Duckweed Evolution: from Land back to Water.

Terrestialization is an important evolutionary process that plants experienced. However, little is known about how land plants acquired aquatic growth behaviors. Here we integrate multiproxy evidence to elucidate the evolution of the aquatic plant duckweed. Three genera of duckweed show chronologically gradual degeneration in root structure and stomatal function and decrease in lignocellulose content, accompanied by gradual contraction in the number of relevant genes and/or their transcriptional decline. The gene numbers in the main phytohormonal pathway are also gradually decreased. The co-action of genes involved in auxin and rhizoid development causes a gradual decrease in adventitious roots. The significant expansion of the flavonoid pathway is also related to the adaptation of duckweed to floating growth. This study reconstructs the evolution history of the duckweed habitat from land back to water, reversing that of early land plants.