Changes in Protein Interactions of Cell Cycle-Related Genes during the Dormancy-to-Growth Transition in Pea Axillary Buds

Apical dominance is a phenomenon in which a terminal bud growspredominantly and the growth of the axillary buds is suppressed.Here, we investigated the molecular mechanisms associated withcell cycle control that occur in pea axillary buds as a resultof decapitation. Proliferating cell nuclear antigen (PCNA) proteinwas detected in both dormant and growing buds, while PCNA mRNAwas absent in dormant buds. Pissa;CycBl;2 and Cdc2 proteinswere undetectable during dormancy. To analyze an interactionbetween PCNA and Pissa;CycD3;l, we performed anti-PCNA immunoaffinitycolumn chromatography. Pissa;CycD3;l protein was detected inthe eluate prepared from the dormant buds, but not in the eluateprepared from the growing buds. Furthermore, we performed anti-Pissa;CycD3;limmunoaffinity column chromatography. PCNA protein was detectedin the eluate prepared from the dormant buds, but not in theeluate prepared from the growing buds. These results indicatedthat PCNA associated with Pissa;CycD3;l only during dormancy.In addition, the interaction between PCNA and Pissa;CycD3;lwas confirmed by a yeast two-hybrid system. (Received April 8, 1998; Accepted August 5, 1998)     

14-3-3 regulates the G2/M transition in the basidiomycete Ustilago maydis

14-3-3 proteins are a family of highly conserved polypeptides that function as small adaptors that facilitate a diverse array of cellular processes by binding phosphorylated target proteins. One of these processes is the regulation of the cell cycle. Here we characterized the role of Bmh1, a 14-3-3 protein, in the cell cycle regulation of the fungus Ustilago maydis. We found that this protein is essential in U. maydis and that it has roles during the G2/M transition in this organism. The function of 14-3-3 in U. maydis seems to mirror the proposed role for this protein during Schizosaccharomyces pombe cell cycle regulation. We provided evidence that in U. maydis 14-3-3 protein binds to the mitotic regulator Cdc25. Comparison of the roles of 14-3-3 during cell cycle regulation in other fungal system let us to discuss the connections between morphogenesis, cell cycle regulation and the evolutionary role of 14-3-3 proteins in fungi.     

Analysis of Cycles of Dormancy and Growth in Pea Axillary Buds Based on mRNA Accumulation Patterns of Cell Cycle-Related Genes

Axillary buds of pea (Pisum sativum L. cv. Alaska) do not growon intact plants. Dormant axillary buds can be stimulated togrow rapidly after decapitation. Here, we isolated cDNAs ofPCNA, cyclinB, cyclinD, and cdc2 from pea. The mRNA expressionlevels of these genes were very low in dormant axillary buds,whereas they remarkably increased after decapitation. Basedon the mRNA accumulation patterns of these genes, we found thatmost cells in dormant axillary buds are arrested at the G₁ phasein the cell cycle. There are four buds at the second node onpea seedlings. After decapitation, mRNAs became abundant inthe large and small buds and were kept during the following3 d. After 4 d, mRNAs were still present in the large bud, butnot in the small bud. However, after removal of the large bud,the mRNA levels started to increase again in the small bud.These mRNA accumulation patterns were the same as those afterthe first decapitation. These results suggested that most cellsin axillary buds at the second node are arrested at the G₁]phase again and have the capacity to undergo multiple cyclesof dormancy and growth. Moreover, in situ hybridization analysesdemonstrated that PCNA mRNA increased in all parts of the axillarybuds after decapitation. (Received October 31, 1997; Accepted December 11, 1997)     

Cloning of a pea cDNA encoding a polypeptide of the light-harvesting complex associated with photosystem I using a monoclonal antibody

A monoclonal antibody (MAb UB42) is described that binds to thylakoids in pea chloroplasts, as shown by EM-immunogold labelling. The antibody recognised proteins of ca. 23–29 kDa in western blots of a pea leaf homogenate. A cDNA library was prepared from pea epidermal cells in the vector ZAP II, and immunoscreening of the library with UB42 led to the isolation of a clone, pUB42. This was sequenced and had an open reading frame of 269 codons encoding a predicted polypeptide of 28.9 kDa. The sequence showed extensive homology with three closely related polypeptides belonging to a family of chlorophyll a/b-binding proteins from the light harvesting complex of photosytem I (LHCI). Collectively, the results suggest that MAb UB42 recognises an epitope on the type II chlorophyll a/b-binding protein from LHCI and that clone pUB42 encodes this protein.     

