Multiple Events of Allopolyploidy in the Evolution of the Racemose Lineages in Prunus (Rosaceae) Based on Integrated Evidence from Nuclear and Plastid Data

Prunus is an economically important genus well-known for cherries, plums, almonds, and peaches. The genus can be divided into three major groups based on inflorescence structure and ploidy levels: (1) the diploid solitary-flower group (subg. Prunus, Amygdalus and Emplectocladus); (2) the diploid corymbose group (subg. Cerasus); and (3) the polyploid racemose group (subg. Padus, subg. Laurocerasus, and the Maddenia group). The plastid phylogeny suggests three major clades within Prunus: Prunus-Amygdalus-Emplectocladus, Cerasus, and Laurocerasus-Padus-Maddenia, while nuclear ITS trees resolve Laurocerasus-Padus-Maddenia as a paraphyletic group. In this study, we employed sequences of the nuclear loci At103, ITS and s6pdh to explore the origins and evolution of the racemose group. Two copies of the At103 gene were identified in Prunus. One copy is found in Prunus species with solitary and corymbose inflorescences as well as those with racemose inflorescences, while the second copy (II) is present only in taxa with racemose inflorescences. The copy I sequences suggest that all racemose species form a paraphyletic group composed of four clades, each of which is definable by morphology and geography. The tree from the combined At103 and ITS sequences and the tree based on the single gene s6pdh had similar general topologies to the tree based on the copy I sequences of At103, with the combined At103-ITS tree showing stronger support in most clades. The nuclear At103, ITS and s6pdh data in conjunction with the plastid data are consistent with the hypothesis that multiple independent allopolyploidy events contributed to the origins of the racemose group. A widespread species or lineage may have served as the maternal parent for multiple hybridizations involving several paternal lineages. This hypothesis of the complex evolutionary history of the racemose group in Prunus reflects a major step forward in our understanding of diversification of the genus and has important implications for the interpretation of its phylogeny, evolution, and classification.

Carbohydrate metabolism changes in Prunus persica gummosis infected with Lasiodiplodia theobromae

Peach gummosis represents a significant global disease of stone fruit trees and a major disease in the south peach production area of the Yangtze River of China. In this study, the carbohydrate composition of peach shoots during infection by Lasiodiplodia theobromae was examined. The expression of genes related to metabolic enzymes was also investigated. Control wounded and noninoculated tissue, lesion tissue, and wounded and inoculated surrounding lesion tissue of peach shoots were analyzed. Soluble sugars, glucose, mannose, arabinose, and xylose significantly increased in inoculated tissues of peach shoots compared with control tissues at different times after inoculation. Accumulation of polysaccharides was also observed by section observation and periodic acid Schiff's reagent staining during infection. Analysis using quantitative reverse-transcription polymerase chain reaction revealed that the abundance of key transcripts on the synthesis pathway of uridine diphosphate (UDP)-D-glucuronate, UDP-D-galactose, and UDP-D-arabinose increased but the synthesis of L-galactose and guanosine diphosphate-L-galactose were inhibited. After inoculation, the transcript levels of sugar transport-related genes (namely, SUT, SOT, GMT, and UGT) was induced. These changes in sugar content and gene expression were directly associated with peach gum polysaccharide formation and may be responsible for the symptoms of peach gummosis.

Heat treatment of peach fruit: modifications in the extracellular compartment and identification of novel extracellular proteins

