Identification of putative homologs of Larix decidua to BABYBOOM (BBM), LEAFY COTYLEDON1 (LEC1), WUSCHEL-related HOMEOBOX2 (WOX2) and SOMATIC EMBRYOGENESIS RECEPTOR-like KINASE (SERK) during somatic embryogenesis

Embryogenesis-related genes ( LdBBM, LdLEC1, LdWOX2 and LdSERK ) were confirmed in sequence and expression abundance for Larix decidua —these findings are valid for somatic as well as for zygotic embryo development.S omatic embryogenesis is a reliable source of high-quality genotypes as it presents an advantageous alternative for conifers in forestry, independent from seed production. Although this propagation method is already being applied, molecular factors initiating and controlling the process remain to be understood. The embryogenesis-associated genes BABYBOOM (BBM), LEAFY COTYLEDON1 (LEC1), WUSCHEL-related HOMEOBOX2 (WOX2) and SOMATIC EMBRYOGENESIS RECEPTOR-like KINASE (SERK) were identified and analyzed in somatic embryos of the European larch, L. decidua Mill. Subsequent comparisons with annotated sequences displayed similarities with angiosperm homologs. Transcript accumulation of the identified genes during embryogenesis has been analyzed. LdLEC1 and LdWOX2 are mainly expressed during early embryogenesis, whereas LdBBM and LdSERK reveal increased expression during later development. Temporal and spatial expression studies revealed a specific LdLEC1 signal in the outer cell layer of young embryo heads, whereas mature embryos showed a homogeneous expression. The overexpression of LdLEC1 in Arabidopsis influences germination and cotyledon formation, thus indicating the interspecific importance of LEC1 for proper embryo and specifically cotyledon development. Our data support a conserved role of principal regulators during plant embryogenesis that may be used as molecular markers for embryogenicity and to further determine initiating processes of somatic embryogenesis.

Effect of light conditions on anatomical and biochemical aspects of somatic and zygotic embryos of hybrid larch (Larix × marschlinsii)

BACKGROUND AND AIMS

In conifers, mature somatic embryos and zygotic embryos appear to resemble one another physiologically and morphologically. However, phenotypes of cloned conifer embryos can be strongly influenced by a number of in vitro factors and in some instances clonal variation can exceed that found in nature. This study examines whether zygotic embryos that develop within light-opaque cones differ from somatic embryos developing in dark/light conditions in vitro. Embryogenesis in larch is well understood both in situ and in vitro and thus provides a suitable system for addressing this question.

METHODS

Features of somatic and zygotic embryos of hybrid larch, Larix × marschlinsii, were quantified, including cotyledon numbers, protein concentration and phenol chemistry. Somatic embryos were placed either in light or darkness for the entire maturation period. Embryos at different developmental stages were embedded and sectioned for histological analysis.

KEY RESULTS

Light, and to a lesser degree abscisic acid (ABA), influenced accumulation of protein and phenolic compounds in somatic and zygotic embryos. Dark-grown mature somatic embryos had more protein (91·77 ± 11·26 µg protein mg(-1) f.wt) than either dark-grown zygotic embryos (62·40 ± 5·58) or light-grown somatic embryos (58·15 ± 10·02). Zygotic embryos never accumulated phenolic compounds at any stage, whereas somatic embryos stored phenolic compounds in the embryonal root caps and suspensors. Light induced the production of quercetrin (261·13 ± 9·2 µg g(-1) d.wt) in somatic embryos. Mature zygotic embryos that were removed from seeds and placed on medium in light rapidly accumulated phenolics in the embryonal root cap and hypocotyl. Delaying germination with ABA delayed phenolic compound accumulation, restricting it to the embryonal root cap.

CONCLUSIONS

In larch embryos, light has a negative effect on protein accumulation, but a positive effect on phenol accumulation. Light did not affect morphogenesis, e.g. cotyledon number. Somatic embryos produced different amounts of phenolics, such as quercetrin, depending on light conditions. The greatest difference was seen in the embryonal root cap in all embryo types and conditions.