Protein phosphorylation in plant mitochondria

Protein kinase activity was detected in osmotically lysed mitochondria isolated from etiolated seedlings of corn, pea, soybean, and wheat, as well as from potato tubers. Ther kinase(s) phosphorylated both endogenous polypeptides and exogenous, nonmitochondrial proteins when supplied with ATP and Mg²⁺. Eight to fifteen endogenous mitochondrial polypeptides were phosphorylated. The major mitochondrial polypeptide labeled in all species migrated during denaturing electrophoresis with an apparent monomeric molecular weight of 47,000. Incorporation of phosphate into endogenous proteins appeared to be biphasic, being most rapid during the first 1 to 2 minutes but slower thereafter. The kinase activity was greatest at neutral and alkaline pH values and utilized ATP with a K_m of approximately 200 micromolar. The kinase was markedly inhibited by CaCl₂ but was essentially unaffected by NaF, calmodulin, oligomycin, or cAMP. These data suggest that plant mitochondrial protein phosphorylation may be similar to protein phosphorylation in animal mitochondria.     

Trehalose 6‐phosphate is involved in triggering axillary bud outgrowth in garden pea (Pisum sativum L.)

Trehalose 6‐phosphate (Tre6P) is a signal of sucrose availability in plants, and has been implicated in the regulation of shoot branching by the abnormal branching phenotypes of Arabidopsis (Arabidopsis thaliana) and maize (Zea mays) mutants with altered Tre6P metabolism. Decapitation of garden pea (Pisum sativum) plants has been proposed to release the dormancy of axillary buds lower down the stem due to changes in sucrose supply, and we hypothesized that this response is mediated by Tre6P. Decapitation led to a rapid and sustained rise in Tre6P levels in axillary buds, coinciding with the onset of bud outgrowth. This response was suppressed by simultaneous defoliation that restricts the supply of sucrose to axillary buds in decapitated plants. Decapitation also led to a rise in amino acid levels in buds, but a fall in phosphoenolpyruvate and 2‐oxoglutarate. Supplying sucrose to stem node explants in vitro triggered a concentration‐dependent increase in the Tre6P content of the buds that was highly correlated with their rate of outgrowth. These data show that changes in bud Tre6P levels are correlated with initiation of bud outgrowth following decapitation, suggesting that Tre6P is involved in the release of bud dormancy by sucrose. Tre6P might also be linked to a reconfiguration of carbon and nitrogen metabolism to support the subsequent growth of the bud into a new shoot.     

Effect of epibrassinolide on tyrosine phosphorylation of the calvin cycle enzymes

Two-dimensional electrophoresis was used to separate proteins from crude extracts of pea (Pisum sativum L.) leaves, and thus isolated proteins were subjected to Western blot analysis with monoclonal antibodies against PY20 phosphotyrosine polypeptides. This analysis revealed 44 polypeptides phosphorylated on tyrosine residues. Phosphorylation of some of these proteins was changed under the action of epibrassinolide. Some of these polypeptides were identified by means of MALDI-TOF MS analysis. The results indicate that eight of these proteins belong to the Calvin cycle enzymes, namely, the isoforms of Rubisco large and small subunits, fructose-1,6-phosphate aldolases 1 and 2, and the precursor of α-subunit of Rubisco-binding protein. The observed changes in phosphorylation of these proteins may partly explain the effects of brassinosteroids on photosynthesis. The tyrosine phosphorylation sites were identified in silico for the fragments of polypeptides examined.     