Ripening of peach (Prunus persica L. Batsch) fruit is accompanied by dramatic cell wall changes that lead to softening. Post-harvest heat treatment is effective in delaying softening and preventing some chilling injury symptoms that this fruit exhibits after storage at low temperatures. In the present work, the levels of twelve transcripts encoding proteins involved in cell wall metabolism, as well as the differential extracellular proteome, were examined after a post-harvest heat treatment (HT; 39 °C for 3 days) of "Dixiland" peach fruit. A typical softening behaviour, in correlation with an increase in 1-aminocyclopropane-1-carboxylic acid oxidase-1 (PpACO1), was observed for peach maintained at 20 °C for 3 days (R3). Six transcripts encoding proteins involved in cell wall metabolism significantly increased in R3 with respect to peach at harvest, while six showed no modification or even decreased. In contrast, after HT, fruit maintained their firmness, exhibiting low PpACO1 level and significant lower levels of the twelve cell wall-modifying genes than in R3. Differential proteomic analysis of apoplastic proteins during softening and after HT revealed a significant decrease of DUF642 proteins after HT; as well as an increase of glyceraldehyde-3-phosphate dehydrogenase (GAPC) after softening. The presence of GAPC in the peach extracellular matrix was further confirmed by in situ immunolocalization and transient expression in tomato fruit. Though further studies are required to establish the function of DUF642 and GAPC in the apoplast, this study contributes to a deeper understanding of the events during peach softening and after HT with a focus on this key compartment.

The peach (Prunus persica L. Batsch) genome harbours 10 KNOX genes, which are differentially expressed in stem development, and the class 1 KNOPE1 regulates elongation and lignification during primary growth

The KNOTTED-like (KNOX) genes encode homeodomain transcription factors and regulate several processes of plant organ development. The peach (Prunus persica L. Batsch) genome was found to contain 10 KNOX members (KNOPE genes); six of them were experimentally located on the Prunus reference map and the class 1 KNOPE1 was found to link to a quantitative trait locus (QTL) for the internode length in the peach×Ferganensis population. All the KNOPE genes were differentially transcribed in the internodes of growing shoots; the KNOPE1 mRNA abundance decreased progressively from primary (elongation) to secondary growth (radial expansion). During primary growth, the KNOPE1 mRNA was localized in the cortex and in the procambium/metaphloem zones, whereas it was undetected in incipient phloem and xylem fibres. KNOPE1 overexpression in the Arabidopsis bp4 loss-of-function background (35S:KNOPE1/bp genotype) restored the rachis length, suggesting, together with the QTL association, a role for KNOPE1 in peach shoot elongation. Several lignin biosynthesis genes were up-regulated in the bp4 internodes but repressed in the 35S:KNOPE1/bp lines similarly to the wild type. Moreover, the lignin deposition pattern of the 35S:KNOPE1/bp and the wild-type internodes were the same. The KNOPE1 protein was found to recognize in vitro one of the typical KNOX DNA-binding sites that recurred in peach and Arabidopsis lignin genes. KNOPE1 expression was inversely correlated with that of lignin genes and lignin deposition along the peach shoot stems and was down-regulated in lignifying vascular tissues. These data strongly support that KNOPE1 prevents cell lignification by repressing lignin genes during peach stem primary growth.

Prunasin hydrolases during fruit development in sweet and bitter almonds

Amygdalin is a cyanogenic diglucoside and constitutes the bitter component in bitter almond (Prunus dulcis). Amygdalin concentration increases in the course of fruit formation. The monoglucoside prunasin is the precursor of amygdalin. Prunasin may be degraded to hydrogen cyanide, glucose, and benzaldehyde by the action of the β-glucosidase prunasin hydrolase (PH) and mandelonitirile lyase or be glucosylated to form amygdalin. The tissue and cellular localization of PHs was determined during fruit development in two sweet and two bitter almond cultivars using a specific antibody toward PHs. Confocal studies on sections of tegument, nucellus, endosperm, and embryo showed that the localization of the PH proteins is dependent on the stage of fruit development, shifting between apoplast and symplast in opposite patterns in sweet and bitter cultivars. Two different PH genes, Ph691 and Ph692, have been identified in a sweet and a bitter almond cultivar. Both cDNAs are 86% identical on the nucleotide level, and their encoded proteins are 79% identical to each other. In addition, Ph691 and Ph692 display 92% and 86% nucleotide identity to Ph1 from black cherry (Prunus serotina). Both proteins were predicted to contain an amino-terminal signal peptide, with the size of 26 amino acid residues for PH691 and 22 residues for PH692. The PH activity and the localization of the respective proteins in vivo differ between cultivars. This implies that there might be different concentrations of prunasin available in the seed for amygdalin synthesis and that these differences may determine whether the mature almond develops into bitter or sweet.

