Moving reproductive siRNAs
When thinking about plant reproduction you might be forgiven thinking that all the action happens in flowers. Small interference RNA, or siRNA for short, that originated elsewhere in the plant also play a role. In fact, without them pollen are largely not viable. The one exception, at least so scientists thought, where the brassica species. Now a new study shows that they siRNAs as well, only they produce them slightly different.
In most plants the siRNAs required for reproduction are called phasiRNAs. They are made via a miRNA route. They are absent in brassica species. However, there were hints that reproductive siRNAs still might have a role to play. Therefore, a group of international researchers decided to find out.
Knowing that an alternative way to produce siRNAs occurs via Polymerase IV the researchers looked at pollen viability of a Capsella rubella plant without a working Polymerase IV. In those plants pollen development stopped halfway through.
Moving siRNAs
Now to find out if this arrested pollen development was due to a signal from the rest of the plant, the researchers grafted the roots of a normal plant to the shoots of a Polymerase IV missing plant. Finding that pollen development was partially restored. Next, they looked at the siRNA levels in those grafted plants. Finding that the siRNA levels were restored in the pollen.
Now the question was, what are those siRNAs doing in the pollen cells. There are two possible options, the first is that they function as guides for DNA methylation, marks on the DNA that tell the cell not to express those genes. The other is that siRNAs influence cell development through functioning as guides earmarking messenger RNAs for breakdown, in this way siRNAs can prevent messenger RNA being translated into proteins.
Influencing protein translation
Looking to methylation levels in pollen from the grafted plants the researchers found that plants with normal roots but shoots in which Polymerase IV was missing did not restore methylation levels to that in normal plants.
However, looking at the gene expression level, or messenger RNA levels to be more precise, the researchers did find that plants with normal roots but shoots in which Polymerase IV was missing had mostly comparable gene expression levels as normal plants. Suggesting that the in Capsella rubella Polymerase IV is needed for the production of reproductive siRNAs, and that these siRNAs work at the regulating gene expression on messenger RNA level.
While the grafting experiments indicate that these reproductive siRNAs are produced in the roots. Genetic analysis suggests that other non-reproductive tissues like shoots and leaves also produce these siRNAs.
Now you can imagen that the environment has some influence on the targets for these reproductive siRNAs. In this way the plant can tailor gene expression of its offspring towards the environment it likely encounters.
Literature
Kai Bartusch, Tina B Schreier, Michaela Fischer-Stettler, Samuel C Zeeman, Elisabeth Truernit, Grafting Reveals Organ-Autonomous and Feedback Roles of Root Phloem Development in Source-Sink Dynamics, Journal of Experimental Botany, 2026;, erag042, https://doi.org/10.1093/jxb/erag042
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