In search of markers for somatic embryo maturation in hybrid larch (Larix × eurolepis): global DNA methylation and proteomic analyses

A global DNA methylation and proteomics approach was used to investigate somatic embryo maturation in hybrid larch. Each developmental step during somatic embryogenesis was associated with a distinct and significantly different global DNA methylation level: from 45.8% mC for undifferentiated somatic embryos (1-week proliferation) to 61.5% mC for immature somatic embryos (1-week maturation), while maturation was associated with a decrease in DNA methylation to 53.4% for mature cotyledonary somatic embryos (8-weeks maturation). The presence of 5-azacytidine (hypo-methylating agent) or hydroxyurea (hyper-methylating agent) in the maturation medium altered the global DNA methylation status of the embryogenic cultures, and significantly reduced both their relative growth rate and embryogenic potential, suggesting an important role for DNA methylation in embryogenesis. Maturation was also assessed by examining changes in the total protein profile. Storage proteins, identified as legumin- and vicilin-like, appeared at the precotyledonary stage. In the proteomic study, total soluble proteins were extracted from embryos after 1 and 8 weeks of maturation, and separated by two-dimensional gel electrophoresis. There were 147 spots which showed significant differences between the stages of maturation; they were found to be involved mainly in primary metabolism and the stabilization of the resulting metabolites. This indicated that the somatic embryo was still metabolically active at 8 weeks of maturation. This is the first report of analyses of global DNA methylation (including the effects of hyper- and hypo-treatments) and proteome during somatic embryogenesis in hybrid larch, and thus provides novel insights into maturation of conifer somatic embryos.

[Somatic embryogenesis in in vitro culture of three larch species]

Embryogenic callus formation in different larch species from Siberia (Larix sibirica, L. gmelinii, and L. sukaczewii) was carried out on MSGm medium supplemented with growth regulators (2.4-D and BAP) and followed one and the same scheme: elongation of somatic cells and their asymmetric division with formation of initial and tube cells. The cells of embryo initial underwent sequential divisions and formed embryonic globules which caused the formation of somatic embryos. Somatic embryos became mature and germinated by addition of ABA and PEG into the medium. Long-term proliferating cell lines and regenerant plants were obtained in Sukachev larch and its hybrid with Siberian larch. The success of somatic embryogenesis depended on the genotype of the donor tree.

Transcriptome profiling and in silico analysis of somatic embryos in Japanese larch (Larix leptolepis)

Japanese larch (Larix leptolepis) is an ecologically and economically important species mainly grown in northeastern China, Japan and Europe. However, erratic flowering and poor germplasm resources caused by high embryo abortion rates have hampered breeding of Larix species. Somatic embryogenesis (SE) is an effective tool for the production of L. leptolepis with desirable characteristics, such as expression of totipotency, preparation of synthetic seeds, and genetic transformation. However, public genomic resources for this species are limited. We sequenced 591,759 raw expressed sequence tags (ESTs) from a 454 sequencing cDNA library of L. leptolepis somatic embryos, resulting in 572,403 high-quality reads. These reads were assembled into 70,927 unique sequences (UniGenes), including 32,321 contigs and 38,606 singletons. After removal of low-quality sequences, 65,115 UniGenes were annotated using the UniProtKB program. Based on their sequence similarity with known proteins, the matched 30,372 sequences from 664 species were estimated to represent approximately 19,000 unique genes. Gene ontology analysis revealed 21,324 UniGenes assigned to 51 categories. By Kyoto Encyclopedia of Genes and Genomes mapping, 25,773 transcripts were associated with 160 biochemical pathways. Further analysis screened four signal transduction pathways represented by 337 enzymes and 17 secondary metabolites. In silico analysis reveals that 207 UniESTs in Larix are homologous to MAPKs genes identified from other model plants, which may be involved in regulating SE development. This study provides an initial insight into the Larix transcriptomes of the pro-embryogenic mass and is a sound basis for future studies.

