Phytosulfokine Stimulates Somatic Embryogenesis in Cryptomeria japonica

Response surface methodology mediated optimization of phytosulfokine and plant growth regulators for enhanced protoplast division, callus induction, and somatic embryogenesis in Angelica Gigas Nakai

BACKGROUND

Angelica Gigas (Purple parsnip) is an important medicinal plant that is cultivated and utilized in Korea, Japan, and China. It contains bioactive substances especially coumarins with anti-inflammatory, anti-platelet aggregation, anti-cancer, anti-diabetic, antimicrobial, anti-obesity, anti-oxidant, immunomodulatory, and neuroprotective properties. This medicinal crop can be genetically improved, and the metabolites can be obtained by embryonic stem cells. In this context, we established the protoplast-to-plant regeneration methodology in Angelica gigas.

RESULTS

In the present investigation, we isolated the protoplast from the embryogenic callus by applying methods that we have developed earlier and established protoplast cultures using Murashige and Skoog (MS) liquid medium and by embedding the protoplast in thin alginate layer (TAL) methods. We supplemented the culture medium with growth regulators namely 2,4-dichlorophenoxyaceticacid (2,4-D, 0, 0.75, 1.5 mg L- 1), kinetin (KN, 0, 0.5, and 1.0 mg L- 1) and phytosulfokine (PSK, 0, 50, 100 nM) to induce protoplast division, microcolony formation, and embryogenic callus regeneration. We applied central composite design (CCD) and response surface methodology (RSM) for the optimization of 2,4-D, KN, and PSK levels during protoplast division, micro-callus formation, and induction of embryogenic callus stages. The results revealed that 0.04 mg L- 1 2,4-D + 0.5 mg L- 1 KN + 2 nM PSK, 0.5 mg L- 1 2,4-D + 0.9 mg L- 1 KN and 90 nM PSK, and 1.5 mg L- 1 2,4-D and 1 mg L- 1 KN were optimum for protoplast division, micro-callus formation and induction embryogenic callus. MS basal semi-solid medium without growth regulators was good for the development of embryos and plant regeneration.

CONCLUSIONS

This study demonstrated successful protoplast culture, protoplast division, micro-callus formation, induction embryogenic callus, somatic embryogenesis, and plant regeneration in A. gigas. The methodologies developed here are quite useful for the genetic improvement of this important medicinal plant.

DNA- and Selectable-Marker-Free Genome-Editing System Using Zygotes from Recalcitrant Maize Inbred B73

Genome-editing tools such as the clustered regularly interspaced short palindromic repeats/Cas9 (CRISPR/Cas9) system have become essential tools for increasing the efficiency and accuracy of plant breeding. Using such genome-editing tools on maize, one of the most important cereal crops of the world, will greatly benefit the agriculture and the mankind. Conventional genome-editing methods typically used for maize involve insertion of a Cas9-guide RNA expression cassette and a selectable marker in the genome DNA; however, using such methods, it is essential to eliminate the inserted DNA cassettes to avoid legislative concerns on gene-modified organisms. Another major hurdle for establishing an efficient and broadly applicable DNA-free genome-editing system for maize is presented by recalcitrant genotypes/cultivars, since cell/tissue culture and its subsequent regeneration into plantlets are crucial for producing transgenic and/or genome-edited maize. In this study, to establish a DNA-free genome-editing system for recalcitrant maize genotypes/cultivars, Cas9-gRNA ribonucleoproteins were directly delivered into zygotes isolated from the pollinated flowers of the maize-B73 cultivar. The zygotes successfully developed and were regenerated into genome-edited plantlets by co-culture with phytosulfokine, a peptide phytohormone. The method developed herein made it possible to obtain DNA- and selectable-marker-free genome-edited recalcitrant maize genotypes/cultivars with high efficiency. This method can advance the molecular breeding of maize and other important cereals, regardless of their recalcitrant characteristics.

Application of Developmental Regulators for Enhancing Plant Regeneration and Genetic Transformation

Establishing plant regeneration systems and efficient genetic transformation techniques plays a crucial role in plant functional genomics research and the development of new crop varieties. The inefficient methods of transformation and regeneration of recalcitrant species and the genetic dependence of the transformation process remain major obstacles. With the advancement of plant meristematic tissues and somatic embryogenesis research, several key regulatory genes, collectively known as developmental regulators, have been identified. In the field of plant genetic transformation, the application of developmental regulators has recently garnered significant interest. These regulators play important roles in plant growth and development, and when applied in plant genetic transformation, they can effectively enhance the induction and regeneration capabilities of plant meristematic tissues, thus providing important opportunities for improving genetic transformation efficiency. This review focuses on the introduction of several commonly used developmental regulators. By gaining an in-depth understanding of and applying these developmental regulators, it is possible to further enhance the efficiency and success rate of plant genetic transformation, providing strong support for plant breeding and genetic engineering research.

Phytosulfokine alpha enhances regeneration of transformed and untransformed protoplasts of Brassica oleracea

Phytosulfokine-α (PSK-α) is a disulfated pentapeptide (YIYTQ) acting as an intercellular signal peptide and growth factor. It was originally isolated from conditioned medium of asparagus mesophyll cell cultures in 1996 and later characterized as a hormone-like signal molecule with important roles in numerous processes of in vivo plant growth and development. It is currently becoming a valuable mitogenic factor in plant breeding and biotechnology due to its stimulatory effect on in vitro cell elongation, proliferation and differentiation. The focus of our work was to review current knowledge about the roles of PSK-α in plant biotechnology and to evaluate its influence on the regeneration of protoplasts of four Brassica oleracea cultivars (two cauliflower and two cabbage) cultured under two distinctive protocols and with different protoplast densities. Protoplast regeneration was studied due to its high value for plant genome editing, which is generally limited by the inefficient regeneration of treated protoplasts of numerous important plant genotypes. Our hypothesis was that the stress related to PEG-mediated protoplast transformation and the following decrease in viable protoplast density in culture could be alleviated by the addition of PSK-α to the culture medium. We therefore tested whether PSK-α could increase cell division at the early stages of culture (5 and 15 days after protoplast isolation) and stimulate the formation of microcallus colonies up to the 30st day of culture and to evaluate its influence on callus organogenesis leading to shoot regeneration. The PSK-α showed a strong stimulatory effect on untransformed protoplast regeneration already during the first days of culture, accelerating cell division up to 5.3-fold and the formation of multicellular microcallus colonies up to 37.0-fold. The beneficial influence was retained at later stages of regeneration, when PSK improved shoot organogenesis even if it was present only during the first 10 days of culture. The highest numbers of shoots, however, were regenerated when PSK was present during the first days of culture and later in solid shoot regeneration medium. Finally, the addition of PSK-α to PEG-transformed protoplasts significantly enhanced their division rate and the formation of microcallus colonies in selection media, up to 44.0-fold.

