Auxin biosynthesis maintains embryo identity and growth during BABY BOOM-induced somatic embryogenesis

Molecular Mechanism of ARF5-AHL15-Mediated Auxin-Induced Embryogenic Cell Formation in Apples

The somatic embryo (SE) has bipolar characteristics, which is an ideal material for large-scale microproduction of woody plants represented by apples, and the somatic embryo is also an excellent receptor for genetic transformation. The formation of embryogenic cells is a prerequisite for somatic embryogenesis to occur. The embryogenic cells of apples cannot be obtained without induction of exogenous auxin, but how the auxin pathway regulates this process remains unknown. In this study, via RNA sequencing, MdARF5 and MdAHL15 were identified as differentially expressed genes involved in this process. Overexpression of MdARF5 and MdAHL15 induced the formation and proliferation of embryogenic cells and thus substantially shortened the induction cycle and improved the somatic embryo proliferation efficiency. A yeast one-hybrid assay showed that MdARF5 can directly bind to the promoter of MdAHL15. β-Glucuronidase (GUS) and dual-luciferase reporter assays revealed that MdARF5 activation of MdAHL15 transcription was substantial. In conclusion, our results suggest that MdAHL15 is induced by auxin and promotes the formation of embryogenic cells in early somatic embryogenesis via the positive regulation of MdARF5 in apples. The results will provide a theoretical basis for somatic embryogenesis-based development, reproduction, and transgenic breeding in apples.

Genome-Wide Identification and Characterization of the RWP-RK Proteins in Zanthoxylum armatum

Apomixis is a common reproductive characteristic of Zanthoxylum plants, and RWP-RKs are plant-specific transcription factors known to regulate embryonic development. However, the genome-wide analysis and function prediction of RWP-RK family genes in Z. armatum are unclear. In this study, 36 ZaRWP-RK transcription factors were identified in the genome of Z. armatum, among which 15 genes belonged to the RKD subfamily and 21 belonged to the NLP subfamily. Duplication events of ZaRWP-RK genes were mainly segmental duplication, and synteny analysis revealed a close phylogenetic relationship between Z. armatum and Arabidopsis. The analysis of cis-elements indicated that ZaRWP-RK genes may be involved in the regulation of the embryonic development of Z. armatum by responding to plant hormones such as abscisic acid, auxin, and gibberellin. Results of a real-time PCR showed that the expression levels of most ZaRWP-RK genes were significantly increased from flowers to young fruits. Protein-protein interaction network analysis further revealed the potential roles of the ZaRWP-RK proteins in apomixis. Collectively, this study is expected to improve our understanding of ZaRWP-RK transcription factors and provide a theoretical basis for future investigations into the ZaRWP-RK genes and their regulatory mechanisms in the apomixis process of Z. armatum.

Organelle Ca2+/CAM1-SELTP confers somatic cell embryogenic competence acquisition and transformation in plant regeneration

Somatic cell totipotency in plant regeneration represents the forefront of the compelling scientific puzzles and one of the most challenging problems in biology. How somatic embryogenic competence is achieved in regeneration remains elusive. Here, we discover uncharacterized organelle-based embryogenic differentiation processes of intracellular acquisition and intercellular transformation, and demonstrate the underlying regulatory system of somatic embryogenesis-associated lipid transfer protein (SELTP) and its interactor calmodulin1 (CAM1) in cotton as the pioneer crop for biotechnology application. The synergistic CAM1 and SELTP exhibit consistent dynamical amyloplast-plasmodesmata (PD) localization patterns but show opposite functional effects. CAM1 inhibits the effect of SELTP to regulate embryogenic differentiation for plant regeneration. It is noteworthy that callus grafting assay reflects intercellular trafficking of CAM1 through PD for embryogenic transformation. This work originally provides insight into the mechanisms responsible for embryogenic competence acquisition and transformation mediated by the Ca2+/CAM1-SELTP regulatory pathway, suggesting a principle for plant regeneration and cell/genetic engineering.

H2 supplied via ammonia borane stimulates lateral root branching via phytomelatonin signaling

A reliable and stable hydrogen gas (H2) supply will benefit agricultural laboratory and field trials. Here, we assessed ammonia borane (AB), an efficient hydrogen storage material used in the energy industry, and determined its effect on plant physiology and the corresponding mechanism. Through hydroponics and pot experiments, we discovered that AB increases tomato (Solanum lycopersicum) lateral root (LR) branching and this function depended on the increased endogenous H2 level caused by the sustainable H2 supply. In particular, AB might trigger LR primordia initiation. Transgenic tomato and Arabidopsis (Arabidopsis thaliana) expressing hydrogenase1 (CrHYD1) from Chlamydomonas reinhardtii not only accumulated higher endogenous H2 and phytomelatonin levels but also displayed pronounced LR branching. These endogenous H2 responses achieved by AB or genetic manipulation were sensitive to the pharmacological removal of phytomelatonin, indicating the downstream role of phytomelatonin in endogenous H2 control of LR formation. Consistently, extra H2 supply failed to influence the LR defective phenotypes in phytomelatonin synthetic mutants. Molecular evidence showed that the phytomelatonin-regulated auxin signaling network and cell-cycle regulation were associated with the AB/H2 control of LR branching. Also, AB and melatonin had little effect on LR branching in the presence of auxin synthetic inhibitors. Collectively, our integrated approaches show that supplying H2 via AB increases LR branching via phytomelatonin signaling. This finding might open the way for applying hydrogen storage materials to horticultural production.

