Carbon metabolism of peach fruit after harvest: changes in enzymes involved in organic acid and sugar level modifications

Peach (Prunus persica L. Batsch) is a climacteric fruit that ripens after harvest, prior to human consumption. Organic acids and soluble sugars contribute to the overall organoleptic quality of fresh peach; thus, the integrated study of the metabolic pathways controlling the levels of these compounds is of great relevance. Therefore, in this work, several metabolites and enzymes involved in carbon metabolism were analysed during the post-harvest ripening of peach fruit cv 'Dixiland'. Depending on the enzyme studied, activity, protein level by western blot, or transcript level by quantitative real time-PCR were analysed. Even though sorbitol did not accumulate at a high level in relation to sucrose at harvest, it was rapidly consumed once the fruit was separated from the tree. During the ripening process, sucrose degradation was accompanied by an increase of glucose and fructose. Specific transcripts encoding neutral invertases (NIs) were up-regulated or down-regulated, indicating differential functions for each putative NI isoform. Phosphoenolpyruvate carboxylase was markedly induced, and may participate as a glycolytic shunt, since the malate level did not increase during post-harvest ripening. The fermentative pathway was highly induced, with increases in both the acetaldehyde level and the enzymes involved in this process. In addition, proteins differentially expressed during the post-harvest ripening process were also analysed. Overall, the present study identified enzymes and pathways operating during the post-harvest ripening of peach fruit, which may contribute to further identification of varieties with altered levels of enzymes/metabolites or in the evaluation of post-harvest treatments to produce fruit of better organoleptic attributes.

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.

[Effects of waterlogging on root respiration intensity and respiratory enzyme activities of sweet cherry]

Taking Meizao/Dongbeishanyingtao (Prunus serrulata G. Don) and Meizao/Mahaleb (P. mahaleb L.) as test materials, the respiration intensity and respiratory enzyme activities of their growing roots and brown lignified roots were determined under waterlogging. The results showed that under waterlogging, the root respiration intensity of the two kind rootstocks had a decreasing trend, with more decrement for their growing roots. For P. serrulata, the decrement of the respiration intensity of its growing roots and brown lignified roots was 1.47 and 1.36 times as much as that of P. mahaleb, respectively. The pyruvate decarboxylase (PDC) and lactate dehydrogenase (LDH) activities of the two kind roots had a decrease after an initial increase. Alcohol dehydrogenase (ADH) activity in growing roots also had a decrease after an initial increase, but that in brown lignified roots had an increasing trend. The test three enzyme activities changed more in growing roots than in brown lignified roots. The increment of ADH and LDH activities was higher for P. serrulata than for P. mahaleb, while PDC activity was in adverse. The malate dehydrogenase (MDH) activity of the two kind roots decreased, and the decrement was larger in growing roots than in brown lignified roots. P. serrulata had a larger decrement of root MDH activity than P. mahaleb To the waterlogging stress, growing roots was more sensitive than brown lignified roots, and P. serrulata was more sensitive than P. mahaleb.

Carbohydrate availability affects growth and metabolism in peach fruit

Along with sucrose, sorbitol represents the main photosynthetic product and form of translocated carbon in peach. This study aimed at determining whether peach fruit carbohydrate metabolism is affected by changes in source-sink balance, and specifically whether sorbitol or sucrose availability regulates fruit enzyme activities and growth. In various trials, different levels of assimilate availability to growing fruits were induced in vivo by varying crop load of entire trees, leaf : fruit ratio (L:F) of fruiting shoots, or by interrupting the phloem stream (girdling) to individual fruits. In vitro, fruit tissue was incubated in presence/absence of sorbitol and sucrose. Relative growth rate (RGR), enzyme activities and carbohydrates were measured at different fruit growth stages of various peach cultivars in different years. At stage III, high crop load induced higher acid invertase (AI, EC 3.2.1.26) activities and hexose : sucrose ratios. Both sorbitol and sucrose contents were proportional to L:F, while sorbitol dehydrogenase (SDH, EC 1.1.1.14) activity was the only enzyme activity directly related to L:F in both fruit growth stages. Girdling reduced fruit RGR and all major carbohydrates after 4 days and SDH activity already after 48 h, but it did not affect sucrose synthase (SS, EC 2.4.1.13), AI and neutral invertase (NI, EC 3.2.1.27). Fruit incubation in sorbitol for 24 h induced higher SDH activities than in buffer alone. In general, assimilate availability affected both sorbitol and sucrose metabolism in peach fruit, and sorbitol may function as a signal for modulating SDH activity. Under highly competitive conditions, AI activity may be enhanced by assimilate depletion, providing a mechanism to increase fruit sink strength by increasing hexose concentrations.

