Reasons for lower transformation efficiency in indica rice using Agrobacterium tumefaciens-mediated transformation: lessons from transformation assays and genome-wide expression profiling

Establishment of a Rapid and Effective Agrobacterium-Mediated Genetic Transformation System of Oxalis triangularis 'Purpurea'

Oxalis triangularis 'Purpurea' has significant ornamental value in landscaping. There is a critical necessity to elucidate the gene functions of O. triangularis 'Purpurea' and dissect the molecular mechanisms governing key ornamental traits. However, a reliable genetic transformation method remains elusive. In this study, our investigation revealed that various transformation parameters, including recipient material (petioles), pre-culture time (2-5 days), acetosyringone (AS) concentration (100-400 μM), Agrobacterium concentrations (OD600 = 0.4-1.0), infection time (5-20 min), and co-culture time (2-5 days), significantly impacted the stable genetic transformation in O. triangular 'Purpurea'. Notably, the highest genetic transformation rate was achieved from the leaf discs pre-cultured for 3 days, treated with 200 μM AS infected with Agrobacterium for 11 min at OD600 of 0.6, and subsequently co-cultured for 3 days. This treatment resulted in a genetic transformation efficiency of 9.88%, and it only took 79 days to produce transgenic plants. Our transformation protocol offers advantages of speed, efficiency, and simplicity, which will greatly facilitate genetic transformation for O. triangular 'Purpurea' and gene function studies.

In Vitro Induction of Interspecific Hybrid and Polyploidy Derived from Oryza officinalis Wall

Oryza officinalis Wall is a potential genetic resource for rice breeding; however, its distant genome limits its crossing ability with cultivated rice. The interspecific hybridization of O. officinalis and cultivated rice, establishment of its tissue culture, and induction of polyploidy are ways to improve O. officinalis's poor crossability. We developed an interspecific hybrid and studied its reproductive pollen development process in this work, and the results showed that abortive pollens (81.94%) and embryo sac abnormalities (91.04%) were the key causes of its high sterility. In order to induce callus formation in interspecific hybrid explants, two different culture media, namely Chu's N-6 medium (N6) and 1/2 Murashig and Skoog medium (1/2 MS), were employed. Additionally, two plant growth regulators (PGRs), namely 2,4-dichlorophenoxyacetic acid (2,4-D) and 6-benzylaminopurine (BA), along with L-proline (Pro) and acid hydrolyzed casein, were utilized in the experiment. The optimal N6 medium, supplemented with 2.0 mg·L-1 2,4-D, produced the highest induction rate (80.56 ± 5.44)%. For callus differentiation and proliferation, the MS medium supplemented with 2.0 mg·L-1 BA + 0.2 mg·L-1 NAA produced the highest differentiation rate (75.00 ± 4.97)% and seedling emergence ratio (28.97 ± 4.67)%. The optimal combination for seedling rooting was the 1/2 MS medium supplemented with 2.0 mg L-1 NAA and 0.2 mg L-1 BA, which produced an average of 13.95 roots per plant. For polyploidy induction in the interspecific hybrid, the concentration of colchicine treatment at 400 mg·L-1 for three days is an ideal protocol. We devised tissue culture and interspecific hybrid polyploidy induction to overcome O. officinalis' poor crossability and introduce its favorable features into cultivated rice.

An Efficient Transformation System for Fast Production of VcCHS Transgenic Blueberry Callus and Its Expressional Analysis

The Agrobacterium tumefaciens-mediated transformation for blueberries remains less efficient than is desirable. A new leaf callus regeneration and genetic transformation system was investigated in blueberries in this study. The leaf explants of cv. 'Legacy' and 'Northland' were used to establish the stable callus induction system when placed on the woody plant medium (WPM) supplemented with 1.0 mg·L-1 2, 4-D, 0.4 mg·L-1 6-BA for 30 d; then, the callus was sub-cultured in the proliferation medium supplemented with 1.5 mg·L-1 2, 4-D, 0.4 mg·L-1 6-BA in the darkness at 25 °C every 30 days. The co-cultivation of callus with A. tumefaciens was operated on WPM plus 100 μM acetosyringone for 4 days; then, the transferred callus was grown in WPM supplemented with 1.5 mg·L-1 2,4-D, 0.4 mg·L-1 6-BA, 50 mg·L-1 hygromycin, and 200 mg·L-1 cefotaxime. The VcCHS transgenic blueberry callus with both GFP signal and Hyg resistance was obtained from the transformed callus of cv. 'Northland'. The rate of GFP signal detected in the transformed callus was as high as 49.02%, which was consistent with the PCR assay. Collectively, this study provides a highly efficient genetic transformation system in blueberry callus and a powerful approach for the molecular breeding of blueberries.

