Plasma membrane receptor complexes

CDC48A, an interactor of WOX2, is required for embryonic patterning in Arabidopsis thaliana

KEY MESSAGE

Interactor of WOX2, CDC48A, is crucial for early embryo patterning and shoot meristem stem cell initiation, but is not required for WOX2 protein turnover or subcellular localization. During Arabidopsis embryo patterning, the WUSCHEL HOMEOBOX 2 (WOX2) transcription factor is a major regulator of protoderm and shoot stem cell initiation. Loss of WOX2 function results in aberrant protodermal cell divisions and, redundantly with its paralogs WOX1, WOX3, and WOX5, compromised shoot meristem formation. To elucidate the molecular basis for WOX2 function, we searched for protein interactors by IP-MS/MS from WOX2-overexpression roots displaying reprogramming toward shoot-like cell fates. Here, we report that WOX2 directly interacts with the type II AAA ATPase molecular chaperone CELL DIVISION CYCLE 48A (CDC48A). We confirmed this interaction with bimolecular fluorescence complementation and co-immunoprecipitation and found that both proteins co-localize in the nucleus. We show that CDC48A loss of function results in protoderm and shoot meristem stem cell initiation defects similar to WOX2 loss of function. We also provide evidence that CDC48A promotes WOX2 activity independently of proteolysis or the regulation of nuclear localization, common mechanisms of CDC48A function in other processes. Our results point to a new role of CDC48A in potentiating WOX2 function during early embryo patterning.

Genome-Wide Identification, Diversification, and Expression Analysis of Lectin Receptor-Like Kinase (LecRLK) Gene Family in Cucumber under Biotic Stress

Members of the lectin receptor-like kinase (LecRLKs) family play a vital role in innate plant immunity. Few members of the LecRLKs family have been characterized in rice and Arabidopsis, respectively. However, little literature is available about LecRLKs and their role against fungal infection in cucumber. In this study, 60 putative cucumber LecRLK (CsLecRLK) proteins were identified using genome-wide analysis and further characterized into L-type LecRLKs (24) and G-type LecRLKs (36) based on domain composition and phylogenetic analysis. These proteins were allocated to seven cucumber chromosomes and found to be involved in the expansion of the CsLecRLK gene family. Subcellular localization of CsaLecRLK9 and CsaLecRLK12 showed green fluorescence signals in the plasma membrane of leaves. The transcriptional profiling of CsLecRLK genes showed that L-type LecRLKs exhibited functional redundancy as compared to G-type LecRLKs. The qRT-PCR results indicated that both L- and G-type LecRLKs showed significant response against plant growth-promoting fungi (PGPF-Trichoderma harzianum Rifai), powdery mildew pathogen (PPM-Golovinomyces orontii (Castagne) V.P. Heluta), and combined (PGPF+PPM) treatments. The findings of this study contribute to a better understanding of the role of cucumber CsLecRLK genes in response to PGPF, PPM, and PGPF+PPM treatments and lay the basis for the characterization of this important functional gene family.

SCFSNIPER7 controls protein turnover of unfoldase CDC48A to promote plant immunity

The unfoldase CDC48 (Cell Division Cycle 48) is highly conserved in eukaryotes, serving as an AAA + ATPase to extract ubiquitinated proteins from large protein complexes and membranes. Although its biochemical properties have been studied extensively in yeast and animal systems, the biological roles and regulations of the plant CDC48s have been explored only recently. Here we describe the identification of a novel E3 ligase from the SNIPER (snc1-influencing plant E3 ligase reverse genetic) screen, which contributes to plant defense regulation by targeting CDC48A for degradation. SNIPER7 encodes an F-box protein and its overexpression leads to autoimmunity. We identified CDC48s as interactors of SNIPER7 through immunoprecipitation followed by mass spectrometry proteomic analysis. SNIPER7 overexpression lines phenocopy the autoimmune mutant Atcdc48a-4. Furthermore, CDC48A protein levels are reduced or stabilized when SNIPER7 is overexpressed or inhibited, respectively, suggesting that CDC48A is the ubiquitination substrate of SCF^SNIPER7 . Taken together, this study reveals a new mechanism where a SCF^SNIPER7 complex regulates CDC48 unfoldase levels and modulates immune output.

