A Unified Agrobacterium-Mediated Transformation Protocol for Alfalfa (Medicago sativa L.) and Medicago truncatula

The P-type ATPase gene AHA5 is involved in proanthocyanidins accumulation in Medicago truncatula

Proanthocyanidins (PAs) are the second most abundant plant phenolic natural products. The proton membrane H+-ATPase (AHA) is required for PA transportation in vacuoles, but it remains unclear which AHA gene(s) encode tonoplast proton pump in M. truncatula. Here, we identified three Tnt1 mutant lines of MtAHA5, resulting in PAs deficit in seeds. MtAHA5 was preferentially expressed in developing seeds, exhibiting its highest transcript levels at early stages. Although MtAHA3, MtAHA4, and MtAHA9 shared similar transcript patterns with MtAHA5 and other structural genes involved in PA biosynthesis, their mutant lines did not exhibit any PA-deficit phenotypes. Subcellular localization analysis demonstrated that MtAHA5 is targeted to the tonoplast in tobacco leaves; conversely, MtAHA3 and MtAHA9 are localized to the cytoplasm, suggesting that MtAHA5 acts as a tonoplast proton pump but not MtAHA3 or MtAHA9. Further genetic analyses revealed that MtAHA5 could complement the PA-deficit phenotype in mtaha5 mutants and ataha10 mutants. Transient transcription assays indicated that MtAHA5 is activated by the MBW complex to regulate the PA accumulation. Collectively, our findings suggest that MtAHA5 serves as a tonoplast proton pump to generate the driving force for MATE1-mediated transport of PA precursors into vacuoles.

Shading increases the susceptibility of alfalfa (Medicago sativa) to Pst. DC3000 by inhibiting the expression of MsIFS1

Shade is a stressful factor for most plants, leading to both morphological and physiological changes, and often resulting in increased susceptibility to diseases and pathogen attacks. Our study revealed that the isoflavonoid synthesis pathway was inhibited in alfalfa under shade, resulting in a significant reduction in disease resistance. Overexpression of MsIFS1, a switch regulator in isoflavonoid synthesis, led to a notable increase in endogenous isoflavonoids and enhanced resistance to Pseudomonas syringae pv. tomato DC3000 (Pst. DC3000). Conversely, MsIFS1-RNAi had the opposite effect. Yeast one-hybrid (Y1H) assays revealed that the shade-responsive transcription factor MsWRKY41 could directly bind to the MsIFS1 promoter. This interaction was confirmed through Dual-Luciferase Reporter (Dual-LUC) and Chromatin Immunoprecipitation coupled with quantitative PCR (ChIP-qPCR) assays, both in vitro and in vivo. Overexpression of MsWRKY41 not only enhanced alfalfa's resistance to Pst. DC3000 but also promoted the accumulation of isoflavonoids. Additionally, yeast two-hybrid (Y2H) assays showed that neither MsWRKY41 nor MsIFS1 physically interacted with the Type III effector (T3SE) HopZ1 secreted by Pst. DC3000, suggesting that the MsWRKY41-MsIFS1 module is not a direct target of HopZ1. These findings provide valuable theoretical insights and genetic resources for the development of shade-tolerant alfalfa with enhanced disease resistance.

Improved Protocol for Efficient Agrobacterium-Mediated Transient Gene Expression in Medicago sativa L

Medicago sativa L. (Alfalfa) is a globally recognized forage legume that has recently gained attention for its high protein content, making it suitable for both human and animal consumption. However, due to its perennial nature and autotetraploid genetics, conventional plant breeding requires a longer timeframe compared to other crops. Therefore, genetic engineering offers a faster route for trait modification and improvement. Here, we describe a protocol for achieving efficient transient gene expression in alfalfa through genetic transformation with the Agrobacterium tumefaciens pCAMBIA1304 vector. This vector contains the reporter genes β-glucuronidase (GUS) and green fluorescent protein (GFP), along with a selectable hygromycin B phosphotransferase gene, all driven by the CaMV 35s promoter. Various transformation parameters-such as different explant types, leaf ages, leaf sizes, wounding types, bacterial concentrations (OD600nm), tissue preculture periods, infection periods, co-cultivation periods, and different concentrations of acetosyringone, silver nitrate, and calcium chloride-were optimized using 3-week-old in vitro-grown plantlets. Results were attained from data based on the semi-quantitative observation of the percentage and number of GUS spots on different days of agro-infection in alfalfa explants. The highest percentage of GUS positivity (76.2%) was observed in 3-week-old, scalpel-wounded, segmented alfalfa leaf explants after 3 days of agro-infection at a bacterial concentration of 0.6, with 2 days of preculture, 30 min of co-cultivation, and the addition of 150 µM acetosyringone, 4 mM calcium chloride, and 75 µM silver nitrate. The transient expression of genes of interest was confirmed via histochemical GUS and GFP assays. The results based on transient reporter gene expression suggest that various factors influence T-DNA delivery in the Agrobacterium-mediated transformation of alfalfa. The improved protocol can be used in stable transformation techniques for alfalfa.