CRISPR-Mediated Base Editing: From Precise Point Mutation to Genome-Wide Engineering in Nonmodel Microbes

Nonmodel microbes with unique and diverse metabolisms have become rising stars in synthetic biology; however, the lack of efficient gene engineering techniques still hinders their development. Recently, the use of base editors has emerged as a versatile method for gene engineering in a wide range of organisms including nonmodel microbes. This method is a fusion of impaired CRISPR/Cas9 nuclease and base deaminase, enabling the precise point mutation at the target without inducing homologous recombination. This review updates the latest advancement of base editors in microbes, including the conclusion of all microbes that have been researched by base editors, the introduction of newly developed base editors, and their applications. We provide a list that comprehensively concludes specific applications of BEs in nonmodel microbes, which play important roles in industrial, agricultural, and clinical fields. We also present some microbes in which BEs have not been fully established, in the hope that they are explored further and so that other microbial species can achieve arbitrary base conversions. The current obstacles facing BEs and solutions are put forward. Lastly, the highly efficient BEs and other developed versions for genome-wide reprogramming of cells are discussed, showing great potential for future engineering of nonmodel microbes.

Comparative transcriptome analysis reveals compatible and recalcitrant genotypic response of barley microspore-derived embryogenic callus toward Agrobacterium infection

BACKGROUND

The Agrobacterium mediated transformation has been routinely used in lots of plant species as a powerful tool to deliver genes of interest into a host plant. However, the transformation of elite and commercially valuable cultivar is still limited by the genotype-dependency, and the efficiency of Agrobacterium infection efficiency is crucial for the success of transformation.

RESULTS

In this study, the microspore-derived embryogenic calli (MDEC) of barley elite cultivars and breeding lines were employed as unique subjects to characterize the genotypic response during Agrobacterium infection process. Our results identified compatible barley genotypes (GanPi 6 and L07, assigned as GP6-L07 group) and one recalcitrant genotype (Hong 99, assigned as H99) for the Agrobacterium strain LBA4404 infection using GUS assay. The accumulation trend of reactive oxygen species (ROS) was similar among genotypes across the time course. The results of RNA-seq depicted that the average expressional intensity of whole genomic genes was similar among barley genotypes during Agrobacterium infection. However, the numbers of differentially expressed genes (DEGs) exhibited significant expressional variation between GP6-L07 and H99 groups from 6 to 12 h post-inoculation (hpi). Gene ontology (GO) enrichment analysis revealed different regulation patterns for the predicted biological processes between the early (up-regulated DEGs overrepresented at 2 hpi) and late stages (down-regulated DEGs overrepresented from 6 to 24 hpi) of infection. KEGG analysis predicted 12 pathways during Agrobacterium infection. Among which one pathway related to pyruvate metabolism was enriched in GP6 and L07 at 6 hpi. Two pathways related to plant hormone signal transduction and DNA replication showed expressional variation between GP6-L07 and H99 at 24 hpi. It was further validated by qRT-PCR assay for seven candidate genes (Aldehyde dehydrogenase, SAUR, SAUR50, ARG7, Replication protein A, DNA helicase and DNA replication licensing factor) involved in the three pathways, which are all up-regulated in compatible while down-regulated in recalcitrant genotypes, suggesting the potential compatibility achieved at later stage for the growth of Agrobacterium infected cells.

CONCLUSIONS

Our findings demonstrated the similarity and difference between compatible and recalcitrant genotypes of barley MDEC upon Agrobacterium infection. Seven candidate genes involved in pyruvate metabolism, hormonal signal transduction and DNA replication were identified, which advocates the genotypic dependency during Agrobacterium infection process.