Toward the understanding of the role of CDC48, a major component of the protein quality control, in plant immunity

The evolutionally conserved chaperone-like protein CDC48 (cell division cycle 48) is a major component of ubiquitin-dependent protein degradation pathways in animal and yeast and, more generally, of the protein quality control machinery. In plants, CDC48 plays essential regulatory functions in development and the possibly that it contributes to protein degradation through the ubiquitin-proteasome system (UPS) and the endoplasmic reticulum-associated protein degradation (ERAD) system has been reported. In this review we described recent findings highlighting a role for CDC48 in plant immunity. First data indicated that CDC48 is S-nitrosylated in plant cells undergoing an immune response, regulates the turnover of immune receptors and mediates the degradation of viral proteins. Furthermore its overexpression was associated to an exacerbated hypersensitive-like cell death. We also designed and reported here the first CDC48 interactome. The corresponding data confirm the closed interaction of CDC48 with components of the UPS and shed light on its putative regulatory function of S-adenosyl-methionine synthesis and metabolism. More generally, these investigations further support the concept that plant cells facing pathogen attack finely regulate the protein quality control machinery.

Arabidopsis thaliana RECEPTOR DEAD KINASE1 Functions as a Positive Regulator in Plant Responses to ABA

Abscisic acid (ABA) is a major phytohormone involved in important stress-related and developmental plant processes. Membrane-delimited ABA signal transduction plays an important role in early ABA signaling, but the molecular mechanisms connecting core signaling components to the plasma membrane remain unclear. Plants have evolved a large number of receptor-like kinases (RLKs) to modulate diverse biological processes by perceiving extracellular stimuli and activating downstream signaling responses. In this study, a putative leucine-rich repeat-RLK gene named RECEPTOR DEAD KINASE1 (AtRDK1) was identified and characterized in Arabidopsis thaliana. RDK1 promoter-GUS analysis revealed that RDK1 is expressed ubiquitously in the various tissues in Arabidopsis, and its expression is mainly induced by ABA. In the presence of ABA, RDK1-deficient rdk1-1 and rdk1-2 lines showed significant resistance in cotyledon greening and root growth, whereas RDK1-overexpressing lines showed enhanced sensitivity. Consistently, the expression of ABA-responsive genes was significantly downregulated in rdk1 mutant seedlings, which were also hypersensitive to drought stress with increased water loss. Interestingly, RDK1 was found to be an atypical kinase localized to the plasma membrane and did not require its kinase activity during ABA-mediated inhibition of seedling development. Accordingly, RDK1 interacted in the plasma membrane with type 2C protein phosphatase ABSCISIC ACID INSENSITIVE1 (ABI1); this interaction was further enhanced by exogenous application of ABA, suggesting that RDK1-mediated recruitment of ABI1 onto the plasma membrane is important for ABA signaling. Taken together, these results reveal an important role for RDK1 in plant responses to abiotic stress conditions in an ABA-dependent manner.

New Insights on Leucine-Rich Repeats Receptor-Like Kinase Orthologous Relationships in Angiosperms

Leucine-Rich Repeats Receptor-Like Kinase (LRR-RLK) genes represent a large and complex gene family in plants, mainly involved in development and stress responses. These receptors are composed of an LRR-containing extracellular domain (ECD), a transmembrane domain (TM) and an intracellular kinase domain (KD). To provide new perspectives on functional analyses of these genes in model and non-model plant species, we performed a phylogenetic analysis on 8,360 LRR-RLK receptors in 31 angiosperm genomes (8 monocots and 23 dicots). We identified 101 orthologous groups (OGs) of genes being conserved among almost all monocot and dicot species analyzed. We observed that more than 10% of these OGs are absent in the Brassicaceae species studied. We show that the ECD structural features are not always conserved among orthologs, suggesting that functions may have diverged in some OG sets. Moreover, we looked at targets of positive selection footprints in 12 pairs of OGs and noticed that depending on the subgroups, positive selection occurred more frequently either in the ECDs or in the KDs.

Structure and functions of the chaperone-like p97/CDC48 in plants

BACKGROUND

The chaperone-like p97 is a member of the AAA+ ATPase enzyme family that contributes to numerous cellular activities. P97 has been broadly studied in mammals (VCP/p97) and yeasts (CDC48: Cell Division Cycle 48/p97) and numerous investigations highlighted that this protein is post-translationally regulated, is structured in homohexamer and interacts with partners and cofactors that direct it to distinct cellular signalization pathway including protein quality control and degradation, cell cycle regulation, genome stability, vesicular trafficking, autophagy and immunity.