Patterns of protein synthesis in dormant and growing vegetative buds of pea

Lateral buds on intact pea plants (Pisum sativum L. cv. Alaska) remain dormant until they are stimulated to develop by decapitating the terminal bud. Using two-dimensional gel electrophoresis, we have examined the protein content of terminal and lateral buds from intact plants and from plants at various times after decapitation. Silver-staining and in-vivo-labeling demonstrated very different sets of proteins. The level of expression of 18 stained and 25 labeled proteins was altered when growth was stimulated; this represents 3.4% and 9.1% of the total proteins detected by each method, respectively. Within 24 h of being stimulated, lateral buds doubled in length and their protein content was qualitatively nearly the same as that of terminal buds. Six hours after decapitation, before the onset of detectable growth, the overall pattern of protein synthesis in lateral buds was more like that of growing lateral buds or of terminal buds than that of dormant lateral buds. Direct application of N⁶-furfurylaminopurine (kinetin) to buds on intact plants stimulated their growth and resulted in the same pattern of protein synthesis as did decapitation. Inhibition of bud growth by addition of indole-3-acetic acid to the stumps of decapitated plants resulted in the synthesis of dormancy-related proteins. Lateral buds at all stages of development incorporated labeled amino acids at similar rates, indicating that metabolic activity is not a component of dormancy in these buds.Abbreviations IAA indole-3-acetic acid - IEF isoelectric focusing - KIN kinetin (N⁶-furfurylaminopurine) - SDS sodium dodecylsulfate - TCA trichloroacetic acid - 2D-PAGE two-dimensional polyacrylamide gel electrophoresis     

Factors regulating the expression of cell cycle genes in individual buds ofPopulus

The early and differential responses of the individual buds along a shoot have remained largely unknown due to the difficulties of analyzing early indicators that allow the monitoring of the effects of subtle changes in the environment on the growth activity of the individual bud. To overcome this problem, we transformed poplar [Populus tremula (L.) xP. alba (L.)] with two chimeric genes,Pcdc2a-gus andPcycl At-gus, the expression of which is closely linked to cell division inArabidopsis thaliana (L.) Heynh. We analyzed the expression levels of both chimeric genes in individual buds of the same tree, and under different conditions known to promote or retard growth in the buds. The expression levels of both chimeric genes were found to reflect closely the growth activity of the buds. After decapitation of the shoot, the expression ofPcdc2a-gus andPcycl At-gus revealed rapid and selective changes in the cell cycle, even when no morphological changes were observed. Furthermore, on the basis of the expression of the chimeric cell cycle genes, different degrees of growth activity and dormancy could be discriminated in the axillary buds. In addition, the expression ofPcycl At-gus was found to be closely associated with the day length, which is critical for dormancy induction in poplar.Abbreviations GUS -glucuronidase - MU methylumbelliferone     

Two novel transcripts expressed in pea dormant axillary buds

To elucidate the molecular mechanism of apical dominance, the expression patterns of genes that are preferentially expressed in dormant axillary buds of pea (Pisum sativum L. cv. Alaska) seedlings were investigated. We isolated two cDNA clones, cPsAD1 and cPsAD2 whose corresponding genes were named PsAD1 and PsAD2, from a cDNA library of dormant axillary buds using the differential display method. The deduced amino acid sequence of PsAD1 contains 87 residues and is rich in glycine residues in the amino terminal region. A search of the protein databases failed to find any sequences similar to PsAD1 protein except for the glycine-rich region. Northern blot analyses showed that PsAD1 mRNA mainly accumulated in dormant axillary buds and that its amount rapidly decreased after decapitation of the terminal bud. In situ hybridization analyses indicated that PsAD1 mRNA was localized in the apical meristem, procambia, and leaf primordia in dormant axillary buds that were competent to grow out but whose growth was temporarily suspended. That is, the expression of the PsAD1 gene is closely associated with the dormancy of axillary buds. The deduced amino acid sequence of PsAD2 contains 98 amino acid residues and is not similar to those of previously characterized proteins. PsAD2 mRNA accumulated in dormant axillary buds, roots, mature leaflets and elongated stems, suggesting that PsAD2 is involved in not only the dormancy of axillary buds but also the non-growing state in various tissues.     

Nuclear protein kinase activities during the cell cycle of HeLa S3 cells

To ascertain the activity and substrate specificity of nuclear protein kinases during various stages of the cell cycle of HeLa S3 cells, a nuclear phospho-protein-enriched sample was extracted from synchronised cells and assayed in vitro in the presence of homologous substrates. The nuclear protein kinases increased in activity during S and G2 phase to a level that was twice that of kinases from early S phase cells. The activity was reduced during mitosis but increased again in G1 phase. When the phosphoproteins were separated into five fractions by cellulose-phosphate chromatography each fraction, though not homogenous, exhibited differences in activity. Variations in the activity of the protein kinase fractions were observed during the cell cycle, similar to those observed for the unfractionated kinases. Sodium dodecyl sulfate polyacrylamide gel electrophoretic analysis of the proteins phosphorylated by each of the five kinase fractions demonstrated a substrate specificity. The fractions also exhibited some cell cycle stage-specific preference for substrates; kinases from G1 cells phosphorylated mainly high molecular weight polypeptides, whereas lower molecular weight species were phosphorylated by kinases from the S, G2 and mitotic stages of the cell cycle. Inhibition of DNA and histone synthesis by cytosine arabinoside had no effect on the activity or substrate specificity of S phase kinases. Some kinase fractions phosphorylated histones as well as non-histone chromosomal proteins and this phosphorylation was also cell cycle stage dependent. The presence of histones in the in vitro assay influenced the ability of some fractions to phosphorylate particular non-histone polypeptides; non-histone proteins also appeared to affect the in vitro phosphorylation of histones.     

