Diversity of Arabidopsis genes encoding precursors for phytosulfokine, a peptide growth factor

Signaling from an altered cell wall to the nucleus mediates sugar-responsive growth and development in Arabidopsis thaliana

Sugars such as glucose function as signal molecules that regulate gene expression, growth, and development in plants, animals, and yeast. To understand the molecular mechanisms of sugar responses, we isolated and characterized an Arabidopsis thaliana mutant, high sugar response8 (hsr8), which enhances sugar-responsive growth and gene expression. Light-grown hsr8 plants exhibited increased starch and anthocyanin and reduced chlorophyll content in response to glucose treatment. Dark-grown hsr8 seedlings showed glucose-hypersensitive hypocotyl elongation and development. The HSR8 gene, isolated using map-based cloning, was allelic to the MURUS4 (MUR4) gene involved in arabinose synthesis. Dark-grown mur1 and mur3 seedlings also exhibited similar sugar responses to hsr8/mur4. The sugar-hypersensitive phenotypes of hsr8/mur4, mur1, and mur3 were rescued by boric acid, suggesting that alterations in the cell wall cause hypersensitive sugar-responsive phenotypes. Genetic analysis showed that sugar-hypersensitive responses in hsr8 mutants were suppressed by pleiotropic regulatory locus1 (prl1), indicating that nucleus-localized PRL1 is required for enhanced sugar responses in hsr8 mutant plants. Microarray analysis revealed that the expression of many cell wall-related and sugar-responsive genes was altered in mur4-1, and the expression of a significant proportion of these genes was restored to wild-type levels in the mur4-1 prl1 double mutant. These findings reveal a pathway that signals changes in the cell wall through PRL1 to altered gene expression and sugar-responsive metabolic, growth, and developmental changes.

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.

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.

Gain-of-function phenotypes of many CLAVATA3/ESR genes, including four new family members, correlate with tandem variations in the conserved CLAVATA3/ESR domain

Secreted peptide ligands are known to play key roles in the regulation of plant growth, development, and environmental responses. However, phenotypes for surprisingly few such genes have been identified via loss-of-function mutant screens. To begin to understand the processes regulated by the CLAVATA3 (CLV3)/ESR (CLE) ligand gene family, we took a systems approach to gene identification and gain-of-function phenotype screens in transgenic plants. We identified four new CLE family members in the Arabidopsis (Arabidopsis thaliana) genome sequence and determined their relative transcript levels in various organs. Overexpression of CLV3 and the 17 CLE genes we tested resulted in premature mortality and/or developmental timing delays in transgenic Arabidopsis plants. Overexpression of 10 CLE genes and the CLV3 positive control resulted in arrest of growth from the shoot apical meristem (SAM). Overexpression of nearly all the CLE genes and CLV3 resulted in either inhibition or stimulation of root growth. CLE4 expression reversed the SAM proliferation phenotype of a clv3 mutant to one of SAM arrest. Dwarf plants resulted from overexpression of five CLE genes. Overexpression of new family members CLE42 and CLE44 resulted in distinctive shrub-like dwarf plants lacking apical dominance. Our results indicate the capacity for functional redundancy of many of the CLE ligands. Additionally, overexpression phenotypes of various CLE family members suggest roles in organ size regulation, apical dominance, and root growth. Similarities among overexpression phenotypes of many CLE genes correlate with similarities in their CLE domain sequences, suggesting that the CLE domain is responsible for interaction with cognate receptors.

The Egg apparatus 1 gene from maize is a member of a large gene family found in both monocots and dicots

The maize ZmEA1 protein was recently postulated to be involved in short-range pollen tube guidance from the embryo sac. To date, EA1-like sequences had only been identified in monocot species. Using a more conserved C-terminal motif found in the monocot species, numerous ZmEA1-like sequences were retrieved in EST databases from dicot species, as well as from unannotated genomic sequences of Arabidopsis thaliana. RT-PCR analyses were produced for these unannotated genes and showed that these were indeed expressed genes. Further structural and phylogenetic analyses revealed that all members of the EA1-like (EAL) gene family shared a conserved 27-29 amino acid motif, termed the EA box near the C-terminal end, and appear to be secretory proteins. Therefore, the EA box proteins defines a new class of small secretory proteins, some of which being possibly involved in pollen tube guidance.