Genotype-Independent Regeneration and Transformation Protocol for Rice Cultivars

Developing an efficient and reproducible plant transformation protocol relies on callus induction and plant regeneration, which is prerequisite for genetic enhancement of crops, especially rice. The present study has been carried out in order to establish a genotype-independent regeneration and biolistic transformation protocol for rice varieties. Putative transgenic rice lines were confirmed by PCR analysis, DNA sequencing, and Southern analysis. The transformation protocol reported here is relatively simple and consistent and can be exploited in future biotechnological investigations particularly for gene transformation studies.

Transient expression of the β-glucuronidase gene in Cannabis sativa varieties

In plant biology, transient expression analysis plays a vital role to provide a fast method to study the gene of interest. In this study, we report a rapid and efficient method for transient expression in Cannabis sativa seedlings using Agrobacterium tumefaciens-mediated transformation. A. tumefaciens strain EHA105 carrying the pCAMBIA1301 construct with uidA gene was used to transform cannabis seedlings and the GUS assay (a measurement of β-glucuronidase activity) was used to detect the uidA expression. In the current study, we have also established a rapid germination protocol for cannabis seeds. The all three steps seed sterilization, germination and seedlings development were carried out in a 1% H2O2 solution. Transient transformation revealed that both cotyledons and young true leaves are amenable to transformation. Compared with tobacco (Nicotiana benthamiana), cannabis seedlings were less susceptible to transformation with A. tumefaciens. Susceptibility to Agrobacterium transformation also varied with the different cannabis varieties. The method established in this study has the potential to be an important tool for gene function studies and genetic improvement in cannabis.

Effects of Medium Composition and Genetic Background on Agrobacterium-Mediated Transformation Efficiency of Lentinula edodes

The establishment of genetic transformation method is crucial for the functional genomics research in filamentous fungi. Although the transformation method has been developed in several types of fungi, a highly efficient and convenient transformation system is desperately needed in Lentinula edodes. Present work established the Agrobacterium-mediated transformation (ATMT) of basidiomycete L. edodes in both monokaryon and dikaryon mycelia by using constructed binary plasmid pCAMBIA-1300-GFP. Then, the transformation efficiency of ATMT was evaluated by using different mediums for recipient incubation and different varieties of L. edodes. The results showed that in dikaryon strain W1, the positive hygromycin-resistant transformants was observed in all medium with the positive frequency of selected transformants that ranged from 0 to 30%. While in the monokaryon strain W1-26, only the millet medium group obtained positive transformants with a positive frequency of 75.48%. Moreover, three dikaryotic wild strains (YS55, YS3334, and YS3357) and two dikaryotic cultivated strains (W1 and S606) showed the highest transformation efficiency, with 32.96% of the germination frequency, and 85.12% of positive frequency for hygromycin-resistant transformants. This work demonstrated that Agrobacterium-mediated transformation was successfully performed in L. edodes, and the genotype of recipients as well as the medium for mycelial incubation were suggested to play key roles in determining the transformation efficiency. These findings may provide new avenues for the genetic modification of edible mushroom and may extend the cognition of DNA-mediated transformation in filamentous fungi.

Characterisation of HvVIP1 and expression profile analysis of stress response regulators in barley under Agrobacterium and Fusarium infections

Arabidopsis thaliana's VirE2-Interacting Protein 1 (VIP1) interacts with Agrobacterium tumefaciens VirE2 protein and regulates stress responses and plant immunity signaling occurring downstream of the Mitogen-Activated Protein Kinase (MPK3) signal transduction pathway. In this study, a full-length cDNA of 972bp encoding HvVIP1 was obtained from barley (Hordeum vulgare L.) leaves. A corresponding 323 amino acid poly-peptide was shown to carry the conserved bZIP (Basic Leucine Zipper) domain within its 157th and 223rd amino acid residue. 13 non-synonymous SNPs were spotted within the HvVIP1 bZIP domain sequence when compared with AtVIP1. Moreover, minor differences in the bZIP domain locations and lengths were noted when comparing Arabidopsis thaliana and Hordeum vulgare VIP1 proteins through the 3D models, structural domain predictions and disorder prediction profiling. The expression of HvVIP1 was stable in barley tissues infected by pathogen (whether Agrobacterium tumefaciens or Fusarium culmorum), but was induced at specific time points. We found a strong correlation between the transcript accumulation of HvVIP1 and barley PR- genes HvPR1, HvPR4 and HvPR10, but not with HvPR3 and HvPR5, probably due to low induction of those particular genes. In addition, a gene encoding for a member of the barley MAPK family, HvMPK1, showed significantly higher expression after pathogenic infection of barley cells. Collectively, our results might suggest that early expression of PR genes upon infection in barley cells play a pivotal role in the Agrobacterium-resistance of this plant.

