How epigenetics encodes plant sperm

An anther. There are two ovals that are red on the inside, with yellow lining the red, and black in the middle
An anther. The sperm grows in the black area. CLSY3 (fused with a yellow fluorescent protein) is specifically expressed in the tapetal cells surrounding the germ cells. The red cells do not have CLSY3. Credit: John Innes Centre.

When we think of changing plant traits, we often think it involves genetic code, but a team of researchers from the John Innes Centre, UK, has found how inherited traits can also be altered without changing any genes.

Typically, traits are inherited by passing genes on to offspring, but through a process called epigenetics a trait is sometimes inherited because of changes to the shape of DNA molecule instead of the genetic code.

CLSY3 (fused with a yellow fluorescent protein) is specifically expressed in the tapetal cells surrounding the germ cells. Credit: John Innes Centre.

One way plant epigenetics can happen is through DNA methylation of the sperm genome, a process that adds a methyl molecule to the DNA, which changes how often a gene is turned on or off. But how this process happens has been unclear.

In the epigenetics study, published in Science, the team revealed how this happens to the DNA in plant sperm.

Plant sperm develop inside the plant’s reproductive parts, called the anthers. When the sperm cells are developing, they are surrounded by nourishing cells – called tapetal cells – that make sure the sperm get everything they need to grow.

The team found that the tapetal cells had a protein called CLSY3, which helped make small RNAs. These RNAs moved up to the developing sperms cells and added new methyl molecules to the DNA.

These new molecules acted almost like sticky notes on some useful genes, flagging when they should be switched on or off to help the sperm develop.

Read more: Worms inherit epigenetic traits

“Our work demonstrates that paternal epigenetic inheritance is determined by tapetal cells, which drive reprogramming at a scale unprecedented in plants,” says lead author James Walker.

“The molecular mechanism our work revealed pushes our understanding of de novo DNA methylation to the next level, showing how new methyl marks are established at specific sites in specific cells.”

Interestingly, the new methyl molecules also prevented some genes from jumping around, which meant they stayed put and didn’t compromise the integrity of the DNA.

These RNAs were very important in helping sperm grow and ensuring that offspring received good DNA, meaning this research could have wide-reaching implications for the improvement of crops. There may be ways to utilise the RNAs without having to alter genes.

“Our study could open a new avenue of crop biotechnology,” says first author Jingeng Long. “For example, through the manipulation of small RNA–directed DNA methylation of the cells that directly contribute to seed formation and the breeding process.”

Corresponding author Xiaoqi Feng adds that the discovery “changes the way we think about epigenetic inheritance across generations in plants.

“Small RNAs produced by germline nurse cells can determine the DNA methylome in the sperm. The key role played by these small RNAs in determining the inherited DNA methylome indicates convergent functional evolution between plant and animal reproduction.”

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