The effect of phytosulfokine alpha on haploid embryogenesis and gene expression of Brassica napus microspore cultures

Microspore embryogenesis (ME) is the most powerful tool for creating homozygous lines in plant breeding and molecular biology research. It is still based mainly on the reprogramming of microspores by temperature, osmotic and/or nutrient stress. New compounds are being sought that could increase the efficiency of microspore embryogenesis or even induce the formation of haploid embryos from recalcitrant genotypes. Among these, the mitogenic factor phytosulfokine alpha (PSK-α) is promising due to its broad spectrum of activity in vivo and in vitro. The aim of our study was to investigate the effect of PSK-α on haploid embryogenesis from microspores of oilseed rape (Brassica napus L., DH4079), one of the most important oil crops and a model plant for studying the molecular mechanisms controlling embryo formation. We tested different concentrations (0, 0.01, 0.1 and 1 µM) of the peptide and evaluated its effect on microspore viability and embryo regeneration after four weeks of culture. Our results showed a positive correlation between addition of PSK-α and cultured microspore viability and a positive effect also on the number of developed embryos. The analysis of transcriptomes across three time points (day 0, 2 and 4) with or without PSK-α supplementation (15 RNA libraries in total) unveiled differentially expressed genes pivotal in cell division, microspore embryogenesis, and subsequent regeneration. PCA grouped transcriptomes by RNA sampling time, with the first two principal components explaining 56.8% variability. On day 2 with PSK, 45 genes (15 up- and 30 down-regulated) were differentially expressed when PSK-α was added and their number increased to 304 by day 4 (30 up- and 274 down-regulated). PSK, PSKR, and PSI gene expression analysis revealed dynamic patterns, with PSK2 displaying the highest increase and overall expression during microspore culture at days 2 and 4. Despite some variations, only PSK1 showed significant differential expression upon PSK-α addition. Of 16 ME-related molecular markers, 3 and 15 exhibited significant differential expression in PSK-supplemented cultures at days 2 and 4, respectively. Embryo-specific markers predominantly expressed after 4 days of culture, with higher expression in medium without PSK, while on day 0, numerous sporophyte-specific markers were highly expressed.

PHGD: An integrative and user-friendly database for plant hormone-related genes

Plant Hormone Gene Database (PHGD) database platform construction pipeline. First, we collected all reported hormone-related genes in the model plant Arabidopsis thaliana, and combined with the existing experimental background, mapped the hormone-gene interaction network to provide a blueprint. Next, we collected 469 high-quality plant genomes. Then, bioinformatics was used to identify hormone-related genes in these plants. Finally, these genetic data were programmed to be stored in a database and a platform website PHGD was built. PHGD was divided into eight modules, namely Home, Browse, Search, Resources, Download, Tools, Help, and Contact. We provided data resources and platform services to facilitate the study of plant hormones.

Phytosulfokine peptides, their receptors, and functions

Phytosulfokines (PSKs) are a class of disulfated pentapeptides and are regarded as plant peptide hormones. PSK-α, -γ, -δ, and -ϵ are four bioactive PSKs that are reported to have roles in plant growth, development, and immunity. In this review, we summarize recent advances in PSK biosynthesis, signaling, and function. PSKs are encoded by precursor genes that are widespread in higher plants. PSKs maturation from these precursors requires a sulfation step, which is catalyzed by a tyrosylprotein sulfotransferase, as well as proteolytic cleavage by subtilisin serine proteases. PSK signaling is mediated by plasma membrane-localized receptors PSKRs that belong to the leucine-rich repeat receptor-like kinase family. Moreover, multiple biological functions can be attributed to PSKs, including promoting cell division and cell growth, regulating plant reproduction, inducing somatic embryogenesis, enhancing legume nodulation, and regulating plant resistance to biotic and abiotic stress. Finally, we propose several research directions in this field. This review provides important insights into PSKs that will facilitate biotechnological development and PSK application in agriculture.

Less Frequently Used Growth Regulators in Plant Tissue Culture

Plant growth regulators are routinely added to in vitro culture media to foster the growth and differentiation of the cells, tissues, and organs. However, while the literature on usage of the more common auxins, cytokinins, gibberellins, abscisic acid, and ethylene is vast, other compounds that also have shown a growth-regulating activity have not been studied as frequently. Such substances are also capable of modulating the responses of plant cells and tissues in vitro by regulating their growth, differentiation, and regeneration competence, but also by enhancing their responses toward biotic and abiotic stress agents and improving the production of secondary metabolites of interest. This chapter will discuss the in vitro effects of several of such less frequently added plant growth regulators, including brassinosteroids (BRS), strigolactones (SLs), phytosulfokines (PSKs), methyl jasmonate, salicylic acid (SA), sodium nitroprusside (SNP), hydrogen sulfite, various plant growth retardants and inhibitors (e.g., ancymidol, uniconazole, flurprimidol, paclobutrazol), and polyamines.

Genome-wide association study for boll weight in Gossypium hirsutum races

High yield has always been an essential target in almost all of the cotton breeding programs. Boll weight (BW) is a key component of cotton yield. Numerous linkage mapping and genome-wide association studies (GWAS) have been performed to understand the genetic mechanism of BW, but information on the markers/genes controlling BW remains limited. In this study, we conducted a GWAS for BW using 51,268 high-quality single-nucleotide polymorphisms (SNPs) and 189 Gossypium hirsutum accessions across five different environments. A total of 55 SNPs significantly associated with BW were detected, of which 29 and 26 were distributed in the A and D subgenomes, respectively. Five SNPs were simultaneously detected in two environments. For TM5655, TM8662, TM36371, and TM50258, the BW grouped by alleles of each SNP was significantly different. The ± 550 kb regions around these four key SNPs contained 262 genes. Of them, Gh_A02G1473, Gh_A10G1765, and Gh_A02G1442 were expressed highly at 0 to 1 days post-anthesis (dpa), - 3 to 0 dpa, and - 3 to 0 dpa in ovule of TM-1, respectively. They were presumed as the candidate genes for fiber cell differentiation, initiation, or elongation based on gene annotation of their homologs. Overall, these results supplemented valuable information for dissecting the genetic architecture of BW and might help to improve cotton yield through molecular marker-assisted selection breeding and molecular design breeding.

The Research Process of PSK Biosynthesis, Signaling Transduction, and Potential Applications in Brassica napus

Phytosulfokine (PSK) is a disulfated pentapeptide that acts as a growth regulator to control plant growth and development as well as adaptability to biotic and abiotic stress. In the last three decades, PSK has drawn increasing attention due to its various functions. Preproproteins that have been tyrosine sulfonylated and then cleaved by specific enzymes contribute to mature PSK. To transfer a signal from the apoplast to the inner cells, the PSK peptide must bind to the PSK receptors (PSKR1 and PSKR2) at the cell surface. The precise mechanism of PSK signal transduction is still unknown, given that PSKR combines receptor and kinase activity with a capacity to bind calmodulin (CaM). The binding of PSK and PSKR stimulates an abundance of cGMP downstream from PSKR, further activating a cation-translocating unit composed of cyclic nucleotide-gated channel 17 (CNGC17), H+-ATPases AHA1 and AHA2, and BRI-associated receptor kinase 1 (BAK1). Recently, it has been revealed that posttranslational ubiquitination is closely related to the control of PSK and PSKR binding. To date, the majority of studies related to PSK have used Arabidopsis. Given that rapeseed and Arabidopsis share a close genetic relationship, the relevant knowledge obtained from Arabidopsis can be further applied to rapeseed.

Somatic Embryogenesis in Conifers: One Clade to Rule Them All?

Somatic embryogenesis (SE) in conifers is usually characterized as a multi-step process starting with the development of proembryogenic cell masses and followed by histodifferentiation, somatic embryo development, maturation, desiccation, and plant regeneration. Our current understanding of conifers' SE is mainly derived from studies using Pinaceae species as a model. However, the evolutionary relationships between conifers are not clear. Some hypotheses consider conifers as a paraphyletic group and Gnetales as a closely related clade. In this review, we used an integrated approach in order to cover the advances in knowledge on SE in conifers and Gnetales, discussing the state-of-the-art and shedding light on similarities and current bottlenecks. With this approach, we expect to be able to better understand the integration of these clades within current studies on SE. Finally, the points discussed raise an intriguing question: are non-Pinaceae conifers less prone to expressing embryogenic competence and generating somatic embryos as compared to Pinaceae species? The development of fundamental studies focused on this morphogenetic route in the coming years could be the key to finding a higher number of points in common between these species, allowing the success of the SE of one species to positively affect the success of another.

