The mystery of the compression of plant sperm

Sperm cells have tight bundles of DNA, but until now, how and why the nuclei of sperm cells in flowering plants clotted has been a mystery
.

Professor Xiaoqi Feng's team discovered the mechanism of sperm compression in flowering plants and gathered clues
about why it is needed.

How do flowering plants compress DNA in sperm cells?

Researchers in Prof.
Xiaoqi Feng's team showed how in flowering plants, sperm chromatin (a complex of DNA and proteins) is compacted by a special histone that spontaneously aggregates itself, like oil droplets in water, a phenomenon known as phase separation
.

Flowering plants use different mechanisms
than animals and non-seed plants such as ferns and mosses.
In these other organisms, sperm chromatin undergoes an almost complete replacement of histone by protamine, which is highly dense with DNA
.

The compaction mechanism of flowering plants is unclear, as they do not have protamine but instead maintain histone-based chromatin
.

Professor Feng's research team used super-resolution microscopy, comparative proteomics, single-cell epigenomic sequencing and 3D genomic mapping to study the mystery
.

The team examined Arabidopsis thaliana's sperm, vegetative nuclei, and leaf nuclei using super-resolution microscopy and found the histone variant H2B.
8
.
Specific expression
in sperm nuclei by comparative proteomics.

H2B.
8 has a long intrinsic disordered region (IDR), a feature that often separates proteins
.
The study found that almost all flowering plant species contained homologous copies (copies) of H2B.
8, all containing an IDR indicating important functions
.

Through imaging, epigenomic sequencing, and 3D genomic mapping, the researchers found that H2B.
8 concentrates sperm DNA by inducing phase separation and aggregation of chromatin, the relatively dense and transcriptionally active portion
of chromatin.

Since chromatin accounts for most of the nuclear volume, its aggregation is an efficient nuclear agglomeration mechanism
.

They also showed that its coagulation function in inactive normochromatin does not adversely affect
the transcription and activity of genes due to the specific localization of H2B.
8.

Why do flowering plants coagulate their DNA in this way?

Many organisms have highly concentrated sperm
.
For example, mammals produce swimming sperm, which benefit from tight DNA bundles in the sperm nucleus, forming a small, hydrodynamic sperm head that helps increase swimming speed
.

The pollen produced by flowering plants does not swim, which raises the question: "Why does DNA compression occur in the sperm of flowering plants?"

The team concluded that H2B.
8-mediated sperm clotting is important
for male fertility.
The researchers speculate that sperm clotting is important for flowering plants, where sperm cells need to travel through a long pollen tube to reach egg cells
buried deep in the maternal tissue.

Consistent with this view, gymnosperms, a group of seed plants without flowering (e.
g.
conifers, cycads), have exposed oovirulers, have non-coagulating sperm nuclei, and lack H2B.
8
.

Dr Toby Buttress, first author of the study, said: "We believe that H2B.
8 is an evolutionary innovation of flowering plants that achieves moderate levels of nuclear coagulation compared to protamine, which achieves super compression
at the expense of transcription.
H2B.
8-mediated coagulation is sufficient to quiesce sperm and is compatible with
gene activity.
”

The team also speculates that this nuclear coagulation mechanism may occur in transcriptionally active cells outside of flowering plants, which tend to favor smaller nuclei
.

Dr.
Buttress continued: "We have found the first example of
a specific core histone variant capable of influencing chromatin phase separation properties.
We demonstrate an exciting new mechanism for genome compression that does not affect gene activity
.
”