Transcriptome Profiling of Plant Genes in Response to Agrobacterium tumefaciens-Mediated Transformation

Agrobacterium tumefaciens is a plant pathogen that causes crown gall disease. During infection of the host plant, Agrobacterium transfers T-DNA from its Ti plasmid into the host cell, which can then be integrated into the host genome. This unique genetic transformation capability has been employed as the dominant technology for producing genetically modified plants for both basic research and biotechnological applications. Agrobacterium has been well studied as a disease-causing agent. The Agrobacterium-mediated transformation process involves early attachment of the bacterium to the host's surface, followed by transfer of T-DNA and virulence proteins into the plant cell. Throughout this process, the host plants exhibit dynamic gene expression patterns at each infection stage or in response to Agrobacterium strains with varying pathogenic capabilities. Shifting host gene expression patterns throughout the transformation process have effects on transformation frequency, host morphology, and metabolism. Thus, gene expression profiling during the Agrobacterium infection process can be an important approach to help elucidate the interaction between Agrobacterium and plants. This review highlights recent findings on host plant differential gene expression patterns in response to A. tumefaciens or related elicitor molecules.

Transcriptomic and Hormonal Analyses Reveal that YUC-Mediated Auxin Biogenesis Is Involved in Shoot Regeneration from Rhizome in Cymbidium

Cymbidium, one of the most important orchid genera in horticulture, can be classified into epiphytic and terrestrial species. Generally, epiphytic Cymbidium seedlings can be easily propagated by tissue culture, but terrestrial seedlings are difficult to propagate. To date, the molecular mechanisms underlying the differences in the ease with which terrestrial and epiphytic cymbidiums can be propagated are largely unknown. Using RNA-sequencing, quantitative reverse transcription PCR and enzyme-linked immunosorbent assay, Cymbidium 'Xiaofeng' (CXF), which can be efficiently micropropagated, and terrestrial Cymbidium sinense 'Qijianbaimo' (CSQ), which has a low regeneration ability, were used to explore the molecular mechanisms underlying the micropropagation ability of Cymbidium species. To this end, 447 million clean short reads were generated, and 31,264 annotated unigenes were obtained from 10 cDNA libraries. A total of 1,290 differentially expressed genes (DEGs) were identified between CXF and CSQ during shoot induction. Gene ontology (GO) enrichment analysis indicated that the DEGs were significantly enriched in auxin pathway-related GO terms. Further analysis demonstrated that YUC and GH3 family genes, which play crucial roles in the regulation of auxin/IAA (indole-3-acetic acid) metabolism, acted quickly in response to shoot induction culture in vitro and were closely correlated with variation in shoot regeneration between CXF and CSQ. In addition, the study showed that IAA accumulated rapidly and significantly during shoot induction in CXF compared to that in CSQ; in contrast, no significant changes in other hormones were observed between CXF and CSQ. Furthermore, shoot regeneration in CXF was inhibited by a yucasin-auxin biosynthesis inhibitor, indicating that increased IAA level is required for high-frequency shoot regeneration in CXF. In conclusion, our study revealed that YUC-mediated auxin biogenesis is involved in shoot regeneration from rhizome in Cymbidium.

An Agrobacterium tumefaciens Strain with Gamma-Aminobutyric Acid Transaminase Activity Shows an Enhanced Genetic Transformation Ability in Plants

Agrobacterium tumefaciens has the unique ability to mediate inter-kingdom DNA transfer, and for this reason, it has been utilized for plant genetic engineering. To increase the transformation frequency in plant genetic engineering, we focused on gamma-aminobutyric acid (GABA), which is a negative factor in the Agrobacterium-plant interaction. Recent studies have shown contradictory results regarding the effects of GABA on vir gene expression, leading to the speculation that GABA inhibits T-DNA transfer. In this study, we examined the effect of GABA on T-DNA transfer using a tomato line with a low GABA content. Compared with the control, the T-DNA transfer frequency was increased in the low-GABA tomato line, indicating that GABA inhibits T-DNA transfer. Therefore, we bred a new A. tumefaciens strain with GABA transaminase activity and the ability to degrade GABA. The A. tumefaciens strain exhibited increased T-DNA transfer in two tomato cultivars and Erianthus arundinacues and an increased frequency of stable transformation in tomato.