Phytosulfokine peptide optimizes plant growth and defense via glutamine synthetase GS2 phosphorylation in tomato

Phytosulfokine (PSK) is a plant pentapeptide hormone that fulfills a wide range of functions. Although PSK has frequently been reported to function in the inverse regulation of growth and defense in response to (hemi)biotrophic pathogens, the mechanisms involved remain largely unknown. Using the tomato (Solanum lycopersicum) and Pseudomonas syringae pv. tomato (Pst) DC3000 pathogen system, we present compelling evidence that the PSK receptor PSKR1 interacts with the calcium-dependent protein kinase CPK28, which in turn phosphorylates the key enzyme of nitrogen assimilation glutamine synthetase GS2 at two sites (Serine-334 and Serine-360). GS2 phosphorylation at S334 specifically regulates plant defense, whereas S360 regulates growth, uncoupling the PSK-induced effects on defense responses and growth regulation. The discovery of these sites will inform breeding strategies designed to optimize the growth-defense balance in a compatible manner.

The plant peptide hormone phytosulfokine promotes somatic embryogenesis by maintaining redox homeostasis in Cunninghamia lanceolata

Somatic embryogenesis (SE) is widely used for studying the mechanisms of embryo development. However, little is known about the underlying mechanisms, especially in woody plants. Previous studies have established an SE system for Chinese fir (Cunninghamia lanceolata), but this system is genotype-dependent, which limits its application in practice. Here, we found that phytosulfokine (PSK), a plant peptide hormone, can not only increase SE efficiency, but also establish SE in recalcitrant genotypes of C. lanceolata. Proembryogenic mass (PEM) browning and determination of hydrogen peroxide (H₂ O₂ ) content by 2',7'-dichlorofluorescein staining indicated that a reactive oxygen species (ROS) burst occurred rapidly after PEMs were transferred to SE induction medium. Transcriptome analysis and quantitative reverse transcriptase-PCR validation showed that PSK treatment helped to maintain ROS homeostasis by decreasing the activity of peroxidases in early SE induction. This PSK-regulated redox microenvironment might be helpful to induce expression of SE-related genes like WOX2 in early SE induction. Further analyses suggested that PSK promotes SE induction in C. lanceolata partially through decreasing H₂ O₂ levels, which is necessary but not sufficient for SE induction in recalcitrant genotypes of C. lanceolata. Furthermore, heterologous overexpression of ClPSK in Arabidopsis led to enhanced SE induction and resistance to H₂ O₂ stress. Taken together, our study reveals a biological function for the plant peptide hormone PSK, extends our knowledge about SE in woody trees, and provides a valuable tool for establishing an efficient and genotype-independent SE system in C. lanceolata and other coniferous trees.

OsPSKR15, a phytosulfokine receptor from rice enhances abscisic acid response and drought stress tolerance

Abscisic acid (ABA) is a major phytohormone that acts as stimuli and plays an important role in plant growth, development, and environmental stress responses. Membrane-localized receptor-like kinases (RLKs) help to detect extracellular stimuli and activate downstream signaling responses to modulate a variety of biological processes. Phytosulfokine receptor (PSKR), a Leu-rich repeat (LRR)-RLK, has been characterized for its role in growth, development and biotic stress. Here, we observed that OsPSKR15, a rice PSKR, was upregulated by ABA in Oryza sativa. We demonstrated OsPSKR15 is a positive regulator in plant response to ABA. Ectopic expression of OsPSKR15 in Arabidopsis thaliana increased the sensitivity to ABA during germination, growth and stomatal closure. Consistently, the expression of ABA-inducible genes was significantly upregulated in these plants. OsPSKR15 also regulated reactive oxygen species (ROS)-mediated ABA signaling in guard cells, thereby governing stomatal closure. Furthermore, the constitutive expression of OsPSKR15 enhanced drought tolerance by reducing the transpirational water loss in Arabidopsis. We also reported that OsPSKR15 directly interacts with AtPYL9 and its orthologue OsPYL11 of rice through its kinase domain in the plasma membrane and nucleus. Altogether, these results reveal an important role of OsPSKR15 in plant response toward abiotic stress in an ABA-dependent manner.

Insight into early diversification of leucine-rich repeat receptor-like kinases provided by the sequenced moss and hornwort genomes

Identification of the subfamily X leucine-rich repeat receptor-like kinases in the recently sequenced moss and hornwort genomes points to their diversification into distinct groups during early evolution of land plants. Signal transduction mediated through receptor-ligand interactions plays key roles in controlling developmental and physiological processes of multicellular organisms, and plants employ diverse receptors in signaling. Leucine-rich repeat receptor-like kinases (LRR-RLKs) represent one of the largest receptor classes in plants and are structurally classified into subfamilies. LRR-RLKs of the subfamily X are unique in the variety of their signaling roles; they include receptors for steroid or peptide hormones as well as negative regulators of signaling through binding to other LRR-RLKs, raising a question as to how they diversified. However, our understanding of diversification processes of LRR-RLKs has been hindered by the paucity of genomic data in non-seed plants and limited taxa sampling in previous phylogenetic analyses. Here we analyzed the phylogeny of LRR-RLK X sequences collected from all major land plant lineages and show that this subfamily diversified into six major clades before the divergence between bryophytes and vascular plants. Notably, we have identified homologues of the brassinosteroid receptor, BRASSINOSTEROID INSENSITIVE 1 (BRI1), in the genomes of Sphagnum mosses, hornworts, and ferns, contrary to earlier reports that postulate the origin of BRI1-like LRR-RLKs in the seed plant lineage. The phylogenetic distribution of major clades illustrates that the current receptor repertoire was shaped through lineage-specific gene family expansion and independent gene losses, highlighting dynamic changes in the evolution of LRR-RLKs.

Proteins, Small Peptides and Other Signaling Molecules Identified as Inconspicuous but Possibly Important Players in Microspores Reprogramming Toward Embryogenesis

In this review, we describe and integrate the latest knowledge on the signaling role of proteins and peptides in the stress-induced microspore embryogenesis (ME) in some crop plants with agricultural importance (i.e., oilseed rape, tobacco, barley, wheat, rice, triticale, rye). Based on the results received from the most advanced omix analyses, we have selected some inconspicuous but possibly important players in microspores reprogramming toward embryogenic development. We provide an overview of the roles and downstream effect of stress-related proteins (e.g., β-1,3-glucanases, chitinases) and small signaling peptides, especially cysteine—(e.g., glutathione, γ-thionins, rapid alkalinization factor, lipid transfer, phytosulfokine) and glycine-rich peptides and other proteins (e.g., fasciclin-like arabinogalactan protein) on acclimation ability of microspores and the cell wall reconstruction in a context of ME induction and haploids/doubled haploids (DHs) production. Application of these molecules, stimulating the induction and proper development of embryo-like structures and green plant regeneration, brings significant improvement of the effectiveness of DHs procedures and could result in its wider incorporation on a commercial scale. Recent advances in the design and construction of synthetic peptides–mainly cysteine-rich peptides and their derivatives–have accelerated the development of new DNA-free genome-editing techniques. These new systems are evolving incredibly fast and soon will find application in many areas of plant science and breeding.

Root meristem growth factor RGF, a sulfated peptide hormone in plants

In recent decades, small secreted peptides have been recognized as a new class of intercellular signaling phytohormones in plants. Tyrosine sulfation plays crucial roles in peptide hormone bioactivities in plants. The Arabidopsis tyrosylprotein sulfotransferase mutant tpst-1 causes severe abnormalities in the root tip due to deficiency in the biosynthesis of all functional tyrosine-sulfated peptides. Root meristem growth factor RGF, a sulfated peptide hormone specifically expressed in the root tip, was found to complement tpst-1 root defects. This review summarizes the history of the identification of RGF, the characteristics of RGF, the identification of RGF receptors, and the target of RGF. In brief, RGF is a 13 amino acid sulfated peptide. The RGF peptide mutant rgf1,2,3 exhibited a reduced size of the root apical meristem, indicating that RGF maintains cell proliferation activity in the root apical meristem. RGF receptors were identified by comprehensive binding analysis with a custom-made receptor expression library. The RGF receptor mutant rgfr1,2,3 showed a phenotype of reduced root length due to a reduction in the root apical meristem and was insensitive to RGF. The signaling cascade through RGF-RGF receptor pairs regulates the gradient formation of PLETHORA (PLT), which is known as the master regulator of root formation. In the peptide mutant rgf1,2,3 and receptor mutant rgfr1,2,3, the gradient of PLT proteins disappeared, indicating that RGF defines the PLT protein gradient to ensure robust root growth and root development. Recent studies of the downstream signaling of RGF-RGF receptor pairs are also described in this review.