Multifaceted roles of transcription factors during plant embryogenesis

Transcription factors (TFs) are diverse groups of regulatory proteins. Through their specific binding domains, TFs bind to their target genes and regulate their expression, therefore TFs play important roles in various growth and developmental processes. Plant embryogenesis is a highly regulated and intricate process during which embryos arise from various sources and undergo development; it can be further divided into zygotic embryogenesis (ZE) and somatic embryogenesis (SE). TFs play a crucial role in the process of plant embryogenesis with a number of them acting as master regulators in both ZE and SE. In this review, we focus on the master TFs involved in embryogenesis such as BABY BOOM (BBM) from the APETALA2/Ethylene-Responsive Factor (AP2/ERF) family, WUSCHEL and WUSCHEL-related homeobox (WOX) from the homeobox family, LEAFY COTYLEDON 2 (LEC2) from the B3 family, AGAMOUS-Like 15 (AGL15) from the MADS family and LEAFY COTYLEDON 1 (LEC1) from the Nuclear Factor Y (NF-Y) family. We aim to present the recent progress pertaining to the diverse roles these master TFs play in both ZE and SE in Arabidopsis, as well as other plant species including crops. We also discuss future perspectives in this context.

Epigenetic modifications and miRNAs determine the transition of somatic cells into somatic embryos

KEY MESSAGE

This review discusses the epigenetic changes during somatic embryo (SE) development, highlights the genes and miRNAs involved in the transition of somatic cells into SEs as a result of epigenetic changes, and draws insights on biotechnological opportunities to study SE development. Somatic embryogenesis from somatic cells occurs in a series of steps. The transition of somatic cells into somatic embryos (SEs) is the most critical step under genetic and epigenetic regulations. Major regulatory genes such as SERK, WUS, BBM, FUS3/FUSA3, AGL15, and PKL, control SE steps and development by turning on and off other regulatory genes. Gene transcription profiles of somatic cells during SE development is the result of epigenetic changes, such as DNA and histone protein modifications, that control and decide the fate of SE formation. Depending on the type of somatic cells and the treatment with plant growth regulators, epigenetic changes take place dynamically. Either hypermethylation or hypomethylation of SE-related genes promotes the transition of somatic cells. For example, the reduced levels of DNA methylation of SERK and WUS promotes SE initiation. Histone modifications also promote SE induction by regulating SE-related genes in somatic cells. In addition, miRNAs contribute to the various stages of SE by regulating the expression of auxin signaling pathway genes (TIR1, AFB2, ARF6, and ARF8), transcription factors (CUC1 and CUC2), and growth-regulating factors (GRFs) involved in SE formation. These epigenetic and miRNA functions are unique and have the potential to regenerate bipolar structures from somatic cells when a pluripotent state is induced. However, an integrated overview of the key regulators involved in SE development and downstream processes is lacking. Therefore, this review discusses epigenetic modifications involved in SE development, SE-related genes and miRNAs associated with epigenetics, and common cis-regulatory elements in the promoters of SE-related genes. Finally, we highlight future biotechnological opportunities to alter epigenetic pathways using the genome editing tool and to study the transition mechanism of somatic cells.

Identification and functional analysis of LpNAC37 associated with somatic embryogenesis in Lilium pumilum DC. Fisch. based on transcriptome analysis

Somatic embryogenesis (SE) is important for Lilium bulb propagation, germplasm conservation, and genetic transformation. The transition of somatic cells to embryonic cells is a critical step in SE, but the associated regulatory mechanisms have not been fully elucidated. Lilium pumilum DC. Fisch has a high regenerative capacity, and this study clarifies the critical timing of embryonic cell appearance in Lilium SE. Transcriptome sequencing using RNA-seq technology was performed on 5 representative samples from the early stage of Lilium SE. The 15 established cDNA libraries yielded 91.47 GB of valid data, and a total of 11,155 genes were consistently differentially expressed in the early stages of Lilium SE. GO annotation and KEGG pathway analysis of differentially expressed genes (DEGs) suggested that transcriptional regulation, hormone signaling, and stress response pathways play essential roles in the early stages of Lilium SE. WOX8, WOX11, SHR2, NAC37, AHP2, ANT, PIN1C, LAX2, LBD4, ACS12, YUC4, NFYB3, WRKY28, SAUR50, PYL9, and WRKY39 may be candidate genes for regulating early SE in Lilium. We further cloned LpNAC37, one of the key DEGs obtained from WGCNA and screening. LpNAC37 encodes a protein of 303 amino acids with a conserved NAM structural domain. The protein is a nuclear transcription factor with the highest homology to carrot DcNAC37. Overexpression of LpNAC37 suggested that LpNAC37 promotes embryonic callus formation in Arabidopsis. These results will help reveal the molecular mechanisms of the early stages of Lilium SE and advance the application of SE in Lilium propagation and genetic transformation.