Functional classification of rice flanking sequence tagged genes using MapMan terms and global understanding on metabolic and regulatory pathways affected by dxr mutant having defects in light response

BACKGROUND

Rice is one of the most important food crops for humans. To improve the agronomical traits of rice, the functions of more than 1,000 rice genes have been recently characterized and summarized. The completed, map-based sequence of the rice genome has significantly accelerated the functional characterization of rice genes, but progress remains limited in assigning functions to all predicted non-transposable element (non-TE) genes, estimated to number 37,000-41,000.

RESULTS

The International Rice Functional Genomics Consortium (IRFGC) has generated a huge number of gene-indexed mutants by using mutagens such as T-DNA, Tos17 and Ds/dSpm. These mutants have been identified by 246,566 flanking sequence tags (FSTs) and cover 65 % (25,275 of 38,869) of the non-TE genes in rice, while the mutation ratio of TE genes is 25.7 %. In addition, almost 80 % of highly expressed non-TE genes have insertion mutations, indicating that highly expressed genes in rice chromosomes are more likely to have mutations by mutagens such as T-DNA, Ds, dSpm and Tos17. The functions of around 2.5 % of rice genes have been characterized, and studies have mainly focused on transcriptional and post-transcriptional regulation. Slow progress in characterizing the function of rice genes is mainly due to a lack of clues to guide functional studies or functional redundancy. These limitations can be partially solved by a well-categorized functional classification of FST genes. To create this classification, we used the diverse overviews installed in the MapMan toolkit. Gene Ontology (GO) assignment to FST genes supplemented the limitation of MapMan overviews.

CONCLUSION

The functions of 863 of 1,022 known genes can be evaluated by current FST lines, indicating that FST genes are useful resources for functional genomic studies. We assigned 16,169 out of 29,624 FST genes to 34 MapMan classes, including major three categories such as DNA, RNA and protein. To demonstrate the MapMan application on FST genes, transcriptome analysis was done from a rice mutant of 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR) gene with FST. Mapping of 756 down-regulated genes in dxr mutants and their annotation in terms of various MapMan overviews revealed candidate genes downstream of DXR-mediating light signaling pathway in diverse functional classes such as the methyl-D-erythritol 4-phosphatepathway (MEP) pathway overview, photosynthesis, secondary metabolism and regulatory overview. This report provides a useful guide for systematic phenomics and further applications to enhance the key agronomic traits of rice.

Unmasking host and microbial strategies in the Agrobacterium-plant defense tango

Coevolutionary forces drive adaptation of both plant-associated microbes and their hosts. Eloquently captured in the Red Queen Hypothesis, the complexity of each plant-pathogen relationship reflects escalating adversarial strategies, but also external biotic and abiotic pressures on both partners. Innate immune responses are triggered by highly conserved pathogen-associated molecular patterns, or PAMPs, that are harbingers of microbial presence. Upon cell surface receptor-mediated recognition of these pathogen-derived molecules, host plants mount a variety of physiological responses to limit pathogen survival and/or invasion. Successful pathogens often rely on secretion systems to translocate host-modulating effectors that subvert plant defenses, thereby increasing virulence. Host plants, in turn, have evolved to recognize these effectors, activating what has typically been characterized as a pathogen-specific form of immunity. Recent data support the notion that PAMP-triggered and effector-triggered defenses are complementary facets of a convergent, albeit differentially regulated, set of immune responses. This review highlights the key players in the plant's recognition and signal transduction pathways, with a focus on the aspects that may limit Agrobacterium tumefaciens infection and the ways it might overcome those defenses. Recent advances in the field include a growing appreciation for the contributions of cytoskeletal dynamics and membrane trafficking to the regulation of these exquisitely tuned defenses. Pathogen counter-defenses frequently manipulate the interwoven hormonal pathways that mediate host responses. Emerging systems-level analyses include host physiological factors such as circadian cycling. The existing literature indicates that varying or even conflicting results from different labs may well be attributable to environmental factors including time of day of infection, temperature, and/or developmental stage of the host plant.

