Ancient Grass Genomes Trace the Origins of Starch and Lignin Pathways

Metabolism is often drawn as a static map, a fixed network of reactions inherited intact from some deep ancestral cell. In reality, metabolic networks are historical documents. They have been rewired, duplicated, and repurposed across hundreds of millions of years, and the evidence of that history is written into the genomes of living organisms. A new analysis highlighted in Nature Plants by Esther A. Harding, Alisdair R. Fernie, and Mustafa Bulut argues that some of the most revealing pages of that document can be read at evolutionary branchpoints, the moments when major lineages split and their biochemical fates began to diverge. By sampling genomes around decisive branches in the evolution of grasses, the underlying study, published in Science by Takeda-Kimura and colleagues, pinpoints when two of the most consequential metabolic systems in the plant kingdom, distinct starch and lignin pathways, first came into being.

The strategic logic behind the approach is what makes it compelling. Rather than surveying genomes uniformly across the plant tree of life, the researchers concentrated their sampling on lineages that sit immediately on either side of key evolutionary forks in grass history. This technique, sometimes described as phylogenetic bracketing, allows investigators to infer the timing of a biochemical innovation by asking a simple comparative question: which organisms on one side of the branch possess the pathway, and which on the other side lack it? If a complete set of pathway genes appears in every descendant of one branch but in none of the closest relatives on the other, the origin of that pathway can be placed, with considerable confidence, in the interval between the two divergence events.

Applied to grasses, this logic yielded a dated origin for distinct starch metabolism. Starch is the dominant storage carbohydrate in plants and the principal caloric source for humanity, yet not all starch is built the same way. Grasses, which include the cereal crops that feed the bulk of the world’s population, assemble their storage and transitory starch with a characteristic suite of enzymes that differs in its composition and regulation from the starch systems of many other flowering plants. By mapping the presence of these enzyme sets onto the grass phylogeny, the study could show that the distinctive grass starch pathway arose in the ancestor of a specific branch of the grass family tree, and not earlier. The innovation, in other words, is not an ancient inheritance from the first flowering plants but a later acquisition that coincided with the diversification of the lineage that would eventually give rise to wheat, rice, maize, and their relatives.

The second gateway the study illuminated concerns lignin, the phenolic polymer that stiffens plant cell walls and enabled land plants to stand upright, conduct water under tension, and colonize dry terrestrial environments. Lignin is the product of the phenylpropanoid pathway, a metabolic assembly line that begins with the deamination of the aromatic amino acid phenylalanine by the enzyme phenylalanine ammonia lyase, universally abbreviated PAL. This reaction strips the amino group from phenylalanine and commits the carbon skeleton to the phenylpropanoid pipeline, from which lignin monomers, flavonoids, and thousands of specialized metabolites flow. PAL is therefore the gatekeeper of the entire pathway, the enzyme whose substrate preference determines which amino acids can enter the aromatic metabolism of the cell.

Here the study made its most mechanistically striking discovery. The genomic neighbourhood of ancestral PAL duplicates, examined across the sampled grass lineages, revealed that the PAL gene family had undergone duplication events whose timing could be anchored to the same phylogenetic framework used for the starch analysis. Duplicate genes are the raw material of metabolic evolution, because one copy can preserve the original function while the other accumulates mutations and explores new chemistry. But whether a duplicate actually acquires a new function depends on the specific amino acid changes that alter the shape and chemistry of the enzyme’s active site, and demonstrating this requires more than sequence comparison alone.

To close that gap, the researchers combined their genomic survey with structural and biochemical analysis of the ancestral and derived PAL enzymes. Structural modelling of the enzyme, informed by known PAL architectures, identified candidate positions where substitutions could plausibly change substrate handling. Biochemical characterization of the reconstructed enzymes then tested those predictions directly, measuring what each enzyme version could and could not do. The result was remarkably clean: two amino acid substitutions were sufficient to convert the substrate preference of the enzyme, opening a route by which tyrosine, rather than exclusively phenylalanine, could feed into phenylpropanoid metabolism.

This finding matters because the two aromatic amino acids are not interchangeable feedstocks. Phenylalanine and tyrosine sit at adjacent positions in the aromatic amino acid biosynthetic pathway, but they carry different side chains, and an ammonia lyase that accepts tyrosine produces a different immediate product, one that can be channeled toward particular classes of phenolic compounds. A tyrosine route into phenylpropanoid metabolism therefore represents a genuine expansion of the metabolic gateway, a new door into the pathway that did not exist in ancestors whose PAL enzymes were restricted to phenylalanine. In grasses, this expanded access has implications for the composition of lignin itself, since grass lignins are notable for incorporating substantial quantities of phenolic constituents derived through routes that other plants use only sparingly.

The evolutionary narrative that emerges from combining these strands is one of coordinated innovation. At a decisive branchpoint in grass ancestry, the lineage did not merely accumulate random sequence drift. It acquired a new starch metabolism with its own enzyme configuration, and it acquired an expanded aromatic gateway through PAL duplication and functional divergence. Both innovations would have reshaped the physiology of the plants that carried them. A distinctive starch metabolism alters how carbon is stored and mobilized across the day-night cycle and during development, while an enlarged phenylpropanoid input changes the raw material available for cell wall reinforcement and chemical defense. Together, these changes could have contributed to the ecological success of the grass lineage, which today covers roughly a third of the terrestrial land surface and supplies the majority of human caloric intake.

Methodologically, the study exemplifies a broader shift in how evolutionary metabolism is investigated. The classical approach, exemplified by landmark work on benzoxazinoid biosynthesis in grasses and by decades of comparative enzymology, identified pathways one at a time through careful biochemistry. The new approach inverts the logic: it starts from a well-resolved phylogeny, samples genomes strategically at branchpoints, identifies gene families whose duplication history coincides with metabolic transitions, and then moves into the laboratory to reconstruct and test the ancestral enzymes. Each step constrains the others. Phylogeny tells biochemistry where to look, and biochemistry confirms or refutes what the phylogeny suggests. The identification of two substitutions that opened the tyrosine route is a case study in how this iterative loop can convert a comparative pattern into a causal molecular mechanism.

The implications extend beyond evolutionary theory. Knowing when and how grasses acquired their distinctive starch and lignin systems provides a rational framework for crop improvement, because it identifies the enzymes and substitutions that define the grass-specific versions of these pathways. Engineers seeking to modify lignin for more efficient biofuel production, or to alter starch quality for food and industrial uses, can now consult an evolutionary record that indicates which enzyme versions are ancestral, which are derived, and which specific amino acid changes produced new functions. The branchpoints of grass evolution, once visible only as splits in a species tree, have thus become annotated waypoints on a metabolic map, marking the precise moments when the gateways of plant chemistry were opened, and pointing the way toward deliberate redesign of the pathways that feed and shelter the modern world.

Subject of Research: Evolutionary origins of grass starch and lignin metabolic pathways revealed through phylogenetic genome sampling and ancestral phenylalanine ammonia lyase enzyme analysis

Article Title: Branchpoints reveal metabolic gateways

Article References: Harding, E. A., Fernie, A. R., & Bulut, M. (2026). Branchpoints reveal metabolic gateways. Nature Plants. https://doi.org/10.1038/s41477-026-02434-1

Image Credits: AI Generated

DOI: 10.1038/s41477-026-02434-1

Keywords: grass evolution, starch metabolism, lignin, phenylpropanoid pathway, phenylalanine ammonia lyase, gene duplication, tyrosine, phylogenetic bracketing, plant metabolism, genome evolution, ancestral enzyme reconstruction, cell wall biosynthesis