The Genetics of Plant Metabolism.

Plant metabolic studies have traditionally focused on the role and regulation of the enzymes catalyzing key reactions within specific pathways. Within the past 20 years, reverse genetic approaches have allowed direct determination of the effects of the deficiency, or surplus, of a given protein on the biochemistry of a plant. In parallel, top-down approaches have also been taken, which rely on screening broad, natural genetic diversity for metabolic diversity. Here, we compare and contrast the various strategies that have been adopted to enhance our understanding of the natural diversity of metabolism. We also detail how these approaches have enhanced our understanding of both specific and global aspects of the genetic regulation of metabolism. Finally, we discuss how such approaches are providing important insights into the evolution of plant secondary metabolism.

The Semi-Enzymatic Origin of Metabolic Pathways: Inferring a Very Early Stage of the Evolution of Life.

The early evolution of life is a period with many important events with a lot of big and open questions. One of them is the evolution of metabolic pathways, which means the origin and assembly of enzymes that act together. The retrograde hypothesis was the first attempt to explain the origin and evolutionary history of metabolic pathways; Norman Horowitz developed this first significant hypothesis. This idea was followed by relevant proposals developed by Sam Granick, who proposed the "forward direction hypothesis," and then the successful idea of "Patchwork" assembly proposed independently by Martynas Ycas and Roy Jensen. Since then, a few new hypotheses were proposed; one of the most influential was made by Antonio Lazcano and Stanley Miller in the Journal Molecular Evolution, the "semi-enzymatic origin" of metabolic pathways. This article was cited more than 160 times, including in most papers published about the early evolution of metabolism, placing it as influential work in the field. The ideas proposed in this work and their effects on studying the origin and early evolution of life are analyzed.

The Coevolution of Cellularity and Metabolism Following the Origin of Life.

The emergence of cellular organisms occurred sometime between the origin of life and the evolution of the last universal common ancestor and represents one of the major transitions in evolutionary history. Here we describe a series of artificial life simulations that reveal a close relationship between the evolution of cellularity, the evolution of metabolism, and the richness of the environment. When environments are rich in processing energy, a resource that the digital organisms require to both process their genomes and replicate, populations evolve toward a state of non-cellularity. But when processing energy is not readily available in the environment and organisms must produce their own processing energy from food puzzles, populations always evolve both a proficient metabolism and a high level of cellular impermeability. Even between these two environmental extremes, the population-averaged values of cellular impermeability and metabolic proficiency exhibit a very strong correlation with one another. Further investigations show that non-cellularity is selectively advantageous when environmental processing energy is abundant because it allows organisms to access the available energy, while cellularity is selectively advantageous when environmental processing energy is scarce because it affords organisms the genetic fidelity required to incrementally evolve efficient metabolisms. The selection pressures favoring either non-cellularity or cellularity can be reversed when the environment transitions from one of abundant processing energy to one of scarce processing energy. These results have important implications for when and why cellular organisms evolved following the origin of life.

Evolutionary History of Plant Metabolism.

Tremendous chemical diversity is the hallmark of plants and is supported by highly complex biochemical machinery. Plant metabolic enzymes originated and were transferred from eukaryotic and prokaryotic ancestors and further diversified by the unprecedented rates of gene duplication and functionalization experienced in land plants. Unlike microbes, which have frequent horizontal gene transfer events and multiple inputs of energy and organic carbon, land plants predominantly rely on organic carbon generated from CO2 and have experienced very few, if any, gene transfers during their recent evolutionary history. As such, plant metabolic networks have evolved in a stepwise manner and on existing networks under various evolutionary constraints. This review aims to take a broader view of plant metabolic evolution and lay a framework to further explore evolutionary mechanisms of the complex metabolic network. Understanding the underlying metabolic and genetic constraints is also an empirical prerequisite for rational engineering and redesigning of plant metabolic pathways.

The Structure and Function of Major Plant Metabolite Modifications.

Plants produce a myriad of structurally and functionally diverse metabolites that play many different roles in plant growth and development and in plant response to continually changing environmental conditions as well as abiotic and biotic stresses. This metabolic diversity is, to a large extent, due to chemical modification of the basic skeletons of metabolites. Here, we review the major known plant metabolite modifications and summarize the progress that has been achieved and the challenges we are facing in the field. We focus on discussing both technical and functional aspects in studying the influences that various modifications have on biosynthesis, degradation, transport, and storage of metabolites, as well as their bioactivity and toxicity. Finally, we discuss some emerging insights into the evolution of metabolic pathways and metabolite functionality.

