Science1 publisher2 min readPublished
A single protein wires RAB5 trafficking to ROP7 signaling in Arabidopsis pollen
A team at Japan's National Institute for Basic Biology reports that REAP1/SWAP70 brings the plant-specific GTPase ROP7 onto RAB5 endosomes, and that weakening both GTPases together disturbs pollen at the one-cell stage.
The Scientist · Science desk
What happened
- A team led by Emi Ito and Takashi Ueda at Japan's National Institute for Basic Biology reports a RAB5-REAP1-ROP7 pathway in Arabidopsis that matters for normal pollen development.
- The protein sits on endosomes, and that position depends on its interactions with RAB5 and with specific membrane lipids.
- Overexpressing RAB5 drew ROP7 onto endosomes, and in cells lacking REAP1 the same overexpression did not recruit it.
- Reducing RAB5 and ROP7 function at the same time disturbed the transition from unicellular to bicellular pollen and left more grains unable to mature.
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Why it matters
- capability A REAP1-null background plus an endosome localisation readout gives plant labs a concrete way to test which other trafficking and ROP pairs are connected, one pair at a time.
- constraint The pollen defect appeared only when two GTPases were weakened together, so screens that knock out one gene at a time are poorly matched to finding pathways of this shape.
- decision Plant experiments that perturb endosomal trafficking now have a ROP signalling side effect to control for, because ROP7's position depends on the state of RAB5.
- precedent With one worked plant example, crosstalk between small GTPase families stops being an open question about animals only. Plant signalling papers are now expected to test it.
Overexpressing a trafficking GTPase changes a great deal of traffic at once. The informative half of the recruitment experiment is therefore the half that produced nothing: with REAP1 absent, raising RAB5 did not put ROP7 onto endosomes [8]. That comparison is what takes a correlation between two GTPases and pins it to a named protein in between.
The direction of the wiring is worth stating plainly. RAB5 regulates membrane trafficking, ROP7 belongs to the plant-specific family associated with cell morphogenesis [4], and endosomes are the compartments where trafficking routes converge [16]. Here RAB5 sets where ROP7 sits [8].
REAP1 works with both branches of the RAB5 family, the plant-only ARA6 and the canonical RAB5 proteins conserved across eukaryotes [5].
The reproductive result is a genetic interaction between two GTPases, and the perturbation was to RAB5 and ROP7 [9]. The announcement did not report the fraction of grains affected or the number of plants scored [14]. Read alongside the cell biology, the bridge is the most economical explanation for the pollen defect, and I would want a pollen count from plants lacking REAP1 before calling it the cause.
Emi Ito, the paper's first author and co-corresponding author [11], put the general argument this way: "The fact that different organisms have evolved crosstalk between small GTPases raises the possibility that connecting these molecular switches is a particularly efficient way to coordinate different cellular processes," she said [10].
One species cannot test that. The experiments are in Arabidopsis thaliana [2], ROPs exist only in plants [12], and crosstalk between GTPase families was documented in animal cells while the plant case stayed open [1]. Telling a plant invention apart from an old arrangement that was kept and rebuilt requires bridge proteins identified in lineages beyond Arabidopsis.
The paper establishes something narrower and solid: in one plant, at one step of pollen development, two GTPase families that were studied separately are connected by a protein that binds both [4][9]. The work is published in Nature Plants as "REAP1/AtSWAP70 integrates RAB5 and ROP signaling during sexual reproduction" [3].
What to watch
- Whether the same endosome recruitment assay bridges other ROPs and other RAB5 members, or whether REAP1 is specific to the pair tested.
- Whether ROP7 is GTP-loaded once it reaches the endosome, which would make the compartment a signalling site.
- Whether the bridge operates in vegetative cells, where ROPs are associated with cell morphogenesis.