The sequenced genomes of non-flowering land plants reveal the innovative evolutionary history of peptide signaling

An understanding of land plant evolution is a prerequisite for in-depth knowledge of plant biology. Here we extract and explore information hidden in the increasing number of sequenced plant genomes, from bryophytes to angiosperms, to elucidate a specific biological question - how peptide signaling evolved. To conquer land and cope with changing environmental conditions, plants have gone through transformations that must have required innovations in cell-to-cell communication. We discuss peptides mediating endogenous and exogenous changes by interaction with receptors activating intracellular molecular signaling. Signaling peptides were discovered in angiosperms and operate in tissues and organs such as flowers, seeds, vasculature, and 3D meristems that are not universally conserved across land plants. Nevertheless, orthologs of angiosperm peptides and receptors have been identified in non-angiosperms. These discoveries provoke questions regarding co-evolution of ligands and their receptors, and whether de novo interactions in peptide signaling pathways may have contributed to generate novel traits in land plants. The answers to such questions will have profound implications for the understanding of the evolution of cell-to-cell communication and the wealth of diversified terrestrial plants. Under this perspective we have generated, analyzed, and reviewed phylogenetic, genomic, structural, and functional data to elucidate the evolution of peptide signaling.

Factors Influencing Somatic Embryo Maturation in Sugi (Japanese Cedar, Cryptomeria japonica (Thunb. ex L.f.) D. Don)

This paper presents the results of several experiments identifying basal salts (BS) contained in maturation medium, polyethylene glycol (PEG) concentration, abscisic acid (ABA) concentration, additional supplementation with potassium chloride (KCl), amino acid (AA) concentration, and proliferation culture medium (PCM) as the main culture factors affecting somatic embryo maturation in sugi (Japanese cedar, Cryptomeria japonica, Cupressaceae). Highly efficient embryo maturation was achieved when embryogenic cell lines (ECLs) were cultured on media supplemented with a combination of PEG, ABA, and AAs. More than 1000 embryos per gram of fresh weight (FW) can be produced on EM maturation medium supplemented with 175 g L^-1 PEG, 100 µM ABA, 2 g L^-1 glutamine, 1 g L^-1 asparagine, and 0.5 g L^-1 arginine.

Regeneration of Pinus halepensis (Mill.) through Organogenesis from Apical Shoot Buds

Organogenesis and somatic embryogenesis have been widely applied as the two main regeneration pathways in plant tissue cultures. However, recalcitrance is still the main restriction in the clonal propagation of many woody species, especially in conifers. They undergo a “phase change” that leads to significant loss of vegetative propagation capacity, reducing the aptitude of tissues and organs to be regenerated in vitro beyond this point. In line with this, the in vitro regeneration of mature conifer trees has been a long-cherished goal in many laboratories worldwide. Based on previous works in Pinus species regeneration from adult trees, we now present data about the culture of apical shoot buds in an attempt to induce organogenesis and somatic embryogenesis to clone mature trees of Aleppo pine (Pinus halepensis). Reinvigorated axillary shoots were submitted to conditions usually applied to induce somatic embryogenesis through the manipulation of culture media, including the use of auxins such as 2,4-Dichlorophenoxyacetic acid and 1-Naphthaleneacetic acid, cytokinins (6-benzyladenine and kinetin), and phytosulfokine (50, 100, and 200 nM). Although somatic embryos could not be obtained, an embryogenic-like tissue was produced, followed by the emergence of actively proliferating non-embryogenic calli. Variations in the consistence, texture, and color of non-embryogenic calli were observed; especially those arising in the media containing phytosulfokine. Reinvigorated shoots, induced by 22 or 44 µM 6-benzyladenine, were obtained through organogenesis and acclimatized, and phenotypically normal plants were obtained.

Somatic Embryogenesis and Plant Regeneration from Sugi (Japanese Cedar, Cryptomeria japonica D. Don, Cupressaceae) Seed Families by Marker Assisted Selection for the Male Sterility Allele ms1

One of the possible countermeasures for pollinosis caused by sugi (Cryptomeria japonica), a serious public health problem in Japan, is the use of male sterile plants (MSPs; pollen-free plants). However, the production efficiencies of MSPs raised by conventional methods are extremely poor, time consuming, and resulting in a high seedling cost. Here, we report the development of a novel technique for efficient production of MSPs, which combines marker-assisted selection (MAS) and somatic embryogenesis (SE). SE from four full sib seed families of sugi, carrying the male sterility gene MS1, was initiated using megagametophyte explants that originated from four seed collections taken at one-week intervals during the month of July 2017. Embryogenic cell lines (ECLs) were achieved in all families, with initiation rates varying from 0.6% to 59%. Somatic embryos were produced from genetic marker-selected male sterile ECLs on medium containing maltose, abscisic acid (ABA), polyethylene glycol (PEG), and activated charcoal (AC). Subsequently, high frequencies of germination and plant conversion (≥76%) were obtained on plant growth regulator-free medium. Regenerated plantlets were acclimatized successfully, and the initial growth of male sterile somatic plants was monitored in the field.

Genome-wide analysis and transcript profiling of PSKR gene family members in Oryza sativa

Peptide signalling is an integral part of cell-to-cell communication which helps to relay the information responsible for coordinating cell proliferation and differentiation. Phytosulfokine Receptor (PSKR) is a transmembrane LRR-RLK family protein with a binding site for small signalling peptide, phytosulfokine (PSK). PSK signalling through PSKR promotes normal growth and development and also plays a role in defense responses. Like other RLKs, these PSKRs might have a role in signal transduction pathways related to abiotic stress responses. Genome-wide analysis of phytosulfokine receptor gene family has led to the identification of fifteen putative members in the Oryza sativa genome. The expression analysis of OsPSKR genes done using RNA-seq data, showed that these genes were differentially expressed in different tissues and responded specifically to heat, salt, drought and cold stress. Furthermore, the real-time quantitative PCR for fifteen OsPSKR genes revealed temporally and spatially regulated gene expression corresponding to salinity and drought stress. Our results provide useful information for a better understanding of OsPSKR genes and provide the foundation for additional functional exploration of the rice PSKR gene family in development and stress response.

Cytokinin Signaling Downstream of the His-Asp Phosphorelay Network: Cytokinin-Regulated Genes and Their Functions

The plant hormone cytokinin, existing in several molecular forms, is perceived by membrane-localized histidine kinases. The signal is transduced to transcription factors of the type-B response regulator family localized in the nucleus by a multi-step histidine-aspartate phosphorelay network employing histidine phosphotransmitters as shuttle proteins across the nuclear envelope. The type-B response regulators activate a number of primary response genes, some of which trigger in turn further signaling events and the expression of secondary response genes. Most genes activated in both rounds of transcription were identified with high confidence using different transcriptomic toolkits and meta analyses of multiple individual published datasets. In this review, we attempt to summarize the existing knowledge about the primary and secondary cytokinin response genes in order to try connecting gene expression with the multitude of effects that cytokinin exerts within the plant body and throughout the lifespan of a plant.