A Novel MsEOBI-MsPAL1 Module Enhances Salinity Stress Tolerance, Floral Scent Emission and Seed Yield in Alfalfa

Alfalfa (Medicago sativa L.) is an important and widely cultivated forage legume, yet its yield is constrained by salinity stress. In this study, we characterized an R2R3-MYB transcription factor MsEOBI in alfalfa. Its salt tolerance function and regulatory pathways were investigated. The nuclear-localized MsEOBI functions as a transcriptional activator, enhancing salinity tolerance by promoting the biosynthesis of flavonoids and lignin, as well as facilitating the scavenging of reactive oxygen species (ROS). Additionally, MsEOBI promotes pollinator attraction and increases seed yield by activating the biosynthesis of volatile phenylpropanoids. Yeast one-hybrid (Y1H), dual-luciferase reporter and chromatin immunoprecipitation coupled with quantitative PCR (ChIP-qPCR) assays demonstrated that MsEOBI directly binds to the promoter regions of MsPAL1, a key gene in the phenylpropanoid pathway, thereby activating its expression. Overexpression of MsPAL1 enhances salinity tolerance in alfalfa. These findings elucidate the role of the MsEOBI-MsPAL1 regulatory module and provide valuable genetic resources for the future breeding of salt-tolerant alfalfa varieties.

An inducible CRISPR activation tool for accelerating plant regeneration

The inducible CRISPR activation (CRISPR-a) system offers unparalleled precision and versatility for regulating endogenous genes, making it highly sought after in plant research. In this study, we developed a chemically inducible CRISPR-a tool for plants called ER-Tag by combining the LexA-VP16-ER inducible system with the SunTag CRISPR-a system. We systematically compared different induction strategies and achieved high efficiency in target gene activation. We demonstrated that guide RNAs can be multiplexed and pooled for large-scale screening of effective morphogenic genes and gene pairs involved in plant regeneration. Further experiments showed that induced activation of these morphogenic genes can accelerate regeneration and improve regeneration efficiency in both eudicot and monocot plants, including alfalfa, woodland strawberry, and sheepgrass. Our study expands the CRISPR toolset in plants and provides a powerful new strategy for studying gene function when constitutive expression is not feasible or ideal.

Functional Characterization of the MsFKF1 Gene Reveals Its Dual Role in Regulating the Flowering Time and Plant Height in Medicago sativa L

Alfalfa (M. sativa), a perennial legume forage, is known for its high yield and good quality. As a long-day plant, it is sensitive to changes in the day length, which affects the flowering time and plant growth, and limits alfalfa yield. Photoperiod-mediated delayed flowering in alfalfa helps to extend the vegetative growth period and increase the yield. We isolated a blue-light phytohormone gene from the alfalfa genome that is an ortholog of soybean FKF1 and named it MsFKF1. Gene expression analyses showed that MsFKF1 responds to blue light and the circadian clock in alfalfa. We found that MsFKF1 regulates the flowering time through the plant circadian clock pathway by inhibiting the transcription of E1 and COL, thus suppressing FLOWERING LOCUS T a1 (FTa1) transcription. In addition, transgenic lines exhibited higher plant height and accumulated more biomass in comparison to wild-type plants. However, the increased fiber (NDF and ADF) and lignin content also led to a reduction in the digestibility of the forage. The key genes related to GA biosynthesis, GA20OX1, increased in the transgenic lines, while GA2OX1 decreased for the inactive GA transformation. These findings offer novel insights on the function of MsFKF1 in the regulation of the flowering time and plant height in cultivated M. sativa. These insights into MsFKF1's roles in alfalfa offer potential strategies for molecular breeding aimed at optimizing flowering time and biomass yield.

