'Candidatus Phytoplasma mali', 'Candidatus Phytoplasma pyri' and 'Candidatus Phytoplasma prunorum', the causal agents of apple proliferation, pear decline and European stone fruit yellows, respectively.
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Apple proliferation (AP), pear decline (PD) and European stone fruit yellows (ESFY) are among the most economically important plant diseases that are caused by phytoplasmas. Phylogenetic analyses revealed that the 16S rDNA sequences of strains of each of these pathogens were identical or nearly identical. Differences between the three phytoplasmas ranged from 1.0 to 1.5% of nucleotide positions and were thus below the recommended threshold of 2.5% for assigning species rank to phytoplasmas under the provisional status 'Candidatus'. However, supporting data for distinguishing the AP, PD and ESFY agents at the species level were obtained by examining other molecular markers, including the 16S-23S rDNA spacer region, protein-encoding genes and randomly cloned DNA fragments. The three phytoplasmas also differed in serological comparisons and showed clear differences in vector transmission and host-range specificity. From these results, it can be concluded that the AP, PD and ESFY phytoplasmas are coherent but discrete taxa that can be distinguished at the putative species level, for which the names 'Candidatus Phytoplasma mali', 'Candidatus Phytoplasma pyri' and 'Candidatus Phytoplasma prunorum', respectively, are proposed. Strains AP15R, PD1R and ESFY-G1R were selected as reference strains. Examination of available data on the peach yellow leaf roll (PYLR) phytoplasma, which clusters with the AP, PD and ESFY agents, confirmed previous results showing that it is related most closely to the PD pathogen. The two phytoplasmas share 99.6% 16S rDNA sequence similarity. Significant differences were only observed in the sequence of a gene that encodes an immunodominant membrane protein. Until more information on this phytoplasma is available, it is proposed that the PYLR phytoplasma should be regarded as a subtype of 'Candidatus Phytoplasma pyri'. (+info)
The gene geranylgeranyl reductase of peach (Prunus persica [L.] Batsch) is regulated during leaf development and responds differentially to distinct stress factors.
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Plant geranylgeranyl hydrogenase (CHL P) reduces free geranylgeranyl diphosphate to phytil diphosphate, which provides the side chain to chlorophylls, tocopherols, and plastoquinones. In peach, the single copy gene (PpCHL P) encodes a deduced product of 51.68 kDa, which harbours a transit peptide for cytoplasm-to-chloroplast transport and a nicotinamide binding domain. The PpCHL P message was abundant in chlorophyll-containing tissues and flower organs, but barely detected in the roots and mesocarp of ripening fruits, suggesting that transcription was related to plastid types and maturation. The message was not revealed in shoot apical meristems, but spread thoroughly in leaf cells during the early stages and was located mainly in the palisade of mature leaves, which exhibited higher transcript levels than young ones. Hence, the transcription of PpCHL P was likely to be regulated during leaf development. Gene expression was monitored in leaves responding to natural dark, cold, wounding, stress by imposed darkening, and during the curl disease. Transcription was stimulated by light, but repressed by dark and cold stress. In darkened leaves, the PpCHL P message was augmented concomitantly with that of CATALASE. In wounded leaves, the message decreased, but recovered rapidly, whereas in curled leaves, a reduction in gene expression was related to leaf damage intensity. However, transcript signals increased locally both in cells mechanically wounded by a needle and in those naturally injured by the pathogenic fungus Taphrina deformans. These data suggest that PpCHL P expression was regulated by photosynthetic activity and was possibly involved in the defence response. (+info)
Cell wall metabolism during maturation, ripening and senescence of peach fruit.