TaWRKY68 responses to biotic stresses are revealed by the orthologous genes from major cereals

WRKY transcription factors have been extensively characterized in the past 20 years, but in wheat, studies on WRKY genes and their function are lagging behind many other species. To explore the function of wheat WRKY genes, we identified a TaWRKY68 gene from a common wheat cultivar. It encodes a protein comprising 313 amino acids which harbors 19 conserved motifs or active sites. Gene expression patterns were determined by analyzing microarray data of TaWRKY68 in wheat and of orthologous genes from maize, rice and barley using Genevestigator. TaWRKY68 orthologs were identified and clustered using DELTA-BLAST and COBALT programs available at NCBI. The results showed that these genes, which are expressed in all tissues tested, had relatively higher levels in the roots and were up-regulated in response to biotic stresses. Bioinformatics results were confirmed by RT-PCR experiments using wheat plants infected by Agrobacterium tumefaciens and Blumeria graminis, or treated with Deoxynivalenol, a Fusarium graminearum-induced mycotoxin in wheat or barley. In summary, TaWRKY68 functions differ during plant developmental stages and might be representing a hub gene function in wheat responses to various biotic stresses. It was also found that including data from major cereal genes in the bioinformatics analysis gave more accurate and comprehensive predictions of wheat gene functions.

Agrobacterium infection and plant defense-transformation success hangs by a thread

The value of Agrobacterium tumefaciens for plant molecular biologists cannot be appreciated enough. This soil-borne pathogen has the unique capability to transfer DNA (T-DNA) into plant systems. Gene transfer involves both bacterial and host factors, and it is the orchestration of these factors that determines the success of transformation. Some plant species readily accept integration of foreign DNA, while others are recalcitrant. The timing and intensity of the microbially activated host defense repertoire sets the switch to "yes" or "no." This repertoire is comprised of the specific induction of mitogen-activated protein kinases (MAPKs), defense gene expression, production of reactive oxygen species (ROS) and hormonal adjustments. Agrobacterium tumefaciens abuses components of the host immunity system it mimics plant protein functions and manipulates hormone levels to bypass or override plant defenses. A better understanding of the ongoing molecular battle between agrobacteria and attacked hosts paves the way toward developing transformation protocols for recalcitrant plant species. This review highlights recent findings in agrobacterial transformation research conducted in diverse plant species. Efficiency-limiting factors, both of plant and bacterial origin, are summarized and discussed in a thought-provoking manner.

Global analysis of differentially expressed genes and proteins in the wheat callus infected by Agrobacterium tumefaciens

Agrobacterium-mediated plant transformation is an extremely complex and evolved process involving genetic determinants of both the bacteria and the host plant cells. However, the mechanism of the determinants remains obscure, especially in some cereal crops such as wheat, which is recalcitrant for Agrobacterium-mediated transformation. In this study, differentially expressed genes (DEGs) and differentially expressed proteins (DEPs) were analyzed in wheat callus cells co-cultured with Agrobacterium by using RNA sequencing (RNA-seq) and two-dimensional electrophoresis (2-DE) in conjunction with mass spectrometry (MS). A set of 4,889 DEGs and 90 DEPs were identified, respectively. Most of them are related to metabolism, chromatin assembly or disassembly and immune defense. After comparative analysis, 24 of the 90 DEPs were detected in RNA-seq and proteomics datasets simultaneously. In addition, real-time RT-PCR experiments were performed to check the differential expression of the 24 genes, and the results were consistent with the RNA-seq data. According to gene ontology (GO) analysis, we found that a big part of these differentially expressed genes were related to the process of stress or immunity response. Several putative determinants and candidate effectors responsive to Agrobacterium mediated transformation of wheat cells were discussed. We speculate that some of these genes are possibly related to Agrobacterium infection. Our results will help to understand the interaction between Agrobacterium and host cells, and may facilitate developing efficient transformation strategies in cereal crops.