SCOPE OF REVIEW

p97 is also conserved in plants (CDC48) but its functions are less understood. In the present review we intended to present the state of the art of the structure, regulation and functions of CDC48 in plants.

MAJOR CONCLUSIONS

Evidence accumulated underline that CDC48 plays a crucial role in development, cell cycle regulation and protein turnover in plants. Furthermore, its involvement in plant immunity has recently emerged and first interacting partners have been identified, shedding light on its putative cellular activities.

GENERAL SIGNIFICANCE

Identification of emerging functions of CDC48 in plants opens new roads of research in immunity and provides new insights into the mechanisms of protein quality control.

Identification and expression analysis of the LRR-RLK gene family in tomato (Solanum lycopersicum) Heinz 1706

As the largest subfamily of receptor-like kinases (RLKs), leucine-rich repeat receptor-like kinases (LRR-RLKs) regulate the growth, development, and stress responses of plants. Through a reiterative process of sequence analysis and re-annotation, 234 LRR-RLK genes were identified in the genome of tomato (Solanum lycopersicum) ‘Heinz 1706’, which were further grouped into 10 major groups based on their sequence similarity. In comparison to the significant role of tandem duplication in the expansion process of this gene family in other species, only approximately 12% (29 out of 234) of SlLRR-RLK genes arose from tandem duplication. Using the multiple expectation maximization for motif elicitation (MEME) method, the motif composition and arrangement were found to be variably conserved within each SlLRR-RLK group, indicating their different extent of functional divergence. Expression profiling analyses by qRT-PCR data revealed that SlLRR-RLK genes were differentially expressed in various tomato organs and tissues, and some SlLRR-RLK genes exhibited preferential expression in fruits at distinct developmental stages, suggesting that SlLRR-RLK may take important roles in fruit development and ripening process. The results of this study provide an overview of the LRR-RLK gene family in tomato Heinz 1706, one important species of Solanaceae, and will be helpful for future functional analysis of this important protein family in fleshy fruit-bearing species.

Different Gene Expressions of Resistant and Susceptible Hop Cultivars in Response to Infection with a Highly Aggressive Strain of Verticillium albo-atrum

Verticillium wilt has become a serious threat to hop production in Europe due to outbreaks of lethal wilt caused by a highly virulent strain of Verticillium albo-atrum. In order to enhance our understanding of resistance mechanisms, the fungal colonization patterns and interactions of resistant and susceptible hop cultivars infected with V. albo-atrum were analysed in time course experiments. Quantification of fungal DNA showed marked differences in spatial and temporal fungal colonization patterns in the two cultivars. Two differential display methods obtained 217 transcripts with altered expression, of which 84 showed similarity to plant proteins and 8 to fungal proteins. Gene ontology categorised them into cellular and metabolic processes, response to stimuli, biological regulation, biogenesis and localization. The expression patterns of 17 transcripts with possible implication in plant immunity were examined by real-time PCR (RT-qPCR). Our results showed strong expression of genes encoding pathogenesis-related (PR) proteins in susceptible plants and strong upregulation of genes implicated in ubiquitination and vesicle trafficking in the incompatible interaction and their downregulation in susceptible plants, suggesting the involvement of these processes in the hop resistance reaction. In the resistant cultivar, the RT-qPCR expression patterns of most genes showed their peak at 20 dpi and declined towards 30 dpi, comparable to the gene expression pattern of in planta detected fungal protein and coinciding with the highest fungal biomass in plants at 15 dpi. These expression patterns suggest that the defence response in the resistant cultivar is strong enough at 20 dpi to restrict further fungus colonization.

GRIM REAPER peptide binds to receptor kinase PRK5 to trigger cell death in Arabidopsis

Recognition of extracellular peptides by plasma membrane-localized receptor proteins is commonly used in signal transduction. In plants, very little is known about how extracellular peptides are processed and activated in order to allow recognition by receptors. Here, we show that induction of cell death in planta by a secreted plant protein GRIM REAPER (GRI) is dependent on the activity of the type II metacaspase METACASPASE-9. GRI is cleaved by METACASPASE-9 in vitro resulting in the release of an 11 amino acid peptide. This peptide bound in vivo to the extracellular domain of the plasma membrane-localized, atypical leucine-rich repeat receptor-like kinase POLLEN-SPECIFIC RECEPTOR-LIKE KINASE 5 (PRK5) and was sufficient to induce oxidative stress/ROS-dependent cell death. This shows a signaling pathway in plants from processing and activation of an extracellular protein to recognition by its receptor.

