To cope with adverse environments, plants rely on detection followed by signaling leading to developmental responses. While many signals are local, others require phloem-mediated long-distant transport. We have identified lipids and their putative protein partners in the phloem. These proteins could function by releasing the lipid from the membrane, transporting the lipid, acting as a receptor, or acting as the signal itself. I focused on three lipid-binding proteins: a putative GDSL-lipase that may release lipids into the phloem; PIG-P, a potential receptor, and PLAFP, a protein of unknown function. These proteins respond to abiotic stress and bind lipids: GDSL is downregulated by drought mimics and binds diacylgylcerol (DAG) and phosphatidic acid (PA). PIG-P is unaffected by stress and binds phosphatidylserine (PS), PA, and phosphatidylinositol-4-phosphate (PIP). PLAFP is induced by drought mimics/signals and binds PA. PA is a known secondary signal in response to drought and ABA. Since both PA and PLAFP increase in response to ABA and drought, we propose that PLAFP-PA could function in an ABA-related long-distance signaling path. Changes in PLAFP levels affect root and vasculature development, plant size, seed yield, and drought tolerance. Computational modeling suggests two possible PLAFP-PA interaction models: 1) PA binds solely with the head group—where the lipid likely remains inserted in the membrane or 2) PA fits into the hydrophobic groove on the protein—where PA could be mobile. Initial experiments suggest specificity of PLAFP for ligands with unsaturated acyl chains further supporting model 2 and thus long-distance signaling.
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