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Cell wall changes were examined in fruit of a melting flesh peach (Prunus persica L.) allowed to ripen on the tree. Three phases to softening were noted, the first of which began prior to the completion of flesh colour change and an increase in ethylene evolution. Softening in young mature fruit, prior to ripening, was associated with a depolymerization of matrix glycans both loosely and tightly attached to cellulose and a loss of Gal from all cell wall fractions. After the initiation of ripening, but before the melting stage, softening was associated with continuing, progressive depolymerization of matrix glycans. A massive loss of Ara from the loosely bound matrix glycan fraction was observed, probably from side chains of glucuronoarabinoxylan, pectin, or possibly arabinogalactan protein firmly bound into the wall and solubilized in this extract. An increase in the solubilization of polyuronides also occurred during this period, when softening was already well advanced. The extensive softening of the melting period was marked by substantial depolymerization of both loosely and tightly bound matrix glycans, including a loss of Ara from the latter, an increase in matrix glycan extractability, and a dramatic depolymerization of chelator-soluble polyuronides which continued during senescence. Depolymerization of chelator-soluble polyuronides thus occurred substantially after the increase in their solubilization. Ripening-related increases were observed in the activities of exo- and endo-polygalacturonase (EC 3.2.1.67; EC 3.2.1.15), pectin methylesterase (EC 3.1.1.11), endo-1,4-beta-glucanase (EC 3.2.1.4), endo-1,4-beta-mannanase (EC 3.2.1.78), alpha-arabinosidase (EC 3.2.1.55), and beta-galactosidase (EC 3.2.1.23), but the timing and extent of the increases differed between enzymes and was not necessarily related to ethylene evolution. Fruit softening in peach is a continuous process and correlated closely with the depolymerization of matrix glycans, which proceeded throughout development. However, numerous other cell wall changes also took place, such as the deglycosylation of particular polymers and the solubilization and depolymerization of chelator-soluble polyuronides, but these were transient and occurred only at specific phases of the softening process. Fruit softening and other textural changes in peach appear to have a number of stages, each involving a different set of cell wall modifications. (+info)
Geographically and temporally distant natural recombinant isolates of Plum pox virus (PPV) are genetically very similar and form a unique PPV subgroup.
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Natural recombinant Plum pox virus (PPV) isolates were detected in Albania, Bulgaria, Czech Republic, Germany, Hungary and Slovakia. Despite different geographical origins and dates of isolation, all the recombinant isolates were closely related at the molecular level and shared the same recombination breakpoint as well as a typical signature in their N-terminal coat protein sequence, suggesting a common origin. Biological assays with four recombinant isolates demonstrated their capacity to be aphid-transmitted to various Prunus hosts. One of these isolates had a threonine-to-isoleucine mutation in the conserved PTK motif of its HC-Pro and showed a drastically decreased, although not abolished, aphid transmissibility. The complete genome sequence of one of the recombinant isolates, BOR-3, was determined, as well as some partial sequences in the HC-Pro and P3 genes for additional natural recombinant isolates. Analysis of the phylogenetic relationships between the recombinant isolates and other sequenced PPV isolates confirmed that the recombinant isolates form a phylogenetically homogeneous lineage. In addition, this analysis revealed an ancient recombination event between the PPV-D and M subgroups, with a recombination breakpoint located in the P3 gene. Taken together, these results indicate that recombinant isolates represent an evolutionarily successful, homogeneous group of isolates with a common history and unique founding recombination event. The name PPV-Rec is proposed for this coherent ensemble of isolates. (+info)
Cell wall metabolism during the development of chilling injury in cold-stored peach fruit: association of mealiness with arrested disassembly of cell wall pectins.
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Partially tree-ripened ripe fruit of peach (Prunus persica L.) were stored for 1-4 weeks at 5 degrees C and then ripened at 20 degrees C for 3 d to induce chilling injury. With increasing cold storage the incidence and severity of mealiness symptoms increased progressively, manifested as reduced quantities of free juice and internal flesh browning. Relative to juicy fruit, tissue of mealy fruit showed altered intercellular adhesion when examined by microscopy and, upon crushing, a higher proportion of cells remained intact and did not release cellular contents. Substantial alterations in the metabolism of cell wall polysaccharides were observed. Chelator-soluble polyuronides from mealy fruit were partially depolymerized during cold storage in a manner dissimilar to that in unripe or ripe juicy fruit, and were not depolymerized further during the ripening period. The solubility of these high molecular weight pectins remained low, and did not show the increase characteristic of juicy fruit. Furthermore, in mealy fruit the dramatic decline in the polymeric Ara content of base-soluble, matrix glycan-enriched fractions occurring during normal ripening was absent, indicating diminished disassembly of an arabinan-rich polysaccharide firmly attached to cellulose. A corresponding rise in the polymeric Ara content of the most soluble pectin fraction was also absent, as was a decline in the Gal content of this extract. The depolymerization of matrix glycans showed only minor differences between juicy and mealy fruit. After cold storage and ripening, the activities of endo-1,4-beta-glucanase (EC 3.2.1.4), endo-1,4-beta-mannanase (EC 3.2.1.78), beta-galactosidase (EC 3.2.1.23), alpha-arabinosidase (EC 3.2.1.55), and particularly endo-polygalacturonase (EC 3.2.1.15) were lower in mealy fruit than in juicy fruit, whereas pectin methylesterase activity (EC 3.1.1.11) was lower in slightly mealy and higher in very mealy fruit. The data suggest that cold storage affects the activities of numerous cell wall-modifying enzymes, with important consequences for pectin metabolism. These changes alter the properties of the primary wall and middle lamella, resulting in tissue breakage along enlarged air spaces, rather than across cells, which reduces the amount and availability of free juice upon tissue fragmentation. (+info)
The role of polar auxin transport through pedicels of Prunus avium L. in relation to fruit development and retention.
