic stresses and to enhance the nutritional values.

Competency for shoot regeneration from Arabidopsis root explants is regulated by DNA methylation

Plants exhibit high capacity to regenerate in three alternative pathways: tissue repair, somatic embryogenesis and de novo organogenesis. For most plants, de novo organ initiation can be easily achieved in tissue culture by exposing explants to auxin and/or cytokinin, yet the competence to regenerate varies among species and within tissues from the same plant. In Arabidopsis, root explants incubated directly on cytokinin-rich shoot inducing medium (SIM-direct), are incapable of regenerating shoots, and a pre-incubation step on auxin-rich callus inducing medium (CIM) is required to acquire competency to regenerate on the SIM. However the mechanism underlying competency acquisition still remains elusive. Here we show that the chromomethylase 3 (cmt3) mutant which exhibits significant reduction in CHG methylation, shows high capacity to regenerate on SIM-direct and that regeneration occurs via direct organogenesis. In WT, WUSCHEL (WUS) promoter, an essential gene for shoot formation, is highly methylated, and its expression on SIM requires pre-incubation on CIM. However, in cmt3, WUS expression induced by SIM-direct. We propose that pre-incubation on CIM is required for the re-activation of cell division. Following the transfer of roots to SIM, the intensive cell division activity continues, and in the presence of cytokinin leads to a dilution in DNA methylation that allows certain genes required for shoot regeneration to respond to SIM, thereby advancing shoot formation.

Cytokinin, auxin, and abscisic acid affects sucrose metabolism conduce to de novo shoot organogenesis in rice (Oryza sativa L.) callus

BACKGROUND

Shoot regeneration frequency in rice callus is still low and highly diverse among rice cultivars. This study aimed to investigate the association of plant hormone signaling and sucrose uptake and metabolism in rice during callus induction and early shoot organogenesis. The immatured seeds of two rice cultivars, Ai-Nan-Tsao 39 (ANT39) and Tainan 11 (TN11) are used in this study.

RESULTS

Callus formation is earlier, callus fresh weight is higher, but water content is significant lower in ANT39 than in TN11 while their explants are inoculated on callus induction medium (CIM). Besides, the regeneration frequency is prominently higher in ANT39 (~80%) compared to TN11 callus (0%). Levels of glucose, sucrose, and starch are all significant higher in ANT39 than in TN11 either at callus induction or early shoot organogenesis stage. Moreover, high expression levels of Cell wall-bound invertase 1, Sucrose transporter 1 (OsSUT1) and OsSUT2 are detected in ANT39 at the fourth-day in CIM but it cannot be detected in TN11 until the tenth-day. It suggested that ANT39 has higher callus growth rate and shoot regeneration ability may cause from higher activity of sucrose uptake and metabolism. As well, the expression levels of ORYZA SATIVA RESPONSE REGULATOR 1 (ORR1), PIN-formed 1 and Late embryogenesis-abundant 1, representing endogenous cytokinin, auxin and ABA signals, respectively, were also up-regulated in highly regenerable callus, ANT39, but only ORR1 was greatly enhanced in TN11 at the tenth-day in CIM.

CONCLUSION

Thus, phytohormone signals may affect sucrose metabolism to trigger callus initiation and further de novo shoot regeneration in rice culture.