A Protoplast Transient Expression System to Enable Molecular, Cellular, and Functional Studies in Phalaenopsis orchids

Diverse geotropic responses in the orchid family

In epiphytes, aerial roots are important to combat water-deficient, nutrient-poor, and high-irradiance microhabitats. However, whether aerial roots can respond to gravity and whether auxin plays a role in regulating aerial root development remain open-ended questions. Here, we investigated the gravitropic response of the epiphytic orchid Phalaenopsis aphrodite. Our data showed that aerial roots of P. aphrodite failed to respond to gravity, and this was correlated with a lack of starch granules/statolith sedimentation in the roots and the absence of the auxin efflux carrier PIN2 gene. Using an established auxin reporter, we discovered that auxin maximum was absent in the quiescent center of aerial roots of P. aphrodite. Also, gravity failed to trigger auxin redistribution in the root caps. Hence, loss of gravity sensing and gravity-dependent auxin redistribution may be the genetic factors contributing to aerial root development. Moreover, the architectural and functional innovations that achieve fast gravitropism in the flowering plants appear to be lost in both terrestrial and epiphytic orchids, but are present in the early diverged orchid subfamilies. Taken together, our findings provide physiological and molecular evidence to support the notion that epiphytic orchids lack gravitropism and suggest diverse geotropic responses in the orchid family.

Breeding of ornamental orchids with focus on Phalaenopsis: current approaches, tools, and challenges for this century

Ornamental orchid breeding programs have been conducted to develop commercially valuable cultivars with improved characteristics of commercial interest, such as size, flower color, pattern, shape, and resistance to pathogens. Conventional breeding, including sexual hybridization followed by selection of desirable characteristics in plants, has so far been the main method for ornamental breeding, but other techniques, including mutation induction by polyploidization and gamma irradiation, and biotechnological techniques, such as genetic transformation, have also been studied and used in ornamental breeding programs. Orchids are one of the most commercially important families in floriculture industry, having very particular reproductive biology characteristics and being a well-studied group of ornamentals in terms of genetic improvement. The present review focuses on the conventional and biotechnological techniques and approaches specially employed in breeding Phalaenopsis orchids, the genus with highest worldwide importance as an ornamental orchid, highlighting the main limitations and strengths of the approaches. Furthermore, new opportunities and future prospects for ornamental breeding in the CRISPR/Cas9 genome editing era are also discussed. We conclude that conventional hybridization remains the most used method to obtain new cultivars in orchids. However, the emergence of the first biotechnology-derived cultivars, as well as the new biotechnological tools available, such as CRISPR-Cas9, rekindled the full potential of biotechnology approaches and their importance for improve ornamental orchid breeding programs.

Preparation of leaf protoplasts from Populus (Populus × xiaohei T. S. Hwang et Liang) and establishment of transient expression system

Poplar, as a typical woody plant, is an ideal raw material for the production of lignocellulose biofuel. However, the longer life cycle is not conducive to the rapid identification of poplar genes. At present, protoplasts have been used for gene function identification and high-throughput analysis in many model plants. In this paper, a simplified and efficient protoplast isolation and transient expression system of Populus (Populus × xiaohei T. S. Hwang et Liang) is described. Firstly, we proposed an efficient enzyme hydrolysis method for isolating protoplasts from leaves of Populus × xiaohei. Secondly, we optimized the conditions of protoplast transformation mediated by PEG, and established an efficient transient expression system of protoplasts of Populus × xiaohei. Finally, the subcellular localization of three identified Dof transcription factors (PnDof19, PnDof20 and PnDof30) was also observed in the nucleus by using this scheme, which proved that the method was feasible. In general, this efficient method of protoplast isolation and transformation can be used for the study of protein subcellular localization and can be applied to other fields of molecular biology, such as protein interaction, gene activation and so on.

Development of a protoplast isolation system for functional gene expression and characterization using petals of Camellia Oleifera

Protoplasts preparation and purification have been frequently used in plant genetics and breeding studies, whereas application of protoplasts in woody plants is still in its infancy. Although transient gene expression using purified protoplasts is well-documented and widely used in model plants and agriculture crops, no instance of either stable transformation or transient gene expression in the woody plant Camellia Oleifera has as of yet been reported. Here, we developed a protoplast preparation and purification method using C. oleifera petals by optimizing osmotic condition with D-mannitol and polysaccharide-degrading enzyme concentrations for petal cell wall digestion, to reach a high efficiency of protoplast productivity and viability. The achieved protoplasts yield was approximately 1.42 × 107 cells per gram of petal material and the viability of protoplasts was up to 89%. In addition, we explored influencing factors of protoplast transformation, including concentrations of PEG4000 and plasmid DNA. The transformation efficiency of 81% could be reached under the optimized condition. This protoplast isolation and transient expression system were deployed to further identify the functional regulation of C. oleifera related genes and the subcellular distribution of their encoded products. In summary, the protoplast isolation and transient expression system we established using oil-tea tree petals is an efficient, versatile and time-saving system, being suitable for gene function characterization and molecular mechanism analysis.