Protein abundance changes and ubiquitylation targets identified after inhibition of the proteasome with syringolin A

As proteins are the main effectors inside cells, their levels need to be tightly regulated. This is partly achieved by specific protein degradation via the Ubiquitin-26S proteasome system (UPS). In plants, an exceptionally high number of proteins are involved in Ubiquitin-26S proteasome system-mediated protein degradation and it is known to regulate most, if not all, important cellular processes. Here, we investigated the response to the inhibition of the proteasome at the protein level treating leaves with the specific inhibitor Syringolin A (SylA) in a daytime specific manner and found 109 accumulated and 140 decreased proteins. The patterns of protein level changes indicate that the accumulating proteins cause proteotoxic stress that triggers various responses. Comparing protein level changes in SylA treated with those in a transgenic line over-expressing a mutated ubiquitin unable to form polyubiquitylated proteins produced little overlap pointing to different response pathways. To distinguish between direct and indirect targets of the UPS we also enriched and identified ubiquitylated proteins after inhibition of the proteasome, revealing a total of 1791 ubiquitylated proteins in leaves and roots and 1209 that were uniquely identified in our study. The comparison of the ubiquitylated proteins with those changing in abundance after SylA-mediated inhibition of the proteasome confirmed the complexity of the response and revealed that some proteins are regulated both at transcriptional and post-transcriptional level. For the ubiquitylated proteins that accumulate in the cytoplasm but are targeted to the plastid or the mitochondrion, we often found peptides in their target sequences, demonstrating that the UPS is involved in controlling organellar protein levels. Attempts to identify the sites of ubiquitylation revealed that the specific properties of this post-translational modification can lead to incorrect peptide spectrum assignments in complex peptide mixtures in which only a small fraction of peptides is expected to carry the ubiquitin footprint. This was confirmed with measurements of synthetically produced peptides and calculating the similarities between the different spectra.

Analysis of expression profiles of selected genes associated with the regenerative property and the receptivity to gene transfer during somatic embryogenesis in Triticum aestivum L

The physiological, biochemical and molecular mechanisms regulating the initiation of a regenerative pathway remain partially unknown. Efforts to identify the biological features that confer transformation ability, or the tendency of some cells to induce transgene silencing, would help to improve plant genetic engineering. The objective of our study was to monitor the evolution of plant cell competencies in relation to both in vitro tissue culture regeneration and the genetic transformation properties. We used a simple wheat regeneration procedure as an experimental model for studying the regenerative capacity of plant cells and their receptivity to direct gene transfer over the successive steps of the regenerative pathway. Target gene profiling studies and biochemical assays were conducted to follow some of the mechanisms triggered during the somatic-to-embryogenic transition (i.e. dedifferentiation, cell division activation, redifferentiation) and affecting the accessibility of plant cells to receive and stably express the exogenous DNA introduced by bombardment. Our results seem to indicate that the control of cell-cycle (S-phase) and host defense strategies can be crucial determinants of genetic transformation efficiency. The results from studies conducted at macro-, micro- and molecular scales are then integrated into a holistic approach that addresses the question of tissue culture and transgenesis competencies more broadly. Through this multilevel analysis we try to establish functional links between both regenerative capacity and transformation receptiveness, and thereby to provide a more global and integrated vision of both processes, at the core of defense/adaptive mechanisms and survival, between undifferentiated cell proliferation and organization.

Control of Tobacco mosaic virus movement protein fate by CELL-DIVISION-CYCLE protein48

Like many other viruses, Tobacco mosaic virus replicates in association with the endoplasmic reticulum (ER) and exploits this membrane network for intercellular spread through plasmodesmata (PD), a process depending on virus-encoded movement protein (MP). The movement process involves interactions of MP with the ER and the cytoskeleton as well as its targeting to PD. Later in the infection cycle, the MP further accumulates and localizes to ER-associated inclusions, the viral factories, and along microtubules before it is finally degraded. Although these patterns of MP accumulation have been described in great detail, the underlying mechanisms that control MP fate and function during infection are not known. Here, we identify CELL-DIVISION-CYCLE protein48 (CDC48), a conserved chaperone controlling protein fate in yeast (Saccharomyces cerevisiae) and animal cells by extracting protein substrates from membranes or complexes, as a cellular factor regulating MP accumulation patterns in plant cells. We demonstrate that Arabidopsis (Arabidopsis thaliana) CDC48 is induced upon infection, interacts with MP in ER inclusions dependent on the MP N terminus, and promotes degradation of the protein. We further provide evidence that CDC48 extracts MP from ER inclusions to the cytosol, where it subsequently accumulates on and stabilizes microtubules. We show that virus movement is impaired upon overexpression of CDC48, suggesting that CDC48 further functions in controlling virus movement by removal of MP from the ER transport pathway and by promoting interference of MP with microtubule dynamics. CDC48 acts also in response to other proteins expressed in the ER, thus suggesting a general role of CDC48 in ER membrane maintenance upon ER stress.