An improved Agrobacterium-mediated transformation of recalcitrant indica rice (Oryza sativa L.) cultivars

Agrobacterium-mediated transformation of indica rice varieties has been quite difficult as these are recalcitrant to in vitro responses. In the present study, we established a high-efficiency Agrobacterium tumefaciens-mediated transformation system of rice (Oryza sativa L. ssp. indica) cv. IR-64, Lalat, and IET-4786. Agrobacterium strain EHA-101 harboring binary vector pIG121-Hm, containing a gene encoding for β-glucuronidase (GUS) and hygromycin resistance, was used in the transformation experiments. Manipulation of different concentrations of acetosyringone, days of co-culture period, bacterial suspension of different optical densities (ODs), and the concentrations of L-cysteine in liquid followed by solid co-culture medium was done for establishing the protocol. Among the different co-culture periods, 5 days of co-culture with bacterial cells (OD600 nm = 0.5-0.8) promoted the highest frequency of transformation (83.04 %) in medium containing L-cysteine (400 mg l(-1)). Putative transformed plants were analyzed for the presence of a transgene through genomic PCR and GUS histochemical analyses. Our results also suggest that different cultural conditions and the addition of L-cysteine in the co-culture medium improve the Agrobacterium-mediated transformation frequencies from an average of 12.82 % to 33.33 % in different indica rice cultivars.

Identification of QTLs associated with tissue culture response through sequencing-based genotyping of RILs derived from 93-11 × Nipponbare in rice (Oryza sativa)

KEY MESSAGE : The performance of callus induction and callus differentiation was evaluated by 9 indices for 140 RILs; 2 major QTLs associated with plant regeneration were identified. In order to investigate the genetic mechanisms of tissue culture response, 140 recombinant inbred lines (RILs) derived from 93-11 (Oryza sativa ssp. indica) × Nipponbare (Oryza sativa ssp. japonica) and a high quality genetic map based on the SNPs generated from deep sequencing of the RIL genomes, were used to identify the quantitative trait loci (QTLs) associated with in vitro tissue culture response (TCR) from mature seed in rice. The performance of callus induction was evaluated by indices of induced-callus color (ICC), induced-callus size (ICS), induced-callus friability (ICF) and callus induction rate (CIR), respectively, and the performance of callus differentiation was evaluated by indices of callus proliferation ability (CPA), callus browning tendency (CBT), callus greening ability (CGA), the average number of regenerated shoots per callus (NRS) and regeneration rate (%, RR), respectively. A total of 25 QTLs, 2 each for ICC, ICS, ICF, CIR and CBA, 3 for CPA, 4 each for CGA, NRS and RR, respectively, were detected and located on 8 rice chromosomes. Significant correlations were observed among the traits of CGA, NRS and RR, and QTLs identified for these three indices were co-located on chromosomes 3 and 7, and the additive effects came from both Nipponbare and 93-11, respectively. The results obtained from this study provide guidance for further fine mapping and gene cloning of the major QTL of TCR and the knowledge of the genes underlying the traits investigated would be very helpful for revealing the molecular bases of tissue culture response.

Sequential monitoring of transgene expression following Agrobacterium-mediated transformation of rice

Although Agrobacterium-mediated transformation technology is now used widely in rice, many varieties of indica-type rice are still recalcitrant to Agrobacterium-mediated transformation. It was reported recently that T-DNA integration into the rice genome could be the limiting step in this method. Here, we attempted to establish an efficient sequential monitoring system for stable transformation events by visualizing stable transgene expression using a non-destructive and highly sensitive visible marker. Our results demonstrate that click beetle luciferase (ELuc) is an excellent marker allowing the observation of transformed cells in rice callus, exhibiting a sensitivity >30-fold higher than that of firefly luciferase. Since we have previously shown that green fluorescent protein (GFP) is a useful visual marker with which to follow transient and/or stable expression of transgenes in rice, we constructed an enhancer trap vector using both the gfbsd2 (GFP fused to the N-terminus of blasticidin S deaminase) and eluc genes. In this vector, the eluc gene is under the control of the Cauliflower mosaic virus 35S minimal promoter, while the gfbsd2 gene is under the control of the full-length rice elongation factor gene promoter. Observation of transformed callus under a dissecting microscope demonstrated that the level of ELuc luminescence reflected exclusively stable transgene expression, and that both transient and stable expression could be monitored by the level of GFP fluorescence. Moreover, we show that our system enables sequential quantification of transgene expression via differential measurement of ELuc luminescence and GFP fluorescence.