Using A Protoplast Transformation System to Enable Functional Studies in Mangifera indica L

Mangoes (Mangifera indica L.) are an important kind of perennial fruit tree, but their biochemical testing method and transformation technology were insufficient and had not been rigorously explored. The protoplast technology is an excellent method for creating a rapid and effective tool for transient expression and transformation assays, particularly in plants that lack an Agrobacterium-mediated plant transformation system. This study optimized the conditions of the protoplast isolation and transformation system, which can provide a lot of help in the gene expression regulation study of mango. The most beneficial protoplast isolation conditions were 150 mg/mL of cellulase R-10 and 180 mg/mL of macerozyme R-10 in the digestion solution at pH 5.6 and 12 h of digestion time. The 0.16 M and 0.08 M mannitol in wash solution (WI) and suspension for counting (MMG), respectively, were optimal for the protoplast isolation yield. The isolated leaf protoplasts (~5.4 × 105 cells/10 mL) were transfected for 30 min mediated by 40% calcium-chloride-based polyethylene glycol (PEG)-4000-CaCl2, from which 84.38% of the protoplasts were transformed. About 0.08 M and 0.12 M of mannitol concentration in MMG and transfection solutions, respectively, were optimal for protoplast viability. Under the florescence signal, GFP was seen in the transformed protoplasts. This showed that the target gene was successfully induced into the protoplast and that it can be transcribed and translated. Experimental results in this paper show that our high-efficiency protoplast isolation and PEG-mediated transformation protocols can provide excellent new methods for creating a rapid and effective tool for the molecular mechanism study of mangoes.

Optimization of Isolation and Transformation of Protoplasts from Uncaria rhynchophylla and Its Application to Transient Gene Expression Analysis

Protoplast-based engineering has become an important tool for basic plant molecular biology research and developing genome-edited crops. Uncaria rhynchophylla is a traditional Chinese medicinal plant with a variety of pharmaceutically important indole alkaloids. In this study, an optimized protocol for U. rhynchophylla protoplast isolation, purification, and transient gene expression was developed. The best protoplast separation protocol was found to be 0.8 M D-mannitol, 1.25% Cellulase R-10, and 0.6% Macerozyme R-10 enzymolysis for 5 h at 26 °C in the dark with constant oscillation at 40 rpm/min. The protoplast yield was as high as 1.5 × 10⁷ protoplasts/g fresh weight, and the survival rate of protoplasts was greater than 90%. Furthermore, polyethylene glycol (PEG)-mediated transient transformation of U. rhynchophylla protoplasts was investigated by optimizing different crucial factors affecting transfection efficiency, including plasmid DNA amount, PEG concentration, and transfection duration. The U. rhynchophylla protoplast transfection rate was highest (71%) when protoplasts were transfected overnight at 24 °C with the 40 µg of plasmid DNA for 40 min in a solution containing 40% PEG. This highly efficient protoplast-based transient expression system was used for subcellular localization of transcription factor UrWRKY37. Finally, a dual-luciferase assay was used to detect a transcription factor promoter interaction by co-expressing UrWRKY37 with a UrTDC-promoter reporter plasmid. Taken together, our optimized protocols provide a foundation for future molecular studies of gene function and expression in U. rhynchophylla.

Co-immunoprecipitation for Assessing Protein-Protein Interactions in Agrobacterium-Mediated Transient Expression System in Nicotiana benthamiana

The characterization of protein-protein interactions (PPI) often provides functional information about a target protein. Yeast-two-hybrid (Y2H) and luminescence/fluorescence-based detections, therefore, have been widely utilized for assessing PPI. In addition, a co-immunoprecipitation (co-IP) method has also been adopted with transient protein expression in Nicotiana benthamiana (N. benthamiana) infiltrated with Agrobacterium tumefaciens. Herein, we describe a co-IP procedure in which structural maintenance of chromosome 1 (SMC1), identified from a Y2H screening, was verified as an interacting partner for microchidia 1 (MORC1), a protein well known for its function in plant immunity and epigenetics. SMC1 and MORC1 were transiently expressed in N. benthamiana when infiltrated by Agrobacterium with the respective genes. From this approach, we identified a region of SMC1 responsible for interacting with MORC1. The co-IP method, of which outputs are mainly from immunoblot analysis, provided information about target protein expression as well, which is often useful for troubleshooting. Using this feature, we showcased a PPI confirmation from our SMC1-MORC1 study in which a full-length SMC1 protein was not detectable, and, therefore, a subsequent truncated mutant analysis had to be employed for PPI verification.

Protoplast isolation and transient gene expression in different petunia cultivars

The protocol optimized for Petunia hybrida cv. Mirage Rose produced high protoplast yields in 3 out of other 11 cultivars (Damask White, Dreams White, and Opera Supreme White). Factors optimized in the protoplast transfection process showed that the best transfection efficiency (80%) was obtained using 2.5 × 10⁵ protoplast density, 40% polyethylene glycol (PEG) concentration, 10 µg plasmid DNA, and 15 min of transfection time. Assessing the usability of the protocol for other cultivars (Damask White, Dreams White, and Opera Supreme White), a reasonable protoplast transfection efficiency (⁓50%) was observed in the cultivars Dreams White and Opera Supreme White, with lower efficiency (⁓50%) observed in the cv. Damask White. The transient expression of enhanced green fluorescent protein (eGFP) in the nucleus of the transfected protoplasts of all cultivars was confirmed using PCR. This system could be valuable for genome editing of unwanted genes in petunias using the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated 9 (Cas9) technology. Furthermore, it could contribute to other studies on protein subcellular localization, protein-protein interactions, and functional gene expression in the petunias.