Interrogating metabolism as an electron flow system.

Metabolism is generally considered as a neatly organised system of modular pathways, shaped by evolution under selection for optimal cellular growth. This view falls short of explaining and predicting a number of key observations about the structure and dynamics of metabolism. We highlight these limitations of a pathway-centric view on metabolism and summarise studies suggesting how these could be overcome by viewing metabolism as a thermodynamically and kinetically constrained, dynamical flow system. Such a systems-level, first-principles based view of metabolism can open up new avenues of metabolic engineering and cures for metabolic diseases and allow better insights to a myriad of physiological processes that are ultimately linked to metabolism. Towards further developing this view, we call for a closer interaction among physical and biological disciplines and an increased use of electrochemical and biophysical approaches to interrogate cellular metabolism together with the microenvironment in which it exists.

Exploring the Diversity of Plant Metabolism.

Plants produce a huge array of metabolites, far more than those produced by most other organisms. Unraveling this diversity and its underlying genetic variation has attracted increasing research attention. Post-genomic profiling platforms have enabled the marriage and mining of the enormous amount of phenotypic and genetic diversity. We review here achievements to date and challenges remaining that are associated with plant metabolic research using multi-omic strategies. We focus mainly on strategies adopted in investigating the diversity of plant metabolism and its underlying features. Recent advances in linking metabotypes with phenotypic and genotypic traits are also discussed. Taken together, we conclude that exploring the diversity of metabolism could provide new insights into plant evolution and domestication.

Conditional iron and pH-dependent activity of a non-enzymatic glycolysis and pentose phosphate pathway.

Little is known about the evolutionary origins of metabolism. However, key biochemical reactions of glycolysis and the pentose phosphate pathway (PPP), ancient metabolic pathways central to the metabolic network, have non-enzymatic pendants that occur in a prebiotically plausible reaction milieu reconstituted to contain Archean sediment metal components. These non-enzymatic reactions could have given rise to the origin of glycolysis and the PPP during early evolution. Using nuclear magnetic resonance spectroscopy and high-content metabolomics that allowed us to measure several thousand reaction mixtures, we experimentally address the chemical logic of a metabolism-like network constituted from these non-enzymatic reactions. Fe(II), the dominant transition metal component of Archean oceanic sediments, has binding affinity toward metabolic sugar phosphates and drives metabolism-like reactivity acting as both catalyst and cosubstrate. Iron and pH dependencies determine a metabolism-like network topology and comediate reaction rates over several orders of magnitude so that the network adopts conditional activity. Alkaline pH triggered the activity of the non-enzymatic PPP pendant, whereas gentle acidic or neutral conditions favored non-enzymatic glycolytic reactions. Fe(II)-sensitive glycolytic and PPP-like reactions thus form a chemical network mimicking structural features of extant carbon metabolism, including topology, pH dependency, and conditional reactivity. Chemical networks that obtain structure and catalysis on the basis of transition metals found in Archean sediments are hence plausible direct precursors of cellular metabolic networks.

An evolutionary perspective on the Crabtree effect.

The capability to ferment sugars into ethanol is a key metabolic trait of yeasts. Crabtree-positive yeasts use fermentation even in the presence of oxygen, where they could, in principle, rely on the respiration pathway. This is surprising because fermentation has a much lower ATP yield than respiration (2 ATP vs. approximately 18 ATP per glucose). While genetic events in the evolution of the Crabtree effect have been identified, the selective advantages provided by this trait remain controversial. In this review we analyse explanations for the emergence of the Crabtree effect from an evolutionary and game-theoretical perspective. We argue that an increased rate of ATP production is likely the most important factor behind the emergence of the Crabtree effect.

Chemical and genomic evolution of enzyme-catalyzed reaction networks.

There is a tendency that a unit of enzyme genes in an operon-like structure in the prokaryotic genome encodes enzymes that catalyze a series of consecutive reactions in a metabolic pathway. Our recent analysis shows that this and other genomic units correspond to chemical units reflecting chemical logic of organic reactions. From all known metabolic pathways in the KEGG database we identified chemical units, called reaction modules, as the conserved sequences of chemical structure transformation patterns of small molecules. The extracted patterns suggest co-evolution of genomic units and chemical units. While the core of the metabolic network may have evolved with mechanisms involving individual enzymes and reactions, its extension may have been driven by modular units of enzymes and reactions.