Novel Prunus rootstock somaclonal variants with divergent ability to tolerate waterlogging

Plants require access to free water for nutrient uptake, but excess water surrounding the roots can be injurious or even lethal because it blocks the transfer of free oxygen between the soil and the atmosphere. Genetic improvement efforts in this study were focused on the increased tolerance in roots to waterlogging. Among a pool of clones generated in vitro from leaf explants of rootstock Mr.S.2/5 of Prunus cerasifera L., the S.4 clone was flood tolerant whereas the S.1 clone was sensitive. The S.4 clone formed adventitious roots on exposure to flooding. Moreover, the chlorophyll content and mitochondrial activity in the leaf and root, soluble sugar content, alcohol dehydrogenase activity and ethylene content were different between the clones. The sorbitol transporter gene (SOT1) was up-regulated during hypoxia, the alcohol dehydrogenase genes (ADH1 and ADH3) were up-regulated in the leaves and down-regulated in the roots of the S.4 clone during hypoxia, and the 1-aminocyclopropane-1-oxidase gene (ACO1) was up-regulated in the leaves and roots of the S.4 clone during hypoxia and down-regulated in the wild-type roots. In addition, in the S.4 root, hypoxia induced significant down-regulation of a glycosyltransferase-like gene (GTL), which has a yet-undefined role. Although the relevant variation in the S.4 genome has yet to be determined, genetic alteration clearly conferred a flooding-tolerant phenotype. The isolation of novel somaclonals with the same genomic background but with divergent tolerance to flooding may offer new insights in the elucidation of the genetic machinery of resistance to flooding and aid in the selection of new Prunus rootstocks to be used in various adverse environments.

Stamen development and winter dormancy in apricot (Prunus armeniaca)

BACKGROUND AND AIMS

In temperate woody perennials, flower bud development is halted during the winter, when the buds enter dormancy. This dormant period is a prerequisite for adequate flowering, is genetically regulated, and plays a clear role in possibly adapting species and cultivars to climatic areas. However, information on the biological events underpinning dormancy is lacking. Stamen development, with clear differentiated stages, appears as a good framework to put dormancy in a developmental context. Here, stamen developmental changes are characterized in apricot (Prunus armeniaca) and are related to dormancy.

METHODS

Stamen development was characterized cytochemically from the end of August to March, over 4 years. Developmental changes were related to dormancy, using the existing empirical information on chilling requirements.

KEY RESULTS

Stamen development continued during the autumn, and the flower buds entered dormancy with a fully developed sporogenous tissue. Although no anatomical changes were observed during dormancy, breaking of dormancy occurred following a clear sequence of events. Starch accumulated in particular places, pre-empting further development in those areas. Vascular bundles developed and pollen mother cells underwent meiosis followed by microspore development.

CONCLUSIONS

Dormancy appears to mark a boundary between the development of the sporogenous tissue and the occurrence of meiosis for further microspore development. Breaking of dormancy occurs following a clear sequence of events, providing a developmental context in which to study winter dormancy and to evaluate differences in chilling requirements among genotypes.

New insights into the properties of pubescent surfaces: peach fruit as a model

The surface of peach (Prunus persica 'Calrico') is covered by a dense indumentum, which may serve various protective purposes. With the aim of relating structure to function, the chemical composition, morphology, and hydrophobicity of the peach skin was assessed as a model for a pubescent plant surface. Distinct physicochemical features were observed for trichomes versus isolated cuticles. Peach cuticles were composed of 53% cutan, 27% waxes, 23% cutin, and 1% hydroxycinnamic acid derivatives (mainly ferulic and p-coumaric acids). Trichomes were covered by a thin cuticular layer containing 15% waxes and 19% cutin and were filled by polysaccharide material (63%) containing hydroxycinnamic acid derivatives and flavonoids. The surface free energy, polarity, and work of adhesion of intact and shaved peach surfaces were calculated from contact angle measurements of water, glycerol, and diiodomethane. The removal of the trichomes from the surface increased polarity from 3.8% (intact surface) to 23.6% and decreased the total surface free energy chiefly due to a decrease on its nonpolar component. The extraction of waxes and the removal of trichomes led to higher fruit dehydration rates. However, trichomes were found to have a higher water sorption capacity as compared with isolated cuticles. The results show that the peach surface is composed of two different materials that establish a polarity gradient: the trichome network, which has a higher surface free energy and a higher dispersive component, and the cuticle underneath, which has a lower surface free energy and higher surface polarity. The significance of the data concerning water-plant surface interactions is discussed within a physiological context.