Comparative transcriptome analysis of high- and low-embryogenic Hevea brasiliensis genotypes reveals involvement of phytohormones in somatic embryogenesis

BACKGROUND

Rubber plant (Hevea brasiliensis) is one of the major sources of latex. Somatic embryogenesis (SE) is a promising alterative to its propagation by grafting and seed. Phytohormones have been shown to influence SE in different plant species. However, limited knowledge is available on the role of phytohormones in SE in Hevea. The anther cultures of two Hevea genotypes (Yunyan 73477-YT and Reken 628-RT) with contrasting SE rate were established and four stages i.e., anthers (h), anther induced callus (y), callus differentiation state (f), and somatic embryos (p) were studied. UPLC-ESI-MS/MS and transcriptome analyses were used to study phytohormone accumulation and related expression changes in biosynthesis and signaling genes.

RESULTS

YT showed higher callus induction rate than RT. Of the two genotypes, only YT exhibited successful SE. Auxins, cytokinins (CKs), abscisic acid (ABA), jasmonic acid (JA), salicylic acid (SA), gibberellins (GAs), and ethylene (ETH) were detected in the two genotypes. Indole-3-acetic acid (IAA), CKs, ABA, and ETH had notable differences in the studied stages of the two genotypes. The differentially expressed genes identified in treatment comparisons were majorly enriched in MAPK and phytohormone signaling, biosynthesis of secondary metabolites, and metabolic pathways. The expression changes in IAA, CK, ABA, and ETH biosynthesis and signaling genes confirmed the differential accumulation of respective phytohormones in the two genotypes.

CONCLUSION

These results suggest potential roles of phytohormones in SE in Hevea.

The phenomenon of autonomous endosperm in sexual and apomictic plants

Endosperm is a key nutritive tissue that supports the developing embryo or seedling, and serves as a major nutritional source for human and livestock feed. In sexually-reproducing flowering plants, it generally develops after fertilization. However, autonomous endosperm (AE) formation (i.e. independent of fertilization) is also possible. Recent findings of AE loci/ genes and aberrant imprinting in native apomicts, together with a successful initiation of parthenogenesis in rice and lettuce, have enhanced our understanding of the mechanisms bridging sexual and apomictic seed formation. However, the mechanisms driving AE development are not well understood. This review presents novel aspects related to AE development in sexual and asexual plants underlying stress conditions as the primary trigger for AE. Both application of hormones to unfertilized ovules and mutations that impair epigenetic regulation lead to AE development in sexual Arabidopsis thaliana, which may point to a common pathway for both phenomena. Apomictic-like AE development under experimental conditions can take place due to auxin-dependent gene expression and/or DNA methylation.

Transcriptomic and Metabolomic Investigation on Leaf Necrosis Induced by ZmWus2 Transient Overexpression in Nicotiana benthamiana

WUSCHEL (WUS) is a crucial transcription factor in regulating plant stem cell development, and its expression can also improve genetic transformation. However, the ectopic expression of WUS always causes pleiotropic effects during genetic transformation, making it important to understand the regulatory mechanisms underlying these phenomena. In our study, we found that the transient expression of the maize WUS ortholog ZmWus2 caused severe leaf necrosis in Nicotiana benthamiana. We performed transcriptomic and non-target metabolomic analyses on tobacco leaves during healthy to wilted states after ZmWus2 transient overexpression. Transcriptomic analysis revealed that ZmWus2 transformation caused active metabolism of inositol trisphosphate and glycerol-3-phosphate, while also upregulating plant hormone signaling and downregulating photosystem and protein folding pathways. Metabolomic analysis mainly identified changes in the synthesis of phenylpropanoid compounds and various lipid classes, including steroid synthesis. In addition, transcription factors such as ethylene-responsive factors (ERFs), the basic helix-loop-helix (bHLH) factors, and MYBs were found to be regulated by ZmWus2. By integrating these findings, we developed a WUS regulatory model that includes plant hormone accumulation, stress responses, lipid remodeling, and leaf necrosis. Our study sheds light on the mechanisms underlying WUS ectopic expression causing leaf necrosis and may inform the development of future genetic transformation strategies.

