Benzaldehyde dehydrogenase-driven phytoalexin biosynthesis in elicitor-treated Pyrus pyrifolia cell cultures

Multi-omics revealed molecular mechanism of biphenyl phytoalexin formation in response to yeast extract-induced oxidative stress in Sorbus aucuparia suspension cells

KEY MESSAGE

Yeast extract-induced oxidative stress in Sorbus aucuparia suspension cells leads to the biosynthesis of various hormones, which activates specific signaling pathways that augments biphenyl phytoalexin production. Pathogen incursions pose a significant threat to crop yield and can have a pronounced effect on agricultural productivity and food security. Biphenyl phytoalexins are a specialized group of secondary metabolites that are mainly biosynthesized by Pyrinae plants as a defense mechanism against various pathogens. Despite previous research demonstrating that biphenyl phytoalexin production increased dramatically in Sorbus aucuparia suspension cells (SASCs) treated with yeast extract (YE), the underlying mechanisms remain poorly understood. To address this gap, we conducted an in-depth, multi-omics analysis of transcriptome, proteome, and metabolite (including biphenyl phytoalexins and phytohormones) dynamics in SASCs exposed to YE. Our results indicated that exposure to YE-induced oxidative stress in SASCs, leading to the biosynthesis of a range of hormones, including jasmonic acid (JA), jasmonic acid isoleucine (JA-ILE), gibberellin A4 (GA4), indole-3-carboxylic acid (ICA), and indole-3-acetic acid (IAA). These hormones activated specific signaling pathways that promoted phenylpropanoid biosynthesis and augmented biphenyl phytoalexin production. Moreover, reactive oxygen species (ROS) generated during this process also acted as signaling molecules, amplifying the phenylpropanoid biosynthesis cascade through activation of the mitogen-activated protein kinase (MAPK) pathway. Key genes involved in these signaling pathways included SaBIS1, SaBIS2, SaBIS3, SaPAL, SaB4H, SaOMT, SaUGT1, SaLOX2, SaPR1, SaCHIB1, SaCHIB2 and SaCHIB3. Collectively, this study provided intensive insights into biphenyl phytoalexin accumulation in YE-treated SASCs, which would inform the development of more efficient disease-resistance strategies in economically significant cultivars.

PusALDH1 gene confers high levels of volatile aroma accumulation in both pear and tomato fruits

Aroma is an important commercial trait that determines fruit quality and has an important influence on the overall flavor of fruits. Plant ALDH genes have been implicated in diverse pathways and play crucial roles in physiological activities. In this study, via genome resequencing we identified one gene PusALDH1 (Pbr034873.1) related to aroma biosynthesis that can respond to the induction of methyl jasmonate. Transient transformation of pear fruits and heterologous stable transformation of tomato further confirmed the function of PusALDH1 in aroma accumulation. The content of ALDH precursor substance, benzaldehyde, was reduced in the overexpressing pear and tomato fruits, and the content of ALDH product, benzoic acid and benzoic acid derivatives, was increased in the pear fruits. Meanwhile, transgenic tomato fruits with PusALDH1 overexpression exhibited a greater area of yellow placenta, indicating that the gene may be related to the growth and development of the fruit. Taken together, PusALDH1 could act as a strong candidate gene in aroma synthesis.

Biphenyls and dibenzofurans of the rosaceous subtribe Malinae and their role as phytoalexins

MAIN CONCLUSION

Biphenyl and dibenzofuran phytoalexins are differentially distributed among species of the rosaceous subtribe Malinae, which includes apple and pear, and exhibit varying inhibitory activity against phytopathogenic microorganisms. Biphenyls and dibenzofurans are specialized metabolites, which are formed in species of the rosaceous subtribe Malinae upon elicitation by biotic and abiotic inducers. The subtribe Malinae (previously Pyrinae) comprises approximately 1000 species, which include economically important fruit trees such as apple and pear. The present review summarizes the current status of knowledge of biphenyls and dibenzofurans in the Malinae, mainly focusing on their role as phytoalexins. To date, 46 biphenyls and 41 dibenzofurans have been detected in 44 Malinae species. Structurally, 54 simple molecules, 23 glycosidic compounds and 10 miscellaneous structures were identified. Functionally, 21 biphenyls and 21 dibenzofurans were demonstrated to be phytoalexins. Furthermore, their distribution in species of the Malinae, inhibitory activities against phytopathogens, and structure-activity relationships were studied. The most widely distributed phytoalexins of the Malinae are the three biphenyls aucuparin (3), 2'-methoxyaucuparin (7), and 4'-methoxyaucuparin (9) and the three dibenzofurans α-cotonefuran (47), γ-cotonefuran (49), and eriobofuran (53). The formation of biphenyl and dibenzofuran phytoalexins appears to be an essential defense weapon of the Malinae against various stresses. Manipulating phytoalexin formation may enhance the disease resistance in economically important fruit trees. However, this approach requires an extensive understanding of how the compounds are formed. Although the biosynthesis of biphenyls was partially elucidated, formation of dibenzofurans remains largely unclear. Thus, further efforts have to be made to gain deeper insight into the distribution, function, and metabolism of biphenyls and dibenzofurans in the Malinae.