Expression of PiABP19, Picdc2 and PiSERK3 during induction of somatic embryogenesis in leaflets of Prunus incisa (Thunb.)

Somatic embryogenesis is a useful tool of plant breeding. In this context, a procedure for inducing somatic embryogenesis in Prunus incisa leaf explants had been previously developed. The original in vitro protocol relies on picloram treatments and exposure to darkness as inductive conditions, the best frequency of embryogenesis being obtained on the second leaf (F(2)) exposed to 4 μM picloram during 30 days. The morphological and biochemical changes observed during somatic embryogenesis occur in response to alterations in gene expression regulation patterns. A molecular study was conducted in order to provide deeper insight into the fundamental biological factors involved in the induction of this process using a gene candidate strategy and semi-quantitative reverse transcription polymerase chain reaction analysis. So far, no sequence data related to somatic embryogenesis has been available in cherry. In the present study, we cloned and sequenced cDNA fragments of putative genes encoding auxin-binding protein, cell cycle regulator and somatic embryogenesis receptor kinase. Time-course differential transcript accumulations were observed for all investigated genes in leaves or derived callus tissues during the observation period (first month of culture). Their possible involvement in the sequential steps of the embryogenic pathway (dedifferentiation, cell proliferation, differentiation through somatic embryogenesis) is presented and discussed.

A model for the biosynthesis and transport of plasma membrane-associated signaling receptors to the cell surface

Intracellular protein transport is emerging as critical in determining the outcome of receptor-activated signal transduction pathways. In plants, relatively little is known about the nature of the molecular components and mechanisms involved in coordinating receptor synthesis and transport to the cell surface. Recent advances in this field indicate that signaling pathways and intracellular transport machinery converge and coordinate to render receptors competent for signaling at their plasma membrane (PM) activity sites. The biogenesis and transport to the cell surface of signaling receptors appears to require both general trafficking and receptor-specific factors. Several molecular determinants, residing or associated with compartments of the secretory pathway and known to influence aspects in receptor biogenesis, are discussed and integrated into a predictive cooperative model for the functional expression of signaling receptors at the PM.

Quantification of the brassinosteroid insensitive1 receptor in planta

In plants, green fluorescent protein (GFP) is routinely used to determine the subcellular location of fusion proteins. Here, we show that confocal imaging can be employed to approximate the number of GFP-labeled protein molecules present in living Arabidopsis (Arabidopsis thaliana) root cells. The technique involves calibration with soluble GFP to provide a usable protein concentration range within the confocal volume of the microscope. As a proof of principle, we quantified the Brassinosteroid Insensitive1 (BRI1) receptor fused to GFP, under control of its own promoter. The number of BRI1-GFP molecules per root epidermal cell ranges from 22,000 in the meristem and 130,000 in the elongation zone to 80,000 in the maturation zone, indicating that up to 6-fold differences in BRI1 receptor content exist. In contrast, when taking into account differences in cell size, BRI1-GFP receptor density in the plasma membrane is kept constant at 12 receptors μm⁻² in all cells throughout the meristem and elongation zone. Only the quiescent center and columella cells deviate from this pattern and have 5 to 6 receptors μm⁻². Remarkably, root cell sensitivity toward brassinosteroids appears to coincide with uniform meristem receptor density.

