Abstract
Energy partitioning and plant growth are mediated in part by a type I H+-pumping pyrophosphatase (H+-PPase). A canonical role for this transporter has been demonstrated at the tonoplast where it serves a job-sharing role with V-ATPase in vacuolar acidification. Here, we investigated whether the plant H+-PPase from Arabidopsis also functions in "reverse mode" to synthesize PPi using the transmembrane H+ gradient. Using patch-clamp recordings on Arabidopsis vacuoles, we observed inward currents upon Pi application on the cytosolic side. These currents were strongly reduced in vacuoles from two independent H+-PPase mutant lines (vhp1-1 and fugu5-1) lacking the classical PPi-induced outward currents related to H+ pumping, whereas they were significantly larger in vacuoles with engineered heightened expression of the H+-PPase. Current amplitudes related to reverse-mode H+ transport depended on the membrane potential, cytosolic Pi concentration, and magnitude of the pH gradient across the tonoplast. Of note, experiments on vacuolar membrane-enriched vesicles isolated from yeast expressing the Arabidopsis H+-PPase (AVP1) demonstrated Pi-dependent PPi synthase activity in the presence of a pH gradient. Our work establishes that a plant H+-PPase can operate as a PPi synthase beyond its canonical role in vacuolar acidification and cytosolic PPi scavenging. We propose that the PPi synthase activity of H+-PPase contributes to a cascade of events that energize plant growth.
Highlights
Energy partitioning and plant growth are mediated in part by a type I H؉-pumping pyrophosphatase (H؉-PPase)
We propose that the phosphoanhydride bond of pyrophosphate (PPi) synthase activity of H؉-PPase contributes to a cascade of events that energize plant growth
The appearance of inward currents in response to Pi application is consistent with cytosol-directed Hϩ flux. To test whether these inward currents are related to Hϩ-PPase operating in reverse, we determined the amplitudes of Pi-induced currents in several Arabidopsis lines differing in their Hϩ-PPase expression
Summary
V-PPase, vacuolar pyrophosphatase; BTP, bistris propane (1,3-bis(tris(hydroxymethyl)methylamino)propane); Hϩ-PPase, Hϩ-pumping pyrophosphatase; PM, plasma membrane; SE-CC, sieve element– companion cell; Suc, sucrose; V-ATPase, vacuolar Hϩ-ATPase; pF, picofarad; vac, vacuole; cyt, cytosol; KF, potassium fluoride; ACMA, 9-amino-6-chloro-2-methoxyacridine; CAX1, cation exchanger 1; Col-0, Columbia-0. Recent studies show that in fugu lines Hϩ-PPase is essential for maintaining adequate PPi levels and together with cytosolic pyrophosphatase (PPa) isozymes prevents increases in PPi concentrations to toxic levels [14] These studies support a robust model that in metabolically active tissues like the mesophyll AVP1 plays an important role in PPi homeostasis. The phloem-specific expression of AVP1 phenocopies the higher reduced carbon translocation capacity seen when AVP1 is constitutively expressed using the CaMV35S promoter [24] This synthase model is further buttressed by the thermodynamical feasibility for Hϩ-PPases to synthesize PPi if a steep proton gradient is present [25]. We propose that these functions, modulated by different environmental and spatial cues, contribute to the cascade of events that energize plant growth
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