Xanthone Biosynthetic Pathway in Plants: A Review

Xanthones are secondary metabolites rich in structural diversity and possess a broad array of pharmacological properties, such as antitumor, antidiabetic, and anti-microbes. These aromatic compounds are found in higher plants, such as Clusiaceae, Hypericaceae, and Gentianaceae, yet their biosynthetic pathways have not been comprehensively updated especially within the last decade (up to 2021). In this review, plant xanthone biosynthesis is detailed to illuminate their intricacies and differences between species. The pathway initially involves the shikimate pathway, either through L-phenylalanine-dependent or -independent pathway, that later forms an intermediate benzophenone, 2,3',4,6-tetrahydoxybenzophenone. This is followed by a regioselective intramolecular mediated oxidative coupling to form xanthone ring compounds, 1,3,5-trihydroxyxanthone (1,3,5-THX) or 1,3,7-THX, the core precursors for xanthones in most plants. Recent evidence has shed some lights onto the enzymes and reactions involved in this xanthone pathway. In particular, several biosynthetic enzymes have been characterized at both biochemical and molecular levels from various organisms including Hypericum spp., Centaurium erythraea and Garcinia mangostana. Proposed pathways for a plethora of other downstream xanthone derivatives including swertianolin and gambogic acid (derived from 1,3,5-THX) as well as gentisin, hyperixanthone A, α-mangostin, and mangiferin (derived from 1,3,7-THX) have also been thoroughly covered. This review reports one of the most complete xanthone pathways in plants. In the future, the information collected here will be a valuable resource for a more directed molecular works in xanthone-producing plants as well as in synthetic biology application.

Cytosolic aromatic aldehyde dehydrogenase provides benzoic acid for xanthone biosynthesis in Hypericum

Benzoic acid is a building block of a multitude of well-known plant natural products, such as paclitaxel and cocaine. Its simple chemical structure contrasts with its complex biosynthesis. Hypericum species are rich in polyprenylated benzoic acid-derived xanthones, which have received attention due to their biological impact on human health. The upstream biosynthetic sequence leading to xanthones is still incomplete. To supply benzoic acid for xanthone biosynthesis, Hypericum calycinum cell cultures use the CoA-dependent non-β-oxidative pathway, which starts with peroxisomal cinnamate CoA-ligase (HcCNL). Here, we use the xanthone-producing cell cultures to identify the transcript for benzaldehyde dehydrogenase (HcBD), a pivotal player in the non-β-oxidative pathways. In addition to benzaldehyde, the enzyme efficiently catalyzes the oxidation of trans-cinnamaldehyde in vitro. The enzymatic activity is strictly dependent on the presence of NAD⁺ as co-factor. HcBD is localized to the cytosol upon ectopic expression of reporter fusion constructs. HcBD oxidizes benzaldehyde, which moves across the peroxisome membrane, to form benzoic acid. Increases in the HcCNL and HcBD transcript levels precede the elicitor-induced xanthone accumulation. The current work addresses a crucial step in the yet incompletely understood CoA-dependent non-β-oxidative route of benzoic acid biosynthesis. Addressing this step may offer a new biotechnological tool to enhance product formation in biofactories.

Recent Development on Plant Aldehyde Dehydrogenase Enzymes and Their Functions in Plant Development and Stress Signaling

Abiotic and biotic stresses induce the formation of reactive oxygen species (ROS), which subsequently causes the excessive accumulation of aldehydes in cells. Stress-derived aldehydes are commonly designated as reactive electrophile species (RES) as a result of the presence of an electrophilic α, β-unsaturated carbonyl group. Aldehyde dehydrogenases (ALDHs) are NAD(P)⁺-dependent enzymes that metabolize a wide range of endogenous and exogenous aliphatic and aromatic aldehyde molecules by oxidizing them to their corresponding carboxylic acids. The ALDH enzymes are found in nearly all organisms, and plants contain fourteen ALDH protein families. In this review, we performed a critical analysis of the research reports over the last decade on plant ALDHs. Newly discovered roles for these enzymes in metabolism, signaling and development have been highlighted and discussed. We concluded with suggestions for future investigations to exploit the potential of these enzymes in biotechnology and to improve our current knowledge about these enzymes in gene signaling and plant development.