Probing the roles of LRR RLK genes in Arabidopsis thaliana roots using a custom T-DNA insertion set

Leucine-rich repeat receptor-like protein kinases (LRR RLKs) represent the largest group of Arabidopsis RLKs with approximately 235 members. A minority of these LRR RLKs have been assigned to diverse roles in development, pathogen resistance and hormone perception. Using a reverse genetics approach, a collection of homozygous T-DNA insertion lines for 69 root expressed LRR RLK genes was screened for root developmental defects and altered response after exposure to environmental, hormonal/chemical and abiotic stress. The obtained data demonstrate that LRR RLKs play a role in a wide variety of signal transduction pathways related to hormone and abiotic stress responses. The described collection of T-DNA insertion mutants provides a valuable tool for future research into the function of LRR RLK genes.

The SERK1 receptor-like kinase regulates organ separation in Arabidopsis flowers

Through a sensitized screen for novel components of pathways regulating organ separation in Arabidopsis flowers, we have found that the leucine-rich repeat receptor-like kinase SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE1 (SERK1) acts as a negative regulator of abscission. Mutations in SERK1 dominantly rescue abscission in flowers without functional NEVERSHED (NEV), an ADP-ribosylation factor GTPase-activating protein required for floral organ shedding. We previously reported that the organization of the Golgi apparatus and location of the trans-Golgi network (TGN) are altered in nev mutant flowers. Disruption of SERK1 restores Golgi structure and the close association of the TGN in nev flowers, suggesting that defects in these organelles may be responsible for the block in abscission. We have also found that the abscission zones of nev serk1 flowers are enlarged compared to wild-type. A similar phenotype was previously observed in plants constitutively expressing a putative ligand required for organ separation, INFLORESCENCE DEFICIENT IN ABSCISSION (IDA), suggesting that signalling through IDA and its proposed receptors, HAESA and HAESA-LIKE2, may be deregulated in nev serk1 abscission zone cells. Our studies indicate that in addition to its previously characterized roles in stamen development and brassinosteroid perception, SERK1 plays a unique role in modulating the loss of cell adhesion that occurs during organ abscission.

The pollen S-determinant in Papaver: comparisons with known plant receptors and protein ligand partners

Cell-cell communication is vital to multicellular organisms and much of it is controlled by the interactions of secreted protein ligands (or other molecules) with cell surface receptors. In plants, receptor-ligand interactions are known to control phenomena as diverse as floral abscission, shoot apical meristem maintenance, wound response, and self-incompatibility (SI). SI, in which 'self' (incompatible) pollen is rejected, is a classic cell-cell recognition system. Genetic control of SI is maintained by an S-locus, in which male (pollen) and female (pistil) S-determinants are encoded. In Papaver rhoeas, PrsS proteins encoded by the pistil S-determinant interact with incompatible pollen to effect inhibition of pollen growth via a Ca(2+)-dependent signalling network, resulting in programmed cell death of 'self' pollen. Recent studies are described here that identified and characterized the pollen S-determinant of SI in P. rhoeas. Cloning of three alleles of a highly polymorphic pollen-expressed gene, PrpS, which is linked to pistil-expressed PrsS revealed that PrpS encodes a novel approximately 20 kDa transmembrane protein. Use of antisense oligodeoxynucleotides provided data showing that PrpS functions in SI and is the pollen S-determinant. Identification of PrpS represents a milestone in the SI field. The nature of PrpS suggests that it belongs to a novel class of 'receptor' proteins. This opens up new questions about plant 'receptor'-ligand pairs, and PrpS-PrsS have been examined in the light of what is known about other receptors and their protein-ligand pairs in plants.

Molecular aspects of somatic-to-embryogenic transition in plants

Somatic embryogenesis (SE) is a model system for understanding the physiological, biochemical, and molecular biological events occurring during plant embryo development. Plant somatic cells have the ability to undergo sustained divisions and give rise to an entire organism. This remarkable feature is called plant cell totipotency. SE is a notable illustration of plant totipotency and involves reprogramming of development in somatic cells toward the embryogenic pathway. Plant growth regularities, especially auxins, are key components as their exogenous application recapitulates the embryogenic potential of the mitotically quiescent somatic cells. It has been observed that there are genetic and also physiological factors that trigger in vitro embryogenesis in various types of plant somatic cells. Analysis of the proteome and transcriptome has led to the identification and characterization of certain genes involved in SE. Most of these genes, however, are upregulated only in the late developmental stages, suggesting that they do not play a direct role in the vegetative-to-embryogenic transition. However, the molecular bases of those triggering factors and the genetic and biochemical mechanisms leading to in vitro embryogenesis are still unknown. Here, we describe the plant factors that participate in the vegetative-to-embryogenic transition and discuss their possible roles in this process.