How Grasses Supercharged Their Starch and Biomass Production

From towering stalks of maize to floods of rice paddies and waves of golden-colored wheat, grass plants cover ecosystems around the world and provide more than 40 percent of the calories people consume.¹ Researchers have long wondered how grasses became such successful plants and outcompeted other species to dominate the natural and agricultural landscape.

Now, in a new paper published in Science, scientists led by plant biologists Hiroshi Maeda at the University of Wisconsin-Madison and James Leebens-Mack at the University of Georgia used comparative genomics to reveal that while grasses evolved the ability to synthesize starch via two pathways, non-grasses only have one.² They also discovered that grasses’ two metabolic pathways to create lignin, a component of plant biomass, evolved before their split from related non-grasses, and they identified the two mutations that enabled this to happen. The findings open up the potential to engineer other plants to become stronger and more energy-rich, improving agriculture and plant biotechnology.

One roadblock researchers have faced in understanding grass evolution was the lack of reference-quality genomes for closely related non-grass species that diverged from grasses about 100 million years ago. So, the researchers began by sequencing four different plant species—one grass (Pharus latifolius) and three closely related non-grass species (Joinvillea ascendens, Ecdeiocolea monostachya, and Typha latifolia)—and assembled reference-quality and annotated genomes for each of them.

All plants can produce starch in their plastids, which are membrane-bound organelles that help produce energy and other molecules for plant cells, but grasses are unique in that they can also produce starch in the cytosol. To investigate the evolution of this starch metabolic pathway, the team analyzed 20 different gene families involved in cytosolic starch biosynthesis in the grass and non-grass genomes. They found that the metabolic pathway to make starch in the cytosol must have arisen as a consequence of the rho whole genome duplication (ρWGD) event, which doubled the genome in a common ancestor of all grasses.

“That likely gave a competitive advantage to grasses to grow in open habitat, where a lot of plants would love to grow because of all the sun,” said Maeda in a statement. “That's one potential reason why grasses ended up maintaining this highly efficient starch pathway. That same metabolic trait was also very beneficial to agriculture.”

The researchers then looked at the lignin biosynthesis pathways, of which there are two in grasses. Lignin aids plants with structural support, protection against pests, and water transport. To the team’s surprise, closely related non-grass species also encoded the genes for the two biosynthesis pathways. By comparing the genomic sequences of the different plant species, they found that a duplication of the gene for the enzyme phenylalanine ammonia lyase (PAL) occurred before the ρWGD event split grasses into their own lineage. This duplicated gene for PAL evolved into phenylalanine/tyrosine ammonia lyase (PTAL), which enabled grasses and non-grasses to synthesize lignin from phenylalanine and tyrosine. Using structural analysis and site-directed mutagenesis, the researchers then zeroed in on two amino acid residues that were sufficient to change PAL activity into that of PTAL.

“So, that means, we can actually introduce similar mutations in other plants to create this second lignin pathway,” said Maeda in the statement.

These findings open the potential for scientists to engineer grasses and other crops to produce more energy via starch and greater resilience and strength with increased lignin production, with potential to improve agriculture and make advances in plant biotechnology research.