KEY MESSAGE

We constructed a large, full-length 454 sequencing cDNA library of Larix leptolepis during somatic embryogenesis. More than 590,000 sequences were obtained and a deep-coverage EST database was constructed.

Genome-wide identification of microRNAs in larch and stage-specific modulation of 11 conserved microRNAs and their targets during somatic embryogenesis

MicroRNAs (miRNAs) are emerging as essential regulators of biological processes. Somatic embryogenesis is one of the most important techniques for gymnosperm-breeding programs, but there is little understanding of its underlying mechanism. To investigate the roles of miRNAs during somatic embryogenesis in larch, we constructed a small RNA library from somatic embryos. High-throughput sequencing of the library identified 83 conserved miRNAs from 35 families, 16 novel miRNAs, and 14 plausible miRNA candidates, with a high proportion specific to larch or gymnosperms. qRT-PCR analysis demonstrated that both the conserved and novel or candidate miRNAs were expressed in larch. Several miRNA precursor sequences were obtained via RACE. We predicted 110 target genes using bioinformatics, and validated 9 of them by 5' RACE. 11 conserved miRNA families including 17 miRNAs with critical functions in plant development and six target mRNAs were detected by qRT-PCR in the larch SE. Stage-specific expression of miRNAs and their targets indicate their possible modulation on SE of larch: miR171a/b might exert function on PEMs, while miR171c acts in the induction process of larch SE; miR397 and miR398 mainly involved in modulation of PEM propagation and transition to single embryo; miR162 and miR168 exert their regulatory function during total SE process, especially during stages 5-8; miR156, miR159, miR160, miR166, miR167, and miR390 might play regulatory roles during cotyledonary embryo development. These findings indicate that larch and possibly other gymnosperms have complex mechanisms of gene regulation involving specific and common miRNAs operating post-transcriptionally during embryogenesis.

Expression patterns of LmAP2L1 and LmAP2L2 encoding two-APETALA2 domain proteins during somatic embryogenesis and germination of hybrid larch (Larix x marschlinsii)

Two APETALA2 domain transcription factors were characterized first in angiosperms, and, recently, in several gymnosperms. These proteins are involved in several processes, from flowering to embryogenesis in Arabidopsis thaliana. We extrapolated this result to hybrid larch (Larixxmarschlinsii Coaz) resulting from a cross between European (Larix decidua) and Japanese (Larix kaempferi) larches. Somatic embryogenesis is well described and controlled for this Pinaceae. We characterized two-AP2 domain genes: LmAP2L1 and LmAP2L2. Phylogenetic analysis confirmed that LmAP2L1 and LmAP2L2 were orthologous to Norway spruce PaAP2L1 and PaAP2L2 and that L1 forms appeared to be specific to Pinaceae. RT-PCR analysis showed that larch APETALA2 was differentially expressed during late somatic embryogenesis and during the first steps of germination. Whereas LmAP2L2 was constitutively expressed during this process, LmAP2L1 expression appeared only during late somatic embryogenesis, when embryos were able to germinate. Further, LmAP2L1 appeared to be the preferentially expressed form during embryo germination. Thus, LmAP2L1 seems to be a valuable molecular marker for hybrid larch late somatic embryogenesis and could play a role during post-embryonic development.

[Patterns of somatic embryo formation in Siberian larch: embryological aspects]

Somatic embryogenesis was induced in Siberian larch by in vitro culturing zygotic embryos at different developmental stages. Cultures were grown in modified Murashige and Skoog medium supplemented with hormones 2,4-dichlorophenoxyacetic acid (2 mg/l) and 6-benzylaminopurine (0.5-1 mg/l). The success of somatic embryogenesis in this species depended on the tree genotype and developmental stage of embryos used for culturing. Somatic embryogenesis from immature zygotic embryos at the stage of cotyledon initiation was most active. After 5-10 days, such embryos formed the embryogenic tissue including two cell types--elongated highly vacuolated embryonic tubes and small embryonic cells. Somatic embryos were isolated from proliferating embryogenic tissues after 2 months of culture.