Changes in aquaporin gene expression and magnetic resonance imaging of water status in peach tree flower buds during dormancy

The movement of cellular water accompanies changes in growth within dormant buds. To further understand this process, accumulation of tonoplast deltaTIP1 and plasma membrane PIP2 aquaporin transcripts was measured by quantitative reverse transcriptase-polymerase chain reaction and the water dynamics in dormant peach (Prunus persica L.) flower buds was studied by magnetic resonance imaging. Proton density (PD), spin-spin relaxation time (T(2)) and apparent diffusion coefficient (ADC) were used to observe water dynamics during dormancy. The expression of deltaTIP1 and PIP2 aquaporins, PD and T(2) in the upper part of the bud including primordia, in the basal part of the bud and the bud trace increased earlier in the low-chill cultivar 'Coral' than in the high-chill cultivar 'Kansuke Hakuto,' reflecting the difference in timing for the end of endodormancy in the two cultivars. deltaTIP1 mRNA accumulated mainly in the basal part of the bud, whereas PIP2 mRNA was detected mainly in the upper part. These findings may reflect the activation of inter- and intracell communication through membrane transport properties of aquaporins resulting in a gradual increase in water content to that required for bud activity at the end of endodormancy. An apparent decrease in the expression of deltaTIP1 and PIP2 mRNAs was, however, observed in late winter in some portions of the buds of both cultivars just before sprouting.

Evaluation of the hydroxynitrile lyase activity in cell cultures of capulin (Prunus serotina)

Enzymatic preparations obtained from young plants and cell cultures of capulin were screened for hydroxynitrile lyase activity. The three week old plants, grown under sterile conditions, were used to establish a solid cell culture. Crude preparations obtained from this plant material were evaluated for the transformation of benzaldehyde to the corresponding cyanohydrin (mandelonitrile). The results show that the crude material from roots, stalks, and leaves of young plants and calli of roots, stalks, internodes and petioles biocatalyzed the addition of hydrogen cyanide (HCN) to benzaldehyde with a modest to excellent enantioselectivity.

Differential induction of phenylalanine ammonia-lyase gene expression in response to in vitro callus unions of Prunus spp

Phenylalanine ammonia-lyase (PAL) is a key enzyme in the synthesis of phenolic compounds, which play a prominent role in graft union formation, including the marked effects of their accumulation in incompatibility response. The purpose of this study was to assess changes in the abundance of PAL mRNA during graft union development. Partial cDNA clones encoding the enzyme were isolated from in vitro callus tissue in the apricot (Prunus armeniaca L.) cultivar Moniqui and the plum rootstock Marianna 2624 (Prunus munsoniana x Prunus cerasifera). The deduced partial amino acid sequence showed high homology with PAL genes from other plant species. We monitored PAL expression 5, 10, 15 and 20 days after the establishment of in vitro callus unions. The levels of PAL mRNA increased 5 days after grafting in both compatible and incompatible unions. Nevertheless, significant differences were observed at the transcript level through both types of combinations from the second week. The results showed a higher level of PAL transcription in graft unions of incompatible partners, where a lack of adaptation between stock and scion takes place. The level of scion-stock compatibility was related to the PAL expression pattern. In addition, cell walls of the callus cells were not stained by phloroglucinol-HCl, indicating that the increased PAL expression did not result in the formation of lignin. However, staining with Naturstoff reagent A confirmed the highest accumulation of soluble and wall-bound phenolic compounds at the graft interface of incompatible unions.