Opposite Auxin Dynamics Determine the Gametophytic and Embryogenic Fates of the Microspore

The microspore can follow two different developmental pathways. In vivo microspores follow the gametophytic program to produce pollen grains. In vitro, isolated microspores can be reprogrammed by stress treatments and follow the embryogenic program, producing doubled-haploid embryos. In the present study, we analyzed the dynamics and role of endogenous auxin in microspore development during these two different scenarios, in Brassica napus. We analyzed auxin concentration, cellular accumulation, the expression of the TAA1 auxin biosynthesis gene, and the PIN1-like efflux carrier gene, as well as the effects of inhibiting auxin biosynthesis by kynurenine on microspore embryogenesis. During the gametophytic pathway, auxin levels and TAA1 and PIN1-like expression were high at early stages, in tetrads and tapetum, while they progressively decreased during gametogenesis in both pollen and tapetum cells. In contrast, in microspore embryogenesis, TAA1 and PIN1-like genes were upregulated, and auxin concentration increased from the first embryogenic divisions. Kynurenine treatment decreased both embryogenesis induction and embryo production, indicating that auxin biosynthesis is required for microspore embryogenesis initiation and progression. The findings indicate that auxin exhibits two opposite profiles during these two microspore developmental pathways, which determine the different cell fates of the microspore.

Riboflavin mediates m6A modification targeted by miR408, promoting early somatic embryogenesis in longan

Plant somatic embryogenesis (SE) is an in vitro biological process wherein bipolar structures are induced to form somatic cells and regenerate into whole plants. MicroRNA (miRNA) is an essential player in plant SE. However, the mechanism of microRNA408 (miR408) in SE remains elusive. Here, we used stable transgenic technology in longan (Dimocarpus longan) embryogenic calli to verify the mechanism by which miR408 promotes cell division and differentiation of longan early SE. dlo-miR408-3p regulated riboflavin biosynthesis by targeting nudix hydrolase 23 (DlNUDT23), a previously unidentified gene mediating N6-methyladenosine (m6A) modification and influencing RNA homeostasis and cell cycle gene expression during longan early SE. We showed that DlMIR408 overexpression (DlMIR408-OE) promoted 21-nt miRNA biosynthesis. In DlMIR408-OE cell lines, dlo-miR408-3p targeted and downregulated DlNUDT23, promoted riboflavin biosynthesis, decreased flavin mononucleotide (FMN) accumulation, promoted m6A level, and influenced miRNA homeostasis. DNA replication, glycosylphosphatidylinositol (GPI)-anchor biosynthesis, the pentose phosphate pathway, and taurine and hypotaurine metabolism were also closely associated with riboflavin metabolism. In a riboflavin feeding assay, dlo-miR408-3p and pre-miR408 were upregulated and DlNUDT23 was downregulated, increasing the m6A level and cell division and differentiation in longan globular embryos. When riboflavin biosynthesis was inhibited, dlo-miR408-3p was downregulated and DlNUDT23 was upregulated, which decreased m6A modification and inhibited cell division but did not inhibit cell differentiation. FMN artificial demethylated m6A modification affected the homeostasis of precursor miRNA and miRNA. Our results revealed a mechanism underlying dlo-miR408-3p-activated riboflavin biosynthesis in which DlNUDT23 is targeted, m6A modification is dynamically mediated, and cell division is affected, promoting early SE in plants.

Efficient Plant Regeneration System from Leaf Explant Cultures of Daphne genkwa via Somatic Embryogenesis

This study aimed to establish an efficient plant regeneration system from leaf-derived embryogenic structure cultures of Daphne genkwa. To induce embryogenic structures, fully expanded leaf explants of D. genkwa were cultured on Murashige and Skoog (MS) medium supplemented with 0, 0.1, 0.5, 1, 2, and 5 mg·L^-1 2,4-dichlorophenoxyacetic acid (2,4-D), respectively. After 8 weeks of incubation, the highest frequency of embryogenic structure formation reached 100% when the leaf explants were cultivated on MS medium supplemented with 0.1 to 1 mg·L^-1 2,4-D. At higher concentrations of 2,4-D (over 2 mg·L^-1 2,4-D), the frequency of embryogenic structure formation significantly declined. Similar to 2,4-D, indole butyric acid (IBA) and α-naphthaleneacetic acid (NAA) treatments were also able to form embryogenic structures. However, the frequency of embryogenic structure formation was lower than that of 2,4-D. In particular, the yellow embryonic structure (YES) and white embryonic structure (WES) were simultaneously developed from the leaf explants of D. genkwa on culture medium containing 2,4-D, IBA, and NAA, respectively. Embryogenic calluses (ECs) were formed from the YES after subsequent rounds of subculture on MS medium supplemented with 1 mg·L^-1 2,4-D. To regenerate whole plants, the embryogenic callus (EC) and the two embryogenic structures (YES and WES) were transferred onto MS medium supplemented with 0.1 mg·L^-1 6-benzyl aminopurine (BA). The YES had the highest plant regeneration potential via somatic embryo and shoot development compared to the EC and WES. To our knowledge, this is the first successful report of a plant regeneration system via the somatic embryogenesis of D. genkwa. Thus, the embryogenic structures and plant regeneration system of D. genkwa could be applied to mass proliferation and genetic modification for pharmaceutical metabolite production in D. genkwa.

