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

摘要

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, CO2 fixation, nutrient acquisition, and stress responses. H⁺-PPases have a well-documented role in hydrolyzing pyrophosphate () and capturing the released energy to pump H⁺ across the tonoplast and endomembranes to create proton motive force (). Recently, an additional role for H⁺-PPases in phloem loading and biomass partitioning was proposed. In companion cells (s) of the phloem, H⁺-PPases localize to the plasma membrane rather than endomembranes, and rather than hydrolyzing to create , is utilized to synthesize . Additional in the s promotes sucrose oxidation and ATP synthesis, which the plasma membrane P-type ATPase in turn uses to create more for phloem loading of sucrose via sucrose-H⁺ symporters. To test this model, transgenic Arabidopsis (Arabidopsis thaliana) plants were generated with constitutive and -specific overexpression of AVP1, encoding type 1 ARABIDOPSIS VACUOLAR PYROPHOSPHATASE1. Plants with both constitutive and -specific overexpression accumulated more biomass in shoot and root systems. ¹⁴C-labeling experiments showed enhanced photosynthesis, phloem loading, phloem transport, and delivery to sink organs. The results obtained with constitutive and -specific promoters were very similar, such that the growth enhancement mediated by AVP1 overexpression can be attributed to its role in phloem s. This supports the model for H⁺-PPases functioning as synthases in the phloem by arguing that the increases in biomass observed with AVP1 overexpression stem from improved phloem loading and transport.