Plastidial phosphate transport in plants

Abstract

Phosphorus is an essential element for all living organisms and is central to the genetics and energetics of life. Inorganic phosphate (Pi) is recurrently involved in protein regulation and signal transduction but also in energy transfer as a component of the ATP-molecule. When cells and cell organelles commence a plethora of energy-demanding processes associated with ATP hydrolysis to ADP and Pi, a balancing of the Pi content between compartments is crucial to prevent the ATP hydrolysis to be stalled from accumulation of Pi. The transport of Pi via specialized protein(s) is therefore essential for cellular Pi homeostasis since biological membranes are impermeable to Pi (Paper I, III). This thesis shows that the plastid-localized Pi transporter PHT4;2 in Arabidopsis thaliana is nearly restricted to roots during vegetative growth, where it regulates plastid homeostasis by a Na+-dependent Pi efflux. The accumulation of Pi in the root plastids of pht4;2 loss-of function-mutants yields a reduced starch accumulation in roots, which is consistent with the inhibition of starch synthesis by a deficient Pi export. However, the pht4;2 mutants display a 40% increased rosette area and a twofold larger shoot biomass as compared to wild type (WT) plants, indicating an involvement of PHT4;2 in signaling between roots and leaves. The larger leaf area and biomass accounts from an increased cell proliferation in pht4;2 mutants compared to the WT plants. Nevertheless, the cell size and the photosynthetic electron transport rate are similar in all genotypes. (Paper I). Another Pi transporter, PHT4;1, is located in the chloroplast thylakoid membrane of Arabidopsis. By using homology modeling, site directed mutagenesis and functional characterization in Escherichia coli, several residues important for Pi transport and its sodium dependency have been identified in PHT4;1 (Paper II). Rosette area and biomass of the pht4:1 mutants are reduced to 70-80% of the WT plants. Absence of PHT4;1 does not affect the relative electron transport rates, pigment composition, and the expression of photosynthesis-related proteins. However, the ΔpH contribution to the protonmotive force across the thylakoid membrane is significantly higher in the pht4;1 mutants as compared to the WT plants. Non-photochemical quenching kinetics in pht4;1 mutants is transiently increased at the initial phase and declines to WT levels during the plateau phase. Moreover, the Pi content is elevated in the pht4;1 mutants whereas the total Phosphor content is similar to the WT (Paper III). This thesis shows that, through their activity, plastidial Pi transporters play role in plant growth and behavior under different environmental conditions. This is a subject still in its cradle of being understood. The data acquired in this work not only strengthen the importance for a normal daily life of plants, but also the relevance of Pi transporters as a research field.