Abstract
Two inorganic phosphate (Pi) uptake mechanisms operate in streptophytes and chlorophytes, the two lineages of green plants. PHOSPHATE TRANSPORTER B (PTB) proteins are hypothesized to be the Na+ /Pi symporters catalysing Pi uptake in chlorophytes, whereas PHOSPHATE TRANSPORTER 1 (PHT1) proteins are the H+ /Pi symporters that carry out Pi uptake in angiosperms. PHT1 proteins are present in all streptophyte lineages. However, Pi uptake in streptophyte algae and marine angiosperms requires Na+ influx, suggesting that Na+ /Pi symporters also function in some streptophytes. We tested the hypothesis that Na+ /Pi symporters exist in streptophytes. We identified PTB sequences in streptophyte genomes. Core PTB proteins are present at the plasma membrane of the liverwort Marchantia polymorpha. The expression of M.polymorpha core PTB proteins in the Saccharomyces cerevisiae pho2 mutant defective in high-affinity Pi transport rescues growth in low-Pi environments. Moreover, levels of core PTB mRNAs of M.polymorpha and the streptophyte alga Coleochaete nitellarum are higher in low-Pi than in Pi-replete conditions, consistent with a role in Pi uptake from the environment. We conclude that land plants inherited two Pi uptake mechanisms - mediated by the PTB and PHT1 proteins, respectively - from their streptophyte algal ancestor. Both systems operate in parallel in extant early diverging land plants.
Highlights
Phosphorus is an essential macronutrient required for structural and metabolic functions in all living organisms
We report that genes encoding PHOSPHATE TRANSPORTER B (PTB) proteins are present in the genomes of streptophytes, and that core PTB proteins pass through the secretory system to the plasma membrane in Marchantia polymorpha
We show that heterologous expression of M. polymorpha core PTB proteins is sufficient to restore Pi uptake in a Saccharomyces cerevisiae mutant with defective high-affinity Pi transport (Pinson et al, 2009)
Summary
Phosphorus is an essential macronutrient required for structural and metabolic functions in all living organisms. Angiosperms absorb Pi through PHOSPHATE TRANSPORTER 1 (PHT1) plasma membrane H+/Pi symporters (Nussaume et al, 2011). The kinetics of Pi uptake in yeast mutants and plant protoplasts expressing plant PHT1 proteins have demonstrated that PHT1 genes encode either high-affinity or low-affinity H+/Pi symporters (Leggewie et al, 1997; Mitsukawa et al, 1997; Daram et al, 1998; Harrison et al, 2002; Rae et al, 2003; Liu et al., 2008). Pi uptake is 87% lower in the Atpht1;1 Atpht1;2 Atpht1;3 Atpht1;4 quadruple mutant than in wild-type (WT) Arabidopsis thaliana, suggesting that the PHT1 family is the major plasma membrane Pi transporter family in angiosperms (Ayadi et al, 2015)
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