Phytochemistry Reviews (v.17, #1)

Procyanidins: a comprehensive review encompassing structure elucidation via mass spectrometry by Emily A. Rue; Michael D. Rush; Richard B. van Breemen (1-16).
Procyanidins are polyphenols abundant in dietary fruits, vegetables, nuts, legumes, and grains with a variety of chemopreventive biological effects. Rapid structure determination of these compounds is needed, notably for the more complex polymeric procyanidins. We review the recent developments in the structure elucidation of procyanidins with a focus on mass spectrometric approaches, especially liquid chromatography-tandem mass spectrometry (LC–MS/MS) and matrix-assisted laser desorption ionization (MALDI) MS/MS.
Keywords: Procyanidins; Oligomeric; Polymeric; Structure elucidation

Plants have the capacity to produce a staggering array of chemically diverse low molecular weight compounds called specialized metabolites. Though they are non-essential for basic cell activities, these molecules are characterized by their role as integral enhancers of plant fitness, and have distinct biological functions including defense against herbivory, immunity, pollinator attraction, molecular signaling, and abiotic stress tolerance. The chemicals are of particular interest because of their pharmacological, industrial, and agricultural usefulness. The inter- or intraspecies variation in the production of specialized metabolites has been widely observed and found to be largely genetically controlled. The natural genetic variation can be used to help identify biosynthetic and regulatory genes, elucidate mechanistic properties of gene function and gene regulation, and explore evolutionary and ecological questions. Recent advances in sequencing and data mining technologies have facilitated the integration of population genetics tools such as quantitative trait loci (QTL) mapping and genome wide association (GWA) with metabolite and/or gene expression profiling to exploit the natural variation for making new discoveries in the model plant Arabidopsis thaliana, as well as in more agriculturally relevant species. Here we highlight key discoveries that were catalyzed by taking advantage of naturally occurring variation, and comment on technologies and resources employed by this approach, in hopes of providing phytochemists an archetype for harnessing the power of natural variation to accelerate discoveries in plant specialized metabolism.
Keywords: Natural variation based discovery; Metabolite-gene association; Biosynthesis and regulation of specialized metabolites; Structure–function relationship; Ecology and evolution

The effects of climate change associated abiotic stresses on maize phytochemical defenses by Martha M. Vaughan; Anna Block; Shawn A. Christensen; Leon Hartwell Allen; Eric A. Schmelz (37-49).
Reliable large-scale maize production is an essential component of global food security; however, sustained efforts are needed to ensure optimized resilience under diverse crop stress conditions. Climate changes are expected to increase the frequency and intensity of both abiotic and biotic stress. Protective phytochemicals play an important role in both abiotic stress resilience and resistance to biotic challenges, yet the concentration and composition of these phytochemicals are also dependent on climate variables. We review the research on the effects of climate change associated abiotic stresses on three classes of maize defense metabolites, including benzoxazinoids, volatile organic compounds, and terpenoid phytoalexins. Despite significant knowledge gaps that still exist, it is evident that climate change will influence maize phytochemicals associated with resilient productivity. While broad generalizations are not yet possible, climate induced changes in phytochemicals are context specific and dependent upon developmental stage and tissue type. Under conditions of drought, maize modulates different classes of defense phytochemicals to protect the above-and belowground tissues. Aboveground the benzoxazinoid defenses are stimulated, but belowground terpenoid phytoalexins are predominantly deployed. Changes in the allocation or distribution of the different classes of defense metabolites or signaling molecules have the potential to further shape the biodiversity and abundance of pests within the maize agroecosystem. A better understanding of the underlying genetics, biosynthetic pathways, regulation and precise biological roles of maize phytochemicals modulated by arrays of climatic conditions will be required to ensure optimal plant resilience and productivity in the face of combined biotic and abiotic stresses.
Keywords: Maize; Abiotic and biotic stress; Benzoxazinoids; Volatile organic compounds; Terpenoid phytoalexins; Climate resilience