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It was investigated whether premature fruit abscission in Prunus avium L. was triggered by a reduction in polar auxin transport (PAT). The capacity of pedicels to transport tritiated IAA ([3H]-IAA) via the PAT pathway was measured at intervals throughout flower and fruit development. The extent of passive diffusion, assessed by concurrent applications of [14C]-benzoic acid ([14C]-BA), was negligible. Transported radioactivity recovered from agar blocks eluted at the same retention time as authentic [3H]-IAA during HPLC fractionation. The capacity for PAT was already high 7 d before anthesis and increased further following the fertilization of flowers at anthesis. PAT intensity was greatest immediately following fertilization and at the beginning of the cell expansion phase of fruit growth; the transport intensity in fruitlets destined to abscind was negligible. The amount of endogenous IAA moving through the PAT pathway was greatest during the first 3 weeks after fertilization and was again high at the beginning of the fruit expansion stage. IAA export in the phloem increased following fertilization then declined below detectable levels. ABA export in the phloem increased markedly during stone formation and at the onset of fruit expansion. TIBA applied to pedicels of fruit in situ promoted fruitlet abscission in 2000 but not in 2001, despite PAT capacity being reduced by over 98% in the treated pedicels. The application of TIBA to pedicels did not affect fruit expansion. The role of PAT and IAA in relation to the development and retention of Prunus avium fruit is discussed. (+info)
Role of cell walls in the bioaccessibility of lipids in almond seeds.
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BACKGROUND: Certain nutrients and phytochemicals in almonds may confer protection against cardiovascular disease, but little is known about factors that influence their bioavailability. A crucial and relevant aspect is the amount of these dietary components available for absorption in the intestine, which is a concept referred to as bioaccessibility. OBJECTIVE: We investigated the role played by cell walls in influencing the bioaccessibility of intracellular lipid from almond seeds. DESIGN: Quantitative analyses of nonstarch polysaccharides (NSPs) and phenolic compounds of cell walls were performed by gas-liquid chromatography and HPLC, respectively. In a series of experiments, the effects of mechanical disruption, chewing, and digestion on almond seed microstructure and intracellular lipid release were determined. In the digestibility study, fecal samples were collected from healthy subjects who had consumed diets with or without almonds. Almond seeds and fecal samples were examined by microscopy to identify cell walls and intracellular lipid. RESULTS: Cell walls were found to be rich in NSPs, particularly arabinose-rich polysaccharides, with a high concentration of phenolic compounds detected in the seed coat cell wall. During disruption of almond tissue by mechanical methods or chewing, only the first layer of cells at the fractured surface was ruptured and able to release lipid. In fecal samples collected from subjects consuming the almond diet, we observed intact cotyledonary cells, in which the cell walls encapsulated intracellular lipid. This lipid appeared susceptible to colonic fermentation once the cotyledonary cell walls were breached by bacterial degradation. CONCLUSION: The cell walls of almond seeds reduce lipid bioaccessibility by hindering the release of lipid available for digestion. (+info)
Molecular basis of pollen-related food allergy: identification of a second cross-reactive IgE epitope on Pru av 1, the major cherry (Prunus avium) allergen.
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Birch (Betula verrucosa) pollen-associated food allergy is a well-characterized syndrome, which is due to the cross-reactivity of IgE antibodies to homologous allergens in various foods. One crossreacting area on the major birch pollen allergen Bet v 1 and its homologue in cherry (Prunus avium) Pru av 1 has already been identified. This is the so-called 'P-loop' region, which encompasses amino acid residues around position 45 and is found on the two virtually identical tertiary protein structures. We tried to determine an additional IgE cross-reacting patch on Pru av 1 and Bet v 1. The putative IgE-binding region on Pru av 1 was localized with a mAb (monoclonal antibody) that was generated against Bet v 1, and cross-reacts with several Bet v 1 homologues in food and inhibits the binding of patients' IgE to Pru av 1. mAb reactivity pattern was analysed and amino acid positions 28 and 108 of Pru av 1 were selected and mutated by site-directed mutagenesis. The Pru av 1 mutants were produced as recombinant proteins and characterized for their folding, mAb- and IgE-binding capacity and allergenic potency with a cellular assay using the humanized rat basophilic leukaemia cell line RBL-25/30. Amino acid position 28 is involved in a second major IgE-binding region on Pru av 1 and probably on Bet v 1. The identification of this second major IgE-binding region is an essential prerequisite to understand the phenomenon of cross-reactivity and its clinical consequences, and to produce hypoallergenic proteins for an improved immunotherapy of type I allergy. (+info)