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.

Immediate-early and delayed cytokinin response genes of Arabidopsis thaliana identified by genome-wide expression profiling reveal novel cytokinin-sensitive processes and suggest cytokinin action through transcriptional cascades

Cytokinins are hormones that regulate many developmental and physiological processes in plants. Recent work has revealed that the cytokinin signal is transduced by two-component systems to the nucleus where target genes are activated. Most of the rapid transcriptional responses are unknown. We measured immediate-early and delayed cytokinin responses through genome-wide expression profiling with the Affymetrix ATH1 full genome array (Affymetrix Inc., Santa Clara, CA, USA). Fifteen minutes after cytokinin treatment of 5-day-old Arabidopsis seedlings, 71 genes were upregulated and 11 genes were downregulated. Immediate-early cytokinin response genes include a high portion of transcriptional regulators, among them six transcription factors that had previously not been linked to cytokinin. Five plastid transcripts were rapidly regulated as well, indicating a rapid transfer of the signal to plastids or direct perception of the cytokinin signal by plastids. After 2 h of cytokinin treatment genes coding for transcriptional regulators, signaling proteins, developmental and hormonal regulators, primary and secondary metabolism, energy generation and stress reactions were over-represented. A significant number of the responding genes are known to regulate light (PHYA, PSK1, CIP8, PAT1, APRR), auxin (Aux/IAA), ethylene (ETR2, EIN3, ERFs/EREBPs), gibberellin (GAI, RGA1, GA20 oxidase), nitrate (NTR2, NIA) and sugar (STP1, SUS1) dependent processes, indicating intense crosstalk with environmental cues, other hormones and metabolites. Analysis of cytokinin-deficient 35S:AtCKX1 transgenic seedlings has revealed additional, long-lasting cytokinin-sensitive changes of transcript abundance. Comparative overlay-analysis with the software tool mapman identified previously unknown cytokinin-sensitive metabolic genes, for example in the metabolism of trehalose-6-phosphate. Taken together, we present a genome-wide view of changes in cytokinin-responsive transcript abundance of genes that might be functionally relevant for the many biological processes that are governed by cytokinins.

Arabidopsis BRS1 is a secreted and active serine carboxypeptidase

The Arabidopsis BRS1 gene encodes a serine carboxypeptidase II-like protein. Its biological role in the brassinosteroid signaling pathway was first established by its capability to specifically suppress a weak brassinosteroid insensitive 1 (bri1) allele, bri1-5, when overexpressed. To gain additional insights into the molecular mechanisms of BRS1 function, the subcellular localization and the biochemical characteristics of BRS1 were determined by using transgenic plants harboring a 35S-BRS1-GFP construct and fusion proteins purified from 35S-BRS1-FLAG transgenic plants. The BRS1-GFP protein was mainly secreted and accumulated in the extracellular space. Immunological data suggest that BRS1 is proteolytically processed by an unknown endoproteinase in planta. Affinity-purified BRS1-FLAG from transgenic plants show strong hydrolytic activity with a broad P1 substrate preference including basic and hydrophobic groups on either side of the scissile bond. The hydrolytic activity of BRS1 can be strongly inhibited by a serine protease inhibitor, phenylmethylsulfonyl fluoride. The pH and temperature optima for the hydrolytic activity of BRS1 are pH 5.5 and 50 degrees C, respectively. These data demonstrate that BRS1 is a secreted and active serine carboxypeptidase, consistent with the hypothesis suggested by our previous genetic evidence that BRS1 may process a protein involved in an early event in the BRI1 signaling pathway.