Isolation, purification and PEG-mediated transient expression of mesophyll protoplasts in Camellia oleifera

BACKGROUND

Camellia oleifera (C. oleifera) is a woody edible oil crop of great economic importance. Because of the lack of modern biotechnology research, C. oleifera faces huge challenges in both breeding and basic research. The protoplast and transient transformation system plays an important role in biological breeding, plant regeneration and somatic cell fusion. The objective of this present study was to develop a highly efficient protocol for isolating and purifying mesophyll protoplasts and transient transformation of C. oleifera. Several critical factors for mesophyll protoplast isolation from C. oleifera, including starting material (leaf age), pretreatment, enzymatic treatment (type of enzyme, concentration and digestion time), osmotic pressure and purification were optimized. Then the factors affecting the transient transformation rate of mesophyll protoplasts such as PEG molecular weights, PEG4000 concentration, plasmid concentration and incubation time were explored.

RESULTS

The in vitro grown seedlings of C. oleifera 'Huashuo' were treated in the dark for 24 h, then the 1st to 2nd true leaves were picked and vacuumed at - 0.07 MPa for 20 min. The maximum yield (3.5 × 10⁷/g·FW) and viability (90.9%) of protoplast were reached when the 1st to 2nd true leaves were digested in the enzymatic solution containing1.5% (w/v) Cellulase R-10, 0.5% (w/v) Macerozyme R-10 and 0.25% (w/v) Snailase and 0.4 M mannitol for 10 h. Moreover, the protoplast isolation method was also applicable to the other two cultivars, the protoplast yield for 'TXP14' and 'DP47' was 1.1 × 10⁷/g·FW and 2.6 × 10⁷/g·FW, the protoplast viability for 'TXP14' and 'DP47' was 90.0% and 88.2%. The purification effect was the best when using W buffer as a cleaning agent by centrifugal precipitation. The maximum transfection efficiency (70.6%) was obtained with the incubation of the protoplasts with 15 µg plasmid and 40% PEG4000 for 20 min.

CONCLUSION

In summary, a simple and efficient system for isolation and transient transformation of C. oleifera mesophyll protoplast is proposed, which is of great significance in various aspects of C. oleifera research, including the study of somatic cell fusion, genome editing, protein function, signal transduction, transcriptional regulation and multi-omics analyses.

In-depth analysis of genomes and functional genomics of orchid using cutting-edge high-throughput sequencing

High-throughput sequencing technology has been facilitated the development of new methodologies and approaches for studying the origin and evolution of plant genomes and subgenomes, population domestication, and functional genomics. Orchids have tens of thousands of members in nature. Many of them have promising application potential in the extension and conservation of the ecological chain, the horticultural use of ornamental blossoms, and the utilization of botanical medicines. However, a large-scale gene knockout mutant library and a sophisticated genetic transformation system are still lacking in the improvement of orchid germplasm resources. New gene editing tools, such as the favored CRISPR-Cas9 or some base editors, have not yet been widely applied in orchids. In addition to a large variety of orchid cultivars, the high-precision, high-throughput genome sequencing technology is also required for the mining of trait-related functional genes. Nowadays, the focus of orchid genomics research has been directed to the origin and classification of species, genome evolution and deletion, gene duplication and chromosomal polyploidy, and flower morphogenesis-related regulation. Here, the progressing achieved in orchid molecular biology and genomics over the past few decades have been discussed, including the evolution of genome size and polyploidization. The frequent incorporation of LTR retrotransposons play important role in the expansion and structural variation of the orchid genome. The large-scale gene duplication event of the nuclear genome generated plenty of recently tandem duplicated genes, which drove the evolution and functional divergency of new genes. The evolution and loss of the plastid genome, which mostly affected genes related to photosynthesis and autotrophy, demonstrated that orchids have experienced more separate transitions to heterotrophy than any other terrestrial plant. Moreover, large-scale resequencing provide useful SNP markers for constructing genetic maps, which will facilitate the breeding of novel orchid varieties. The significance of high-throughput sequencing and gene editing technologies in the identification and molecular breeding of the trait-related genes in orchids provides us with a representative trait-improving gene as well as some mechanisms worthy of further investigation. In addition, gene editing has promise for the improvement of orchid genetic transformation and the investigation of gene function. This knowledge may provide a scientific reference and theoretical basis for orchid genome studies.

