Constitutive and Companion Cell-Specific Overexpression of AVP1, Encoding a Proton-Pumping Pyrophosphatase, Enhances Biomass Accumulation, Phloem Loading, and Long-Distance Transport

Abstract

Plant productivity is determined in large part by the partitioning of assimilates between the sites of production and the sites of utilization. Proton-pumping pyrophosphatases (H + -PPases) are shown to participate in many energetic plant processes, including general growth and biomass accumulation, CO 2 fixation, nutrient acquisition, and stress responses. H + -PPases have a well-documented role in hydrolyzing pyrophosphate (PPi) and capturing the released energy to pump H + across the tonoplast and endomembranes to create proton motive force (pmf). Recently, an additional role for H + -PPases in phloem loading and biomass partitioning was proposed. In companion cells (CCs) of the phloem, H + -PPases localize to the plasma membrane rather than endomembranes, and rather than hydrolyzing PPi to create pmf, pmf is utilized to synthesize PPi. Additional PPi in the CCs promotes sucrose oxidation and ATP synthesis, which the plasma membrane P-type ATPase in turn uses to create more pmf for phloem loading of sucrose via sucrose-H + symporters. To test this model, transgenic Arabidopsis (Arabidopsis thaliana) plants were generated with constitutive and CC-specific overexpression of AVP1, encoding type 1 ARABIDOPSIS VACUOLAR PYROPHOSPHATASE1. Plants with both constitutive and CC-specific overexpression accumulated more biomass in shoot and root systems.14 C-labeling experiments showed enhanced photosynthesis, phloem loading, phloem transport, and delivery to sink organs. The results obtained with constitutive and CC-specific promoters were very similar, such that the growth enhancement mediated by AVP1 overexpression can be attributed to its role in phloem CCs. This supports the model for H + -PPases functioning as PPi synthases in the phloem by arguing that the increases in biomass observed with AVP1 overexpression stem from improved phloem loading and transport.

H+‐pyrophosphatase IbVP1 promotes efficient iron use in sweet potato [Ipomoea batatas(L.) Lam.]

“…H + ‐PPase has been considered as a yield‐enhancing factor (Gonzalez et al ., 2010; Khadilkar et al ., 2016). Increased biomass, especially root system, has been reported in plants overexpressing type I H + ‐PPases, under deficiencies of phosphate (Gaxiola et al ., 2012; Pei et al ., 2012; Yang et al ., 2007, 2014) and nitrogen (Lv et al ., 2015; Paez‐Valencia et al ., 2013).…”

“…Further, the high expression of IbVP1 in young storage root indicates an important function related to storage root development (Figure 2). These results further confirmed that up‐regulation of H + ‐PPase promoted translocation of photosynthates from leaf to root, as hypothesized previously (Gaxiola et al ., 2012; Hermans et al ., 2006; Khadilkar et al ., 2016; Paez‐Valencia et al ., 2011). A recent study also demonstrated that increased sucrose accumulation was required for the regulation of auxin‐mediated Fe deficiency in plants (Lin et al ., 2016).…”

“…Up‐regulated H + ‐PPases induce plant tolerance to various stresses, such as drought, high salinity, and N and Pi deprivation (Arif et al ., 2013; Paez‐Valencia et al ., 2013; Yang et al ., 2007). It also affects auxin and sugar transport and accumulation during root and shoot growth (Gaxiola et al ., 2012; Khadilkar et al ., 2016; Li et al ., 2005; Pasapula et al ., 2011; Pizzio et al ., 2015). …”

Structural basis for the reversibility of proton pyrophosphatase

“…[5][6][7][8][9][10] However, H C -PPase also saliently localizes at the vascular tissues of plants, wherein this protein is predominantly localized at the plasma membrane (PM) of the sieve element companion cell (SE-CC) complexes. 8,[11][12][13] Thermodynamic, 14 genetic, immunohistochemical, and physiological evidence 8,15,16 is consistent with a reverse PPi synthase activity of H C -PPase at the PM in the phloem, where it plays an important role in PPi homeostasis and photosynthate partitioning. 10,[15][16][17] We contend that the magnitude of proton gradients associated with the tonoplast or PM location of this enzyme drives the protein's structurally congruent potential to work in a reversible manner -either in the synthesis or hydrolysis of its substrate, PPi.…”

“…8,[11][12][13] Thermodynamic, 14 genetic, immunohistochemical, and physiological evidence 8,15,16 is consistent with a reverse PPi synthase activity of H C -PPase at the PM in the phloem, where it plays an important role in PPi homeostasis and photosynthate partitioning. 10,[15][16][17] We contend that the magnitude of proton gradients associated with the tonoplast or PM location of this enzyme drives the protein's structurally congruent potential to work in a reversible manner -either in the synthesis or hydrolysis of its substrate, PPi. 10,17 That the proton gradient drives the hydrolytic or synthetic activity of H C -PPase is corroborated by experimental evidence in maize coleoptiles and oranges.…”