Effects of Low Temperature on Pedicel Abscission and Auxin Synthesis Key Genes of Tomato

Cold stress usually causes the abscission of floral organs and a decline in fruit setting rate, seriously reducing tomato yield. Auxin is one of the key hormones that affects the abscission of plant floral organs; the YUCCA (YUC) family is a key gene in the auxin biosynthesis pathway, but there are few research reports on the abscission of tomato flower organs. This experiment found that, under low temperature stress, the expression of auxin synthesis genes increased in stamens but decreased in pistils. Low temperature treatment decreased pollen vigor and pollen germination rate. Low night temperature reduced the tomato fruit setting rate and led to parthenocarpy, and the treatment effect was most obvious in the early stage of tomato pollen development. The abscission rate of tomato pTRV-Slfzy3 and pTRV-Slfzy5 silenced plants was higher than that of the control, which is the key auxin synthesis gene affecting the abscission rate. The expression of Solyc07g043580 was down-regulated after low night temperature treatment. Solyc07g043580 encodes the bHLH-type transcription factor SlPIF4. It has been reported that PIF4 regulates the expression of auxin synthesis and synthesis genes, and is a key protein in the interaction between low temperature stress and light in regulating plant development.

Brassica napus Haploid and Double Haploid Production and Its Latest Applications

Rapeseed is one of the most important oil crops in the world. Increasing demand for oil and limited agronomic capabilities of present-day rapeseed result in the need for rapid development of new, superior cultivars. Double haploid (DH) technology is a fast and convenient approach in plant breeding as well as genetic research. Brassica napus is considered a model species for DH production based on microspore embryogenesis; however, the molecular mechanisms underlying microspore reprogramming are still vague. It is known that morphological changes are accompanied by gene and protein expression patterns, alongside carbohydrate and lipid metabolism. Novel, more efficient methods for DH rapeseed production have been reported. This review covers new findings and advances in Brassica napus DH production as well as the latest reports related to agronomically important traits in molecular studies employing the double haploid rapeseed lines.

Somatic embryo initiation by rice BABY BOOM1 involves activation of zygote-expressed auxin biosynthesis genes

Plant embryogenesis results from the fusion of male and female gametes but can also be induced in somatic cells. The molecular pathways for embryo initiation are poorly understood, especially in monocots. In rice, the male gamete expressed BABY BOOM1 (OsBBM1) transcription factor functions as an embryogenic trigger in the zygote and can also promote somatic embryogenesis when ectopically expressed in somatic tissues. We used gene editing, transcriptome profiling, and chromatin immunoprecipitation to determine the molecular players involved in embryo initiation downstream of OsBBM1. We identify OsYUCCA (OsYUC) auxin biosynthesis genes as direct targets of OsBBM1. Unexpectedly, these OsYUC targets in zygotes are expressed only from the maternal genome, whereas the paternal genome exclusively provides functional OsBBM1 to initiate embryogenesis. Induction of somatic embryogenesis by exogenous auxin requires OsBBM genes and downstream OsYUC targets. Ectopic OsBBM1 initiates somatic embryogenesis without exogenous auxins but requires functional OsYUC genes. Thus, an OsBBM-OsYUC module is a key player for both somatic and zygotic embryogenesis in rice. Zygotic embryo initiation involves a partnership of male and female genomes, through which paternal OsBBM1 activates maternal OsYUC genes. In somatic embryogenesis, exogenous auxin triggers OsBBM1 expression, which then activates endogenous auxin biosynthesis OsYUC genes.

