Salicylaldehyde synthase activity from Venturia inaequalis elicitor-treated cell culture of apple

A peroxisomal β-oxidative pathway contributes to the formation of C6-C1 aromatic volatiles in poplar

Benzenoids (C6-C1 aromatic compounds) play important roles in plant defense and are often produced upon herbivory. Black cottonwood (Populus trichocarpa) produces a variety of volatile and nonvolatile benzenoids involved in various defense responses. However, their biosynthesis in poplar is mainly unresolved. We showed feeding of the poplar leaf beetle (Chrysomela populi) on P. trichocarpa leaves led to increased emission of the benzenoid volatiles benzaldehyde, benzylalcohol, and benzyl benzoate. The accumulation of salicinoids, a group of nonvolatile phenolic defense glycosides composed in part of benzenoid units, was hardly affected by beetle herbivory. In planta labeling experiments revealed that volatile and nonvolatile poplar benzenoids are produced from cinnamic acid (C6-C3). The biosynthesis of C6-C1 aromatic compounds from cinnamic acid has been described in petunia (Petunia hybrida) flowers where the pathway includes a peroxisomal-localized chain shortening sequence, involving cinnamate-CoA ligase (CNL), cinnamoyl-CoA hydratase/dehydrogenase (CHD), and 3-ketoacyl-CoA thiolase (KAT). Sequence and phylogenetic analysis enabled the identification of small CNL, CHD, and KAT gene families in P. trichocarpa. Heterologous expression of the candidate genes in Escherichia coli and characterization of purified proteins in vitro revealed enzymatic activities similar to those described in petunia flowers. RNA interference-mediated knockdown of the CNL subfamily in gray poplar (Populus x canescens) resulted in decreased emission of C6-C1 aromatic volatiles upon herbivory, while constitutively accumulating salicinoids were not affected. This indicates the peroxisomal β-oxidative pathway participates in the formation of volatile benzenoids. The chain shortening steps for salicinoids, however, likely employ an alternative pathway.

A simple "turn-on" fluorescence sensor for salicylaldehyde skeleton based on switch of PET-AIE effect

The selective detection of salicylaldehyde skeleton is of great significance in phytochemistry and biological research but rarely reported. In this research, a simple and highly selective "turn-on" fluorescence sensor (CDB-Am) for salicylaldehyde skeleton was developed based on switch of photoinduced electron transfer (PET) and aggregation-induced emission (AIE). CDB-Am bearing amino-cyanodistyrene structure responded to salicylaldehyde in the range of 3.1 to 40 μM with a detection limit of 0.94 μM. The sensing process of formation of Schiff-base adduct CDB-SA was confirmed by ¹H NMR, MS, and FT-IR spectra, revealing that a recovered AIE property accounted for the turn-on fluorescence response of CDB-Am and the intramolecular hydrogen bonding played a crucial role in the disruption of PET process. This sensing ability was successfully applied for both fluorescence qualitative test of salicylaldehyde skeleton on TLC analysis and quantitative detection of salicylaldehyde skeleton with good accuracy in the root bark of Periploca sepium, suggesting the extensive applications in phytochemistry and traditional Chinese herbal medicine. Furthermore, CDB-Am exhibited the first excellent fluorescence imaging ability in detecting salicylaldehyde skeleton in a living system. This work supplied a new strategy of preparing a novel "turn-on" fluorescence probe for detecting salicylaldehyde skeleton in complex environments and living bodies.

A new enzymatic activity from elicitor-treated pear cell cultures converting trans-cinnamic acid to benzaldehyde

Cell cultures of Asian pear (Pyrus pyrifolia) are known to produce benzoate-derived biphenyl phytoalexins upon elicitor treatment. Although the downstream pathway for biphenyl phytoalexin biosynthesis is almost known, the upstream route of benzoic acid biosynthesis in pear has not been completely elucidated. In the present work, we report benzaldehyde synthase (BS) activity from yeast extract-treated cell suspension cultures of P. pyrifolia. BS catalyzes the in vitro conversion of trans-cinnamic acid to benzaldehyde using a non-oxidative C2 -side chain cleavage mechanism. The enzyme activity was strictly dependent on the presence of a reducing agent, dithiothreitol being preferred. C2 -side chain shortening of the cinnamic acid backbone resembled the mechanisms catalyzed by 4-hydroxybenzaldehyde synthase (HBS) activity in Vanilla planifolia and salicylaldehyde synthase (SAS) activity in tobacco and apple cell cultures. A basal BS activity was also observed in the non-elicited cell cultures. Upon yeast extract-treatment, a 13-fold increase in BS activity was observed when compared to the non-treated control cells. Moreover, feeding of the cell cultures with trans-cinnamic acid, the substrate for BS, resulted in an enhanced level of noraucuparin, a biphenyl phytoalexin. Comparable accumulation of noraucuparin was observed upon feeding of benzaldehyde, the BS product. The preferred substrate for BS was found to be trans-cinnamic acid, for which the apparent Km and Vmax values were 0.5 mM and 50.7 pkat mg-1 protein, respectively. Our observations indicate the contribution of BS to benzoic acid biosynthesis in Asian pear via the CoA-independent and non-β-oxidative route.

Comparative metabolomics of scab-resistant and susceptible apple cell cultures in response to scab fungus elicitor treatment

Apple scab disease caused by the fungus Venturia inaequalis is a devastating disease that seriously affects quality and yield of apples. In order to understand the mechanisms involved in scab resistance, we performed gas chromatography-mass spectrometry based metabolomics analysis of the cell culture of scab resistant cultivar 'Florina' and scab susceptible cultivar 'Vista Bella' both prior -to and -following treatment with V. inaequalis elicitor (VIE). A total 21 metabolites were identified to be altered significantly in 'Florina' cell cultures upon VIE-treatment. Among 21 metabolites, formation of three new specialized metabolites aucuparin, noraucuparin and eriobofuran were observed only in resistant cultivar 'Florina' after the elicitor treatment. The score plots of principal component analysis (PCA) exhibited clear discrimination between untreated and VIE-treated samples. The alteration in metabolite levels correlated well with the changes in the transcript levels of selected secondary metabolite biosynthesis genes. Aucuparin, noraucuparin and eriobofuran isolated from the 'Florina' cultures showed significant inhibitory effect on the conidial germination of V. inaequalis. The results expand our understanding of the metabolic basis of scab-resistance in apple and therefore are of interest in apple breeding programs to fortify scab resistance potential of commercially grown apple cultivars.