CRISPR/Cas9-Mediated Generation of Mutant Lines in Medicago truncatula Indicates a Symbiotic Role of MtLYK10 during Nodule Formation

CRISPR/Cas9 systems are commonly used for plant genome editing; however, the generation of homozygous mutant lines in Medicago truncatula remains challenging. Here, we present a CRISPR/Cas9-based protocol that allows the efficient generation of M. truncatula mutants. Gene editing was performed for the LysM receptor kinase gene MtLYK10 and two major facilitator superfamily transporter genes. The functionality of CRISPR/Cas9 vectors was tested in Nicotiana benthamiana leaves by editing a co-transformed GUSPlus gene. Transformed M. truncatula leaf explants were regenerated to whole plants at high efficiency (80%). An editing efficiency (frequency of mutations at a given target site) of up to 70% was reached in the regenerated plants. Plants with MtLYK10 knockout mutations were propagated, and three independent homozygous mutant lines were further characterized. No off-target mutations were identified in these lyk10 mutants. Finally, the lyk10 mutants and wild-type plants were compared with respect to the formation of root nodules induced by nitrogen-fixing Sinorhizobium meliloti bacteria. Nodule formation was considerably delayed in the three lyk10 mutant lines. Surprisingly, the size of the rare nodules in mutant plants was higher than in wild-type plants. In conclusion, the symbiotic characterization of lyk10 mutants generated with the developed CRISPR/Cas9 protocol indicated a role of MtLYK10 in nodule formation.

Novel underlying regulatory mechanism of the MsDAD2-mediated salt stress response in alfalfa

Alfalfa (Medicago sativa L.), a crucial and widely grown forage legume, faces yield and quality challenges due to salinity stress. The defender against apoptotic death (DAD) gene, recognized initially as an apoptosis suppressor in mammals, plays a pivotal role in catalyzing N-glycosylation, acting as a positive regulator for protein folding and endoplasmic reticulum (ER) export. Here, we found that the MsDAD2 gene was specially induced in the salt-tolerant alfalfa cultivar (DL) under salinity stress, but not in the salt-sensitive cultivar (SD). Overexpression of MsDAD2 enhanced the salinity resistance of transgenic alfalfa by promoting NAD(P)H-quinone oxidoreductase (NQO1) and cytochrome b6f complex subunit (Cyt b6/f) expression, thereby mitigating reactive oxygen species (ROS) production. ChIP-qPCR analysis suggested that the differential expression of MsDAD2 in DL and SD under salinity stress may be linked to dynamic histone modifications in its promoter. Therefore, our findings elucidate a novel regulatory mechanism of MsDAD2 in alfalfa's response to salinity stress, underscoring its significance as a target for alfalfa breeding to enhance salt tolerance.

Co-expression of GR79 EPSPS and GAT generates high glyphosate-resistant alfalfa with low glyphosate residues

Weed competition seriously threatens the yield of alfalfa, the most important forage legume worldwide, thus generating herbicide-resistant alfalfa varieties is becoming a necessary cost-effective strategy to assist farmers for weed control. Here, we report the co-expression of plant codon-optimized forms of GR79 EPSPS (pGR79 EPSPS) and N-acetyltransferase (pGAT) genes, in alfalfa, via Agrobacterium-mediated transformation. We established that the pGR79 EPSPS-pGAT co-expression alfalfa lines were able to tolerate up to tenfold higher commercial usage of glyphosate and produced approximately ten times lower glyphosate residues than the conventional cultivar. Our findings generate an elite herbicide-resistant germplasm for alfalfa breeding and provide a promising strategy for developing high-glyphosate-resistant and low-glyphosate-residue forages.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42994-023-00119-3.

