Phytochemistry Reviews (v.17, #6)

Allelopathy in tropical and subtropical species by Joey K. Ooka; Daniel K. Owens (1225-1237).
The ability of certain plants to synthesize allelochemicals that disrupt the germination, development, reproduction and/or survival of organisms that compete with them for resources has been observed in a variety of environments worldwide. Tropical and subtropical regions are particularly conducive to the evolution of allelopathic survival strategies as the relatively constant temperatures and mild frost-free winters produce a hospitable year-round growing season. This allows for the proliferation of a large variety of species and leads to fierce competition for sunlight, nutrients, water and other resources. Allelopathy provides an advantage to invasive species allowing for increased competitiveness and fitness over native and agricultural species in a variety of different habitats. Herein, the diversity and known action mechanisms of allelopathic compounds with a focus on tropical and subtropical communities is reviewed. Furthermore, the current and future prospect of utilizing and developing these allelopathic chemicals as weed control options is discussed.
Keywords: Allelochemicals; Herbicide; Invasive species; Secondary metabolite; Weed control

At least twelve plant families contain species that synthesize cardiac glycosides as defense against herbivory. These inhibitors of animal Na+, K+-ATPases also have medical uses in treating congestive heart failure and other diseases. However, despite extensive ecological research and centuries of use in both traditional and modern medicine, the complete cardiac glycoside biosynthesis pathway has yet to be elucidated in any plant species. To a large extent, this research deficit results from the fact that cardiac glycosides are produced exclusively by non-model plant species such as Digitalis that have not been amenable to the development of mutagenesis, cloning, and genetic mapping approaches. Recent advances in genome sequencing, transcript profiling, plant transformation, transient expression assays, and plant metabolite analysis have provided new opportunities for the investigation and elucidation of cardiac glycoside biosynthesis pathways. The genetic tools that have been developed for Brassicaceae, in particular Arabidopsis thaliana, may be directly applicable to Erysimum, a Brassicaceae genus that characteristically produces cardiac glycosides as defensive metabolites. We propose that Erysimum cheiranthoides (wormseed wallflower), a rapid-cycling, self-pollinating species with a relatively small, diploid genome, would be a suitable model system to advance research on the biosynthesis of cardiac glycosides in plants.
Keywords: Erysimum cheiranthoides ; Wallflower; Cardiac glycoside; Cardenolide; Model system

Dissecting metabolic flux in C4 plants: experimental and theoretical approaches by Mohammad Mazharul Islam; Adil Al-Siyabi; Rajib Saha; Toshihiro Obata (1253-1274).
C4 photosynthesis is the carbon fixation pathway in specific plant species, so called C4 plants including maize, sorghum and sugarcane. It is characterized by the carboxylation reaction that forms four-carbon (C4) molecules, which are then used to transport CO2 to the proximity of RubisCO in the bundle sheath cells. Since C4 photosynthesis confers high photosynthetic as well as water and nitrogen use efficiency on plants, worldwide efforts have been made to understand the mechanisms of C4 photosynthesis and to properly introduce the pathway into C3 crops. Metabolic flux analysis (MFA) is a research field trying to analyze the metabolic pathway structure and activity (i.e., flux) in vivo. Constraint-based reconstruction and analysis tools theoretically study the distribution of metabolic flux in genome-scale network-based models. Different types of MFA and model-based analyses have been contributing to the discovery of C4 photosynthetic pathways and to analyze its operation in C4 plant species. This article reviews the studies to dissect the operation of C4 photosynthesis and adjacent pathways, from the pioneer studies using radioisotope-based MFA to the recent stable isotope-based MFA and the model-based approaches. These studies indicate complex interconnections among metabolic pathways and the importance of the integration of experimental and theoretical approaches. Perspectives on the integrative approach and major obstacles are also discussed.
Keywords: C4 photosynthesis; Constraint-based reconstruction and analysis; Genome scale metabolic model; Isotope labeling; Metabolic flux analysis