Characterization of DRGs,developmentally regulated GTP-binding proteins,from pea and Arabidopsis

Developmentally regulated GTP-binding proteins (DRGs) from animals and fungi are highly conserved but have no known function. Here we characterize DRGs from pea (PsDRG) and Arabidopsis (AtDRG). Amino acid sequences of AtDRG and PsDRG were 90% identical to each other and about 65% identical to human DRG. Genomic Southern blotting indicated that AtDRG and PsDRG probably are single-copy genes. PsDRG mRNA accumulated preferentially in growing organs (root apices, growing axillary buds and elongating stems) compared with their non-growing counterparts. At DRG mRNA was relatively abundant in Arabidopsis leaves, stems and siliques, less abundant in flowers and flower buds, and barely detectable in roots. Histone mRNAs are known to accumulate predominantly during S phase of the cell cycle and are markers for proliferating cells. The patterns of histone H2A mRNA accumulation in pea and Arabidopsis organs were very similar to those of DRG mRNAs. An antiserum raised against a PsDRG N-terminal fusion protein recognized 43 and 45 kDa proteins. PsDRG proteins were more abundant in growing pea roots and stems than in non-growing organs, but they were equally abundant in growing and dormant axillary buds. After differential centrifugation, PsDRG proteins were found primarily in the microsomal (150 000×g pellet) and soluble (150 000×g supernatant) cell fractions.     

Dormancy-associated gene expression in pea axillary buds.

Pea (Pisum sativum L. cv. Alaska) axillary buds can be stimulated to cycle between dormant and growing states. Dormant buds synthesize unique proteins and are as metabolically active as growing buds. Two cDNAs, PsDRM1 and PsDRM2, were isolated from a dormant bud library. The deduced amino acid sequence of PsDRM1 (111 residues) is 75% identical to that of an auxin-repressed strawberry clone. PsDRM2 encodes a putative protein containing 129 residues, which includes 11 repeats of the sequence [G]-GGGY[H][N] (the bracketed residues may be absent). PsDRM2 is related to cold- and ABA-stimulated clones from alfalfa. Decapitating the terminal bud rapidly stimulates dormant axillary buds to begin growing. The abundance of PsDRM1 mRNA in axillary buds declines 20-fold within 6 h of decapitation; it quickly reaccumulates when buds become dormant again. The level of PsDRM2 mRNA is about three fold lower in growing buds than in dormant buds. Expression of PsDRM1 is enhanced in other non-growing organs (roots root apices; fully-elongated stems >elongating stems), and thus is an excellent “dormancy” marker. In contrast, PsDRM2 expression is not dormancy-associated in other organs. Received: 10 December 1997 / Accepted: 23 January 1998     

Vegetative axillary bud dormancy induced by shade and defoliation signals in the grasses