Phytosulphokine gene regulation during maize (Zea mays L.) reproduction

The sulphated pentapeptide phytosulphokine (PSK) was identified as a substance that promotes cell division in low-density suspension cultures and has been implicated in various aspects of tissue differentiation in plants. The peptide is derived from PSK precursor proteins that are encoded by small gene families. The physiological roles of PSK are still not clearly defined and little is known about expression of members of the PSK precursor gene family in any plant species. In this study, highly regulated tissue and cell type-specific expression are described for four PSK genes from maize (Zea mays L.) in female and male gametophytes, and during seed development. ZmPSK1 and ZmPSK3 were specifically and differentially expressed in cells of female and male gametophytes and in female and male gametes. In anthers ZmPSK1 or ZmPSK3 transcripts were found, for example, at high levels in secretory tapetal cells which support developing microspores. ZmPSK1 mRNA was abundant in mature pollen including sperm cells. ZmPSK1 and ZmPSK3 transcripts were also detected in egg and central cells of the female gametophyte and ZmPSK1 mRNA was present in synergids, indicating that the PSK peptide probably plays a role during gametogenesis, pollen germination, and fertilization. In developing maize kernels all four ZmPSK genes were expressed, albeit with striking differences in their expression patterns. It is proposed here that PSK is required for numerous but defined processes during gametophyte and early sporophyte development. In general, PSK availability appears to be controlled through transcriptional regulation in a tissue and cell type-specific and development-dependent manner.

Generation and analysis of expressed sequence tags from the salt-tolerant mangrove species Avicennia marina (Forsk) Vierh

Salinization poses an increasingly serious problem in coastal and agricultural areas with negative effects on plant productivity and yield. Avicennia marina is a pantropical mangrove species that can survive in highly saline conditions. As a first step towards the characterization of genes that contribute to combating salinity stress, the construction of a cDNA library of A. marina genes is reported here. Random expressed sequence tag (EST) sequencing of 1,841 clones produced 1,602 quality reads. These clones were classified into functional categories, and BLAST: comparisons revealed that 113 clones were homologous to genes earlier implicated in stress responses, of which the dehydrins are the most predominant in this category. Of the ESTs analyzed, 30% showed homology to previously uncharacterized genes in the public plant databases. Of these 30%, 52 clones were selected for reverse Northern analysis: 26 were shown to be up-regulated and five shown to be down-regulated. The results obtained by reverse Northern analysis were confirmed by Northern analysis for three clones.

Leucine-Rich Repeat Receptor Kinases in Plants: Structure, Function, and Signal Transduction Pathways

Leucine-rich repeat receptor kinases (LRR-RKs) comprise the largest subfamily of transmembrane receptor-like kinases in plants, with over 200 members in Arabidopsis. LRR-RKs regulate a wide variety of developmental and defense-related processes including cell proliferation, stem cell maintenance, hormone perception, host-specific as well as non-host-specific defense response, wounding response, and symbiosis. Several studies indicate that LRR-RKs act as dimers, and some may form a receptor complex with leucine-rich repeat receptor-like proteins (LRR-RPs) that lack a cytoplasmic kinase domain. Despite the fact that structural features of LRR-RKs are fairy similar, five available ligand molecules for LRR-RKs are structurally diverse, from steroids (brassinolides) to peptides (phytosulfokine and systemin) and secreted proteins (CLV3). Precise ligand-binding sites of LRR-RKs are not understood. However, the extracellular "island" domain that intercepts the LRR domain in some LRR-RKs may play an important role in ligand binding. Advances in unveiling components of three LRR-RK signaling pathways, namely BRI1 in steroid signaling, CLV1 in meristem maintenance, and FLS2 in bacterial elicitor perception, revealed an intriguing link between plant LRR-RK and animal receptor signaling pathways. Finally, rapid progress made in LRR-RK research beyond the model system Arabidopsis has provided exciting, novel insights into the evolution of the LRR-RK signaling system in plants, such as BRI1 utilized in the wound-responsive signaling pathway in Solanaceae plants and recruitment of CLV1 in nodule development in leguminous plants.