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.

Functional characterization of PaLAX1, a putative auxin permease, in heterologous plant systems

We have isolated the cDNA of the gene PaLAX1 from a wild cherry tree (Prunus avium). The gene and its product are highly similar in sequences to both the cDNAs and the corresponding protein products of AUX/LAX-type genes, coding for putative auxin influx carriers. We have prepared and characterized transformed Nicotiana tabacum and Arabidopsis thaliana plants carrying the gene PaLAX1. We have proved that constitutive overexpression of PaLAX1 is accompanied by changes in the content and distribution of free indole-3-acetic acid, the major endogenous auxin. The increase in free indole-3-acetic acid content in transgenic plants resulted in various phenotype changes, typical for the auxin-overproducing plants. The uptake of synthetic auxin, 2,4-dichlorophenoxyacetic acid, was 3 times higher in transgenic lines compared to the wild-type lines and the treatment with the auxin uptake inhibitor 1-naphthoxyacetic acid reverted the changes caused by the expression of PaLAX1. Moreover, the agravitropic response could be restored by expression of PaLAX1 in the mutant aux1 plants, which are deficient in auxin influx carrier activity. Based on our data, we have concluded that the product of the gene PaLAX1 promotes the uptake of auxin into cells, and, as a putative auxin influx carrier, it affects the content and distribution of free endogenous auxin in transgenic plants.

Bitterness in almonds

Bitterness in almond (Prunus dulcis) is determined by the content of the cyanogenic diglucoside amygdalin. The ability to synthesize and degrade prunasin and amygdalin in the almond kernel was studied throughout the growth season using four different genotypes for bitterness. Liquid chromatography-mass spectrometry analyses showed a specific developmentally dependent accumulation of prunasin in the tegument of the bitter genotype. The prunasin level decreased concomitant with the initiation of amygdalin accumulation in the cotyledons of the bitter genotype. By administration of radiolabeled phenylalanine, the tegument was identified as a specific site of synthesis of prunasin in all four genotypes. A major difference between sweet and bitter genotypes was observed upon staining of thin sections of teguments and cotyledons for beta-glucosidase activity using Fast Blue BB salt. In the sweet genotype, the inner epidermis in the tegument facing the nucellus was rich in cytoplasmic and vacuolar localized beta-glucosidase activity, whereas in the bitter cultivar, the beta-glucosidase activity in this cell layer was low. These combined data show that in the bitter genotype, prunasin synthesized in the tegument is transported into the cotyledon via the transfer cells and converted into amygdalin in the developing almond seed, whereas in the sweet genotype, amygdalin formation is prevented because the prunasin is degraded upon passage of the beta-glucosidase-rich cell layer in the inner epidermis of the tegument. The prunasin turnover may offer a buffer supply of ammonia, aspartic acid, and asparagine enabling the plants to balance the supply of nitrogen to the developing cotyledons.

