Gained specificity

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Gained specificity

Plants have veins to translocate nutrients from one location in the plant to another. Phloem, for the translocation form the leaves to the roots, and xylem for the opposite direction. They are essential for a healthy plant. In their absence plants starve. Still, there is little known about the development of these veins. No researchers from Germany have discovered the how the onset of phloem development is regulated.

The phloem consists out of sieve cells, which you can picture as cells without their regular content, and companion cells, which support the sieve cells to keep them alive. But what is initiating the development of phloem cells was unknown. However, the researchers had a hint: SMXL protein were required.

To discover the actual role of these SMXL proteins, the researchers studied plants that did not make any SMXL4 and SMXL5. These plants, so noticed the researchers, did not turn on genes needed for phloem development. Moreover, the roots of these plants hardly grew at all.

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With SMXL4 and SMXL5 only binding to OBE3 in the phloem precursor cells, only those cells will develop into phloem

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To find the next hind of what SMXL proteins are doing, the researchers analysed which proteins SMXL proteins interacted with. In the interaction assay SMXL5 was found to bind strongly to OBE3. To see if this interaction could happen in the plant, the researchers studied were in the plant these proteins occurred. The researchers observed SMXL4 and SMXL5 in the root only in the precursor cells of the phloem. While OBE3 was observed in more widely in the root tip. In addition, the researchers observed the location of SMLX and OBE3 proteins in the cell. This revealed that both SMXL5 and OBE3 located to the same spots in the nucleus.

As is known that OBE proteins influence the readability of the DNA, the researchers decided to analyse this in plants with and without OBE3, SMXL5 and SMXL4. Hereby they observed that plants in without OBE3 and SMXL5 and plants without SMXL4 and SMXL5 the genes needed for phloem development could not be read. This made it impossible for those plants to develop phloem veins.

Because OBE3 can only bind SMXL4 and SMXL5 in the phloem precursor cells it makes sure that only in those cells the phloem development genes can be read. In this way, the plant ensures that phloem veins can only develop in those predestined cells. Enabling a continuous stream of nutrients from the leaves to the root tips.

Literature

Eva-Sophie Wallner, Nina Tonn, Dongbo Shi, Laura Luzzietti, Friederike Wanke, Pascal Hunziker, Yingqiang Xu, Ilona Jung, Vadir Lopéz-Salmerón, Michael Gebert, Christian Wenzl, Jan U. Lohmann, Klaus Harter, and Thomas Greb (2023) OBERON3 and SUPPRESSOR OF MAX2 1-LIKE proteins form a regulatory module driving phloem development. Nat Commun 14, 2128. https://doi.org/10.1038/s41467-023-37790-5