Germin-like genes are expressed during somatic embryogenesis and early development of conifers

Germins and germin-like proteins (GLPs) are members of a superfamily of proteins widely distributed in plants. Their localization within the extracellular matrix and in some cases their hydrogen peroxide-producing activity suggests that these proteins are involved in cell wall metabolism during stress responses and developmental processes. Several very highly conserved conifer GLPs have been identified in somatic embryo tissues. In order to gain more knowledge on their potential involvement in the development of this particular tissue, we have characterized a new GLP gene, LmGER1 in hybrid larch. Anti-GLP immunserum and in-gel activity analyses suggested the presence of superoxide dismutase activity in apoplastic proteins from larch somatic embryos. These results could indicate a possible role for LmGER1 in this physiological process. The expression of LmGER1 has been followed during the maturation of somatic embryos and in different organs of young plantlets by homologous transformation with a promoter-gus construct. This promoter was activated in the root cap of young embryos and, later on, in the cotyledons and in the vascular procambium and xylem. Furthermore, the importance of this gene in embryo development was evaluated by transforming embryonal masses with a gene construct encoding a hairpin RNA leading to gene silencing. The potential role of LmGER1 in cross-linking of cell wall components is discussed.

Atomic force microscopy and laser confocal scanning microscopy analysis of callose fibers developed from protoplasts of embryogenic cells of a conifer

Efficiency of novel fiber formation was much improved in protoplast culture of embryogenic cells (ECs) of a conifer, Larix leptolepis (Sieb. et Zucc.) Gord., by pre-culturing ECs in a medium containing a high concentration of glutamine (13.7 mM). The fibrillar substructures of large and elongated fibers of protoplasts isolated from Larix ECs were investigated by laser confocal scanning microscopy (LCSM) after Aniline Blue staining and atomic force microscopy (AFM) using a micromanipulator without any pre-treatment. Fibers were composed of bundles of fibrils and subfibrils, whose diameters were defined as 0.7 and 0.17 mum, respectively, by image analysis after LCSM and AFM. These fibers were proven to be composed of callose by using specific degrading enzymes for beta-1,4-glucan and beta-1,3-glucan.

Stable genetic transformation of Larix gmelinii L. by particle bombardment of zygotic embryos

We report a new protocol for the stable transformation of Larix gmelinii. Thirty mature zygotic embryos precultured for 3 days on solid medium supplemented with benzyladenine were bombarded with plasmids pUC-GHG (GUS, HPT, and GFP genes) or pBI221-HPT (HPT and GUS genes). After a 2-month culture on selection medium, hygromycin-resistant calli appeared on the surfaces of the necrotic embryos. The frequencies of embryos with resistant calli were 18.4% and 17.4% in the transformations with pUC-GHG and pBI221-HPT DNA, respectively. More than 20 adventitious shoots formed from each of the transgenic calli. Of 17 elongated shoots selected for culturing on a rooting medium, five shoots rooted after 2 months. Expression of the GFP and GUS genes was detected in the resistant tissues by microscopic observations and by a histological GUS activity assay, respectively. PCR and Southern analysis confirmed the stable insertion of the introduced DNA into the genome.

