Genes identified in early land plant descendant also found in modern crops
Since their arrival on earth, plants have probably used the same genetic tools to regulate their growth or reproduction. The discovery was made using the liver, a descendant of the first plants to emerge from the ancient oceans and settle on land.
Liverworts grow all over the world and look like moss, spreading over moist soil in the shade. The male and female versions of the hepatic are recognized by unique umbrella-shaped structures at the base of the plant.
"Liverworts have maximum power with the least structure possible," said Professor Yuichiro Watanabe of the Department of Life Sciences at the University of Tokyo, an expert in plant molecular biology.
The hepatic genome is structurally simple compared to flowering plants commonly used in research laboratories, such as tobacco and thalassic watercress (Arabidopsis). Flowering plants are evolutionary plants that are younger than hepatics, with gene duplications and redundancies that make it more difficult to study their genomes.
Despite this simplicity, the hepatic genome seems to have the same life cycle stages and the same powers to regulate them.
Genomic similarities
The entire genome of the Marchantia polymorpha liverwort was first sequenced in 2017 by an international team of several researchers who also participated in the recently published genetic analysis.
Liverworts normally germinate distinct male (top row, left) and female (bottom row, left) structures when they reproduce. When researchers at the University of Tokyo genetically modified plants to lack microRNA156/529, plants developed reproductive organs on their vegetative structures, called thalli. Normal thalli (centre) are plain green with smooth edges. MicroRNA156/529 male thalli knockout (top right) were transparent on the edges and microRNA156/529 female thalli knockout (bottom right) developed irregular edges. initially published by Current Biology, DOI : 10.1016/j.cub.2019.07.084 Credit : Tsuzuki and others, 2019, CC-BY-SA
When they examined the entire genome, the researchers discovered that even the simple liver has about 100 different types of a small molecule, called microRNA, that regulate the activity of other genes.
About eight of the microRNAs in the liver were almost identical to the known microRNAs in Thale watercress. These eight microRNAs fascinated researchers because the ancestral plants that evolved into modern liverworts and modern thalassotherapy cress separated more than 450 million years ago.
"So why keep them? We want to know what these shared microRNAs are doing, and the liverworts are now a practical model for us to investigate," says Watanabe.
Enlarge or reproduce
Most mammals, including humans, are born with the cells they will need in adulthood to produce their own offspring. However, plants do not develop their reproductive cells until after the vegetative stage, when they develop new leaves or grow, at the reproductive stage.
One of the microRNAs that help flowering plants control the transition to the reproductive stage is also one of eight microRNAs shared by Thale cress and the liver. This microRNA is known to researchers as microRNA156/529.
"It was incredible for us. These liverworts have skipped part of the reproductive process and the body itself becomes the reproductive organ," says Watanabe.
Identification of the same molecule with a similar role in the transition from vegetation to reproduction in such different plant species reveals that microRNA156/529 and the other molecules with which it interacts are part of an important control module potentially used by all terrestrial plants to regulate their reproductive cycle.
Watanabe imagines that in the future, farmers could measure the amount of microRNA156/529 in crops to predict harvest periods.
"We hope that our results will inspire others to develop new applications for plant breeding," said Watanabe.