DNA Free CRISPR/DCAS9 Based Transcriptional Activation System for UGT76G1 Gene in Stevia rebaudiana Bertoni Protoplasts

The UDP-glycosyltransferase 76G1 (UGT76G1) is responsible for the conversion of stevioside to rebaudioside A. Four single guide RNAs (sgRNAs) were designed from the UGT76G1 proximal promoter region of stevia by using the online-based tool, benchling. The dCas9 fused with VP64 as a transcriptional activation domain (TAD) was produced and purified for the formation of ribonucleoproteins (RNPs) by mixing with the in vitro transcribed sgRNAs. Protoplast yield was the highest from leaf mesophyll of in vitro grown stevia plantlets (3.16 × 10⁶/g of FW) using ES5 (1.25% cellulase R-10 and 0.75% macerozyme R-10). The RNPs were delivered into the isolated protoplasts through the Polyethylene glycol (PEG)-mediated transfection method. The highest endogenous activation of the UGT76G1 gene was detected at 27.51-fold after 24 h of transfection with RNP30 consisting of CRISPR/dCas9-TAD with sgRNA30 and a similar activation level was obtained using RNP18, RNP33, and RNP34, produced using sgRNA18, sgRNA33, and sgRNA34, respectively. Activation of UGT76G1 by RNP18 led to a significant increase in the expression of the rate-limiting enzyme UGT85C2 by 2.37-fold and there was an increasing trend in the expression of UGT85C2 using RNP30, RNP33, and RNP34. Successful application of CRISPR/dCas9-TAD RNP in activating specific genes can avoid the negative integration effects of introduced DNA in the host genome.

Protoplast technology enables the identification of efficient multiplex genome editing tools in Phalaenopsis

Phalaenopsis orchids are popular ornamental plants worldwide. The application and optimization of efficient CRISPR-Cas genome editing toolkits in Phalaenopsis greatly accelerate the development of orchid gene function and breeding research. However, these methods are greatly hindered by the deficiency of a rapid screening system. In this study, we established a fast and convenient Phalaenopsis protoplast technology for the identification of functional genome editing tools. Two multiplex genome editing tools, PTG-Cas9-HPG (PTG, polycistronic tRNA-gRNA) system and RMC-Cpf1-HPG (RMC, ribozyme-based multi-crRNA) system, were developed for Phalaenopsis genome editing and further evaluated by established protoplast technology. We successfully detected various editing events comprising substitution and indel at designed target sites of the PDS gene and MADS gene, showing that both PTG-Cas9-HPG and RMC-Cpf1-HPG multiplex genome editing systems are functional in Phalaenopsis. Additionally, by optimizing the promoter that drives Cpf1 expression, we found that Super promoter can significantly improve the editing efficiency of the RMC-Cpf1-HPG system. Altogether, we successfully developed two efficient multiplex genome editing systems, PTG-Cas9-HPG and RMC-Cpf1-HPG, for Phalaenopsis, and the established protoplast-based screening technology provides a valuable foundation for developing more diverse and efficient genome editing toolkits and facilitating the development of orchid precision breeding.

Co-option of the SHOOT MERISTEMLESS network regulates protocorm-like body development in Phalaenopsis aphrodite

The protocorm is a structure that is formed upon germination of an orchid seed. It lacks cotyledons and is ovoid in shape. The protocorm-like body (PLB), on the other hand, is a protocorm-like organ induced from somatic tissues. PLBs have been widely used for orchid micropropagation. Because of its unique structure and its application in the orchid industry, PLB development has drawn considerable interest from orchid and developmental biologists. Our previous genome-wide comparative transcriptome study demonstrated that protocorms and PLBs share similar molecular signatures and suggested that SHOOT MERISTEMLESS (STM)-dependent organogenesis is important for PLB development. Here, we show that overexpression of Phalaenopsis aphrodite STM (PaSTM) greatly enhances PLB regeneration from vegetative tissue-based explants of Phalaenopsis orchids, confirming its regulatory role in PLB development. Expression of PaSTM restored shoot meristem function of the Arabidopsis (Arabidopsis thaliana) stm-2 mutant. Moreover, we identified class S11 MYB transcription factors (TFs) as targets downstream of PaSTM. A cis-acting element, TTGACT, identified in the promoters of S11 MYB TFs was found to be important for PaSTM binding and activation. Overexpression of PaSTM or its downstream targets, PaMYB13, PaMYB14, and PaMYB17, enhanced de novo shoot regeneration in Arabidopsis, indicating the active role of the PaSTM-S11 PaMYB module in organogenesis. In summary, our data demonstrate that PaSTM is important for PLB development. The STM-S11 MYB regulatory module is evolutionarily conserved and may regulate shoot or shoot-related organ development in plants.

Development of a petal protoplast transfection system for Sinningia speciosa

PREMISE

Transient gene expression systems are powerful tools for studying gene interactions in plant species without available or stable genetic transformation protocols. We optimized a petal protoplast transformation protocol for Sinningia speciosa, a model plant, to study the development of floral symmetry.

METHODS AND RESULTS

A high yield of petal protoplasts was obtained using a 6-h enzyme digestion in a solution of 1.5% cellulase and 0.4% macerozyme. Modest transfection efficiency (average 41.4%) was achieved. The viability of the transfected protoplasts remained at more than 90%. A fusion of green fluorescent protein and CYCLOIDEA (SsCYC), the Teosinte branched 1/Cincinnata/Proliferating cell factor transcription factor responsible for floral symmetry, was subcellularly localized inside the nuclei of the protoplasts. Transiently overexpressing SsCYC indicates the success of this system, which resulted in the predicted increased (but nonsignificant) expression of its known target RADIALIS (SsRAD1), consistent with gene network expectations.