Phytosulfokine Stimulates Somatic Embryogenesis in Cryptomeria japonica

Phytosulfokine (PSK), which has been identified as a plant growth factor, had a dramatic stimulatory effect on the formation of somatic embryos of sugi (Cryptomeria japonica) in the presence of polyethylene glycol. The resultant somatic embryos germinated with synchronous sprouting of cotyledons, hypocotyls and roots, and most of the seedlings grew normally. A cDNA clone for the precursor to the PSK peptide of C. japonica was identified in an expressed sequence tags database. Our results support the existence of a PSK signaling pathway in C. japonica.

Ligand-receptor pairs in plant peptide signaling

Extensive studies on plant signaling molecules over the past decade indicate that plant cell-to-cell communication, as is the case with animal systems, makes use of small peptide signals and specific receptors. To date, four peptide-ligand-receptor pairs have been identified and shown to be involved in a variety of processes. Systemin and phytosulfokine (PSK), the first and second signaling peptides identified in plants, were isolated by biochemical purification based on their biological activities. Furthermore, their receptors have been biochemically purified from plasma membranes on the basis of specific ligand-receptor interactions. By contrast, the two other peptide signals, CLAVATA3 (CLV3) and the pollen S determinant SCR/SP11, were genetically identified during searches for specific ligands for receptors that had already been cloned. Systemin functions in the plant wound response, whereas PSK appears to cooperate with auxin and cytokinin to regulate cellular dedifferentiation and redifferentiation. CLV3 is important for meristem organization, binding to a heterodimeric receptor comprising the CLV1 and CLV2 proteins. SCR/SP11 instead plays a role in self-incompatibility, where it activates a signalling cascade that leads to rejection of pollen with the same S haplotype. These ligands all seem to bind to receptors that possess intrinsic kinase activity, and al least two of them are generated by proteolytic processing of larger precursor proteins.

Polypeptide hormones: signaling molecules in plants

Biochemical and genetic studies have identified peptides that play crucial roles in plant defense, growth, and development. The number of known, functionally active, peptides is currently small, but genome sequencing has revealed many potential peptide-encoding genus. A major challenge of the post-genomic era is to determine the function of these molecules.

An LRR receptor kinase involved in perception of a peptide plant hormone, phytosulfokine

The sulfated peptide phytosulfokine (PSK) is an intercellular signal that plays a key role in cellular dedifferentiation and proliferation in plants. Using ligand-based affinity chromatography, we purified a 120-kilodalton membrane protein, specifically interacting with PSK, from carrot microsomal fractions. The corresponding complementary DNA encodes a 1021-amino acid receptor kinase that contains extracellular leucine-rich repeats, a single transmembrane domain, and a cytoplasmic kinase domain. Overexpression of this receptor kinase in carrot cells caused enhanced callus growth in response to PSK and a substantial increase in the number of tritium-labeled PSK binding sites, suggesting that PSK and this receptor kinase act as a ligand-receptor pair.

Peptide signals and their receptors in higher plants

At least four peptides play a vital role in plant cell-cell communication by means of their specific receptors. Two of these receptors have been identified as receptor kinases, which form a large family of receptor molecules in plants. These findings highlight the significance of receptor-mediated peptide signaling in various physiological events in plants, and predict the existence of further peptide-signal-interacting receptor kinases. Some candidates have been found in plant genomes. Here, we outline recent progress and future challenges in the signaling peptide analysis, which began with systemin, phytosulfokine, CLAVATA3 and S-locus cysteine-rich protein (also called S-locus protein 11).

Of weeds and men: what genomes teach us about plant cell biology

It has generally been assumed that fundamental cellular processes are conserved at the molecular level. Genome comparisons, however, suggest that the molecular mechanisms underlying programmed cell death, defense, adaptation and development may differ considerably between the plant and animal kingdoms. Phylogenetic analyses have revealed a great deal of novelty in the plant genes that are implicated in conserved processes such as transcription, cytoskeletal dynamics and vesicle trafficking. The Arabidopsis genome highlights the highly dynamic and regulated nature of the plant cell, which is fine-tuned to light, water, nutrient availability, temperature, touch and wind.