2023-12-01 02:23:46
The move from single-celled organisms capable of photosynthesis to multicellular plants was accompanied by a change in signal processing. At the threshold of multicellularity, cells switched to polarized growth and remained together after division. Molecular biologists at the ÖAW were able to show that from this point in plant evolution – regardless of the step on land – ROP proteins for a universal signaling chain were present in plants. The study was published in Current Biology.
The transition from unicellular to multicellular life forms in plants required mechanisms that limited cell growth and division to a single direction and held the resulting cells together. The genetic basis for this transition to multicellular filamentous plants is being intensively researched internationally. Hugh Mulvey, postdoctoral researcher, and Liam Dolan, senior group leader at the GMI – Gregor Mendel Institute for Molecular Plant Biology of the Austrian Academy of Sciences (ÖAW), looked at the sequence and function of the ROP signaling protein. They published their results in the journal Current Biology.
The two GMI researchers have been focusing for some time on the signaling chain proteins that belong to the RHO-GTPase family and are specific to plants as ROP proteins. They knew from previous experiments that ROP proteins in land plants are essential for regulating three-dimensional tissue development and organ formation and are specifically involved in the control of polarized cell growth and the direction of cell division. In the current study, they wanted to find out at what point in plant phylogeny these important proteins were first detectable.
Does the ROP signal coincide with multicellularity or stepping on land?
“We asked ourselves whether ROP only occurs in land plants or whether this signaling protein had already emerged in the algal ancestors of land plants,” says Hugh Mulvey, explaining the starting point of the study. The two researchers now compared the land-dwelling fountain liverwort Marchantia with a series of green algae of varying complexity: with the unicellular Mesostigma, with the single-cell colony Chlorokybus, with the multicellular filaments of Klebsormidium and the Coleochaete consisting of many filaments. The last two groups live in freshwater like their unicellular algae ancestors, but are among the immediate ancestors of land plants.
This means that ROP signaling coincides with the development of multicellularity in plants.
The ROP sequence of fountain liverwort was actually very similar to that of Coleochaete and Klebsormidium, the multicellular ancestors of land plants, and differed from that of the unicellular or cell pack-growing species.
Similarity in sequence and function
The researchers next asked themselves whether only the sequence of the protein is conserved or whether ROP proteins from multicellular green algae and land plants also function similarly. A question like this can be answered with a complementation experiment. The gene in focus is replaced by the homologous gene from another species. When it comes to ROP, the fountain liverwort Marchantia is a perfect model for this type of study because – unlike other model plants – it only has a single ROP gene.
Mulvey and Dolan created Marchantia mutants equipped with different ROP homologues from green algae and compared the growth functions of the mutants. The picture from the sequence analyzes was confirmed: the mutants with the signaling protein from multicellular green algae caused the well liverwort to grow normally; Mutants with a single-cell ROP did not develop normally. By combining phylogenetic and genetic complementation studies, Mulvey and Dolan were able to confirm that ROP signaling is highly conserved in the evolutionary branch leading from aquatic multicellular green algae to present-day land plants. “Because the ROP proteins in land plants control polarized cell growth and the direction of cell division, we hypothesize that the evolution of ROP signaling in this lineage has contributed to the evolution of multicellularity and the morphological transition to a multicellular body plan,” it said Summary of the two GMI researchers.
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#Plant #signaling #protein #threshold #multicellularity