CONCLUSIONS

The transient transfection system presented herein can be effectively used to study gene-regulatory interactions in Gesneriaceae species.

CRISPR/SpCas9-mediated double knockout of barley Microrchidia MORC1 and MORC6a reveals their strong involvement in plant immunity, transcriptional gene silencing and plant growth

The Microrchidia (MORC) family proteins are important nuclear regulators in both animals and plants with critical roles in epigenetic gene silencing and genome stabilization. In the crop plant barley (Hordeum vulgare), seven MORC gene family members have been described. While barley HvMORC1 has been functionally characterized, very little information is available about other HvMORC paralogs. In this study, we elucidate the role of HvMORC6a and its potential interactors in regulating plant immunity via analysis of CRISPR/SpCas9-mediated single and double knockout (dKO) mutants, hvmorc1 (previously generated and characterized by our group), hvmorc6a, and hvmorc1/6a. For generation of hvmorc1/6a, we utilized two different strategies: (i) successive Agrobacterium-mediated transformation of homozygous single mutants, hvmorc1 and hvmorc6a, with the respective second construct, and (ii) simultaneous transformation with both hvmorc1 and hvmorc6a CRISPR/SpCas9 constructs. Total mutation efficiency in transformed homozygous single mutants ranged from 80 to 90%, while upon simultaneous transformation, SpCas9-induced mutation in both HvMORC1 and HvMORC6a genes was observed in 58% of T0 plants. Subsequent infection assays showed that HvMORC6a covers a key role in resistance to biotrophic (Blumeria graminis) and necrotrophic (Fusarium graminearum) plant pathogenic fungi, where the dKO hvmorc1/6a showed the strongest resistant phenotype. Consistent with this, the dKO showed highest levels of basal PR gene expression and derepression of TEs. Finally, we demonstrate that HvMORC1 and HvMORC6a form distinct nucleocytoplasmic homo-/heteromers with other HvMORCs and interact with components of the RNA-directed DNA methylation (RdDM) pathway, further substantiating that MORC proteins are involved in the regulation of TEs in barley.

Establishing an efficient protoplast transient expression system for investigation of floral thermogenesis in aroids

Floral thermogenesis is an important reproductive strategy for attracting pollinators. We developed essential biological tools for studying floral thermogenesis using two species of thermogenic aroids, Symplocarpus renifolius and Alocasia odora. Aroids contain many species with intense heat-producing abilities in their inflorescences. Several genes have been proposed to be involved in thermogenesis of these species, but biological tools for gene functional analyses are lacking. In this study, we aimed to develop a protoplast-based transient expression (PTE) system for the study of thermogenic aroids. Initially, we focused on skunk cabbage (Symplocarpus renifolius) because of its ability to produce intense as well as durable heat. In this plant, leaf protoplasts were isolated from potted and shoot tip-cultured plants with high efficiency (ca. 1.0 × 10⁵/g fresh weight), and more than half of these protoplasts were successfully transfected. Using this PTE system, we determined the protein localization of three mitochondrial energy-dissipating proteins, SrAOX, SrUCPA, and SrNDA1, fused to green fluorescent protein (GFP). These three GFP-fused proteins were localized in MitoTracker-stained mitochondria in leaf protoplasts, although the green fluorescent particles in protoplasts expressing SrUCPA-GFP were significantly enlarged. Finally, to assess whether the PTE system established in the leaves of S. renifolius is applicable for floral tissues of thermogenic aroids, inflorescences of S. renifolius and another thermogenic aroid (Alocasia odora) were used. Although protoplasts were successfully isolated from several tissues of the inflorescences, PTE systems worked well only for the protoplasts isolated from the female parts (slightly thermogenic or nonthermogenic) of A. odora inflorescences. Our developed system has a potential to be widely used in inflorescences as well as leaves in thermogenic aroids and therefore may be a useful biological tool for investigating floral thermogenesis.

Optimization of Protoplast Isolation from Leaf Mesophylls of Chinese Cabbage (Brassica rapa ssp. pekinensis) and Subsequent Transfection with a Binary Vector

Chinese cabbage is an important dietary source of numerous phytochemicals, including glucosinolates and anthocyanins. The selection and development of elite Chinese cabbage cultivars with favorable traits is hindered by a long breeding cycle, a complex genome structure, and the lack of an efficient plant transformation protocol. Thus, a protoplast transfection-based transformation method may be useful for cell-based breeding and functional studies involving Chinese cabbage plants. In this study, we established an effective method for isolating Chinese cabbage protoplasts, which were then transfected with the pCAMBIA1303 binary vector according to an optimized PEG-based method. More specifically, protoplasts were isolated following a 4 h incubation in a solution comprising 1.5% (v/v) cellulase, 0.25% (v/v) macerozyme, 0.25% (v/v) pectinase, 0.5 M mannitol, 15 mM CaCl₂, 25 mM KCl, 0.1% BSA, and 20 mM MES buffer, pH 5.7. This method generated 7.1 × 10⁶ protoplasts, 78% of which were viable. The gfp reporter gene in pCAMBIA1303 was used to determine the transfection efficiency. The Chinese cabbage protoplast transfection rate was highest (68%) when protoplasts were transfected with the 40 μg binary vector for 30 min in a solution containing 40% PEG. The presence of gusA and hptII in the protoplasts was confirmed by PCR. The methods developed in this study would be useful for DNA-free genome editing as well as functional and molecular investigations of Chinese cabbage.