Cunninghamia lanceolata PSK Peptide Hormone Genes Promote Primary Root Growth and Adventitious Root Formation

Phytosulfokine-α (PSK-α) is a newly discovered short peptide that acts as a phytohormone in various plants. Previous studies have shown that PSK-α is critical for many biological processes in plants, such as cell division and differentiation, somatic embryogenesis, pollen germination and plant resistance. In this study, we cloned two PSK homolog genes from Cunninghamia lanceolata (Lamb.) Hook (Chinese fir), ClPSK1 and ClPSK2, and characterized their function in root development. Quantitative RT-PCR analyses showed that both ClPSK1 and ClPSK2 were expressed in vegetative organs, mainly in roots. Transgenic Arabidopsis plants overexpressing ClPSK1 or ClPSK2 showed a higher frequency of adventitious root formation and increased root length. The expression of genes in Arabidopsis that are involved in stem cell activity (PLT1, PLT2 and WOX5), radial organization of the root (SHR and SCR) and cell cycle (CYCB1;1, CYCD4;1, CDKB1;1 and RBR) were significantly up-regulated, which may contribute to the elongation of the primary root and the formation of adventitious root in transgenic lines. Our results suggest that ClPSKs play an important role during root growth and development.

Updates on molecular mechanisms in the development of branched trichome in Arabidopsis and nonbranched in cotton

Trichomes are specialized epidermal cells and a vital plant organ that protect plants from various harms and provide valuable resources for plant development and use. Some key genes related to trichomes have been identified in the model plant Arabidopsis thaliana through glabrous mutants and gene cloning, and the hub MYB-bHLH-WD40, consisting of several factors including GLABRA1 (GL1), GL3, TRANSPARENT TESTA GLABRA1 (TTG1), and ENHANCER OF GLABRA3 (EGL3), has been established. Subsequently, some upstream transcription factors, phytohormones and epigenetic modification factors have also been studied in depth. In cotton, a very important fibre and oil crop globally, in addition to the key MYB-like factors, more important regulators and potential molecular mechanisms (e.g. epigenetic modifiers, distinct metabolic pathways) are being exploited during different fibre developmental stages. This occurs due to increased cotton research, resulting in the discovery of more complex regulation mechanisms from the allotetraploid genome of cotton. In addition, some conservative as well as specific mediators are involved in trichome development in other species. This study summarizes molecular mechanisms in trichome development and provides a detailed comparison of the similarities and differences between Arabidopsis and cotton, analyses the possible reasons for the discrepancy in identification of regulators, and raises future questions and foci for understanding trichome development in more detail.

Sulfated plant peptide hormones

Sulfated peptides are plant hormones that are active at nanomolar concentrations. The sulfation at one or more tyrosine residues is catalysed by tyrosylprotein sulfotransferase (TPST), which is encoded by a single-copy gene. The sulfate group is provided by the co-substrate 3´-phosphoadenosine 5´-phosphosulfate (PAPS), which links synthesis of sulfated signaling peptides to sulfur metabolism. The precursor proteins share a conserved DY-motif that is implicated in specifying tyrosine sulfation. Several sulfated peptides undergo additional modification such as hydroxylation of proline and glycosylation of hydroxyproline. The modifications render the secreted signaling molecules active and stable. Several sulfated signaling peptides have been shown to be perceived by leucine-rich repeat receptor-like kinases (LRR-RLKs) but have signaling pathways that, for the most part, are yet to be elucidated. Sulfated peptide hormones regulate growth and a wide variety of developmental processes, and intricately modulate immunity to pathogens. While basic research on sulfated peptides has made steady progress, their potential in agricultural and pharmaceutical applications has yet to be explored.

Expression of a novel PSK-encoding gene from soybean improves seed growth and yield in transgenic plants

MAIN CONCLUSION

Expression of GmPSKγ1 , a novel PSK-encoding gene from soybean, increases seed size and yield in transgenic plants by promoting cell expansion. Phytosulfokine-α (PSK-α), a sulfated pentapeptide hormone with the sequence YIYTQ, plays important roles in many aspects of plant growth and development. In this study, we identified a pair of putative precursor genes in soybean, GmPSKγ1 and -2, encoding a PSK-like peptide: PSK-γ. Similar to PSK-α in amino acid composition, the sequence of PSK-γ is YVYTQ, and the tyrosines undergo sulfonylation. Treatment of Arabidopsis seedlings with synthetic sulfated PSK-γ significantly enhanced root elongation, indicating that PSK-γ might be a functional analog of PSK-α. Expression pattern analysis revealed that the two GmPSKγ genes, especially GmPSKγ1, are primarily expressed in developing soybean seeds. Heterologous expression of GmPSKγ1 under the control of a seed-specific promoter markedly increased seed size and weight in Arabidopsis, and this promoting effect of PSK-γ on seed growth was further confirmed in transgenic tobacco constitutively expressing GmPSKγ1. Cytological analysis of transgenic Arabidopsis seeds revealed that PSK-γ promotes seed growth by inducing embryo cell expansion. In addition, expression analysis of downstream candidate genes suggested that PSK-γ signaling might regulate cell wall loosening to promote cell expansion in Arabidopsis seeds. Overall, our results shed light on the mechanism by which PSK-γ promotes seed growth, paving the way for the use of this new peptide for biotechnological improvement of crop seed/grain size and yield.

Exploring peptide hormones in plants: identification of four peptide hormone-receptor pairs and two post-translational modification enzymes

The identification of hormones and their receptors in multicellular organisms is one of the most exciting research areas and has lead to breakthroughs in understanding how their growth and development are regulated. In particular, peptide hormones offer advantages as cell-to-cell signals in that they can be synthesized rapidly and have the greatest diversity in their structure and function. Peptides often undergo post-translational modifications and proteolytic processing to generate small oligopeptide hormones. In plants, such small post-translationally modified peptides constitute the largest group of peptide hormones. We initially explored this type of peptide hormone using bioassay-guided fractionation and later by in silico gene screening coupled with biochemical peptide detection, which led to the identification of four types of novel peptide hormones in plants. We also identified specific receptors for these peptides and transferases required for their post-translational modification. This review summarizes how we discovered these peptide hormone-receptor pairs and post-translational modification enzymes, and how these molecules function in plant growth, development and environmental adaptation.

Overexpression of phytosulfokine-α induces male sterility and cell growth by regulating cell wall development in Arabidopsis

Over-production of functional PSK-α in Arabidopsis caused increases in both plant cell growth and biomass and induced male sterility by regulating cell wall development. Phytosulfokine-α (PSK-α) is a novel disulfated pentapeptide hormone that is involved in promoting plant cell growth. Although a role for PSK-α in stimulating protoplast expansion has been suggested, how PSK-α regulates cell growth in planta remains poorly understood. In this study, we found that overexpression of the normal PSK-α precursor gene AtPSK4, which resulted in high levels of PSK-α, caused longer roots and larger leaves with enlarged cells. As expected, these changes were not observed in transgenic plants overexpressing mutated AtPSK4, which generated unsulfated PSK-α. These findings confirmed the role of PSK-α in promoting plant cell growth. Furthermore, we found that overexpressing AtPSK4, but not mutated AtPSK4, induced a phenotype of male sterility that resulted from the failure of fibrous cell wall development in the endothecium. In addition, overexpressing AtPSK4 enhanced expression of a number of genes encoding expansins, which are involved in cell wall loosening. Accordingly, in addition to its role in cell growth, we propose a novel function for PSK-α signaling in the modulation of plant male sterility via regulation of cell wall development.

