Dissecting the Role of Type I Proton Pyrophosphatase in Arabidopsis thaliana

The partitioning of photosynthates between their sites of production (source) and their sites of utilization (sink) is a major determinant of crop yield and the potential of regulating this translocation promises substantial opportunities for yield increases. Ubiquitous overexpression of the plant type I proton pyrophosphatase (H+-PPase) in crops improves several valuable traits including salt tolerance and drought resistance, nutrient and water use efficiencies, and increased root biomass and yield. Originally, type I H+-PPases were described as pyrophosphate (PPi)-dependent proton pumps localized exclusively in vacuoles of mesophyll and meristematic tissues. It has been proposed that in the meristematic tissues, the role of this enzyme would be hydrolyzing PPi originated in biosynthetic reactions and favoring sink strength. Interestingly, this enzyme has been also localized at the plasma membrane of companion cells in the phloem which load and transport photosynthates from source leaves to sinks. Of note, the plasma membrane-localized H+-PPase could only function as a PPi-synthase in these cells due to the steep proton gradient between the apoplast and cytosol. The generated PPi would favor active sucrose loading through the sucrose/proton symporter in the phloem by promoting sucrose hydrolysis through the Sucrose Synthase pathway and providing the ATP required to maintain the proton gradient. To better understand these two different roles of type I H+-PPases, a series of Arabidopsis thaliana transgenic plants were generated. By expressing soluble pyrophosphatases in companion cells of Col-0 ecotype and H+-PPase mutants, impaired photosynthates partitioning was observed, suggesting phloem-localized H+-PPase could generate the PPi required for sucrose loading. Col-0 plants expressed with either phloem- or meristem-specific AVP1 overexpression cassette and the cross between the two tissue specific lines (Cross) were generated. The results showed that the phloem-specific AVP1-overexpressing plants had increased root hair elongation under limited nutrient conditions and both phloem- and meristem-overexpression of AVP1 contributed to improved rhizosphere acidification and drought resistance. It was concluded that H+-PPases localized in both sink and source tissues regulate plant growth and performance under stress through its versatile enzymatic functions (PPi hydrolase and synthase).