Biphenyl Phytoalexin in Sorbus pohuashanensis Suspension Cell Induced by Yeast Extract

Biphenyls are unique phytoalexins de novo synthesized in plants in response to pathogen attack. These compounds are found in Maloideae, a subfamily of the Rosaceae. The anti-microbial activities of biphenyls have been reported in a number of studies and they appear to represent an important defense strategy against pathogens common in the Maloideae, such as species in Malus, Pyrus, Sorbus, and Chaenomeles. Here, cell suspension cultures of Sorbus pohuashanensis were established to study biphenyl phytoalexins formation after yeast extract (YE) treatment. An ultra-performance liquid chromatography (UPLC) method coupled with quadrupole time of flight mass spectrometry (Q-TOF-MS) LC−MS/MS was applied to determine the time course of these biphenyl biomarkers accumulation in YE-treated S. pohuashanensis suspension cells. The results of quantitative analyses show the content of Noraucuparin, 2′-Hydroxyaucuparin, and their glycosides initially increased, then decreased over time. The Noraucuparin content reached its highest (225.76 μg·g⁻¹) at 18 h after treatment, 6 hours earlier than that of Noraucuparin 5-O-β-d-glucopyranoside. The content of 2′-Hydroxyaucuparin reached its highest (422.75 μg·g⁻¹) at 30 h after treatment, also earlier than that of its glycoside. The understanding of phytoalexin metabolism in this study may provide a basis for improving Maloideae resistance to pathogens.

Seasonal patterns of leaf photosynthetic and secondary xylem vascular traits in current-year stems of three Sorbus species with contrasting growth habits

Seasonal effects of environmental variables on photosynthetic activity and secondary xylem formation provide data to demonstrate how environmental factors together with leaf ageing during the season control tree growth. In this study, we assessed physiological responses in photosynthetic behaviour to seasonal climate changes, and also identified seasonal differences in vascular traits within differentiating secondary xylem tissue from three diploid species of the taxonomically complex genus Sorbus. From sampling day 150, a clear physiological segregation of S. chamaemespilus from S. torminalis and S. aria was evident. The shrubby species S. chamaemespilus could be distinguished by a higher photosynthetic capacity between days 150 and 206. This was reflected in its associations with net CO2 assimilation rate (PN), maximum photochemical efficiency of PSII (F(v)/F(m)), variable-to-initial fluorescence ratio (F(v)/F(0)), potential electron acceptor capacity ('area') in multivariate space, and also its associations with log-transformed vessel area and log-transformed relative conductivity between days 239 and 268. The maximum segregation and differentiation among the examined Sorbus species was on sampling day 206. The largest differences between S. torminalis and S. aria were found on day 115, when the latter species clearly showed closer associations with high values of vessel density and transpiration (E). Sampling day clusters were arranged along an arch-like gradient that reflected the positioning of the entire growing season in multivariate space. This arch-like pattern was most apparent in the case of S. chamaemespilus, but was also observed in S. torminalis and S. aria.

Leaf traits in parental and hybrid species of Sorbus (Rosaceae)

PREMISE OF THE STUDY

Knowledge of functional leaf traits can provide important insights into the processes structuring plant communities. In the genus Sorbus, the generation of taxonomic novelty through reticulate evolution that gives rise to new microspecies is believed to be driven primarily by a series of interspecific hybridizations among closely related taxa. We tested hypotheses for dispersion of intermediacy across the leaf traits in Sorbus hybrids and for trait linkages with leaf area and specific leaf area.

METHODS

Here, we measured and compared the whole complex of growth, vascular, and ecophysiological leaf traits among parental (Sorbus aria, Sorbus aucuparia, Sorbus chamaemespilus) and natural hybrid (Sorbus montisalpae, Sorbus zuzanae) species growing under field conditions. A recently developed atomic force microscopy technique, PeakForce quantitative nanomechanical mapping, was used to characterize the topography of cell wall surfaces of tracheary elements and to map the reduced Young's modulus of elasticity.