Differential regulation of four members of the ACC synthase gene family in plum

The regulation of ACC synthase (ACS) genes was studied in early ('Early Golden') and late ('Shiro') Japanese plum cultivars (Prunus salicina L.) in order to determine the role of this gene family in fruit ripening. Of the four Ps-ACS cDNAs isolated, two (Ps-ACS1 and -3) showed differential expression between the two cultivars. Ps-ACS1 accumulated during fruit ripening of 'Early Golden' ('EG') and 'Shiro' ('SH') in ethylene-dependent and -independent manners, respectively. Ps-ACS3a transcripts accumulated throughout fruit development and during 'EG' fruit ripening. Ps-ACS3b was detected only during ripening of 'SH' fruit. Furthermore, Ps-ACS3a transcript accumulation was negatively regulated by ethylene, whereas Ps-ACS3b was positively induced by the hormone. In both cultivars, the expression of Ps-ACS4 and -5 is under positive and negative feedback control by ethylene, respectively. Genetic analyses of 'EG' and 'SH' cultivars demonstrated that 'EG' is homozygous for Ps-ACS3a whereas 'SH' is heterozygous for Ps-ACS3 (a/b). The role of ethylene-overproducer 1-like in delaying fruit ripening by interacting with Ps-ACS proteins was also studied. The effect of the plant hormones, auxin, gibberellin, and cytokinin, in regulating ethylene production by promoting the induction of the different Ps-ACS mRNAs in plum was investigated. A model is presented in which differences in Ps-ACS alleles and gene expression between early and late plums are critical in determining the ripening behaviour of the cultivars.

Partial purification of latent polyphenol oxidase from peach (Prunus persica L. Cv. Catherina). Molecular properties and kinetic characterization of soluble and membrane-bound forms

This paper analyzes the kinetic and structural characteristics of polyphenol oxidase (PPO) from peach cv. Catherina. The PPO was obtained in a latent state in both the soluble and membrane-bound forms, and both forms were activated by acid shock and the detergent SDS. Plant defense is the main function assigned to PPO, which would be activated by the acid environment resulting from tissue damage. On the other hand, it has been suggested that, physiologically, the role played by SDS may be fulfilled by lipids. Native isoelectric focusing identified two acid isoforms of pI 5.7 and 5.8 for the soluble form and one isoform with pI 5.7 for the membrane-bound form. A partially denaturing SDS-PAGE revealed two very close bands of activity in both cases, but the Western blot performed on a totally denaturing SDS-PAGE, using polyclonal antibodies against bean PPO, revealed a single band in the membrane-bound fraction with a molecular mass of 60 kDa.

[Stabilizing effect of Azospirillum lectins on beta-glucosidase activity]

Lectins from the surface of Azospirillum brasilense Sp7 and Azospirillumn brasilense Sp7.2.3 (a mutant with impaired lectin activity) were shown to induce a stabilizing effect on the activity of almond beta-glucosidase under conditions of thermoinactivation and proteolytic enzyme treatment. Differences were revealed in the influence of lectins with various antigenic properties. Our results indicate that the effects of lectins on the catalytic activity of the enzyme are mainly associated with conformational changes in lectin molecules during mutagenesis, but not with carbohydrate specificity (general property). These data should be taken into account in evaluating the role of lectins in the formation of nitrogen-fixing associations.