Protoplast: A Valuable Toolbox to Investigate Plant Stress Perception and Response

Plants are constantly facing abiotic and biotic stresses. To continue to thrive in their environment, they have developed many sophisticated mechanisms to perceive these stresses and provide an appropriate response. There are many ways to study these stress signals in plant, and among them, protoplasts appear to provide a unique experimental system. As plant cells devoid of cell wall, protoplasts allow observations at the individual cell level. They also offer a prime access to the plasma membrane and an original view on the inside of the cell. In this regard, protoplasts are particularly useful to address essential biological questions regarding stress response, such as protein signaling, ion fluxes, ROS production, and plasma membrane dynamics. Here, the tools associated with protoplasts to comprehend plant stress signaling are overviewed and their potential to decipher plant defense mechanisms is discussed.

The ancestral duplicated DL/CRC orthologs, PeDL1 and PeDL2, function in orchid reproductive organ innovation

Orchid gynostemium, the fused organ of the androecium and gynoecium, and ovule development are unique developmental processes. Two DROOPING LEAF/CRABS CLAW (DL/CRC) genes, PeDL1 and PeDL2, were identified from the Phalaenopsis orchid genome and functionally characterized. Phylogenetic analysis indicated that the most recent common ancestor of orchids contained the duplicated DL/CRC-like genes. Temporal and spatial expression analysis indicated that PeDL genes are specifically expressed in the gynostemium and at the early stages of ovule development. Both PeDLs could partially complement an Arabidopsis crc-1 mutant. Virus-induced gene silencing (VIGS) of PeDL1 and PeDL2 affected the number of protuberant ovule initials differentiated from the placenta. Transient overexpression of PeDL1 in Phalaenopsis orchids caused abnormal development of ovule and stigmatic cavity of gynostemium. PeDL1, but not PeDL2, could form a heterodimer with Phalaenopsis equestris CINCINNATA 8 (PeCIN8). Paralogous retention and subsequent divergence of the gene sequences of PeDL1 and PeDL2 in P. equestris might result in the differentiation of function and protein behaviors. These results reveal that the ancestral duplicated DL/CRC-like genes play important roles in orchid reproductive organ innovation.

Highly Efficient Leaf Base Protoplast Isolation and Transient Expression Systems for Orchids and Other Important Monocot Crops

Versatile protoplast platforms greatly facilitate the development of modern botany. However, efficient protoplast-based systems are still challenging for numerous horticultural plants and crops. Orchids are globally cultivated ornamental and medicinal monocot plants, but few efficient protoplast isolation and transient expression systems have been developed. In this study, we established a highly efficient orchid protoplast isolation protocol by selecting suitable source materials and optimizing the enzymatic conditions, which required optimal D-mannitol concentrations (0.4-0.6 M) combined with optimal 1.2% cellulose and 0.6% macerozyme, 5 μM of 2-mercaptoethanol and 6 h digestion. Tissue- and organ-specific protoplasts were successfully isolated from young leaves [∼3.22 × 106/g fresh weight (FW)], flower pedicels (∼5.26 × 106/g FW), and young root tips (∼7.66 × 105/g FW) of Cymbidium orchids. This protocol recommends the leaf base tissues (the tender part of young leaves attached to the stem) as better source materials. High yielding viable protoplasts were isolated from the leaf base of Cymbidium (∼2.50 × 107/g FW), Phalaenopsis (1.83 × 107/g FW), Paphiopedilum (1.10 × 107/g FW), Dendrobium (8.21 × 106/g FW), Arundina (3.78 × 106/g FW) orchids, and other economically important monocot crops including maize (Zea mays) (3.25 × 107/g FW) and rice (Oryza sativa) (4.31 × 107/g FW), which showed marked advantages over previous mesophyll protoplast isolation protocols. Leaf base protoplasts of Cymbidium orchids were used for polyethylene glycol (PEG)-mediated transfection, and a transfection efficiency of more than 80% was achieved. This leaf base protoplast system was applied successfully to analyze the CsDELLA-mediated gibberellin signaling in Cymbidium orchids. We investigated the subcellular localization of the CsDELLA-green fluorescent protein fusion and analyzed the role of CsDELLA in the regulation of gibberellin to flowering-related genes via efficient transient overexpression and gene silencing of CsDELLA in Cymbidium protoplasts. This protoplast isolation and transient expression system is the most efficient based on the documented results to date. It can be widely used for cellular and molecular studies in orchids and other economically important monocot crops, especially for those lacking an efficient genetic transformation system in vivo.