AT-Hook Transcription Factors Show Functions in Liriodendron chinense under Drought Stress and Somatic Embryogenesis

AT-hook motif nuclear localized (AHL) is a transcription factor that can directly induce plant somatic embryogenesis without adding exogenous hormones. One of its functional domains, the AT-hook motif, has a chromatin-modifying function and participates in various cellular processes, including DNA replication and repair and gene transcription leading to cell growth. Liriodendron chinense (Hemsl.) Sargent is an important ornamental and timber tree in China. However, its low drought-resistant ability further leads to a low natural growth rate of its population. Based on bioinformatics analysis, this study identified a total of 21 LcAHLs in L. chinense. To explore the expression pattern of the AHL gene family under drought and somatic embryogenesis, we performed a systematic analysis including basic characteristics, gene structure, chromosome localization, replication event, cis-acting elements and phylogenetic analyses. According to the phylogenetic tree, the 21 LcAHL genes are divided into three separate clades (Clade I, II, and III). Cis-acting element analysis indicated the involvement of the LcAHL genes in drought, cold, light, and auxin regulation. In the generated drought stress transcriptome, a total of eight LcAHL genes showed increased expression levels, with their expression peaking at 3 h and leveling off after 1 d. Nearly all LcAHL genes were highly expressed in the process of somatic embryogenesis. In this study, we performed a genome-wide analysis of the LcAHL gene family and found that LcAHLs take part in resistance to drought stress and the development of somatic embryos. These findings will provide an important theoretical basis for understanding of the LcAHL gene function.

Endogenous auxin maintains embryonic cell identity and promotes somatic embryo development in Arabidopsis

Somatic embryogenesis (SE), or embryo development from in vitro cultured vegetative explants, can be induced in Arabidopsis by the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) or by overexpression of specific transcription factors, such as AT-HOOK MOTIF NUCLEAR LOCALIZED 15 (AHL15). Here, we explored the role of endogenous auxin [indole-3-acetic acid (IAA)] during 2,4-D and AHL15-induced SE. Using the pWOX2:NLS-YFP reporter, we identified three distinct developmental stages for 2,4-D and AHL15-induced SE in Arabidopsis, with these being (i) acquisition of embryo identity; (ii) formation of pro-embryos; and (iii) somatic embryo patterning and development. The acquisition of embryo identity coincided with enhanced expression of the indole-3-pyruvic acid auxin biosynthesis YUCCA genes, resulting in an enhanced pDR5:GFP-reported auxin response in the embryo-forming tissues. Chemical inhibition of the indole-3-pyruvic acid pathway did not affect the acquisition of embryo identity, but significantly reduced or completely inhibited the formation of pro-embryos. Co-application of IAA with auxin biosynthesis inhibitors in the AHL15-induced SE system rescued differentiated somatic embryo formation, confirming that increased IAA levels are important during the last two stages of SE. Our analyses also showed that polar auxin transport, with AUXIN/LIKE-AUX influx and PIN-FORMED1 efflux carriers as important drivers, is required for the transition of embryonic cells to proembryos and, later, for correct cell fate specification and differentiation. Taken together, our results indicate that endogenous IAA biosynthesis and its polar transport are not required for the acquisition of embryo identity, but rather to maintain embryonic cell identity and for the formation of multicellular proembryos and their development into histodifferentiated embryos.

Effects of Ascorbic Acid on Physiological Characteristics during Somatic Embryogenesis of Fraxinus mandshurica

Fraxinus mandshurica is one of the precious tree species in northeast China and has important economic and ecological value. Ascorbic acid (ASA) is a strong antioxidant that can significantly improve plant photosynthetic efficiency and stress resistance and participate widely in plant growth and development. In this study, we investigated the development process of mature zygotic embryos of F. mandshurica under different concentrations of ASA and found that 100 mg·L^-1 exogenous ASA was the optimal concentration and that the induction rate of somatic embryos (SEs) was the highest at 72.89%, which was 7.13 times higher than that of the control group. The polyphenol content, peroxidase (POD) activity, nitric oxide (NO) content, nitrate reductase (NR) activity, total ascorbic acid (T-ASA) content, ASA content, ASA/Dehydroascorbic acid (DHA) ratio, GSH/GSSG ratio, and ascorbate peroxidase (APX) activity were significantly increased under the application of exogenous ASA in explants, whereas the polyphenol oxidase (PPO) activity, phenylalanine ammonia-lyase (PAL) activity, superoxide dismutase (SOD) activity, and catalase (CAT) activity, malondialdehyde (MDA) content and nitric oxide synthase (NOS) activity were decreased. At the same time, the content of T-ASA and ASA, T-GSH and GSSG, and PAL and SOD had the same change pattern in the control group and the treatment group. These results suggested that high or low concentrations of ASA could not promote the somatic embryogenesis of F. mandshurica and that exogenous ASA had significant effects on the physiology of F. mandshurica explants. ASA was also highly related to somatic embryogenesis and the explant browning of F. mandshurica. Our results could provide a reference for further study on the browning mechanism of F. mandshurica explants and lay the foundation for optimizing the condition of somatic embryogenesis in F. mandshurica.