Salt effects on beta-glucosidase: pH-profile narrowing

Salts inhibit the activity of sweet almond beta-glucosidase. For cations (Cl(-) salts) the effectiveness follows the series: Cu(+2), Fe(+2)>Zn(+2)>Li(+)>Ca(+2)>Mg(+2)>Cs(+)>NH(4)(+)>Rb(+)>K(+)>Na(+) and for anions (Na(+) salts) the series is: I(-)>ClO(4)(-)>(-)SCN>Br(-) approximately NO(3)(-)>Cl(-) approximately (-)OAc>F(-) approximately SO(4)(-2). The activity of the enzyme, like that of most glycohydrolases, depends on a deprotonated carboxylate (nucleophile) and a protonated carboxylic acid for optimal activity. The resulting pH-profile of k(cat)/K(m) for the beta-glucosidase-catalyzed hydrolysis of p-nitrophenyl glucoside is characterized by a width at half height that is strongly sensitive to the nature and concentration of the salt. Most of the inhibition is due to a shift in the enzymic pK(a)s and not to an effect on the pH-independent second-order rate constant, (k(cat)/K(m))(lim). For example, as the NaCl concentration is increased from 0.01 M to 1.0 M the apparent pK(a1)increases (from 3.7 to 4.9) and the apparent pK(a2)decreases (from 7.2 to 5.9). With p-nitrophenyl glucoside, the value of the pH-independent (k(cat)/K(m))(lim) (=9 x 10(4) M(-1) s(-1)) is reduced by less than 4% as the NaCl concentration is increased. There is a similar shift in the pK(a)s when the LiCl concentration is increased to 1.0 M. The results of these salt-induced pK(a) shifts rule out a significant contribution of reverse protonation to the catalytic efficiency of the enzyme. At low salt concentration, the fraction of the catalytically active monoprotonated enzyme in the reverse protonated form (i.e., proton on the group with a pK(a) of 3.7 and dissociated from the group with a pK(a) of 7.2) is very small ( approximately 0.03%). At higher salt concentrations, where the two pK(a)s become closer, the fraction of the monoprotonated enzyme in the reverse protonated form increases over 300-fold. However, there is no increase in the intrinsic reactivity, (k(cat)/K(m))(lim), of the monoprotonated species. For other enzymes which may show such salt-induced pK(a) shifts, this provides a convenient test for the role of reverse protonation.

Thioglycoside hydrolysis catalyzed by β-glucosidase

Sweet almond beta-glucosidase (EC 3.2.1.21) has been shown to have significant thioglycohydrolase activity. While the Km values for the S- and O-glycosides are similar, the k(cat) values are about 1000-times lower for the S-glycosides. Remarkably, the pH-profile for k(cat)/Km for hydrolysis of p-nitrophenyl thioglucoside (pNPSG) shows the identical dependence on a deprotonated carboxylate (pKa 4.5) and a protonated group (pKa 6.7) as does the pH-profile for hydrolysis of the corresponding O-glycoside. Not surprisingly, in spite of the requirement for the presence of this protonated group in catalytically active beta-glucosidase, thioglucoside hydrolysis does not involve general acid catalysis. There is no solvent kinetic isotope effect on the enzyme-catalyzed hydrolysis of pNPSG.

Self-(in)compatibility of the almonds P. dulcis and P. webbii: detection and cloning of ‘wild-type S f ’ and new self-compatibility alleles encoding inactive S-RNases

Prunus dulcis, the almond, is a predominantly self-incompatible (SI) species with a gametophytic self-incompatibility system mediated by S-RNases. The economically important allele Sf, which results in self-compatibility in P. dulcis, is said to have arisen by introgression from Prunus webbii in the Italian region of Apulia. We investigated the range of self-(in)compatibility alleles in Apulian material of the two species. About 23 cultivars of P. dulcis (14 self-compatible (SC) and nine SI) and 33 accessions of P. webbii (16 SC, two SI and 15 initially of unknown status), all from Apulia, were analysed using PCR of genomic DNA to amplify S-RNase alleles and, in most cases, IEF and staining of stylar protein extracts to detect S-RNase activity. Some amplification products were cloned and sequenced. The allele Sf was present in nearly all the SC cultivars of P. dulcis but, surprisingly, was absent from nearly all SC accessions of P. webbii. And of particular interest was the presence in many SI cultivars of P. dulcis of a new active allele, labelled S30, the sequence of which showed it to be the wild-type of Sf so that Sf can be regarded as a stylar part mutant S30 degrees . These findings indicate Sf may have arisen within P. dulcis, by mutation. One SC cultivar of P. dulcis, 'Patalina', had a new self-compatibility allele lacking RNase activity, Sn5, which could be useful in breeding programmes. In the accessions of P. webbii, some of which were known to be SC, three new alleles were found which lacked RNase activity but had normal DNA sequences.