Relationship between petal abscission and programmed cell death in Prunus yedoensis and Delphinium belladonna

Depending on the species, the end of flower life span is characterized by petal wilting or by abscission of petals that are still fully turgid. Wilting at the end of petal life is due to programmed cell death (PCD). It is not known whether the abscission of turgid petals is preceded by PCD. We studied some parameters that indicate PCD: chromatin condensation, a decrease in nuclear diameter, DNA fragmentation, and DNA content per nucleus, using Prunus yedoensis and Delphinium belladonna which both show abscission of turgid petals at the end of floral life. No DNA degradation, no chromatin condensation, and no change in nuclear volume was observed in P. yedoensis petals, prior to abscission. In abscising D. belladonna petals, in contrast, considerable DNA degradation was found, chromatin was condensed and the nuclear volume considerably reduced. Following abscission, the nuclear area in both species drastically increased, and the chromatin became unevenly distributed. Similar chromatin changes were observed after dehydration (24 h at 60 degrees C) of petals severed at the time of flower opening, and in dehydrated petals of Ipomoea nil and Petunia hybrida, severed at the time of flower opening. In these flowers the petal life span is terminated by wilting rather than abscission. It is concluded that the abscission of turgid petals in D. belladonna was preceded by a number of PCD indicators, whereas no such evidence for PCD was found at the time of P. yedoensis petal abscission. Dehydration of the petal cells, after abscission, was associated with a remarkable nuclear morphology which was also found in younger petals subjected to dehydration. This nuclear morphology has apparently not been described previously, for any organism.

Changes in free polyamine titers and expression of polyamine biosynthetic genes during growth of peach in vitro callus

In the present paper, correlation between free polyamines and growth of peach (Prunus persica cv. Yuzora) in vitro callus was investigated. Growth of the callus was divided into three phases based on measurement of fresh weight. Free polyamines, putrescine (Put), spermidine (Spd), and spermine (Spm), could be detected during peach callus growth. Changes in free Put titers followed the callus growth rate, as shown by low and stable levels in the first stage, quick increase at the beginning of the second phase, and slow increase in the last phase, whereas fluctuations of Spd and Spm titers were aberrant from that of Put at early stage. Expressions of five key genes involved in polyamine biosynthesis were characterized, in which only the genes leading to Put synthesis, ADC (arginine decarboxylase) and ODC (ornithine decarboxylase), agreed with callus growth and fluctuation of Put titers. Treatment of the callus with D-arginine, an inhibitor of ADC, led to significant growth inhibition and enormous reduction of endogenous Put, coupled with obvious decrease of mRNA levels of ADC and ODC. Exogenous application of Put partially restored the callus growth, along with resumption of endogenous Put and expression levels of ADC and ODC. Spd and Spm titers experienced minor change in comparison to Put. The data presented here suggested that free Put played an important part in peach callus growth. Putative mechanisms or mode of action underlying the role of Put in peach callus growth and different expression patterns of the genes responsible for polyamine biosynthesis are also discussed.

Morphological changes in woody stem of Prunus jamasakura under simulated microgravity

When the four-week-old woody stem of Prunus jamasakura was grown under simulated microgravity condition on a three-dimensional clinostat, it bent at growth, and width of its secondary xylem decreased due to the reduction of fiber cell numbers and a smaller microfibril angle in the secondary cell wall, as reported in our previous paper. Gravity induces the development of the secondary xylem that supports the stem upward against the action of gravity. In this study, morphological changes of the tissues and cells were microscopically observed. Disorder was found in the concentric structure of tissues that organize the stem. The radial arrangement of the cells was also disturbed in the secondary xylem, and in the secondary phloem secondary cell walls of the bast fiber cells were undeveloped. These findings suggest that differentiation and development of the secondary xylem and the bast fiber cells are strongly controlled by terrestrial gravity. These tissue and cells functions to support the stem under the action of gravity. Furthermore, clinorotation induced disorder in the straight joint of vessel elements and the lattice-like structure of radial parenchyma cells, which is responsible for water transportation and storage, respectively. Gravity is an essential factor for keeping the division and differentiation normal in woody stem.