A 3-Ketoacyl-CoA Synthase 10 (KCS10) Homologue from Alfalfa Enhances Drought Tolerance by Regulating Cuticular Wax Biosynthesis

Cuticular wax, forming the first line of defense against adverse environmental stresses, comprises very long-chain fatty acids (VLCFAs) and their derivatives. 3-Ketoacyl-CoA synthase (KCS) is a rate-limiting enzyme for VLCFA biosynthesis. In this study, we isolated KCS10, a KCS gene from alfalfa, and analyzed the effect of gene expression on wax production and drought stress in transgenic plants. MsKCS10 overexpression increased compact platelet-like crystal deposition and promoted primary alcohol biosynthesis through acyl reduction pathways in alfalfa leaves. Overexpression of MsKCS10 induced the formation of coiled-rodlet-like crystals and increased n-alkane content through decarbonylation pathways in tobacco and tomato fruits. Overexpression of MsKCS10 enhanced drought tolerance by limiting nonstomatal water loss, improving photosynthesis, and maintaining osmotic potential under drought stress in transgenic tobacco. In summary, MsKCS10 plays an important role in wax biosynthesis, wax crystal morphology, and drought tolerance, although the mechanisms are different among the plant species. MsKCS10 can be targeted in future breeding programs to improve drought tolerance in plants.

Recalcitrance to transformation, a hindrance for genome editing of legumes

Plant genome editing, a recently discovered method for targeted mutagenesis, has emerged as a promising tool for crop improvement and gene function research. Many genome-edited plants, such as rice, wheat, and tomato, have emerged over the last decade. As the preliminary steps in the procedure for genome editing involve genetic transformation, amenability to genome editing depends on the efficiency of genetic engineering. Hence, there are numerous reports on the aforementioned crops because they are transformed with relative ease. Legume crops are rich in protein and, thus, are a favored source of plant proteins for the human diet in most countries. However, legume cultivation often succumbs to various biotic/abiotic threats, thereby leading to high yield loss. Furthermore, certain legumes like peanuts possess allergens, and these need to be eliminated as these deprive many people from gaining the benefits of such crops. Further genetic variations are limited in certain legumes. Genome editing has the potential to offer solutions to not only combat biotic/abiotic stress but also generate desirable knock-outs and genetic variants. However, excluding soybean, alfalfa, and Lotus japonicus, reports obtained on genome editing of other legume crops are less. This is because, excluding the aforementioned three legume crops, the transformation efficiency of most legumes is found to be very low. Obtaining a higher number of genome-edited events is desirable as it offers the option to genotypically/phenotypically select the best candidate, without the baggage of off-target mutations. Eliminating the barriers to genetic engineering would directly help in increasing genome-editing rates. Thus, this review aims to compare various legumes for their transformation, editing, and regeneration efficiencies and discusses various solutions available for increasing transformation and genome-editing rates in legumes.

Development of two protocols for Agrobacterium-mediated transformation of white clover (Trifolium repens) via the callus system

White clover (Trifolium repens) is one of the most widely cultivated livestock forage plants whose persistence is severely affected by abiotic stresses. For the white clover, efficient regeneration systems is still a great necessity. In this study, inoculating 4-day-old cotyledons into MS media fortified with 0.4 mg·L-1 6-BA and 2 mg·L-1 2,4-D significantly increased the callus induction rate. Roots and cotyledons proved to be better explants, followed by hypocotyls, leaves, and petioles for callus induction. The development of differentiated structures occurred effectively on MS supplemented with 1 mg·L-1 6-BA and 0.1 mg·L-1 NAA. To increase transformation, we investigated various factors affecting the Agrobacterium tumefaciens transformation in white clover. The optimal conditions for root-derived callus and 4-day-old cotyledons were as follows: Agrobacterium suspension density with OD600 of 0.5, 20 mg·L-1 AS, and 4 days of co-cultivation duration. Subsequently, we developed two transformation protocols: transformation after callus induction from 4-day-old roots (Protocol A) and transformation before initiation of callus from cotyledons (Protocol B). The transformation frequencies varied from 1.92 to 3.17% in Protocol A and from 2.76 to 3.47% in Protocol B. We report the possibility to regenerate multiple transgenic white clover plants from a single genetic background. Our research may also contribute to successful genetic manipulation and genome editing in white clover.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03591-2.

Highly Efficient Gene Knockout in Medicago truncatula Genotype R108 Using CRISPR-Cas9 System and an Optimized Agrobacterium Transformation Method

Medicago truncatula is the model plant species for studying symbioses with nitrogen-fixing rhizobia and arbuscular mycorrhizae, where edited mutants are invaluable for elucidating the contributions of known genes in these processes. Streptococcus pyogenes Cas9 (SpCas9)-based genome editing is a facile means of achieving loss of function, including where multiple gene knockouts are desired in a single generation. We describe how the user can customize our vector to target single or multiple genes, then how the vector is used to make M. truncatula transgenic plants containing target site mutations. Finally, obtaining transgene-free homozygous mutants is covered.