Photosynthesis in response to sink-source activity and in relation to end products and activities of metabolic enzymes in peach trees

Fruits were removed (-fruit) or retained (+fruit) on limbs of 'Yanfengyihao' peach (Prunus persica (L.) Batch) trees. In addition to the -fruit and +fruit treatments, leaves on 1-year-old fruit-bearing shoots of 'Bayuecui' peach trees were covered with bags for varying periods of time. Net photosynthetic rate (P(n)), stomatal conductance and transpiration rate around midday significantly decreased in the -fruit treatment and significantly increased in bagged leaves after removal of the bags compared with values in the +fruit treatment. Leaf temperature (T(l)) was higher in the -fruit treatment than in the +fruit treatment, and P(n) decreased in the -fruit treatment, especially when T(l) exceeded 38 and 36 degrees C in 'Yanfengyihao' and 'Bayuecui', respectively. Fruit removal resulted in a significant accumulation of sorbitol and starch in leaves, but it did not decrease activities of enzymes involved in carbon metabolism in leaves of 'Yanfengyihao'. In contrast, activities of aldose-6-phosphate reductase and ADP-glucose pyrophosphorylase sometimes increased in leaves in the -fruit treatment. In 'Bayuecui', fruit removal significantly decreased maximal fluorescence and photochemical efficiency and increased minimal fluorescence around midday compared with values in the +fruit and bag treatments. The photosynthetic apparatus of peach may be damaged when T(l) exceeds a critical temperature under conditions of decreased sink demand. Decreased stomatal aperture and increased T(l) may regulate photosynthesis by affecting electron transport.

Different positively selected sites at the gametophytic self-incompatibility pistil S-RNase gene in the Solanaceae and Rosaceae (Prunus, Pyrus, and Malus)

In this work we perform a comparative study on the location of positively selected sites (those likely responsible for defining specificity differences) at the S-RNase gene, the pistil component of the gametophytic self-incompatibility system. For Plantaginaceae and Rosaceae (Prunus and Pyrus/Malus) this is the first study of this kind. A clear sign of positive selection was observed for 13, 17, and 27 amino acid sites in Solanaceae, Prunus, and Pyrus/Malus, respectively, using two different methodologies. In Plantaginaceae no clear positively selected sites were identified. Possible reasons for this result are discussed. Indirect experimental evidence suggests that the identified positively selected amino acid sites play a role in specificity determination. The percentage of positively selected sites is similar in Solanaceae and Rosaceae but the location of those sites is different.

Syntheses of N-vanillyl-nonanamide glycosides using amyloglucosidase from Rhizopus and beta-glucosidase from sweet almond

Enzymatic syntheses of N-vanillyl-nonanamide, 1, glycosides with D-glucose, 2, D-galactose, 3, D-mannose, 4, D-ribose, 5, maltose, 6, and lactose, 7, were carried out using amyloglucosidase from Rhizopus and beta-glucosidase from sweet almond. The latter catalysed the syntheses of N-vanillyl-nonanamide glycosides (8-13) and exclusively yielded beta-glycosides with carbohydrates 2, 3, 4, 6 and 7, while amyloglucosidase yielded C1-alpha- and beta-glycosides and 6-O-aryl derivatives (8, 9, 11 and 12).