Evolutionarily distant pathogens require the Arabidopsis phytosulfokine signalling pathway to establish disease

Secreted peptides and their specific receptors frequently orchestrate cell-to-cell communication in plants. Phytosulfokines (PSKs) are secreted tyrosine-sulphated peptide hormones, which trigger cellular dedifferentiation and redifferentiation upon binding to their membrane receptor. Biotrophic plant pathogens frequently trigger the differentiation of host cells into specialized feeding structures, which are essential for successful infection. We found that oomycete and nematode infections were characterized by the tissue-specific transcriptional regulation of genes encoding Arabidopsis PSKs and the PSK receptor 1 (PSKR1). Subcellular analysis of PSKR1 distribution showed that the plasma membrane-bound receptor internalizes after binding of PSK-α. Arabidopsis pskr1 knockout mutants were impaired in their susceptibility to downy mildew infection. Impaired disease susceptibility depends on functional salicylic acid (SA) signalling, but not on the massive up-regulation of SA-associated defence-related genes. Knockout pskr1 mutants also displayed a major impairment of root-knot nematode reproduction. In the absence of functional PSKR1, giant cells arrested their development and failed to fully differentiate. Our findings indicate that the observed restriction of PSK signalling to cells surrounding giant cells contributes to the isotropic growth and maturation of nematode feeding sites. Taken together, our data suggest that PSK signalling in Arabidopsis promotes the differentiation of host cells into specialized feeding cells.

Advances in Conifer Somatic Embryogenesis Since Year 2000

This review compiles research results published over the last 14 years on conifer somatic embryogenesis (SE). Emphasis is placed on the newest findings that affect the response of seed embryos (typical explants) and shoot primordia (rare explants) to the induction of SE and long-term culture of early somatic embryos. Much research in recent years has focused on maturation of somatic embryos, with respect to both yield and quality, as an important stage for the production of a large number of vigorous somatic seedlings. Attempts to scale up somatic embryo production numbers and handling have resulted in a few bioreactor designs, the utility of which may prove beneficial for an industrial application. A few simplified cryopreservation methods for embryonal masses (EM) were developed as a means to ensure cost-efficient long-term storage of genotypes during clonal field testing. Finally, recent long-term studies on the growth of somatic trees in the field, including seed production yield and comparison of seed parameters produced by somatic versus seed-derived trees, are described.

Phytosulfokine peptide signalling

Phytosulfokine (PSK) belongs to the group of plant peptide growth factors. It is a disulfated pentapeptide encoded by precursor genes that are ubiquitously present in higher plants, suggestive of universal functions. Processing of the preproprotein involves sulfonylation by a tyrosylprotein sulfotransferase in the trans-golgi and proteolytic cleavage in the apoplast. The secreted peptide is perceived at the cell surface by a membrane-bound receptor kinase of the leucine-rich repeat family. The PSK receptor PSKR1 from Arabidopsis thaliana is an active kinase and has guanylate cyclase activity resulting in dual-signal outputs. Receptor activity is regulated by calmodulin. While PSK may be an autocrine growth factor, it also acts non-cell autonomously by promoting growth of cells that are receptor-deficient. In planta, PSK has multiple functions. It promotes cell growth, acts in the quiescent centre cells of the root apical meristem, contributes to funicular pollen tube guidance, and differentially alters immune responses depending on the pathogen. It has been suggested that PSK integrates growth and defence signals to balance the competing metabolic costs of these responses. This review summarizes our current understanding of PSK synthesis, signalling, and activity.

Somatic Embryogenesis: Identified Factors that Lead to Embryogenic Repression. A Case of Species of the Same Genus

Somatic embryogenesis is a powerful biotechnological tool for the mass production of economically important cultivars. Due to the cellular totipotency of plants, somatic cells under appropriate conditions are able to develop a complete functional embryo. During the induction of somatic embryogenesis, there are different factors involved in the success or failure of the somatic embryogenesis response. Among these factors, the origin of the explant, the culture medium and the in vitro environmental conditions have been the most studied. However, the secretion of molecules into the media has not been fully addressed. We found that the somatic embryogenesis of Coffea canephora, a highly direct embryogenic species, is disrupted by the metabolites secreted from C. arabica, a poorly direct embryogenic species. These metabolites also affect DNA methylation. Our results show that the abundance of two major phenolic compounds, caffeine and chlorogenic acid, are responsible for inhibiting somatic embryogenesis in C. canephora.

A peptide hormone gene, GhPSK promotes fibre elongation and contributes to longer and finer cotton fibre

Cotton fibres, the single-celled trichomes derived from the ovule epidermis, provide the most important natural material for the global textile industry. A number of studies have demonstrated that regulating endogenous hormone levels through transgenic approaches can improve cotton fibre qualities. Phytosulfokine-α (PSK-α) is a novel peptide hormone in plants that is involved in regulating cell proliferation and elongation. However, its potential applications in crop genetic improvement have not been evaluated. In this study, we describe how exogenous PSK-α application promotes cotton fibre cell elongation in vitro. Chlorate, an effective inhibitor of peptide sulfation, suppressed fibre elongation in ovule culture. Exogenously applied PSK-α partly restored the chlorate-induced suppression. A putative PSK gene (GhPSK) was cloned from Gossypium hirsutum. Expression pattern analysis revealed that GhPSK is preferentially expressed in rapidly elongating fibre cells (5-20 days postanthesis). Overexpression of GhPSK in cotton increased the endogenous PSK-α level and promoted cotton fibre cell elongation, resulting in longer and finer fibres. Further results from electrophysiological and physiological analyses suggest that GhPSK affects fibre development through regulation of K(+) efflux. Digital gene expression (DGE) profile analysis of GhPSK overexpression lines indicates that PSK signalling may regulate the respiratory electron-transport chain and reactive oxygen species to affect cotton fibre development. These results imply that peptide hormones are involved in cotton fibre growth and suggest a new strategy for the biotechnological improvement of cotton fibre quality.

Posttranslationally modified small-peptide signals in plants

Cell-to-cell signaling is essential for many processes in plant growth and development, including coordination of cellular responses to developmental and environmental cues. Cumulative studies have demonstrated that peptide signaling plays a greater-than-anticipated role in such intercellular communication. Some peptides act as signals during plant growth and development, whereas others are involved in defense responses or symbiosis. Peptides secreted as signals often undergo posttranslational modification and proteolytic processing to generate smaller peptides composed of approximately 10 amino acid residues. Such posttranslationally modified small-peptide signals constitute one of the largest groups of secreted peptide signals in plants. The location of the modification group incorporated into the peptides by specific modification enzymes and the peptide chain length defined by the processing enzymes are critical for biological function and receptor interaction. This review covers 20 years of research into posttranslationally modified small-peptide signals in plants.

Peptides as triggers of plant defence

Plants are confronted with several biotic stresses such as microbial pathogens and other herbivores. To defend against such attackers, plants possess an array of pattern recognition receptors (PRRs) that sense the danger and consequently initiate a defence programme that prevents further damage and spreading of the pest. Characteristic pathogenic structures, so-called microbe-associated molecular patterns (MAMPs), serve as signals that allow the plant to sense invaders. Additionally, pathogens wound or damage the plant and the resulting release of damage-associated molecular patterns (DAMPs) serves as a warning signal. This review focuses on peptides that serve as triggers or amplifiers of plant defence and thus follow the definition of a MAMP or a DAMP.

MBSJ MCC Young Scientist Award 2010. Recent progress in research on small post-translationally modified peptide signals in plants

Peptide signaling plays a major role in various aspects of plant growth and development, as has been shown in recent biochemical, genetic and bioinformatic studies. There are over a dozen secreted peptides recognized in plants known to regulate cellular functions. To become functional, these secreted peptide signals often undergo post-translational modifications, such as tyrosine sulfation, proline hydroxylation, and hydroxyproline arabinosylation, and successive proteolytic processing. These types of ‘small post-translationally modified peptide signals’ are one of the major groups of peptide signals found in plants. In parallel with the discovery of peptide signals, specific receptors for such peptide signals were identified as being membrane-localized leucine-rich repeat receptor kinases. This short review highlights the recent progress in research on small post-translationally modified peptide signals, including our own research.