KEY RESULTS

Intermediacy was associated predominantly with leaf growth traits, whereas vascular and ecophysiological traits were mainly parental-like and transgressive phenotypes. Larger-leaf species tended to have lower modulus of elasticity values for midrib tracheary element cell walls. Leaves with a biomass investment related to a higher specific leaf area had a lower density. Leaf area- and length-normalized theoretical hydraulic conductivity was related to leaf thickness.

CONCLUSIONS

For the whole complex of examined leaf traits, hybrid microspecies were mosaics of parental-like, intermediate, and transgressive phenotypes. The high proportion of transgressive character expressions found in Sorbus hybrids implies that generation of extreme traits through transgressive segregation played a key role in the speciation process.

Benzaldehyde dehydrogenase from chitosan-treated Sorbus aucuparia cell cultures

Cell cultures of Sorbus aucuparia respond to the addition of chitosan with the accumulation of the biphenyl phytoalexin aucuparin. The carbon skeleton of this inducible defense compound is formed by biphenyl synthase (BIS) from benzoyl-CoA and three molecules of malonyl-CoA. The formation of benzoyl-CoA proceeds via benzaldehyde as an intermediate. Benzaldehyde dehydrogenase (BD), which converts benzaldehyde into benzoic acid, was detected in cell-free extracts from S. aucuparia cell cultures. BD and BIS were induced by chitosan treatment. The preferred substrate for BD was benzaldehyde (K(m)=49 microM). Cinnamaldehyde and various hydroxybenzaldehydes were relatively poor substrates. BD activity was strictly dependent on the presence of NAD(+) as a cofactor (K(m)=67 microM).

Biphenyl synthase, a novel type III polyketide synthase

Biphenyls and dibenzofurans are the phytoalexins of the Maloideae, a subfamily of the economically important Rosaceae. The carbon skeleton of the two classes of antimicrobial secondary metabolites is formed by biphenyl synthase (BIS). A cDNA encoding this key enzyme was cloned from yeast-extract-treated cell cultures of Sorbus aucuparia. BIS is a novel type III polyketide synthase (PKS) that shares about 60% amino acid sequence identity with other members of the enzyme superfamily. Its preferred starter substrate is benzoyl-CoA that undergoes iterative condensation with three molecules of malonyl-CoA to give 3,5-dihydroxybiphenyl via intramolecular aldol condensation. BIS did not accept CoA-linked cinnamic acids such as 4-coumaroyl-CoA. This substrate, however, was the preferential starter molecule for chalcone synthase (CHS) that was also cloned from S. aucuparia cell cultures. While BIS expression was rapidly, strongly and transiently induced by yeast extract treatment, CHS expression was not. In a phylogenetic tree, BIS grouped together closely with benzophenone synthase (BPS) that also uses benzoyl-CoA as starter molecule but cyclizes the common intermediate via intramolecular Claisen condensation. The molecular characterization of BIS thus contributes to the understanding of the functional diversity and evolution of type III PKSs.

Biphenyl synthase from yeast-extract-treated cell cultures of Sorbus aucuparia

Biphenyls and dibenzofurans are the phytoalexins of the Maloideae, a subfamily of the economically important Rosaceae. The biphenyl aucuparin accumulated in Sorbus aucuparia L. cell cultures in response to yeast extract treatment. Incubation of cell-free extracts from challenged cell cultures with benzoyl-CoA and malonyl-CoA led to the formation of 3,5-dihydroxybiphenyl. This reaction was catalysed by a novel polyketide synthase, which will be named biphenyl synthase. The most efficient starter substrate for the enzyme was benzoyl-CoA. Relatively high activity was also observed with 2-hydroxybenzoyl-CoA but, instead of the corresponding biphenyl, the derailment product 2-hydroxybenzoyltriacetic acid lactone was formed.