Unlocking grapevine in vitro regeneration: Issues and perspectives for genetic improvement and functional genomic studies

In vitro plant regeneration is a pivotal process in genetic engineering to obtain large numbers of transgenic, cisgenic and gene edited plants in the frame of functional gene or genetic improvement studies. However, several issues emerge as regeneration is not universally possible across the plant kingdom and many variables must be considered. In grapevine (Vitis spp.), as in other woody and fruit tree species, the regeneration process is impaired by a recalcitrance that depends on numerous factors such as genotype and explant-dependent responses. This is one of the major obstacles in developing gene editing approaches and functional genome studies in grapevine and it is therefore crucial to understand how to achieve efficient regeneration across different genotypes. Further issues that emerge in regeneration need to be addressed, such as somaclonal mutations which do not allow the regeneration of individuals identical to the original mother plant, an essential factor for commercial use of the improved grapevines obtained through the New Breeding Techniques. Over the years, the evolution of protocols to achieve plant regeneration has relied mainly on optimizing protocols for genotypes of interest whilst nowadays with new genomic data available there is an emerging opportunity to have a clearer picture of its molecular regulation. The goal of this review is to discuss the latest information available about different aspects of grapevine in vitro regeneration, to address the main factors that can impair the efficiency of the plant regeneration process and cause post-regeneration problems and to propose strategies for investigating and solving them.

Recent advances in crop transformation technologies

Agriculture is experiencing a technological inflection point in its history, while also facing unprecedented challenges posed by human population growth and global climate changes. Key advancements in precise genome editing and new methods for rapid generation of bioengineered crops promise to both revolutionize the speed and breadth of breeding programmes and increase our ability to feed and sustain human population growth. Although genome editing enables targeted and specific modifications of DNA sequences, several existing barriers prevent the widespread adoption of editing technologies for basic and applied research in established and emerging crop species. Inefficient methods for the transformation and regeneration of recalcitrant species and the genotype dependency of the transformation process remain major hurdles. These limitations are frequent in monocotyledonous crops, which alone provide most of the calories consumed by human populations. Somatic embryogenesis and de novo induction of meristems - pluripotent groups of stem cells responsible for plant developmental plasticity - are essential strategies to quickly generate transformed plants. Here we review recent discoveries that are rapidly advancing nuclear transformation technologies and promise to overcome the obstacles that have so far impeded the widespread adoption of genome editing in crop species.

Towards a hierarchical gene regulatory network underlying somatic embryogenesis

Genome-editing technologies have advanced in recent years but designing future crops remains limited by current methods of improving somatic embryogenesis (SE) capacity. In this Opinion, we provide an update on the molecular event by which the phytohormone auxin promotes the acquisition of plant cell totipotency through evoking massive changes in transcriptome and chromatin accessibility. We propose that the chromatin states and individual totipotency-related transcription factors (TFs) from disparate gene families organize into a hierarchical gene regulatory network underlying SE. We conclude with a discussion of the practical paths to probe the cellular origin of the somatic embryo and the epigenetic landscape of the totipotent cell state in the era of single-cell genomics.

Integrated transcriptomic and metabolic analyses provide insights into the maintenance of embryogenic potential and the biosynthesis of phenolic acids and flavonoids involving transcription factors in Larix kaempferi (Lamb.) Carr

Somatic embryogenesis (SE) techniques have been established for micropropagation or basic research related to plant development in many conifer species. The frequent occurrence of non-embryogenic callus (NEC) during SE has impose constraints on the application of somatic embryogenesis SE in Larix kaempferi (Lamb.) Carr, but the potential regulatory mechanisms are poorly understood. In this study, integrated transcriptomic and metabolomic analyses were performed in embryogenic callus (EC) and NEC originating from a single immature zygotic embryo to better decipher the key molecular and metabolic mechanisms required for embryogenic potential maintenance. The results showed that a total of 13,842 differentially expressed genes (DEGs) were found in EC and NEC, among which many were enriched in plant hormone signal transduction, starch and sucrose metabolism, phenylpropanoid biosynthesis, flavonoid biosynthesis, and the biosynthesis of amino acids pathways. Metabolite profiling showed that 441 differentially accumulated metabolites (DAMs) were identified in EC and NEC. Both EC and NEC had vigorous primary metabolic activities, while most secondary metabolites were upregulated in NEC. Many totipotency-related transcription factor (TF) genes such as BBMs, WUSs, and LEC1 showed higher expression levels in EC compared with NEC, which may result in the higher accumulation of indole 3-acetic acid (IAA) in EC. NEC was characterized by upregulation of genes and metabolites associated with stress responses, such as DEGs involved in jasmonic acid (JA) and ethylene (ETH) biosynthesis and signal transduction pathways, and DEGs and DAMs related to phenylpropanoid and flavonoid biosynthesis. We predicted and analyzed TFs that could target several key co-expressed structural DEGs including two C4H genes, two CcoAOMT genes and three HCT genes involved in phenylpropanoid and flavonoid biosynthesis. Based on the targeted relationship and the co-expression network, two ERFs (Lk23436 and Lk458687), one MYB (Lk34626) and one C2C2-dof (Lk37167) may play an important role in regulating phenolic acid and flavonoid biosynthesis by transcriptionally regulating the expression of these structural genes. This study shows an approach involving integrated transcriptomic and metabolic analyses to obtain insights into molecular events underlying embryogenic potential maintenance and the biosynthesis mechanisms of key metabolites involving TF regulation, which provides valuable information for the improvement of SE efficiency in L. kaempferi.

