From photoprotection to growth: shifts in high-light acclimation strategies associated with molecular evolutionary rates in duckweeds

Duckweed plants, belonging to the Araceae family, exhibit diverse molecular evolutionary rates across their lineages. Molecular evolutionary rates, the speed at which genetic mutations accumulate, can vary widely among organisms, but the underlying mechanisms and their evolutionary implications are still not well understood. In plants, the absence of an early-segregated germline, a common reproductive strategy, implies that growth and environmental factors could influence mutation accumulation and ultimately change mutation substitution rates.

To explore this relationship, researchers integrated various data sets, including phylogenomics, distribution data, growth measurements, chlorophyll fluorescence, RNA sequencing, and metabolomics. Their findings reveal that duckweed lineages display significant branch-length differences, indicating molecular evolutionary rate variation.

However, these differences do not seem to correlate with differences in selective constraints. The study also discovered that geographic range differences among duckweed lineages correlate with anthocyanin (pigment) accumulation, suggesting ecological differentiation based on light environments. When subjected to high light conditions, duckweed lineages without prominent anthocyanin accumulation displayed higher growth rates and better photosynthetic efficiency compared to those with prominent anthocyanin accumulation.

Furthermore, the study identified distinct high-light acclimation strategies between two representative duckweed species. Spirodela polyrhiza, a lineage with prominent anthocyanin accumulation, predominantly induced photoprotective pathways, including anthocyanin-related pathways and non-photochemical quenching. In contrast, Wolffia australiana, a lineage without prominent anthocyanin accumulation, showed enhanced mitochondrial oxidative phosphorylation gene induction and broader changes in primary metabolism and energy/redox-related metabolites.

These findings suggest that duckweed lineages possess different strategies for balancing photoprotection, photochemical energy use, and metabolic capacity under high light conditions. This ecophysiological divergence may be linked to molecular evolutionary rate variation via growth-associated DNA replication and redox-dependent processes that impact mutation accumulation rates.