Rice PPO Gene Family Revealed as a Living Record of Birth and Death Evolution

Rice has quietly been rewriting its own genetic toolkit for millions of years, and one small family of enzymes has now been caught in the act. A new study published in Botanical Studies reports the first comprehensive analysis of the polyphenol oxidase, or PPO, gene family across the genus Oryza, the group that includes cultivated rice and its wild relatives. By combing through the fully sequenced genomes of 21 Oryza species and three close grass relatives, the researchers found that these genes have expanded, fragmented, and vanished in patterns that closely match a classic evolutionary model: birth and death.

Polyphenol oxidases are copper-containing enzymes that catalyze the oxidation of phenolic compounds into quinones, the reactive molecules that polymerize into the brown and black pigments behind enzymatic browning. Anyone who has watched a sliced apple darken has seen PPO at work. In cereal crops, the enzyme has a double identity. On one hand, it contributes to grain browning, a trait that farmers and consumers have long selected against because it diminishes the appearance and palatability of food. On the other hand, PPO activity is implicated in plant defense, generating reactive quinones that participate in protein cross-linking and the formation of antimicrobial compounds after wounding or pathogen attack. During domestication, multiple independent PPO mutations have been selected in different crops, creating an evolutionary trade-off between grain quality and plant fitness.

Until now, most of what was known about PPO in rice came from a single reference genome: Oryza sativa ssp. japonica cultivar Nipponbare. Previous work had identified two full-length PPO loci in rice, Phr1 and Phr1L3, with Phr1 directly responsible for the seed browning phenotype. A loss-of-function allele of Phr1 is predominantly found in japonica cultivars, and population genetic analyses indicate that positive selection favored this allele during japonica domestication, while functional alleles are maintained in indica subspecies through balancing selection. But the Nipponbare genome also harbored additional truncated PPO sequences that hinted the family was more complicated than the two-gene picture suggested.

The new analysis confirms that suspicion. The Nipponbare genome, it turns out, contains six PPO loci in total: the two full-length genes, OsPPO1 on chromosome 4 and OsPPO2 on chromosome 1, plus four truncated variants. The full-length proteins show the canonical plant PPO architecture, with an N-terminal chloroplast transit peptide, a central tyrosinase domain carrying two conserved copper-binding sites called CuA and CuB, and a C-terminal domain that acts as a latency-inducing shield over the active site. Notably, Nipponbare PPO1 itself carries an 18-base-pair in-frame deletion within the KFDV motif of the C-terminal domain, rendering it non-functional. Of the truncated loci, two adjacent genes on chromosome 4, previously annotated as separate paralogs, appear to be fragments of a single ancestral gene that was split apart by two Ty3-gypsy transposable elements.

The copy numbers are strikingly dynamic. Diploid Oryza species carry three to five PPO genes, while the eight tetraploid species possess seven to eleven. Truncated copies were found in nearly all species, with the sole Oryza exceptions being O. meridionalis and O. brachyantha, and their numbers ranged from one to eight per genome, with O. alta holding the record. In roughly half of the surveyed species, truncated PPOs actually outnumber their intact counterparts, a clear signature of ongoing pseudogenization.

Phylogenetic analysis resolved the family into three well-supported types, designated PPO1 through PPO3, with PPO1 further subdivided into two subtypes, PPO1-1 and PPO1-2. The third type, PPO3, is reported for the first time in this study; in Nipponbare it corresponds to Os01g58070, a massively truncated, intronless locus. The subtypes differ not only in sequence but in expression: PPO1-1 is primarily expressed in the palea, lemma, and endosperm, PPO1-2 transcripts appear mainly in roots at low levels, PPO2 is predominantly expressed in the leaf sheath, and PPO3 transcripts are barely detectable across tested tissues. Structurally, PPO1 and PPO3 both carry a thylakoid transfer domain and the twin-arginine motifs needed for delivery into the chloroplast thylakoid lumen, whereas PPO2 lacks the entire transfer domain and likely resides elsewhere in the plastid.