Vegetative axillary bud dormancy and outgrowth is regulated by several hormonal and environmental signals. In perennials, the dormancy induced by hormonal and environmental signals has been categorized as eco-, endo- or para-dormancy. Over the past several decades para-dormancy has primarily been investigated in eudicot annuals. Recently, we initiated a study using the monoculm phyB mutant (phyB-1) and the freely branching near isogenic wild type (WT) sorghum (Sorghum bicolor) to identify molecular mechanisms and signaling pathways regulating dormancy and outgrowth of axillary buds in the grasses. In a paper published in the January 2010 issue of Plant Cell and Environment, we reported the role of branching genes in the inhibition of bud outgrowth by phyB, shade and defoliation signals. Here we present a model that depicts the molecular mechanisms and pathways regulating axillary bud dormancy induced by shade and defoliation signals in the grasses.Key words: axillary bud, dormancy, shade, phytochrome, defoliation, shoot branching, teosinte branched1, MAX2, cell cycle, sorghumThe dormancy and outgrowth of axillary buds is regulated by several plant hormones such as auxin, cytokinins, abscisic acid and strigolactones, and by environmental factors such as light quality, quantity and duration as well as water, temperature and nutrient status.^1–3 Since the fate of an axillary bud is regulated by such diverse hormonal and environmental signals and their interactions, the type of dormancy induced varies. In perennials, three types of bud dormancy have been identified.^4,5 Dormancy mediated by factors within the bud is known as endo-dormancy; while dormancy induced by factors within the plant but outside the bud is called paradormancy or correlative inhibition; the best known example being apical dominance. Dormancy induced due to unfavorable environmental conditions is known as eco-dormancy. Although there is an indepth knowledge about para-dormancy in annuals,⁶ few studies have been conducted on eco-dormancy. Similarly, studies of endo-dormancy have largely been restricted to low-temperature mediated growth-cessation of axillary buds of perennial plants.^7,8 To understand the regulation of dormancy and outgrowth of axillary buds in monocots, we initiated a study on the molecular mechanisms inhibiting bud outgrowth by shade and defoliation signals in sorghum. Our results published in the January 2010 issue of Plant, Cell & Environment indicate that different types of dormancy may be induced in axillary buds of annual grasses by various signals and there may be overlapping and independent molecular mechanisms mediating induction of axillary bud dormancy.     

The use of a multiple antigen peptide to detect a minichromosome maintenance protein in pea (Pisum sativum)

A 13-residue oligopeptide corresponding to a conserved region of the MCM family of proteins was synthesised as a multiple antigen peptide in which eight copies of the peptide were conjugated to an oligo-lysine core. The multiple antigen peptide was used for raising antibodies. Western blots of the polypeptides present in the DNA polymerase–primase complex from pea (Pisum sativum L.) were challenged with the antibodies which, even at a dilution of 1:5000, clearly revealed a polypeptide of approximately 62 kDa.     

Protein profiles in pin and thrum floral organs of distylous Averrhoa carambola L.

Floral organs (tepal, stamen, style, including stigma, and ovary) from immature and mature (1 day prior to anthesis) flower buds of pin and thrum morphs of Averrhoa carambola were subjected to one and two-dimensional IEF/SDS-PAGE gel electrophoresis and visualized by silver staining. One-dimensional gel electrophoresis of organ extracts from mature floral buds showed a number of protein bands common to all organs in both pin and thrum morphs. In the stamen and style, these bands differed in intensity between the two morphs. Under two-dimensional gels, the differences in protein profiles between the two morphs were more distinct in these organs. When compared with polypeptide spots from leaflets, a total of 14 floral organ-specific polypeptides was detected, the majority appearing in the stamen, followed by the style but none in the ovary. In the stamen, most of these polypeptides were detected in both the pin and thrum morphs. However, in the style, a 72-kDa polypeptide was detected exclusively in the pin morph, and this was also the most abundant floral organ-specific polypeptide. Floral organ-specific polypeptides of 45 kDa (detected in stamens of the thrum morph) and 70 kDa (detected in stamen and style of both morphs) were found to bind concanavalin A.     

Synthesis and phosphorylation of pollen proteins during the pollen-stigma interaction in self-compatible Brassica napus L. and self-incompatible Brassica oleracea L.

A technique is described which permits the in vivo study of protein synthesis and phosphorylation in the pollen of Brassica spp. during the early stages of the pollen-stigma interaction. In Brassica napus and B. oleracea, compatible pollination is followed by a dramatic activation of protein synthesis in the pollen involving the synthesis of approximately 40 proteins. After incompatible pollinations in B. oleracea, virtually no newly synthesised polypeptides were detected in the pollen except for a small group of high molecular weight proteins which were not normally synthesised during compatible pollinations. Both compatible and incompatible pollinations were followed by the appearance of newly phosphorylated proteins in the pollen; these fell into four distinct groups. In B. oleracea, the number of phosphorylated proteins and the degree of phosphorylation of individual proteins within the four groups differed between compatible and incompatible pollinations. One group of phosphorylated proteins appeared to correspond with the small group of high molecular weight polypeptides which were synthesised in pollen after incompatible pollinations. These findings are discussed in the perspective of cell signalling during the pollen-stigma interaction in Brassica and also in terms of their possible implication in sporophytic self-incompatibility.