A cell wall-oriented genomic approach reveals a new and unexpected complexity of the softening in peaches

During ripening, fleshy fruits undergo textural changes that lead to loss of tissue firmness and consequent softening. It is a common idea that this process is the consequence of cell wall dismantling carried out by different and orderly expressed enzymes. For this purpose, by using a single enzyme family approach many enzymes and related genes have been characterized in different fruits. In this work, the softening of the climacteric peach fruits (Prunus persica (L.) Batsch.) has been studied by using a genomic approach, and the results obtained are novel and partly unexpected. The genes analysed encode proteins involved in the main metabolic aspects of a primary cell wall: degradation, synthesis, structure. In addition, some genes encoding cell-wall-related proteins with an unknown function have been studied. The gene expression profiles show that the softening actually begins well before the climacteric rise and continues thereafter. Genes whose expression starts before the climacteric rise are mostly down-regulated by ethylene, while genes with a ripening-specific expression are mostly up-regulated by the hormone. A few other genes are apparently insensitive to ethylene. Besides the expected parietal degradation, the softening that results from this study also comprises some repairing of the cell wall performed by enzymes involved in the synthesis of parietal polysaccharides and, especially, by proteins with structural functions. The newly synthesized polysaccharides and the structural proteins would thus help to hold together the fruit cell wall while not preventing the softening.

Nondestructive FTIR monitoring of leaf senescence and elicitin-induced changes in plant leaves

The applicability of the FTIR attenuated total reflectance technique for in situ monitoring of plant physiological processes such as leaf senescence and aging has been examined. Difference spectra obtained by subtracting the spectrum of the young plant leaf from that of the older one revealed positive bands at 1650-1500 cm(-1), indicating a higher relative concentration of phenolics in the older leaves of both black cherry and sweet pepper bush leaves. Prolonged physiological stress of tobacco leaves exhibited a progressive time-dependent increase of the absorbance at around 3475 cm(-1), corresponding to hydroxyl functional groups. Absorption changes were also observed between 1650 and 1500 cm(-1), which are likely to correspond to phenolics. The characteristic changes of the FTIR absorbance spectra resulting from physiological and induced aging were detected also as a response to treatment with a recombinant alpha-elicitin, cinnamomin. This allowed the first quantification of the biological activity of a recombinant elicitin using a spectroscopic method. We suggest that FTIR spectroscopy provides important information about physiological events occurring in plant tissue in vivo, and it could be useful for the in situ characterization of the plant responsiveness to fungal toxins such as elicitins.

[Sedimentable amyloplasts in Japanese weeping cherry]

In branches of the upright type of Japanese cherry reacting on the gravity stimulation, tension wood were formed by the action of gibberellin in the secondary xylem and caused negative gravitropism [correction of gravitorpism]. In the other hand, in branches of the weeping type of Japanese cherry, gibberellin was almost used for the elongation of the tip region and the shortage of gibberellin in the supporting tissue caused on the lack of tension wood. The weeping branches were unable to support their own weight and elongated to downward. It has already reported that both the upright and the weeping types of Japanese cherry have sedimentable amyloplasts in the endodermal starch sheath cells. In this study, the endodermal starch sheath cells were examined to investigate the cause of abnormal gravi-response in branches of the weeping type of Japanese cherry. Current-year branches of both the upright and the weeping types of Prunus spachiana were used as materials. The amyloplasts in the weeping type sedimented toward the base of the branches elongating upward and toward the apex in the branches elongating downward. In both cases, the sedimentation was toward the gravity vector. Then, the amyloplasts of the weeping branches were re-sedimentated toward the vector of gravity after changing branch position mechanically to upward, same as the upright type. In electron microscope studies, it was showed that amyloplasts had the lamella structure and the endodermal starch sheath cells were filled with large vacuoles. Moreover, endoplasmic reticulum, which was noticed in organelle relating to the graviperception, distributed to the cell periphery and was not locally. It was not showed the cell polarity [correction of polality]. The fine structures of the endodermal starch sheath cell of both types of cherry were similar. These results suggest that the abnormality of the gravi-response in the weeping Prunus trees is not due to the abnormal development of gravi-sensor.