Intracellular inorganic orthophosphate (Pi) distribution and homeostasis profoundly affect plant growth and development. However, its distribution patterns remain elusive owing to the lack of efficient cellular Pi imaging methods. Here we develop a rapid colorimetric Pi imaging method, inorganic orthophosphate staining assay (IOSA), that can semi-quantitatively image intracellular Pi with high resolution. We used IOSA to reveal the alteration of cellular Pi distribution caused by Pi starvation or mutations that alter Pi homeostasis in two model plants, rice and Arabidopsis, and found that xylem parenchyma cells and basal node sieve tube element cells play a critical role in Pi homeostasis in rice. We also used IOSA to screen for mutants altered in cellular Pi homeostasis. From this, we have identified a novel cellular Pi distribution regulator, HPA1/PHO1;1, specifically expressed in the companion and xylem parenchyma cells regulating phloem Pi translocation from the leaf tip to the leaf base in rice. Taken together, IOSA provides a powerful method for visualizing cellular Pi distribution and facilitates the analysis of Pi signalling and homeostasis from the level of the cell to the whole plant.

细胞内无机正磷酸盐（Pi）的分布和稳态深刻影响植物的生长和发育。然而，由于缺乏有效的细胞 Pi 成像方法，其分布模式仍然难以捉摸。在这里，我们开发了一种快速比色 Pi 成像方法，即无机正磷酸盐染色测定 (IOSA)，可以对细胞内 Pi 进行高分辨率半定量成像。我们使用IOSA揭示了两种模式植物（水稻和拟南芥）中由Pi饥饿或改变Pi稳态的突变引起的细胞Pi分布的改变，并发现木质部薄壁细胞和基底节筛管元件细胞在Pi稳态中发挥着关键作用在米饭中。我们还使用 IOSA 筛选改变细胞 Pi 稳态的突变体。由此，我们发现了一种新型细胞 Pi 分布调节因子 HPA1/PHO1;1，它在水稻伴生细胞和木质部薄壁细胞中特异性表达，调节韧皮部 Pi 从叶尖到叶基部的易位。综上所述，IOSA 提供了一种强大的方法来可视化细胞 Pi 分布，并有助于从细胞水平到整个植物的 Pi 信号传导和稳态分析。