Despite being the major dietary source for countless insects, plants have not only survived but thrived on earth and represent arguably the largest terrestrial biomass. This is because plants, instead of falling passive victims to the insects, have come to possess numerous defense strategies including production of insect poisons, deterrents, and other anti-nutritive compounds. A significant part of these built-in defenses are inducible and tactfully coordinated with plant growth in a manner that maximizes return on investment. The lipid-derived signal jasmonate (JA) orchestrates many of these inducible defense responses. JA has a similar evolutionary origin as eicosanoids which play critical roles in mammalian wound and inflammatory responses indicating cross-kingdom adoption of lipid-derived signals for use against tissue injuries. The molecular perception and signaling mechanism of JA indicates that the intracellular concentration of a derivative, jasmonoyl-l-isoleucine (JA-Ile), is the major determinant factor for the dynamic regulation of the JA signaling system. Interdisciplinary methods including recombinant enzyme assays, mass spectrometry based hormone profiling, genetics, genomics, and the model plant Arabidopsis thaliana have assisted in elucidating metabolic pathways controlling JA-Ile homeostasis. Along with the relatively well established biosynthetic enzymes, more recently discovered catabolic pathways as well as areas that need new discoveries are discussed herein. Knowledge obtained from the JA-dependent stress adaptive responses are expected to have agricultural and industrial impact in the future toward the development of environmentally friendly ways of managing pests and tapping into a largely unexplored treasure trove of plant-derived specialized metabolites for human use.
Keywords: Hormone metabolism; Jasmonate; Oxylipin; Plant signaling; Wound response

Catalyzing stereo- and regio-specific oxidation of inert hydrocarbon backbones, and a range of more exotic reactions inherently difficult in formal chemical synthesis, cytochromes P450 (P450s) offer outstanding potential for biotechnological engineering. Plants and their dazzling diversity of specialized metabolites have emerged as rich repository for functional P450s with the advances of deep transcriptomics and genome wide discovery. P450s are of outstanding interest for understanding chemical diversification throughout evolution, for gaining mechanistic insights through the study of their structure–function relationship, and for exploitation in Synthetic Biology. In this review, we highlight recent developments and examples in the discovery of plant P450s involved in the biosynthesis of industrially relevant monoterpenoids, sesquiterpenoids, diterpenoids and triterpenoids, throughout 2016 and early 2017. Examples were selected to illustrate the spectrum of value from commodity chemicals, flavor and fragrance compounds to pharmacologically active terpenoids. We focus on a recently emerging theme, where P450s control metabolic bifurcations and chemical diversity of the final product profile, either within a pathway, or through neo-functionalization in related species. The implications may inform approaches for rational assembly of recombinant pathways, biotechnological production of high value terpenoids and generation of novel chemical entities.
Keywords: Terpenoid specialized metabolites; Pathway bifurcation; Regio-specificity; Stereo-specificity; Promiscuity; Orthologs; Biosynthetic; Metabolic diversity; Synthetic Biology; Metabolic engineering; Biotechnology

Plant diterpenoids encompass a diverse group of more than ten thousand specialized (traditionally termed ‘secondary’) metabolites with significant ecological functions and industrial uses. Bioactive diterpenoids form an important source of bio-based pharmaceuticals, as exemplified by the approved anti-cancer drugs paclitaxel and ingenol mebutate. Advanced genomics, metabolomics and enzyme discovery technologies have spawned a new era of exploring traditional medicinal plants for novel or improved therapeutics. Across the plant kingdom numerous diterpene synthase and cytochrome P450 enzymes with key roles in generating diterpenoid chemical diversity have been identified in recent years. This catalog of enzyme catalysts and a deeper knowledge of specialized diterpenoid metabolism can now be applied to modern microbial and photosynthetic production systems, offering alternative avenues for the sustainable manufacture of plant-based medicines important to humanity.
Keywords: Diterpenoids; Plant specialized metabolism; Biopharmaceuticals; Plant natural products; Diterpene synthases