A promiscuous coenzyme A ligase provides benzoyl-coenzyme A for xanthone biosynthesis in Hypericum

Benzoic acid-derived compounds, such as polyprenylated benzophenones and xanthones, attract the interest of scientists due to challenging chemical structures and diverse biological activities. The genus Hypericum is of high medicinal value, as exemplified by H. perforatum. It is rich in benzophenone and xanthone derivatives, the biosynthesis of which requires the catalytic activity of benzoate-coenzyme A (benzoate-CoA) ligase (BZL), which activates benzoic acid to benzoyl-CoA. Despite remarkable research so far done on benzoic acid biosynthesis in planta, all previous structural studies of BZL genes and proteins are exclusively related to benzoate-degrading microorganisms. Here, a transcript for a plant acyl-activating enzyme (AAE) was cloned from xanthone-producing Hypericum calycinum cell cultures using transcriptomic resources. An increase in the HcAAE1 transcript level preceded xanthone accumulation after elicitor treatment, as previously observed with other pathway-related genes. Subcellular localization of reporter fusions revealed the dual localization of HcAAE1 to cytosol and peroxisomes owing to a type 2 peroxisomal targeting signal. This result suggests the generation of benzoyl-CoA in Hypericum by the CoA-dependent non-β-oxidative route. A luciferase-based substrate specificity assay and the kinetic characterization indicated that HcAAE1 exhibits promiscuous substrate preference, with benzoic acid being the sole aromatic substrate accepted. Unlike 4-coumarate-CoA ligase and cinnamate-CoA ligase enzymes, HcAAE1 did not accept 4-coumaric and cinnamic acids, respectively. The substrate preference was corroborated by in silico modeling, which indicated valid docking of both benzoic acid and its adenosine monophosphate intermediate in the HcAAE1/BZL active site cavity.

Benzoate-CoA ligase contributes to the biosynthesis of biphenyl phytoalexins in elicitor-treated pear cell cultures

Benzoate-Coenzyme A ligase enzyme activity catalyzing the conversion of free benzoic acid to benzoyl-CoA was detected and biochemically characterized in the elicitor-treated pear cell cultures. Asian pear (Pyrus pyrifolia) is an economically and nutritionally important fruit-bearing tree of the subtribe Malinae. Upon pathogen attack, pears produce unique benzoate-derived biphenyl phytoalexins. The upstream biosynthesis of the biphenyl in Malinae is still incomplete. Previously, protein preparations from yeast extract-treated pear cultures were able to convert L-phenylalanine to cinnamic acid catalyzed by the activity of the phenylalanine ammonia lyase. The same extract was able to perform a C₂ side-chain cleavage of cinnamic acid to benzaldehyde followed by oxidation of the latter to benzoic acid owing to the molecularly-undefined benzaldehyde synthase and benzaldehyde dehydrogenase activities, respectively. The biosynthesis of biphenyls starts with benzoate-Coenzyme A ligase (BZL), which converts benzoic acid to benzoyl-CoA. Subsequently, the previously-defined biphenyl synthase uses benzoyl-CoA to form the biphenyls. The current study reports the first time detection and characterization of BZL activity in elicitor-treated pear cell cultures. The preferred substrate was benzoic acid (K_m = 62 ± 4 µM). Magnesium or manganese was prerequisite for the activity, which was enhanced by ~ 70% in the presence of potassium. Maximum BZL activity was observed 18 h post elicitation, which is in agreement with the coordinate induction reported for the enzymes in the same pathway. The induced BZL activity preceded the accumulation of biphenyls supporting its involvement in their biosynthesis.

Cinnamate-CoA ligase is involved in biosynthesis of benzoate-derived biphenyl phytoalexin in Malus × domestica 'Golden Delicious' cell cultures

