Phosphorus is an essential nutrient taken up by organisms in the form of inorganic phosphate (Pi). Eukaryotes have evolved sophisticated Pi sensing and signaling cascades, enabling them to stably maintain cellular Pi concentrations. Pi homeostasis is regulated by inositol pyrophosphate signaling molecules (PP-InsPs), which are sensed by SPX domain-containing proteins. In plants, PP-InsP-bound SPX receptors inactivate Myb coiled-coil (MYB-CC) Pi starvation response transcription factors (PHRs) by an unknown mechanism. Here we report that a InsP₈–SPX complex targets the plant-unique CC domain of PHRs. Crystal structures of the CC domain reveal an unusual four-stranded anti-parallel arrangement. Interface mutations in the CC domain yield monomeric PHR1, which is no longer able to bind DNA with high affinity. Mutation of conserved basic residues located at the surface of the CC domain disrupt interaction with the SPX receptor in vitro and in planta, resulting in constitutive Pi starvation responses. Together, our findings suggest that InsP₈ regulates plant Pi homeostasis by controlling the oligomeric state and hence the promoter binding capability of PHRs via their SPX receptors.

磷是有机物以无机磷酸盐（Pi）形式吸收的必需营养素。真核生物已经进化出复杂的Pi感应和信号级联，使其能够稳定地维持细胞Pi的浓度。Pi稳态由肌醇焦磷酸信号分子（PP-InsPs）调节，而SP-InsPs则由含SPX域的蛋白质感知。在植物中，PP-InsP结合的SPX受体通过未知机制使Myb卷曲螺旋（MYB-CC）Pi饥饿反应转录因子（PHRs）失活。在这里我们报告说InsP ₈–SPX复合体针对PHR的工厂唯一CC域。CC结构域的晶体结构揭示了不寻常的四链反平行排列。CC域中的界面突变产生单体PHR1，该单体不再能够以高亲和力结合DNA。位于CC结构域表面的保守碱性残基的突变会破坏体外和植物体内与SPX受体的相互作用，从而导致本构Pi饥饿反应。在一起，我们的发现表明，InsP ₈通过控制寡聚状态并因此控制PHR通过其SPX受体的启动子结合能力来调节植物Pi的体内稳态。