The tyrosine-sulfated peptide receptors PSKR1 and PSY1R modify the immunity of Arabidopsis to biotrophic and necrotrophic pathogens in an antagonistic manner

The tyrosine-sulfated peptides PSKα and PSY1 bind to specific leucine-rich repeat surface receptor kinases and control cell proliferation in plants. In a reverse genetic screen, we identified the phytosulfokine (PSK) receptor PSKR1 as an important component of plant defense. Multiple independent loss-of-function mutants in PSKR1 are more resistant to biotrophic bacteria, show enhanced pathogen-associated molecular pattern responses and less lesion formation after infection with the bacterial pathogen Pseudomonas syringae pv. tomato DC3000. By contrast, pskr1 mutants are more susceptible to necrotrophic fungal infection with Alternaria brassicicola, show more lesion formation and fungal growth which is not observed on wild-type plants. The antagonistic effect on biotrophic and necrotrophic pathogen resistance is reflected by enhanced salicylate and reduced jasmonate responses in the mutants, suggesting that PSKR1 suppresses salicylate-dependent defense responses. Detailed analysis of single and multiple mutations in the three paralogous genes PSKR1, -2 and PSY1-receptor (PSY1R) determined that PSKR1 and PSY1R, but not PSKR2, have a partially redundant effect on plant immunity. In animals and plants, peptide sulfation is catalyzed by a tyrosylprotein sulfotransferase (TPST). Mutants lacking TPST show increased resistance to bacterial infection and increased susceptibility to fungal infection, mimicking the triple receptor mutant phenotypes. Feeding experiments with PSKα in tpst-1 mutants partially restore the defense-related phenotypes, indicating that perception of the PSKα peptide has a direct effect on plant defense. These results suggest that the PSKR subfamily integrates growth-promoting and defense signals mediated by sulfated peptides and modulates cellular plasticity to allow flexible adjustment to environmental changes.

The peptide growth factor, phytosulfokine, attenuates pattern-triggered immunity

Pattern-triggered immunity (PTI) is triggered by recognition of elicitors called microbe-associated molecular patterns (MAMPs). Although immune responses may provide good protection of plants from pathogen attack, excessive immune responses have negative impacts on plant growth and development. Thus, a good balance between positive and negative effects on the immune signaling network is important for plant fitness. However, little information is known about the molecular mechanisms that are involved in attenuation of PTI. Here, we describe a growth-promoting peptide hormone, phytosulfokine (PSK), as attenuating PTI signaling in Arabidopsis. This research was motivated by the observation that expression of the PSK Receptor 1 (PSKR1) gene was induced by MAMP treatment. Plants homozygous for pskr1 T-DNA insertions showed enhanced defense gene expression and seedling growth inhibition triggered by MAMPs. The pskr1 plants also showed enhanced PTI against the bacterial pathogen Pseudomonas syringae. These results indicate that the PSKR-mediated signaling attenuates immune responses. Tyrosyl protein sulfotransferase (TPST) is an enzyme required for production of the mature sulfated PSK. Like pskr1 mutants, a tpst T-DNA insertion line exhibited enhanced MAMP-triggered seedling growth inhibition, which was suppressed by exogenous application of PSK. Thus, PSK signaling mediated by PSKR1 attenuates PTI but stimulates growth.

Small post-translationally modified Peptide signals in Arabidopsis

Recent biochemical, genetic and bioinformatic studies have demonstrated that peptide signaling plays a greater than anticipated role in various aspects of plant growth and development. More than a dozen secreted peptides are now recognized as important signals that mediate cell-to-cell communication. Secreted peptide signals often undergo post-translational modification and proteolytic processing, which are important for their function. Such "small post-translationally modified peptide signals" constitute one of the largest groups of peptide signals in plants. In parallel with the discovery of peptide signals, specific receptors for such peptides were identified as being membrane-localized receptor kinases, the largest family of receptor-like molecules in plants. These findings illustrate the critical roles of small peptide ligand-receptor pairs in plant growth and development. This review outlines recent research into secreted peptide signals in plants by focusing on small post-translationally modified peptides.

Involvement of phytosulfokine in the attenuation of stress response during the transdifferentiation of zinnia mesophyll cells into tracheary elements

Phytosulfokine (PSK) is a sulfated peptide hormone required for the proliferation and differentiation of plant cells. Here, we characterize the physiological roles of PSK in transdifferentiation of isolated mesophyll cells of zinnia (Zinnia elegans 'Canary Bird') into tracheary elements (TEs). Transcripts for a zinnia PSK precursor gene, ZePSK1, show two peaks of expression during TE differentiation; the first accumulation is transiently induced in response to wounding at 24 h of culture, and the second accumulation is induced in the final stage of TE differentiation and is dependent on endogenous brassinosteroids. Chlorate, a potent inhibitor of peptide sulfation, is successfully applied as an inhibitor of PSK action. Chlorate significantly suppresses TE differentiation. The chlorate-induced suppression of TE differentiation is overcome by exogenously applied PSK. In the presence of chlorate, expression of stress-related genes for proteinase inhibitors and a pathogenesis-related protein is enhanced and changed from a transient to a continuous pattern. On the contrary, administration of PSK significantly reduces the accumulation of transcripts for the stress-related genes. Even in the absence of auxin and cytokinin, addition of PSK suppresses stress-related gene expression. Microarray analysis reveals 66 genes down-regulated and 42 genes up-regulated in the presence of PSK. The large majority of down-regulated genes show significant similarity to various families of stress-related proteins, including chitinases, phenylpropanoid biosynthesis enzymes, 1-aminocyclopropane-1-carboxylic acid synthase, and receptor-like protein kinases. These results suggest the involvement of PSK in the attenuation of stress response and healing of wound-activated cells during the early stage of TE differentiation.

Transcriptional profiling of pea ABR17 mediated changes in gene expression in Arabidopsis thaliana

BACKGROUND

Pathogenesis-related proteins belonging to group 10 (PR10) are elevated in response to biotic and abiotic stresses in plants. Previously, we have shown a drastic salinity-induced increase in the levels of ABR17, a member of the PR10 family, in pea. Furthermore, we have also demonstrated that the constitutive expression of pea ABR17 cDNA in Arabidopsis thaliana and Brassica napus enhances their germination and early seedling growth under stress. Although it has been reported that several members of the PR10 family including ABR17 possess RNase activity, the exact mechanism by which the aforementioned characteristics are conferred by ABR17 is unknown at this time. We hypothesized that a study of differences in transcriptome between wild type (WT) and ABR17 transgenic A. thaliana may shed light on this process.

RESULTS

The molecular changes brought about by the expression of pea ABR17 cDNA in A. thaliana in the presence or absence of salt stress were investigated using microarrays consisting of 70-mer oligonucleotide probes representing 23,686 Arabidopsis genes. Statistical analysis identified number of genes which were over represented among up- or down-regulated transcripts in the transgenic line. Our results highlight the important roles of many abscisic acid (ABA) and cytokinin (CK) responsive genes in ABR17 transgenic lines. Although the transcriptional changes followed a general salt response theme in both WT and transgenic seedlings under salt stress, many genes exhibited differential expression patterns when the transgenic and WT lines were compared. These genes include plant defensins, heat shock proteins, other defense related genes, and several transcriptional factors. Our microarray results for selected genes were validated using quantitative real-time PCR.

CONCLUSION

Transcriptional analysis in ABR17 transgenic Arabidopsis plants, both under normal and saline conditions, revealed significant changes in abundance of transcripts for many stress responsive genes, as well as those related to plant growth and development. Our results also suggest that ABR17 may mediate stress tolerance through the modulation of many ABA- and CK-responsive genes and may further our understanding of the role of ABR17 in mediating plant stress responses.

