Scientists reveal the genetic secret of mangroves' stress tolerance

Now, researchers from the Okinawa Graduate School of Science and Technology (OIST) have deciphered the genome of the mangrove Bruguiera gymnorhiza and revealed how this species regulates its genes to respond to stress
.

Their findings were recently published in the journal "New Botanist" and may one day be used to help other plants to withstand stress better

"Mangroves are an ideal model system for studying the molecular mechanisms behind tolerance because they can naturally cope with various stress factors," said Dr.
Martin Miryeganeh, the first author of the study and a researcher in the Department of Plant Epigenetics at OIST
.

Mangroves are an important ecosystem on the planet, protecting coastlines from erosion, filtering pollutants in the water, and serving as nurseries for fish and other species that support coastal livelihoods
.

They also play a vital role in combating global warming.

Despite the importance of mangroves, they are being cut down at an unprecedented rate, and due to human pressure and rising sea levels, they are expected to disappear in just 100 years
.

So far, the genome resources that can help scientists protect these ecosystems are very limited

The mangrove project was originally proposed by Sydney Brenner, one of the founders of OIST, and began in 2016 when a survey of mangrove forests in Okinawa was conducted
.

Scientists have noticed that this mangrove named Bruguiera gymnorhiza shows significant differences between the two types of trees that grow on the seashore with high salinity and those that grow on the upper reaches of the salty river

"These trees are very different; near the ocean, the height of the tree is about 1 to 2 meters, while in the upper reaches of the river, the height of the tree can reach up to 7 meters," said Professor Hidetoshi Saze, senior author and head of the Plant Epigenetics Research Group
.

"But dwarf trees are not unhealthy-they bloom and bear fruit normally-so we think this change is adaptive and may allow salt-stressed plants to invest more resources to deal with harsh environments

Unlike long-term evolutionary adaptation, including changes in genetic sequences, adaptation to the environment occurs during the life cycle of an organism through epigenetic changes
.

These are chemical modifications to DNA that affect the activity of different genes and adjust the response of the genome to different environmental stimuli and stress

Before studying how the genome is regulated, the research team first extracted DNA from the mangrove tree Bruguiera gymnorhiza (Bruguiera gymnorhiza) and decoded the genome of the species
.

They found that the genome contains 309 million base pairs and an estimated 34,403 genes-which is much larger than the genomes of other known mangrove species

When the research team examined the types of repetitive DNA, they found that more than a quarter of the genome is made up of genetic elements called transposons or "jumping genes
.

"

Professor Saze explained: “Active transposons are parasitic genes that can'jump' positions in the genome, just like cut and paste or copy and paste computer functions
.

As more copies of itself are inserted into the genome, repeat DNA will be formed
.
"

Transposons are a major driver of genome evolution, introducing genetic diversity, but they are a double-edged sword
.
The destruction of the genome through the movement of transposons is more likely to cause harm than benefits, especially when plants are already under stress, so mangroves are usually smaller than other plants with smaller genomes, and transposons are suppressed
.

However, this is not the case with olive trees.
Scientists speculate that since this mangrove species is older than other species, it may not have evolved to have an effective means of suppression
.

The research team then examined how the activity of genes, including transposons, changed between individuals upstream of high-salinity seaside and low-salinity salt water
.
They also compared the gene activity of mangroves grown in the laboratory and replicated the seaside and upstream salinity levels under two different conditions
.

In general, regardless of whether it is an individual growing at the seaside or in a laboratory under high salinity conditions, the genes involved in inhibiting transposon activity show higher expression, while the genes that usually promote transposon activity show higher expression.
Low expression
.
In addition, when the research team specifically studied transposons, they found evidence that chemical modifications on their DNA reduced their activity
.

"This shows that an important way to deal with saline pressure involves silent transposons," said Dr.
Miryeganeh
.

The researchers also found that gene activity related to plant stress response increased, including those that are activated when plants lack water
.
Gene activity also shows that the photosynthesis level of stressed plants is lower
.

In future research, the team plans to study how changes in seasons, temperature, and rainfall affect mangrove genome activities
.

"This research is a foundation and provides new insights into how mangroves regulate their genomes under extreme stress," said Professor Saze
.
"More research is needed to understand how these changes in gene activity affect molecular processes in plant cells and tissues, and may one day help scientists create new plant varieties to better cope with stress
.
"

Journal Reference :

1. Matin Miryeganeh, Ferdinand Marlétaz, Daria Gavriouchkina, Hidetoshi Saze.
   De novo genome assembly and in natura epigenomics reveal salinity‐induced DNA methylation in the mangrove tree Bruguiera gymnorhiza .
   New Phytologist , 2021; DOI: 10.
   1111/nph.
   17738 72103.
   htm (accessed October 27, 2021).