When flowering plants returned to the water, they encountered a world of moving currents and shifting forces. Rather than developing stronger tissues, many aquatic plants simplified their body structures by reducing roots, losing stems, and forming thin, soft leaves. This shift is now linked to the widespread loss of expansin (EXP) genes.

Expansins are cell wall proteins that loosen plant cell walls to enable growth, playing essential roles in root, stem, and leaf development in terrestrial plants. In aquatic environments, where buoyancy reduces the need for rigid support and water flow favors flexibility, retaining these genes may no longer be advantageous.

To investigate this phenomenon, researchers from the Wuhan Botanical Garden of the Chinese Academy of Sciences and the VIB-UGent Center for Plant Systems Biology in Belgium conducted a comparative study. They analyzed 19 aquatic plant genomes, representing free-floating, submerged, floating-leaved, and emergent life forms, alongside nine terrestrial plant genomes.

Their findings are clear: submerged and free-floating aquatic plants have significantly fewer EXPA and EXLA expansin genes than their terrestrial relatives. The loss of EXPA genes, which promote root hair development and leaf expansion in land plants, correlates with the reduced root systems and thin, structurally simple leaves observed in submerged and free-floating species.

Similarly, the absence of upright stems in free-floating plants mirrors the loss of EXLA genes, which regulate hypocotyl formation in Arabidopsis. In contrast, emergent and floating-leaved plants, which still experience aerial conditions, retain an expansin repertoire similar to that of terrestrial plants.

Using synteny network analysis, the researchers identified 15 conserved expansin clusters across angiosperms, many of which date back to early-diverging lineages. Notably, clusters associated with root development were absent in free-floating plants, and clusters linked to petal and anther formation were absent in other aquatic groups.

This study shows that gene loss, which is often overlooked in favor of gene gain, can be a powerful driver of evolutionary adaptation. By shedding expansin genes, aquatic plants have streamlined their mechanical organs to better cope with the demands of losing complexity in order to survive in water.

This work was supported by the National Natural Science Foundation of China and it was published in Plant Physiology.