A possible role of prolyl oligopeptidase during Linum usitatissimum (flax) seed development

Involvement of prolyl oligopeptidases (POPs) in the control of several mammalian peptide hormone signalling pathways has been studied extensively in recent years. POPs are ubiquitous enzymes, but little attention has been paid to understanding their function in plants. Using a cDNA-AFLP approach, two flax (Linum usitatissimum) POP ESTs were identified as being specifically expressed in the early stages of flax seed development. This specific expression was confirmed using real time RT-PCR and in situ hybridisation approaches. Seed expression of Arabidopsis POP genes was measured and showed no specificity. Comparison between results obtained with flax and Arabidopsis is discussed in order to address a hypothetic function for POPs during seed formation. These results provide the first insights into POP gene expression and hypothetical function in plants.

Identification of ligand binding site of phytosulfokine receptor by on-column photoaffinity labeling

Phytosulfokine (PSK), an endogenous 5-amino-acid-secreted peptide in plants, affects cellular potential for growth via binding to PSKR1, a member of the leucine-rich repeat receptor kinase (LRR-RK) family. PSK interacts with PSKR1 in a highly specific manner with a nanomolar dissociation constant. However, it is not known which residues in the PSKR1 extracellular domain constitute the ligand binding pocket. Here, we have identified the PSK binding domain of carrot PSKR1 (DcPSKR1) by photoaffinity labeling. We cross-linked the photoactivatable PSK analog [(125)I]-[N(epsilon)-(4-azidosalicyl)Lys(5)]PSK with DcPSKR1 using UV irradiation and mapped the cross-linked region using chemical and enzymatic fragmentation. We also established a novel "on-column photoaffinity labeling" methodology that allows repeated incorporation of the photoaffinity label to increase the efficiency of the photoaffinity cross-linking reactions. We purified a labeled DcPSKR1 tryptic fragment using anti-PSK antibodies and identified a peptide fragment that corresponds to the 15-amino-acid Glu(503)-Lys(517) region of DcPSKR1 by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Deletion of Glu(503)-Lys(517) completely abolishes the ligand binding activity of DcPSKR1. This region is in the island domain flanked by extracellular LRRs, indicating that this domain forms a ligand binding pocket that directly interacts with PSK.

Identification and functional characterization of phytosulfokine receptor using a ligand-based approach

Almost all plant cells, even when fully differentiated, can dedifferentiate and proliferate in vitro to form a callus, in which they can then differentiate to form various organs. These sequential processes can be promoted by exposing the cells to a conditioned medium in which either the same or other cells have previously been grown, indicating the involvement of cell-to-cell communication mediated by a chemical factor. This factor was purified from the conditioned medium and identified as a 5-amino-acid sulfated peptide. The addition of this peptide, named phytosulfokine (PSK), to the culture medium, even at nanomolar concentrations, significantly promotes cellular proliferation and/or cellular differentiation. We purified a membrane receptor for PSK (PSKR1) by ligand-based affinity chromatography and identified it as a member of leucine-rich repeat receptor kinases. The PSK-binding domain of PSKR1 was further identified by ligand photoaffinity labeling using a novel "on-column photoaffinity labeling" methodology that allows repeated incorporation of the photoaffinity label. Analysis of loss-of-function and gain-of-function mutants of the Arabidopsis PSKR1 revealed that PSK signaling affects cellular longevity and potential for growth without interfering with basic plant morphogenesis. These results suggest that PSK represents a new class of hormones that affect the potential for cellular growth and longevity of individual cells via binding to PSKR1, thereby exerting a pleiotropic effect on individual cells in response to environmental conditions.

The cellular and molecular biology of conifer embryogenesis

Gymnosperms and angiosperms are thought to have evolved from a common ancestor c. 300 million yr ago. The manner in which gymnosperms and angiosperms form seeds has diverged and, although broad similarities are evident, the anatomy and cell and molecular biology of embryogenesis in gymnosperms, such as the coniferous trees pine, spruce and fir, differ significantly from those in the most widely studied model angiosperm Arabidopsis thaliana. Molecular analysis of signaling pathways and processes such as programmed cell death and embryo maturation indicates that many developmental pathways are conserved between angiosperms and gymnosperms. Recent genomics research reveals that almost 30% of mRNAs found in developing pine embryos are absent from other conifer expressed sequence tag (EST) collections. These data show that the conifer embryo differs markedly from other gymnosperm tissues studied to date in terms of the range of genes transcribed. Approximately 72% of conifer embryo-expressed genes are found in the Arabidopsis proteome and conifer embryos contain mRNAs of very similar sequence to key genes that regulate seed development in Arabidopsis. However, 1388 loblolly pine (Pinus taeda) embryo ESTs (11.4% of the collection) are novel and, to date, have been found in no other plant. The data imply that, in gymnosperm embryogenesis, differences in structure and development are achieved by subtle molecular interactions, control of spatial and temporal gene expression and the regulating agency of a few unique proteins.

A leaf-based regeneration and transformation system for maize (Zea mays L.)

Efficient methods for in vitro propagation, regeneration, and transformation of plants are of pivotal importance to both basic and applied research. While being the world's major food crops, cereals are among the most difficult-to-handle plants in tissue culture which severely limits genetic engineering approaches. In maize, immature zygotic embryos provide the predominantly used material for establishing regeneration-competent cell or callus cultures for genetic transformation experiments. The procedures involved are demanding, laborious and time consuming and depend on greenhouse facilities. We have developed a novel tissue culture and plant regeneration system that uses maize leaf tissue and thus is independent of zygotic embryos and greenhouse facilities. We report here: (i) a protocol for the efficient induction of regeneration-competent callus from maize leaves in the dark, (ii) a protocol for inducing highly regenerable callus in the light, and (iii) the use of leaf-derived callus for the generation of stably transformed maize plants.

Disruption and overexpression of Arabidopsis phytosulfokine receptor gene affects cellular longevity and potential for growth

Phytosulfokine (PSK), a 5-amino acid sulfated peptide that has been identified in conditioned medium of plant cell cultures, promotes cellular growth in vitro via binding to the membrane-localized PSK receptor. Here, we report that loss-of-function and gain-of-function mutations of the Arabidopsis (Arabidopsis thaliana) PSK receptor gene (AtPSKR1) alter cellular longevity and potential for growth without interfering with basic morphogenesis of plants. Although mutant pskr1-1 plants exhibit morphologically normal growth until 3 weeks after germination, individual pskr1-1 cells gradually lose their potential to form calluses as tissues mature. Shortly after a pskr1-1 callus forms, it loses potential for growth, resulting in formation of a smaller callus than the wild type. Leaves of pskr1-1 plants exhibit premature senescence after bolting. Leaves of AtPSKR1ox plants exhibit greater longevity and significantly greater potential for callus formation than leaves of wild-type plants, irrespective of their age. Calluses derived from AtPSKR1ox plants maintain their potential for growth longer than wild-type calluses. Combined with our finding that PSK precursor genes are more strongly expressed in mature plant parts than in immature plant parts, the available evidence indicates that PSK signaling affects cellular longevity and potential for growth and thereby exerts a pleiotropic effect on cultured tissue in response to environmental hormonal conditions.

Peptide hormones in plants

In recent years, numerous biochemical and genetic studies have demonstrated that peptide signaling plays a greater than anticipated role in various aspects of plant growth and development. A substantial proportion of these peptides are secretory and act as local signals mediating cell-to-cell communication. Specific receptors for several peptides were identified as being membrane-localized receptor kinases, the largest family of receptor-like molecules in plants. These findings illustrate the importance of peptide signaling in the regulation of plant growth, functions that were previously ascribed to the combined action of small lipophilic compounds referred to as "traditional plant hormones." Here, we outline recent advances in the current understanding of biologically active peptides in plants, currently regarded as a new class of plant hormones.