Apple (Malus sp.) and other genera belonging to the sub-tribe Malinae of the Rosaceae family produce unique benzoic acid-derived biphenyl phytoalexins. Cell cultures of Malus domestica cv. 'Golden Delicious' accumulate two biphenyl phytoalexins, aucuparin and noraucuparin, in response to the addition of a Venturia inaequalis elicitor (VIE). In this study, we isolated and expressed a cinnamate-CoA ligase (CNL)-encoding sequence from VIE-treated cell cultures of cv. 'Golden Delicious' (M. domestica CNL; MdCNL). MdCNL catalyses the conversion of cinnamic acid into cinnamoyl-CoA, which is subsequently converted to biphenyls. MdCNL failed to accept benzoic acid as a substrate. When scab-resistant (cv. 'Shireen') and moderately scab-susceptible (cv. 'Golden Delicious') apple cultivars were challenged with the V. inaequalis scab fungus, an increase in MdCNL transcript levels was observed in internodal regions. The increase in MdCNL transcript levels could conceivably correlate with the pattern of accumulation of biphenyls. The C-terminal signal in the MdCNL protein directed its N-terminal reporter fusion to peroxisomes in Nicotiana benthamiana leaves. Thus, this report records the cloning and characterisation of a cinnamoyl-CoA-forming enzyme from apple via a series of in vivo and in vitro studies. Defining the key step of phytoalexin formation in apple provides a biotechnological tool for engineering elite cultivars with improved resistance.

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.

Molecular cloning and functional analysis of a biphenyl phytoalexin-specific O-methyltransferase from apple cell suspension cultures

This manuscript describes the cloning and functional characterization of a biphenyl phytoalexin biosynthetic gene, 3,5-dihydroxybiphenyl O-methyltransferase from elicitor-treated cell cultures of scab resistant apple cultivar 'Florina'. Apples belong to the subtribe Malinae of the Rosaceae family. Biphenyls and dibenzofurans are the specialized phytoalexins of Malinae, of which aucuparin is the most widely distributed biphenyl. The precursor of aucuparin, 3,5-dihydroxybiphenyl, is a benzoate-derived polyketide, which is formed by the sequential condensation of three molecules of malonyl-CoA and one molecule of benzoyl-CoA in a reaction catalyzed by biphenyl synthase (BIS). This 3,5-dihydroxybiphenyl then undergoes sequential 5-O-methylation, 4-hydroxylation, and finally 3-O-methylation to form aucuparin. A cDNA encoding O-methyltransferase (OMT) was isolated and functionally characterized from the cell cultures of scab-resistant apple cultivar 'Florina' (Malus domestica cultivar 'Florina'; MdOMT) after treatment with elicitor prepared from the apple scab causing fungus Venturia inaequalis. MdOMT catalyzed the regiospecific O-methylation of 3,5-dihydroxybiphenyl at the 5-position to form 3-hydroxy-5-methoxybiphenyl. The enzyme showed absolute substrate preference for 3,5-dihydroxybiphenyl. The elicitor-treated apple cell cultures showed transient increases in the MdOMT (GenBank ID MF740747) and MdBIS3 (GenBank ID JQ390523) transcript levels followed by the accumulation of biphenyls (aucuparin and noraucuparin) and dibenzofuran (eriobofuran) phytoalexins. MdOMT fused with N- and C-terminal yellow fluorescent protein showed cytoplasmic localization in the epidermis of Nicotiana benthamiana leaves. In scab inoculated greenhouse-grown 'Florina' plants, the expression of MdOMT was transiently induced in the stem followed by the accumulation of biphenyl phytoalexins.

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.

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

Salicylic acid (SA) is known to trigger a number of plant defense responses upon pathogen attack. It is well known that apple (Malus domestica) plants respond to pathogen invasion by synthesizing SA, but its biosynthesis is not well understood. In this study, we report salicylaldehyde synthase (SAS) activity from Venturia inaequalis elicitor (VIE)-treated cell suspension cultures of apple (Malus domestica 'Florina'). SAS catalyzes non-oxidative C₂-side chain cleavage of 2-coumaric acid to form salicylaldehyde (SALD) in the presence of a reducing agent such as cysteine. The side chain cleavage mechanism was found to be very similar to that of salicylaldehyde synthase activity from tobacco and 4-hydroxybenzaldehyde synthase activity from Vanilla planifolia and Daucus carota. A basal SAS activity was observed in the non-elicited cell cultures, and a 7-fold increase in SAS activity was observed upon elicitation. In parallel to SAS activity, the level of total SA accumulation increased by 5.6-fold after elicitation compared to the untreated control cells. Elicitor treatment further resulted in an 8.7-fold increase in the activity of the phenylalanine ammonia-lyase (PAL) enzyme that preceded the peak of SAS activity and total SA accumulation, suggesting the involvement of the phenylpropanoid pathway in SA metabolism. The preferred substrate for SAS was 2-coumaric acid (Km = 0.35 mM), with cysteine being the preferred reducing agent. In addition, a 1.8-fold enhancement in the SA content and 0.7-fold enhancement in the SALD content was observed when elicited cell cultures were fed with 2-coumaric acid. These observations suggest the involvement of SAS in SALD biosynthesis.