A Comparative Transcriptome Analysis Reveals the Molecular Mechanisms That Underlie Somatic Embryogenesis in Peaonia ostii ‘Fengdan’

Low propagation rate is the primary problem that limits industry development of tree peony. In this study, a highly efficient regeneration system for tree peony using somatic embryogenesis (SE) was established. The transcriptomes of zygotic embryo explants (S0), non-embryonic callus (S1), embryonic callus (S2), somatic embryos (S3), and regenerated shoots (S4) were analyzed to determine the regulatory mechanisms that underlie SE in tree peony. The differentially expressed genes (DEGs) were identified in the pairwise comparisons of S1-vs-S2 and S1-vs-S3, respectively. The enriched DEGs were primarily involved in hormone signal transduction, stress response and the nucleus (epigenetic modifications). The results indicated that cell division, particularly asymmetric cell division, was enhanced in S3. Moreover, the genes implicated in cell fate determination played central roles in S3. Hormone signal pathways work in concert with epigenetic modifications and stress responses to regulate SE. SERK, WOX9, BBM, FUS3, CUC, and WUS were characterized as the molecular markers for tree peony SE. To our knowledge, this is the first study of the SE of tree peony using transcriptome sequencing. These results will improve our understanding of the molecular mechanisms that underly SE in tree peony and will benefit the propagation and genetic engineering of this plant.

BABY BOOM regulates early embryo and endosperm development

The zygote is a totipotent structure that develops into an embryo with all of the cells needed to produce an entire plant. The BABY BOOM (BBM) transcription factor induces spontaneous asexual embryo development on plant organs when ectopically expressed. Although BBM is at the top of a transcriptional network that promotes asexual embryo development, little is known about its expression and role during zygotic embryogenesis. Here we show in Arabidopsis that BBM regulates the progression of zygotic embryo development and embryo patterning, and division and cellularization of the filial endosperm. In line with its role as a totipotency factor, ectopic BBM expression in the egg cell is also sufficient to induce haploid embryo development in Arabidopsis and dicot crops.

The BABY BOOM (BBM) AINTEGUMENTA-LIKE (AIL) AP2/ERF domain transcription factor is a major regulator of plant cell totipotency, as it induces asexual embryo formation when ectopically expressed. Surprisingly, only limited information is available on the role of BBM during zygotic embryogenesis. Here we reexamined BBM expression and function in the model plant Arabidopsis thaliana (Arabidopsis) using reporter analysis and newly developed CRISPR mutants. BBM was expressed in the embryo from the zygote stage and also in the maternal (nucellus) and filial (endosperm) seed tissues. Analysis of CRISPR mutant alleles for BBM (bbm-cr) and the redundantly acting AIL gene PLETHORA2 (PLT2) (plt2-cr) uncovered individual roles for these genes in the timing of embryo progression. We also identified redundant roles for BBM and PLT2 in endosperm proliferation and cellularization and the maintenance of zygotic embryo development. Finally, we show that ectopic BBM expression in the egg cell of Arabidopsis and the dicot crops Brassica napus and Solanum lycopersicon is sufficient to bypass the fertilization requirement for embryo development. Together these results highlight roles for BBM and PLT2 in seed development and demonstrate the utility of BBM genes for engineering asexual embryo development in dicot species.

One-Week Scutellar Somatic Embryogenesis in the Monocot Brachypodium distachyon

Plant somatic embryogenesis (SE) is a natural process of vegetative propagation. It can be induced in tissue cultures to investigate developmental transitions, to create transgenic or edited lines, or to multiply valuable crops. We studied the induction of SE in the scutellum of monocots with Brachypodium distachyon as a model system. Towards the in-depth analysis of SE initiation, we determined the earliest stages at which somatic scutellar cells acquired an embryogenic fate, then switched to a morphogenetic mode in a regeneration sequence involving treatments with exogenous hormones: first an auxin (2,4-D) then a cytokinin (kinetin). Our observations indicated that secondary somatic embryos could already develop in the proliferative calli derived from immature zygotic embryo tissues within one week from the start of in vitro culture. Cell states and tissue identity were deduced from detailed histological examination, and in situ hybridization was performed to map the expression of key developmental genes. The fast SE induction method we describe here facilitates the mechanistic study of the processes involved and may significantly shorten the production of transgenic or gene-edited plants.