Identification of a novel factor, vanillyl benzyl ether, which inhibits somatic embryogenesis of Japanese larch (Larix leptolepis Gordon)

In contrast to angiosperms, some gymnosperms form well-developed suspensors in somatic embryogenesis. This characteristic makes it easy to study suspensor biology. In cultures with high cell densities, somatic embryogenesis of Japanese larch, especially the suspensor development, is strongly inhibited due to factor(s) that are released by the cells into the culture medium. In this study, we purified and identified one of the inhibitory factors present in high-cell-density conditioned medium (HCM) of larch cells. The factor with the strongest inhibitory activity was purified by dialysis, extraction by ethyl acetate, octadecylsilyl (ODS) column chromatography and high-performance liquid chromatography (HPLC). The inhibitory factor was identified as vanillyl benzyl ether (VBE) by physicochemical analysis. This compound was first isolated from natural resources. Authentic VBE inhibited somatic embryo formation in Japanese larch, and the inhibitory effect in the suspensor was stronger than in the embryo proper. Furthermore, quantification of VBE by HPLC demonstrated that VBE accumulates at high concentrations in HCM. These results suggest that VBE is a novel negative regulator of somatic embryogenesis.

Spatially quantitative control of the number of cotyledons in a clonal population of somatic embryos of hybrid larch Larix x leptoeuropaea

BACKGROUND AND AIMS

Many conifer embryos, both in natural seeds and in clonal populations of somatic embryos, display variability in the number of cotyledons. In hybrid larch, Larix x leptoeuropaea (synonymous with L. x marschlinsii Coaz), such variability has previously been reported in somatic embryos, together with a decrease in the average cotyledon number when benzyladenine (BA) is applied exogenously. Described here is a spatially quantitative study with the aim of throwing some light on the way cotyledon number is determined, and hence the mechanism of cotyledon formation.

METHODS

Stock cultures of embryogenic tissue were maintained and later made embryogenically active by standard methods. Development through cotyledon formation was followed by optical microscopy with quantitative measurement of embryo diameter and number of cotyledons. SEMs of representative stages and cotyledon numbers were done for purposes of illustration in this account. Existing mathematics of waveforms on a disc were cast into a form suitable to compare with the quantitative data.

RESULTS

The number of cotyledons is linearly related to the diameter of the apical surface of the embryo (which approximates a circular disc) at the time of first appearance of the cotyledon primordia. This linearity is a constant-spacing phenomenon between adjacent primordia. Addition of BA to the medium restricts the range of apical diameters without changing inter-cotyledon spacing. Slope/intercept ratio of the linear plot matches expectation for initiation of cotyledon pattern as a harmonic waveform on a circular disc.

CONCLUSIONS

The entire pattern of cotyledon primordia arises as a single entity coordinated by a mechanism with wave-forming properties. This is explicable by diverse mechanisms, especially either mechanical buckling ('biophysical') or reaction-diffusion kinetics ('physicochemical').

[The somatic embryogenesis and establishment of transformation experiment system in Larix principis-Rupprechtii]

Larix principis-Rupprechtii is one of the superior afforestation forest trees growing in north China. Embryogenic cultures were initiated from immature zygotic embryos of Larix principis-Rupprechtii on S culture medium containing 2, 4-D 0-2.2 mg/L, KT and BA each at 0-0. 8 mg/L. Embryogenic calli were subcultured and multiplicated on S + B culture medium containing dropping off each hormone concentration. We set up 33 steady-going embryogenic cell lines; We studied on the growth stage and genotype differences of every embryogenic cell lines; and Finded more than 10 high-frequency somatic embryogenesis cell lines such as 2K, 2T, 2I, 2J, 3C etc.. The number of 2T somatic embryos reaches 314/per gram of embryogenic tissue and the number of 3C somatic embryos is 185/per gram of embryogenic tissue. The re-induction method of Larix principis-Rupprechtii from somatic embryos was used to produce renewable embryogenic cultures and steady-going embryogenic cell lines effectively. Mature somatic embryos can germinate and develop further into plantlets when they are isolated and cultured on a hormone-free WPM culture medium. The regeneration plantlets were obtained. Furthermore, the transformation with a truncated gene of Bacillus thuringensis (B. t) were carried out, the PCR showed positive results, because of this, embryogenic cell line of Larix principis-Rupprechtii can be used for transformation experiments to support further breeding in forestry.