About 60–85% of total phosphorus (P) in cereal crops is finally allocated to seeds, where it is required for seed development, germination and early growth. However, little is known about the molecular mechanisms underlying P allocation to seeds. Here, we found that two members (OsPHO1;1 and OsPHO1;2) of the PHO1 gene family are involved in the distribution of P to seeds in rice. Both OsPHO1;1 and OsPHO1;2 were localized to the plasma membrane and showed influx transport activities for inorganic phosphate. At the reproductive stage, both OsPHO1;1 and OsPHO1;2 showed higher expression in node I, the uppermost node connecting to the panicle. OsPHO1;1 was mainly localized at the phloem region of diffuse vascular bundles (DVBs) of node I, while OsPHO1;2 was expressed in the xylem parenchyma cells of the enlarged vascular bundles (EVBs). In addition, they were also expressed in the ovular vascular trace, the outer layer of the inner integument (OsPHO1;1) and in the nucellar epidermis (OsPHO1;2) of caryopses. Knockout of OsPHO1;2, as well as OsPHO1;1 to a lesser extent, decreased the distribution of P to the seed, resulting in decreased seed size and delayed germination. Taken together, OsPHO1;2 expressed in node I is responsible for the unloading of P from the xylem of EVBs, while OsPHO1;1 is involved in reloading P into the phloem of DVBs for subsequent allocation of P to seeds. Furthermore, OsPHO1;1 and OsPHO1;2 expression in the caryopsis is important for delivering P from the maternal tissues to the filial tissues for seed development.

中文翻译：

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水稻种子发育必不可少的磷的节点定位转运蛋白。

谷物作物中总磷（P）的约60-85％最终分配给种子，这是种子发育，发芽和早期生长所必需的。但是，对于将P分配给种子的分子机制知之甚少。在这里，我们发现PHO1基因家族的两个成员（OsPHO1; 1和OsPHO1; 2）参与了P在水稻种子中的分布。OsPHO1; 1和OsPHO1; 2均定位在质膜上，并显示出对无机磷酸盐的流入转运活性。在生殖阶段，OsPHO1; 1和OsPHO1; 2均在节点I（最高的节点连接到穗）上显示较高的表达。OsPHO1; 1OsPHO1; 2主要位于结节I的弥散性血管束（DVBs）韧皮部区域，而OsPHO1; 2在扩大的血管束（EVBs）的木质部实质细胞中表达。此外，它们还在卵形血管的痕迹，内被膜的外层（OsPHO1; 1）和细胞核的表皮（OsPHO1; 2）中表达。OsPHO1; 2以及OsPHO1; 1的敲除在较小程度上降低了P在种子中的分布，导致种子尺寸减小和发芽延迟。综上所述，节点I中表达的OsPHO1; 2负责从EVB木质部卸载P，而OsPHO1; 1参与将P重新加载到DVB的韧皮中，以便随后将P分配给种子。此外，在颖果中的OsPHO1; 1和OsPHO1; 2表达对于将P从母体组织传递到孝子组织进行种子发育很重要。