Tiny aquatic plant offers clues that could enable development of next-generation crops, Salk researchers say

The tiny aquatic plant Wolffia, also known as duckweed, is the fastest-growing plant known.
(Sowjanya Sree, Philomena Chu)

Wolffia, also known as duckweed, is the fastest-growing plant known, but the genetics underlying the strange little plant’s success have long been a mystery to scientists. Now, thanks to advances in genome sequencing, researchers are learning what makes the plant unique and, in the process, are discovering some fundamental principles of plant biology and growth.

An effort led by scientists from the Salk Institute for Biological Studies in La Jolla is providing new findings about the plant’s genome that explain how it’s able to grow so fast.

The research, published in the February issue of Genome Research, will help scientists understand how plants make trade-offs between growth and other functions, such as putting down roots and defending themselves from pests.

The research has implications for designing new plants that are optimized for specific functions, such as increased carbon storage to help address climate change.

“A lot of advancement in science has been made thanks to organisms that are really simple, like yeast, bacteria and worms,” said Todd Michael, first author of the paper and a research professor in Salk’s Plant Molecular and Cellular Biology Laboratory. “The idea here is that we can use an absolutely minimal plant like Wolffia to understand the fundamental workings of what makes a plant a plant.”

Wolffia, which grows in fresh water on every continent except Antarctica, looks like tiny floating green seeds, with each plant the size of a pinhead. It has no roots and only a single fused stem-leaf structure called a frond. It reproduces similar to yeast, when a daughter plant buds off from the mother. With a doubling time of as little as a day, some experts believe Wolffia could become an important source of protein for feeding Earth’s growing population. (It is already eaten in parts of Southeast Asia, where it’s known as khai-nam, which translates as “water eggs.”)

To understand what adaptations in Wolffia’s genome account for its rapid growth, the researchers grew the plants under light/dark cycles, then analyzed them to determine which genes were active at different times of the day. (Most plants’ growth is regulated by the light and dark cycle, with the majority of growth taking place in the morning.)

“Surprisingly, Wolffia only has half the number of genes that are regulated by light/dark cycles compared to other plants,” Michael said. “We think this is why it grows so fast. It doesn’t have the regulations that limit when it can grow.”

The researchers also found that genes associated with other important elements of behavior in plants, such as defense mechanisms and root growth, are not present. “This plant has shed most of the genes that it doesn’t need,” Michael said. “It seems to have evolved to focus only on uncontrolled, fast growth.”

Joseph Ecker, a co-author of the paper who is an investigator with the Maryland-based Howard Hughes Medical Institute and director of Salk’s Genomic Analysis Laboratory, said: “Data about the Wolffia genome can provide important insight into the interplay between how plants develop their body plan and how they grow. This plant holds promise for becoming a new lab model for studying the central characteristics of plant behavior, including how genes contribute to different biological activities.”

Todd Michael (left) and Joseph Ecker authored a paper on research of Wolffia, also known as duckweed.
(Salk Institute for Biological Studies)

One focus of Michael’s lab is learning how to develop new plants from the ground up so they can be optimized for certain behaviors. The current study expands knowledge of basic plant biology as well as offers the potential for improving crops and agriculture.

By making plants better able to store carbon from the atmosphere in their roots — an approach pioneered by Salk’s Harnessing Plants Initiative — scientists can optimize plants to help address the threat of climate change.

Michael plans to continue studying Wolffia to learn more about the genomic architecture of plant development. ◆