Activity and subcellular localization of glucose-6-phosphate dehydrogenase in peach fruits

The subcellular distribution and activity of glucose-6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) were studied in developing peach (Prunus persica L. Batsch cv. Zaoyu) fruit. Fruit tissues were separated by differential centrifugation at 15,000g into plastidic and cytosolic fractions. There was no serious loss of enzyme activity (or activation) during the preparation of fractions. G6PDH activity was found in both the plastidic and cytosolic compartments. Moreover, DTT had no effect on the plastidic G6PDH activities, that is, the redox regulatory mechanism did not play an important role in the peach fleshy tissue. Results from the immunogold electron-microscope localization revealed that G6PDH isoenzymes were mainly present in the cytosol, the secondary wall and plastids (chloroplasts and chromoplasts), but scarcely found in the starch granules or the cell wall. In addition to a decrease in fruit firmness, the G6PDH activity in the cytotolic and plastidic fractions increased, and anthocyanin started to accumulate during fruit maturation. These results suggest that G6PDH, by providing precursors for metabolic processes, might be associated with the red coloration that occurs in peach fruit.

Carotenoid accumulation in Japanese apricot (Prunus mume Siebold & Zucc.): molecular analysis of carotenogenic gene expression and ethylene regulation

To elucidate the regulatory mechanisms of carotenogenesis in Japanese apricot (Prunus mume Siebold & Zucc.), the relationships between carotenoid accumulation and the expression of the carotenogenic genes, phytoene synthase (PmPSY-1), phytoene desaturase (PmPDS), zeta-carotene desaturase (PmZDS), lycopene beta-cyclase (PmLCYb), lycopene epsilon-cyclase (PmLCYe), beta-carotene hydroxylase (PmHYb), and zeaxanthin epoxidase (PmZEP), were analyzed in two cultivars with different ripening traits, 'Orihime' and 'Nanko.' In 'Orihime' fruits, large amounts of carotenoids accumulated on the tree, concomitant with the induction of PmPSY-1 and the downstream carotenogenic genes PmLCYb, PmHYb, and PmZEP. In 'Nanko' fruits, carotenoids accumulated mainly after harvest, correlating with an appreciable induction of PmPSY-1 expression, but the downstream genes were not notably induced, which may explain the lower total carotenoid content in 'Nanko' than in 'Orihime.' In both cultivars, a decrease in PmLCYe expression and increased or constant PmLCYb expression could cause the metabolic shift from beta,epsilon-carotenoid synthesis to beta,beta-carotenoid synthesis that occurs as ripening approaches. Next, the effects of ethylene on the expression of PmPSY-1 and carotenoid accumulation were investigated in 'Nanko' fruits treated with propylene or 1-methylcyclopropene (1-MCP). Propylene treatment induced both ethylene production and carotenoid accumulation. PmPSY-1 was constitutively expressed, but propylene treatment accelerated its induction. 1-MCP treatment caused a slight inhibition of carotenoid accumulation along with the repression, although not complete, of PmPSY-1. Collectively, although PmPSY-1 expression was not exclusively regulated by ethylene, both the notable induction of PmPSY-1 accelerated by ethylene and the subsequent induction of the downstream carotenogenic genes, especially PmLCYb, could be necessary for the massive carotenoid accumulation that occurs during ripening. Furthermore, the switch from PmLCYe expression to PmLCYb expression could cause beta,beta-carotenoid accumulation in both Japanese apricot cultivars.

Stereoselective Biocatalytic Synthesis of (S)-2-Hydroxy-2-Methylbutyric Acid via Substrate Engineering by Using “Thio-Disguised” Precursors and Oxynitrilase Catalysis

3-Tetrahydrothiophenone (4) and 4-phenylthiobutan-2-one (7) were used as masked 2-butanone equivalents to give the corresponding cyanohydrins 5 (79 % yield, 91 % ee) and 8 (95 % yield, 96 % ee) in an enzymatic cyanohydrin reaction applying the hydroxynitrile lyase (HNL) from Hevea brasiliensis. After hydrolysis and desulphurisation the desired intermediate (S)-2-hydroxy-2-methylbutyric acid (10) was obtained with 99 % ee. Interestingly, when applying (R)-selective HNL from Prunus amygdalus again the (S)-cyanohydrin 5 was formed (62 % ee). The absolute configuration of 5 was verified by crystal structure determination of the corresponding hydrolysis derived carboxylate. The fact that both enzymes yield the same enantiomer was analysed and interpreted by molecular modelling calculations.