Aromatic amino acid aminotransferases in plants by Minmin Wang; Hiroshi A. Maeda (131-159).
Aromatic amino acid aminotransferases (AAA-ATs) catalyze the reversible transamination reactions of proteinogenic and non-proteinogenic aromatic amino acids to corresponding keto acids and vice versa. The products of plant AAA-ATs serve as key precursors of many primary and secondary metabolites that are crucial for both plant and human metabolism and physiology. In most microbes, l-tyrosine (Tyr) and l-phenylalanine (Phe) aminotransferases (Tyr and Phe-ATs) catalyze the final steps of Phe and Tyr biosynthesis. On the other hand, plants use different pathways to synthesize Tyr and Phe via arogenate, in which prephenate-specific aminotransferases (PPA-ATs) catalyze the committed step in the plastids. Plant Tyr and Phe-ATs, unlike microbial counterparts, often prefer the reverse reactions and metabolize Tyr and Phe to their respective aromatic keto acids, which serve as precursors of various plant natural products (e.g. benzenoid volatiles, tocochromanols, plastoquinone, and tropane and benzylisoquinoline alkaloids). Unlike plastidic PPA-ATs, plant Tyr/Phe-ATs are localized outside of the plastids, have broad substrate specificity, and interlink Tyr and Phe metabolism. l-Tryptophan (Trp) aminotransferases (Trp-ATs) are involved in biosynthesis of the plant hormone auxin. Although significant advancement has been made on biochemical, molecular, and genetic characterizations of plant AAA-ATs, there are still many critical knowledge gaps, which are highlighted in the current review.
Keywords: Aromatic amino acids; Aminotransferase; Transaminase; Amino acid biosynthesis; Plant natural products; Auxin biosynthesis

Centella asiatica: phytochemistry and mechanisms of neuroprotection and cognitive enhancement by Nora E. Gray; Armando Alcazar Magana; Parnian Lak; Kirsten M. Wright; Joseph Quinn; Jan F. Stevens; Claudia S. Maier; Amala Soumyanath (161-194).
This review describes in detail the phytochemistry and neurological effects of the medicinal herb Centella asiatica (L.) Urban. C. asiatica is a small perennial plant that grows in moist, tropical and sub-tropical regions throughout the world. Phytochemicals identified from C. asiatica to date include isoprenoids (sesquiterpenes, plant sterols, pentacyclic triterpenoids and saponins) and phenylpropanoid derivatives (eugenol derivatives, caffeoylquinic acids, and flavonoids). Contemporary methods for fingerprinting and characterization of compounds in C. asiatica extracts include liquid chromatography and/or ion mobility spectrometry in conjunction with high-resolution mass spectrometry. Multiple studies in rodent models, and a limited number of human studies, support C. asiatica’s traditional reputation as a cognitive enhancer, as well as its anxiolytic and anticonvulsant effects. Neuroprotective effects of C. asiatica are seen in several in vitro models, for example against beta amyloid toxicity, and appear to be associated with increased mitochondrial activity, improved antioxidant status, and/or inhibition of the pro-inflammatory enzyme, phospholipase A2. Neurotropic effects of C. asiatica include increased dendritic arborization and synaptogenesis, and may be due to modulations of signal transduction pathways such as ERK1/2 and Akt. Many of these neurotropic and neuroprotective properties of C. asiatica have been associated with the triterpene compounds asiatic acid, asiaticoside and madecassoside. More recently, caffeoylquinic acids are emerging as a second important group of active compounds in C. asiatica, with the potential of enhancing the Nrf2-antioxidant response pathway. The absorption, distribution, metabolism and excretion of the triterpenes, caffeoylquinic acids and flavonoids found in C. asiatica have been studied in humans and animal models, and the compounds or their metabolites found in the brain. This review highlights the remarkable potential for C. asiatica extracts and derivatives to be used in the treatment of neurological conditions, and considers the further research needed to actualize this possibility.
Keywords: Centella asiatica ; Gotu kola; Brahmi; Phytochemistry; Neuroprotection; Cognitive enhancement; Caffeoylquinic acids; Triterpenes; Flavonoids; Liquid chromatography; High-resolution mass spectrometry