Transcriptome Analysis with Different Leaf Blades Identifies the Phloem-Specific Phosphate Transporter OsPHO1;3 Required for Phosphate Homeostasis in Rice.

Phosphate (Pi) is essential for plant growth and development. One strategy to improve Pi use efficiency is to enhance Pi remobilization among leaves. Using transcriptome analysis with first (top) and fourth (down) leaf blades from rice (Oryza sativa) in Pi-sufficient and deficient conditions, we identified 1384 genes differentially expressed among these leaf blades. These genes were involved in physiological processes, metabolism, transport, and photosynthesis. Moreover, we identified the Pi efflux transporter gene, OsPHO1;3, responding to Pi-supplied conditions among these leaf blades. OsPHO1;3 is highly expressed in companion cells of phloem, but not xylem, in leaf blades and induced by Pi starvation. Mutation of OsPHO1;3 led to Pi accumulation in second to fourth leaves under Pi-sufficient conditions, but enhanced Pi levels in first leaves under Pi-deficient conditions. These Pi accumulations in leaves of Ospho1;3 mutants resulted from induction of OsPHT1;2 and OsPHT1;8 in root and reduction of Pi remobilization in leaf blades, revealed by the decreased Pi in phloem of leaves. Importantly, lack of OsPHO1;3 caused growth defects under a range of Pi-supplied conditions. These results demonstrate that Pi remobilization is essential for Pi homeostasis and plant growth irrespective of Pi-supplied conditions, and OsPHO1;3 plays an essential role in Pi remobilization for normal plant growth.