Counteracting expression deficiencies by anticipating posttranslational modification of PaHNL5-L1Q-A111G by genetic engineering

(R)-2-chloromandelic acid represents a key pharmaceutical intermediate. Its production on large scale was hampered by low turnover rates and moderate enantiomeric excess (ee) using enzyme as well as metal catalysts. The cloning and heterologous overexpression of an (R)-hydroxynitrile lyase from Prunus amygdalus opened a way to large-scale production of this compound. Especially the rationally designed mutation of alanine to glycine at amino acid position 111 of the mature protein tremendously raised the yield for enantioselective conversion of 2-chlorobenzaldehyde to (R)-2-chloromandelonitrile, which can be hydrolysed to the corresponding alpha hydroxy acid. However, expression of this mutein was less efficient than for the unmodified enzyme. Subsequent LC/MS/MS-analysis of the protein sequence revealed that mutation A111G triggered the posttranslational deamidation of the neighbouring residue asparagine (N110) to aspartic acid. This finding on the one hand could explain the decreased secretion efficiency of the mutant as compared to the wildtype enzyme, but on the other hand raised the question which of the two residues was truly accountable for the enhanced conversion. The muteins N110D, A111G and N110DA111G were constructed and compared in terms of protein productivity and performance in chemical syntheses. The expression level of the double mutein was augmented significantly and the enantioselectivity remained high. Reduced protein expression of mutein PaHNL5-L1Q-A111G was remedied by mutational anticipation of posttranslational deamidation.

Serine scanning—A tool to prove the consequences of N-glycosylation of proteins

N-Glycosylation of proteins is a common posttranslational modification in eukaryotes. Often this results in enhanced protein stability through protection by the attached sugar moieties. Due to its 13 potential N-glycosylation motifs (N-X-T/S), recombinant hydroxynitrile lyase isoenzyme 5 from almonds (PaHNL5) is secreted by the heterologous host Pichia pastoris in a massively glycosylated form, and it shows extraordinary stability at low pH. The importance of N-glycosylation in general, and individual glycosylation sites in particular for stability at low pH were investigated. To identify especially important glycosylation sites asparagine from all N-X-S/T-motifs was replaced by serine. Thus, critical sites, which contributed to overall enzyme activity and/or stability, were identified individually. One glycosylation site revealed to be essential for stability at low pH. After enzymatic deglycosylation, leaving only one acetylglucosamine attached to asparagines, PaHNL5 retained most of its stability at low pH. Protonation effects in the active site as well as higher-order aggregational events upon incubation in low pH were excluded. This study provides evidence for the interconnection of N-glycosylation and stability at low pH for PaHNL5. Moreover, serine scanning was proven to be applicable for quick identification of critical glycosylation sites.

Amyloglucosidase-catalyzed synthesis of eugenyl and curcuminyl glycosides

Glycosylation of the phenolic hydroxyl group of the phenyl propanoid systems, eugenol 1 and curcumin 2, using an amyloglucosidase from Rhizopus and a beta-glucosidase from sweet almonds together with carbohydrates (D-glucose 3, D-mannose 4, maltose 5, sucrose 6 and D-mannitol 7) in di-isopropyl ether produced glycosides at 7-52% yields in 72 h. Spectral studies indicated that the reaction occurred between the phenolic OH groups and C-1 and/or 6-O-groups of the carbohydrates with curcumin exhibiting bis glycosylation.