Utilization of Legume-Nodule Bacterial Symbiosis in Phytoremediation of Heavy Metal-Contaminated Soils

Simple Summary

The legume–rhizobium symbiosis is one of the most beneficial interactions with high importance in agriculture, as it delivers nitrogen to plants and soil, thereby enhancing plant growth. Currently, this symbiosis is increasingly being exploited in phytoremediation of metal contaminated soil to improve soil fertility and simultaneously metal extraction or stabilization. Rhizobia increase phytoremediation directly by nitrogen fixation, protection of plants from pathogens, and production of plant growth-promoting factors and phytohormones.

Abstract

With the increasing industrial activity of the growing human population, the accumulation of various contaminants in soil, including heavy metals, has increased rapidly. Heavy metals as non-biodegradable elements persist in the soil environment and may pollute crop plants, further accumulating in the human body causing serious conditions. Hence, phytoremediation of land contamination as an environmental restoration technology is desirable for both human health and broad-sense ecology. Legumes (Fabaceae), which play a special role in nitrogen cycling, are dominant plants in contaminated areas. Therefore, the use of legumes and associated nitrogen-fixing rhizobia to reduce the concentrations or toxic effects of contaminants in the soil is environmentally friendly and becomes a promising strategy for phytoremediation and phytostabilization. Rhizobia, which have such plant growth-promoting (PGP) features as phosphorus solubilization, phytohormone synthesis, siderophore release, production of beneficial compounds for plants, and most of all nitrogen fixation, may promote legume growth while diminishing metal toxicity. The aim of the present review is to provide a comprehensive description of the main effects of metal contaminants in nitrogen-fixing leguminous plants and the benefits of using the legume–rhizobium symbiosis with both wild-type and genetically modified plants and bacteria to enhance an efficient recovery of contaminated lands.

Identification of a gene responsible for seedpod spine formation and other phenotypic alterations using whole-genome sequencing analysis in Medicago truncatula

In nature, some plant species produce seedpods with spines, which is an adaptive biological trait for protecting the seed and helping seed dispersal. However, the molecular mechanism of spine formation is still unclear. While conducting routine tissue culture and transformation in the model legume Medicago truncatula, we identified a smooth seedpod (ssp1) mutant with a suite of other phenotypic changes. Preliminary analysis showed that the mutation was derived from the tissue culture process. Genetic segregation analysis suggested that ssp1 is a recessive mutant. By combining whole-genome sequencing and bioinformatics analysis, we found that the mutant phenotype was caused by a single nucleotide polymorphism and a 30 bp deletion in the gene locus Medtr4g039430, named SSP1. Complementation of the M. truncatula ssp1 and Arabidopsis twd1 mutants showed complete restoration, indicating that SSP1 is an ortholog of Arabidopsis TWD1 which encodes an immunophilin-like FK506-binding protein 42. The formation of spines on seedpods is associated with auxin transport. The method used in this study offers an effective way for detecting genes responsible for somaclonal variations. The results demonstrate, for the first time, that SSP1 plays a crucial role in the determination of spine formation on seedpods.

Genetic transformation of legumes: an update

This review summarizes the recent advances in legume genetic transformation and provides an insight into the critical factors that play a major role in the process. It also sheds light on some of the potential areas which may ameliorate the transformation of legumes. Legumes are an important group of dicotyledonous plants, highly enriched in proteins and minerals. Majority of the legume plants are cultivated in the arid and semi-arid parts of the world, and hence said to be climate resilient. They have the capability of atmospheric nitrogen fixation and thus play a vital role in the ecological sphere. However, the worldwide production of legumes is somehow not up to the mark and the yields are greatly affected by various biotic and abiotic stress factors. Genetic engineering strategies have emerged as a core of plant biology and remarkably facilitate the crop improvement programmes. A significant progress has been made towards the optimization of efficient transformation system for legume plants over the years but this group is still underutilized in comparison to other crops. Among the variety of available DNA delivery systems, Agrobacterium-mediated and particle bombardment have been primarily deployed for optimization and trait improvement. However, recalcitrance and genotype-dependence are some of the major bottlenecks for successful transformation. In this context, the present review summarizes the advances taken place in the area of legume transformation and provides an insight into the present scenario. The challenges and future possibilities for yield improvement have also been discussed.