Cuticular waxes coat aerial plant surfaces to protect tissues against biotic and abiotic stress. The waxes are complex mixtures of fatty-acid-derived lipids formed on modular biosynthetic pathways, with varying chain lengths and oxygen functional groups. The waxes of most plant species contain C26–C32 alcohols, aldehydes, alkanes, and fatty acids together with their alkyl esters, and comparisons between diverse wax mixtures have revealed matching chain length distributions between some of these compound classes. Based on such patterns, the biosynthetic pathways leading to the ubiquitous wax constituents were hypothesized early on, and most of these pathway hypotheses have since been confirmed by biochemical and molecular genetic studies in model species. However, the most abundant wax compounds on many species, including many important crop species, contain secondary functional groups and thus their biosynthesis differs at least in part from the ubiquitous wax compounds with which they co-occur. Here, we survey the chemical structures of these species-specific specialty wax compounds based on a comprehensive CAS SciFinder search and then review relevant reports on wax compositions to help develop and refine hypotheses for their biosynthesis. Across the plant kingdom, specialty wax compounds with one, two, and three secondary functional groups have been identified, with most studies focusing on Angiosperms. Where multiple specialty wax compounds were reported, they frequently occurred as homologous series and/or mixtures of isomers. Among these, it is now possible to recognize series of homologs with predominantly odd- or even-numbered chain lengths, and mixtures of isomers with functional groups on adjacent or on alternating carbon atoms. Using these characteristic molecular geometries of the co-occurring specialty compounds, they can be categorized and, based on the common structural patterns, mechanisms of biosynthesis may be predicted. It seems highly likely that mixtures of isomers with secondary functions on adjacent carbons arise from oxidation catalyzed by P450 enzymes, while mixtures of isomers with alternating group positions are formed by malonate condensation reactions mediated by polyketide synthase or ketoacyl-CoA synthase enzymes, or else by the head-to-head condensation of long-chain acyls. Though it is possible that some enzymes leading to ubiquitous compounds also participate in specialty wax compound biosynthesis, comparisons between co-occurring ubiquitous and specialty wax compounds strongly suggest that, at least in some species, dedicated specialty wax compound machinery exists. This seems particularly true for the diverse species in which specialty wax compounds, most notably nonacosan-10-ol, hentriacontan-16-one (palmitone), and very-long-chain β-diketones, accumulate to high concentrations.
Keywords: Cuticular wax; Wax biosynthesis; Very-long-chain; Polyketides; P450; Fatty acid elongation; Nonacosan-10-ol; Palmitone; β-diketone

Lignin modification in planta for valorization by Toshiaki Umezawa (1305-1327).
Lignocellulose polysaccharides are encrusted by lignin, which has long been considered an obstacle for efficient use of polysaccharides during processes such as pulping and bioethanol fermentation. Hence, numerous transgenic plant lines with reduced lignin contents have been generated, leading to more efficient enzymatic saccharification and forage digestion. However, lignin is also a potential feedstock for aromatic products and an important direct-combustion fuel, or a by-product fuel in polysaccharide utilization such as pulping and bioethanol production. For aromatic feedstock production, the complicated structure of lignin along with its occlusion within polysaccharide matrices makes lignin utilization intractable. To alleviate these difficulties, simplification of the lignin structure is an important breeding objective for future high-value utilization of lignin. In addition, higher lignin contents are beneficial for increasing heating values of lignocellulose, because lignin has much larger heating values than polysaccharides, cellulose and hemicelluloses. Structural modification of lignin may also be effective in increasing heating values of lignocellulose biomass, because the heating value of p-hydroxyphenyl lignin is highest, followed by those of guaiacyl lignin and of syringyl lignin in this order. Herein, recent developments for augmenting lignin contents and for lignin structural modifications, to improve its utilization by metabolic engineering, are outlined.
Keywords: Lignin utilization; Up-regulation; Augmentation; Structural simplification; Grass biomass

Engineering plants for tomorrow: how high-throughput phenotyping is contributing to the development of better crops by Zachary C. Campbell; Lucia M. Acosta-Gamboa; Nirman Nepal; Argelia Lorence (1329-1343).
High-throughput plant phenotyping has been advancing at an accelerated rate as a response to the need to fill the gap between genomic information and the plasticity of the plant phenome. During the past decade, North America has seen a stark increase in the number of phenotyping facilities, and these groups are actively contributing to the generation of high-dimensional, richly informative datasets about the phenotype of model and crop plants. As both phenomic datasets and analysis tools are made publicly available, the key to engineering more resilient crops to meet global demand is closer than ever. However, there are a number of bottlenecks that must yet be overcome before this can be achieved. In this paper, we present an overview of the most commonly used sensors that empower digital phenotyping and the information they provide. We also describe modern approaches to identify and characterize plants that are resilient to common abiotic and biotic stresses that limit growth and yield of crops. Of interest to researchers working in plant biochemistry, we also include a section discussing the potential of these high-throughput approaches in linking phenotypic data with chemical composition data. We conclude by discussing the main bottlenecks that still remain in the field and the importance of multidisciplinary teams and collaboration to overcome those challenges.
Keywords: High-throughput plant phenotyping; Plant phenotypes; Phenomes; Phenomics; Abiotic stress tolerance