Distribution of S-alleles in island populations of flowering cherry, Prunus lannesiana var. speciosa

We surveyed the distribution of S-alleles in natural island populations of Prunus lannesiana var. speciosa sampled from seven sites on the Izu Peninsula and six Izu islands, Japan. The S-genotypes of sampled individuals were determined by Southern analysis of RFLPs generated by restriction enzyme digestion of genomic DNA, using cDNA of the S-RNase gene as a probe. All individuals were heterozygous, as expected under gametophytic self-incompatibility (GSI). Sixty-three S-alleles were observed in the species, but 12 private to the Izu Peninsula population seemed to be derived from related species, giving a total of 75. The estimated number of S-alleles in each population ranged from 26 to 62, and was inversely correlated with the respective population's distance from the Izu Peninsula, the closest point in the mainland to the islands. This geographical cline in the estimated numbers of S-alleles suggests that gene flow to and from the distant island populations was less frequent, and that the studied species has migrated from the mainland to the Izu islands. The genetic relationship at the S-locus among populations also gave an "isolation by distance" pattern. The genetic differentiation at the S-locus among the populations was very low (F(ST) = 0.014, p < 0.001). The number of S-alleles in the species did not seem to depend on genetic differences associated with population subdivisions. This might be due to the greater effective migration rates of S-alleles, as expected under balancing selection in GSI.

9-lipoxygenase metabolism is involved in the almond/Aspergillus carbonarius interaction

Phyto-oxylipins are a group of biologically active molecules that play an important role in plant defence. Their production begins with the oxygenation of polyunsaturated fatty acids by lipoxygenases (LOX) to form 9- or 13-hydroperoxides that are substrates for several enzymes involved in the synthesis of final oxylipins, which can act as signal molecules and/or direct antimicrobials. In the present work, the response of the 9-LOX pathway in the almond/Aspergillus carbonarius (producer of ochratoxin A) interaction was studied. Both LOX gene expression and activity are up-regulated over the course of fungal infection in immature and mature almonds. The biochemical characterization of major LOX and hydroperoxide lyase (HPL) isoforms indicated that 9-LOX metabolism is specifically induced by A. carbonarius. Lipid peroxidation profiling showed that, in infected immature almonds, enzymatically produced 9-hydro(peroxy) fatty acids (HFAs) were the main HFAs and are further metabolized by HPL into C9-aldehydes. Both HPL gene expression and C9-aldehydes increased over the course of fungal infection. In mature almonds infected with A. carbonarius, levels of LOX expression and activity were lower than those found in immature seeds, and 9-HFA represented the minority of total HFA, which consisted of mostly 13- and non-enzymatically produced HFA. In these experimental conditions, no volatile aldehydes were recorded from these samples, even though HPL was up-regulated in infected mature almonds. The effects on the growth of A. carbonarius of the aldehydes produced by these enzymes were also tested in vitro. Results reported here led to the proposal that, in almond seed, the association of 9-LOX and HPL has an important role in seed defence mechanism against pathogen infection.

Detection and transcript expression of S-RNase gene associated with self-incompatibility in apricot (Prunus armeniaca L.)

The identity and expression of S-RNase genotypes in the self-compatible (SC) apricot cultivar 'Katy' and the self-incompatible (SI) cultivar 'Xinshiji' were examined. We used allele specific polymerase chain reaction (AS-PCR) and designated the alleles in 'Katy' and 'Xinshiji' as S(8)Sc and S(9)S(10), respectively. The S-RNase gene was expressed in style at the balloon stage in both genotypes. Using real-time fluorescence quantification RT-PCR technology (FQRT-PCR), spatio-temporal expression patterns of S-RNase gene between 'Katy' and 'Xinshiji' were compared. The results revealed that the expression of the S-RNase gene in 'Katy' and 'Xinshiji' were different. The transcript abundance was distinctly diverse at the key stage (i.e., at 24 h after self-pollination) in both genotypes, and was greater in 'Xinshiji' (SI) than 'Katy' (SC). In addition, the abundance of the S-RNase transcript was higher in upper-half of style than in the lower-half of style or in the ovary. In the SI cultivar 'Xinshiji', the expression of S-RNase reminded a relatively high level after cross-pollination, but it dropped continuously after self-pollination and un-pollination.