The genomic geography of the family tells an equally compelling story. PPO1 genes cluster on chromosome 4, while PPO2 and PPO3 genes co-localize on chromosome 1, and every duplicated copy sits adjacent to its progenitor gene. That arrangement points squarely at tandem duplication as the engine of expansion. Yet the two types expanded in different branches of the genus at different times. PPO2 expansion occurred early, appearing in species with CC, JJ, KK, LL, FF, GG, and HH genomes, while PPO1 duplication arose later, in lineages with AA, BB, CC, DD, or EE genomes. Species with CC and DD genomes retain both expansions, which the authors interpret as a transitional evolutionary state. No PPO3 duplications were observed in any species examined.

The mutation patterns within truncated genes split into two categories. In PPO1-1 and PPO3, the disabling indels arose independently in different species, indicating they originated after speciation. In PPO1-2 and PPO2, by contrast, the mutation sites are conserved across species, implying they were inherited from common ancestors. The domestication fingerprints are unmistakable in the cultivated species. Beyond the known 18-base-pair deletion in Nipponbare PPO1, the Nipponbare PPO1-2.1 locus is disrupted by transposable-element insertions that the authors suggest may be domestication-driven, since transposons are known to have been activated during rice domestication. And in the cultivated African rice Oryza glaberrima, the team discovered a novel truncated PPO1-1 allele caused by a single-base deletion in the first exon. A phenol color reaction assay on grains of the reference accession confirmed the loss-of-function phenotype, and the same allele was found in three of seven additional O. glaberrima cultivar genomes examined.

Taken together, the pattern of sequential tandem duplications followed by frequent frameshift-indel truncations fits the birth-and-death model of multigene family evolution, in which new paralogs are born through duplication while older copies accumulate deleterious mutations and slide into pseudogenhood. The authors also propose an ecological framing, suggesting that early PPO2 expansion may reflect selection on vegetative tissues in shaded forest habitats occupied by basal lineages, while later PPO1 expansion in derived lineages may reflect a shift toward reproductive tissues in open wetland environments. They are careful to flag these habitat-based interpretations as hypothesis-generating, noting that present-day distributions may not mirror ancestral environments and that comparative ecological and functional studies will be needed to establish causal links.

Why would a family that bolsters defense keep losing members? The study points to a balance of costs and benefits. PPO overexpression enhances pathogen and herbivore resistance in species such as strawberry, poplar, and tomato, but in walnut it caused cell death and no viable transgenic lines could be obtained, showing that excessive activity can be harmful. In rice, frequent loss of PPO copies after expansion may reflect a compromise between retaining defensive functions and avoiding costs such as accelerated grain browning. Intriguingly, almost all Oryza species retain at least one gene in both the PPO1 and the PPO2/PPO3 clusters, suggesting that basal PPO activity remains essential even as redundant copies are culled. The result is a gene family that records, in its duplications and its wreckage, the combined pressures of natural selection and human domestication on one of the world’s most important crops.

Subject of Research: Evolution of the polyphenol oxidase gene family across Oryza rice species

Article Title: Birth and death evolution of polyphenol oxidase (PPO) gene family in Oryza species

Article References: Tsai, Y.-C., Chen, Y.-J., Cheng, S.-Y., Li, C.-P., & Huang, M.-D. (2026). Birth and death evolution of polyphenol oxidase (PPO) gene family in Oryza species. Botanical Studies, 67(1), Article 4. https://doi.org/10.1186/s40529-026-00491-5

Image Credits: AI Generated

DOI: 10.1186/s40529-026-00491-5

Keywords: polyphenol oxidase, Oryza, gene family evolution, birth-and-death model, tandem duplication, rice domestication, pseudogenization, Oryza glaberrima, Nipponbare, transposable elements, grain browning, comparative genomics