Increased Ascorbate Biosynthesis Does Not Improve Nitrogen Fixation Nor Alleviate the Effect of Drought Stress in Nodulated Medicago truncatula Plants

Legume plants are able to establish nitrogen-fixing symbiotic relations with Rhizobium bacteria. This symbiosis is, however, affected by a number of abiotic constraints, particularly drought. One of the consequences of drought stress is the overproduction of reactive oxygen (ROS) and nitrogen species (RNS), leading to cellular damage and, ultimately, cell death. Ascorbic acid (AsA), also known as vitamin C, is one of the antioxidant compounds that plants synthesize to counteract this oxidative damage. One promising strategy for the improvement of plant growth and symbiotic performance under drought stress is the overproduction of AsA via the overexpression of enzymes in the Smirnoff-Wheeler biosynthesis pathway. In the current work, we generated Medicago truncatula plants with increased AsA biosynthesis by overexpressing MtVTC2, a gene coding for GDP-L-galactose phosphorylase. We characterized the growth and physiological responses of symbiotic plants both under well-watered conditions and during a progressive water deficit. Results show that increased AsA availability did not provide an advantage in terms of plant growth or symbiotic performance either under well-watered conditions or in response to drought.

Developing Portulaca oleracea as a model system for functional genomics analysis of C4/CAM photosynthesis

Previously regarded as an intriguing photosynthetic curiosity, the occurrence of C4 and Crassulacean acid metabolism (CAM) photosynthesis within a single organism has recently emerged as a source of information for future biotechnological use. Among C4/CAM facultative species, Portulaca oleracea L. has been used as a model for biochemical and gene expression analysis of C4/CAM under field and laboratory conditions. In the present work, we focussed on developing molecular tools to facilitate functional genomics studies in this species, from the optimisation of RNA isolation protocols to a method for stable genetic transformation. Eleven variations of RNA extraction procedures were tested and compared for RNA quantity and quality. Also, 7 sample sets comprising total RNA from hormonal and abiotic stress treatments, distinct plant organs, leaf developmental stages, and subspecies were used to select, among 12 reference genes, the most stable reference genes for RT-qPCR analysis of each experimental condition. Furthermore, different explant sources, Agrobacterium tumefaciens strains, and regeneration and antibiotic selection media were tested in various combinations to optimise a protocol for stable genetic transformation of P. oleracea. Altogether, we provide essential tools for functional gene analysis in the context of C4/CAM photosynthesis, including an efficient RNA isolation method, preferred reference genes for RT-qPCR normalisation for a range of experimental conditions, and a protocol to produce P. oleracea stable transformants using A. tumefaciens.

Salinity stress induces epigenetic alterations to the promoter of MsMYB4 encoding a salt-induced MYB transcription factor

The transcriptomic response of plants to salinity stress is regulated in part by epigenetic alterations to gene promoter sequences. The transcription factor MsMYB4 is an important component of the response of alfalfa to salinity stress, but the involvement of epialleles of its encoding gene has not as yet been explored. Here, the MsMYB4 promoter was isolated using a genome walking approach in order to perform a deletion analysis to identify the region harboring the elements required for its stress inducibility. The analysis showed that these reside in the sequence lying between 739 and 336 nt up stream of the MsMYB4 translation start codon. The methylation status of the sequence around the MsMYB4 translation start site was altered by the imposition of salinity stress. The activation of MsMYB4 was associated with an increased level of histone H3K4 trimethylation and H3K9 acetylation in specific regions of the promoter sequence. Our results suggest a critical role for MsMYB4's activation by DNA methylation and/or histone modifications in response to salinity stress in alfalfa.