Transient Gene Expression is an Effective Experimental Tool for the Research into the Fine Mechanisms of Plant Gene Function: Advantages, Limitations, and Solutions

A large data array on plant gene expression accumulated thanks to comparative omic studies directs the efforts of researchers to the specific or fine effects of the target gene functions and, as a consequence, elaboration of relatively simple and concurrently effective approaches allowing for the insight into the physiological role of gene products. Numerous studies have convincingly demonstrated the efficacy of transient expression strategy for characterization of the plant gene functions. The review goals are (i) to consider the advantages and limitations of different plant systems and methods of transient expression used to find out the role of gene products; (ii) to summarize the current data on the use of the transient expression approaches for the insight into fine mechanisms underlying the gene function; and (iii) to outline the accomplishments in efficient transient expression of plant genes. In general, the review discusses the main and critical steps in each of the methods of transient gene expression in plants; areas of their application; main results obtained using plant objects; their contribution to our knowledge about the fine mechanisms of the plant gene functions underlying plant growth and development; and clarification of the mechanisms regulating complex metabolic pathways.

Protoplast Isolation and Shoot Regeneration from Protoplast-Derived Callus of Petunia hybrida Cv. Mirage Rose

Despite the increasing use of protoplasts in plant biotechnology research, shoot regeneration from protoplasts remains challenging. In this study, we investigated the factors involved in protoplast isolation, callus induction, and shoot regeneration in Petunia hybrida cv. Mirage Rose. The following conditions were found to be most optimal for protoplast yield and viability: 0.6 M mannitol, 2.0% cellulase, and 6 h digestion time. A plating density of 10 × 10⁴ protoplasts/mL under osmoticum condition (0.58 M mannitol) showed high microcolony viability in liquid culture. The Kao and Michayluk medium was found to be appropriate for callus proliferation from microcalli under a 16-h light photoperiod. Calli cultured in Murashige and Skoog medium containing 1.0 mg/L 6-benzylaminopurine and 0.2 mg/L 3-indole butyric acid showed the highest shoot regeneration frequency and number of shoots obtained per explant. Random amplification of polymorphic DNA analysis showed that the protoplast-derived shoots exhibited the same banding patterns as those of donor plants. Collectively, these findings can contribute to solving problems encountered in protoplast isolation and shoot regeneration in other petunia cultivars and related species. As the protocol developed by us is highly reproducible, it can be applied in biotechnology research on P. hybrida cv. Mirage Rose.

Development of a rapid and efficient protoplast isolation and transfection method for chickpea (Cicer arietinum)

Chickpea (Cicer arietinum L.) is the second most important grain legume worldwide. Recent advances in the sequencing of the chickpea genome has provided a new and valuable resource to aid efforts in gene discovery and crop trait improvement. Technical difficulties in stable chickpea transgenics and the lack of a transient expression system for rapid analysis of gene expression and function; however, has limited the usefulness of this genomic resource. As a step toward alleviating this limitation, we report here the development of a simple and efficient transient gene expression protocol. Using leaves from chickpea seedlings, we have established a procedure that enables the generation of large quantities of vital chickpea protoplasts within only a few hours. In addition, we have optimized a PEG-calcium-mediated transfection method to efficiently deliver exogenous DNA into the chickpea protoplast. The current study is the first to present a detailed step-by-step procedures for protoplast isolation, evaluation, transfection, and application in chickpea. In addition, we optimize the transfection efficiency which has not been previously reported. Our protoplast transfection approach provides a platform that will allow rapid high-throughput screening and systematic characterization of gene expression and function. Knowledge gained through such studies will benefit current efforts to improve chickpea production and quality.•Modified enzymatic digestion solution for higher yield and viability.•Optimize transfection of chickpea protoplasts.

Highly Efficient Protoplast Isolation and Transient Expression System for Functional Characterization of Flowering Related Genes in Cymbidium Orchids

Protoplast systems have been proven powerful tools in modern plant biology. However, successful preparation of abundant viable protoplasts remains a challenge for Cymbidium orchids. Herein, we established an efficient protoplast isolation protocol from orchid petals through optimization of enzymatic conditions. It requires optimal D-mannitol concentration (0.5 M), enzyme concentration (1.2 % (w/v) cellulose and 0.6 % (w/v) macerozyme) and digestion time (6 h). With this protocol, the highest yield (3.50 × 10⁷/g fresh weight of orchid tissue) and viability (94.21%) of protoplasts were obtained from flower petals of Cymbidium. In addition, we achieved high transfection efficiency (80%) through the optimization of factors affecting polyethylene glycol (PEG)-mediated protoplast transfection including incubation time, final PEG4000 concentration and plasmid DNA amount. This highly efficient protoplast-based transient expression system (PTES) was further used for protein subcellular localization, bimolecular fluorescence complementation (BiFC) assay and gene regulation studies of flowering related genes in Cymbidium orchids. Taken together, our protoplast isolation and transfection protocol is highly efficient, stable and time-saving. It can be used for gene function and molecular analyses in orchids and other economically important monocot crops.

Identification and Characterization of NPR1 and PR1 Homologs in Cymbidium orchids in Response to Multiple Hormones, Salinity and Viral Stresses

The plant nonexpressor of pathogenesis-related 1 (NPR1) and pathogenesis-associated 1 (PR1) genes play fundamental roles in plant immunity response, as well as abiotic-stress tolerance. Nevertheless, comprehensive identification and characterization of NPR1 and PR1 homologs has not been conducted to date in Cymbidium orchids, a valuable industrial crop cultivated as ornamental and medicinal plants worldwide. Herein, three NPR1-like (referred to as CsNPR1-1, CsNPR1-2, and CsNPR1-3) and two PR1-like (CsPR1-1 and CsPR1-2) genes were genome-widely identified from Cymbidium orchids. Sequence and phylogenetic analysis revealed that CsNPR1-1 and CsNPR1-2 were grouped closest to NPR1 homologs in Zea mays (sharing 81.98% identity) and Phalaenopsis (64.14%), while CsNPR1-3 was classified into a distinct group with Oryza sativaNPR 3 (57.72%). CsPR1-1 and CsPR1-2 were both grouped closest to Phalaenopsis PR1 and other monocot plants. Expression profiling showed that CsNPR1 and CsPR1 were highly expressed in stem/pseudobulb and/or flower. Salicylic acid (SA) and hydrogen peroxide (H₂O₂) significantly up-regulated expressions of CsNPR1-2, CsPR1-1 and CsPR1-2, while CsNPR1-3, CsPR1-1 and CsPR1-2 were significantly up-regulated by abscisic acid (ABA) or salinity (NaCl) stress. In vitro transcripts of entire Cymbidium mosaic virus (CymMV) genomic RNA were successfully transfected into Cymbidium protoplasts, and the CymMV infection up-regulated the expression of CsNPR1-2, CsPR1-1 and CsPR1-2. Additionally, these genes were transiently expressed in Cymbidium protoplasts for subcellular localization analysis, and the presence of SA led to the nuclear translocation of the CsNPR1-2 protein, and the transient expression of CsNPR1-2 greatly enhanced the expression of CsPR1-1 and CsPR1-2. Collectively, the CsNPR1-2-mediated signaling pathway is SA-dependent, and confers to the defense against CymMV infection in Cymbidium orchids.

Genome-wide analysis of small RNAs from Odontoglossum ringspot virus and Cymbidium mosaic virus synergistically infecting Phalaenopsis

Cymbidium mosaic virus (CymMV) and Odontoglossum ringspot virus (ORSV) are the two most prevalent viruses infecting orchids and causing economic losses worldwide. Mixed infection of CymMV and ORSV could induce intensified symptoms as early at 10 days post-inoculation in inoculated Phalaenopsis amabilis, where CymMV pathogenesis was unilaterally enhanced by ORSV. To reveal the antiviral RNA silencing activity in orchids, we characterized the viral small-interfering RNAs (vsiRNAs) from CymMV and ORSV singly or synergistically infecting P. amabilis. We also temporally classified the inoculated leaf-tip tissues and noninoculated adjacent tissues as late and early stages of infection, respectively. Regardless of early or late stage with single or double infection, CymMV and ORSV vsiRNAs were predominant in 21- and 22-nt sizes, with excess positive polarity and under-represented 5'-guanine. While CymMV vsiRNAs mainly derived from RNA-dependent RNA polymerase-coding regions, ORSV vsiRNAs encompassed the coat protein gene and 3'-untranslated region, with a specific hotspot residing in the 3'-terminal pseudoknot. With double infection, CymMV vsiRNAs increased more than 5-fold in number with increasing virus titres. Most vsiRNA features remained unchanged with double inoculation, but additional ORSV vsiRNA hotspot peaks were prominent. The potential vsiRNA-mediated regulation of the novel targets in double-infected tissues thereby provides a different view of CymMV and ORSV synergism. Hence, temporally profiled vsiRNAs from taxonomically distinct CymMV and ORSV illustrate active antiviral RNA silencing in their natural host, Phalaenopsis, during both early and late stages of infection. Our findings provide insights into offence-defence interactions among CymMV, ORSV and orchids.

An Efficient and Economical Protocol for Isolating, Purifying and PEG-Mediated Transient Gene Expression of Chinese Kale Hypocotyl Protoplasts

In this study, we report the isolation and purification of protoplasts from Chinese kale (Brassica oleracea var. alboglabra) hypocotyls, and their transient gene expression transformation and subcellular localization of BaMYB75 (Bol042409). The upshot is that the vintage protocol included 5-d hypocotyls that were enzymatically hydrolyzed for 8 h in enzyme solution (3.0% cellulase, 0.5% pectolase, and 0.5 M mannitol), and the protoplasts were purified by precipitation. The total yield of protoplasts was 8 × 10⁵ protoplast g^-1 fresh weight, and the protoplasts' viability was 90%. The maximum transformation efficiency obtained by using green fluorescent protein (GFP) as a detection gene was approximately 45% when the polyethylene glycol (PEG)4000 concentration was 40% and transformation time was 20 min. In addition, BaMYB75 was ultimately localized in the nucleus of Chinese kale hypocotyl protoplasts, verifying the validity and reliability of this transient transformation system. An effective and economical hypocotyl protoplast isolation, purification, and transformation system was established for Chinese kale in this study. This effectively avoided interference of chloroplast autofluorescence compared to using mesophyll cells, laying the foundation for future research in the molecular biology of Brassica vegetables.