Efficient Generation of CRISPR/Cas9-Mediated Homozygous/Biallelic Medicago truncatula Mutants Using a Hairy Root System

In the process of acquiring mutants mediated by CRISPR/Cas9, plantlets are often regenerated from both mutated and non-mutated cells in a random manner, which increase the odds of chimeric mutated plant. In general, it's necessary to infect more explants or grow to next generation for the need of generating more biallelic or homozygous mutants. In present study, an efficient way of obtaining biallelic or homozygous mutated lines via fast-growing hairy root system without increasing numbers of infected explants or prolonging sexual propagation generation is reported. The fast growing lateral branches of hair roots are originated deep within the parental root from a small number of founder cells at the periphery, and therefore were employed as a library that classify different editing types in different lateral branches in which the homozygous or biallelic lines were screened. Here, MtPDS was employed in a proof-of-concept experiment to evaluate the efficiency of genome editing with our hairy root system. Homozygous/biallelic mutations were found only 1 of the 20 lines in the 1^st generation hairy roots, and 8 lines randomly selected were cultured to obtain their branch roots, homozygous/biallelic mutations were found in 6 of the 8 lines in their branch roots. We also tested the method with MtCOMT gene and got the same result. All of the seedlings regenerated from the homozygous/biallelic hairy root mutation lines of MtPDS displayed albino phenotypes. The entire process from vector design to the recovery of plantlets with homozygous/biallelic mutations took approximately 4.5-6.5 months. The whole process could bring inspiration for efficiently generating homozygous/biallelic mutants through CRISPR/Cas9 system from the hairy root or root system of a chimeric mutated transformants, especially for the rare and endangered plants whose explants sources are very limited or the plants that lack of tissue culture and rapid propagation system.

Brassinosteroids Inhibit Autotropic Root Straightening by Modifying Filamentous-Actin Organization and Dynamics

When positioned horizontally, roots grow down toward the direction of gravity. This phenomenon, called gravitropism, is influenced by most of the major plant hormones including brassinosteroids. Epi-brassinolide (eBL) was previously shown to enhance root gravitropism, a phenomenon similar to the response of roots exposed to the actin inhibitor, latrunculin B (LatB). This led us to hypothesize that eBL might enhance root gravitropism through its effects on filamentous-actin (F-actin). This hypothesis was tested by comparing gravitropic responses of maize (Zea mays) roots treated with eBL or LatB. LatB- and eBL-treated roots displayed similar enhanced downward growth compared with controls when vertical roots were oriented horizontally. Moreover, the effects of the two compounds on root growth directionality were more striking on a slowly-rotating two-dimensional clinostat. Both compounds inhibited autotropism, a process in which the root straightened after the initial gravistimulus was withdrawn by clinorotation. Although eBL reduced F-actin density in chemically-fixed Z. mays roots, the impact was not as strong as that of LatB. Modification of F-actin organization after treatment with both compounds was also observed in living roots of barrel medic (Medicago truncatula) seedlings expressing genetically encoded F-actin reporters. Like in fixed Z. mays roots, eBL effects on F-actin in living M. truncatula roots were modest compared with those of LatB. Furthermore, live cell imaging revealed a decrease in global F-actin dynamics in hypocotyls of etiolated M. truncatula seedlings treated with eBL compared to controls. Collectively, our data indicate that eBL-and LatB-induced enhancement of root gravitropism can be explained by inhibited autotropic root straightening, and that eBL affects this process, in part, by modifying F-actin organization and dynamics.

Molecular Cloning and Functional Identification of a Squalene Synthase Encoding Gene from Alfalfa (Medicago sativa L.)

The quality of alfalfa, a main forage legume worldwide, is of great importance for the dairy industry and is affected by the content of triterpene saponins. These natural terpenoid products of triterpene aglycones are catalyzed by squalene synthase (SQS), a highly conserved enzyme present in eukaryotes. However, there is scare information on alfalfa SQS. Here, an open reading frame (ORF) of SQS was cloned from alfalfa. Sequence analysis showed MsSQS had the same exon/intron composition and shared high homology with its orthologs. Bioinformatic analysis revealed the deduced MsSQS had two transmembrane domains. When transiently expressed, GFP-MsSQS fusion protein was localized on the plasma membrane of onion epidermal cells. Removal of the C-terminal transmembrane domain of MsSQS improved solubility in Escherichia coli. MsSQS was preferably expressed in roots, followed by leaves and stems. MeJA treatment induced MsSQS expression and increased the content of total saponins. Overexpression of MsSQS in alfalfa led to the accumulation of total saponins, suggesting a correlation between MsSQS expression level with saponins content. Therefore, MsSQS is a canonical squalene synthase and contributes to saponin synthesis in alfalfa. This study provides a key candidate gene for genetic manipulation of the synthesis of triterpene saponins, which impact both plant and animal health.