背景回顾：To acquire sufficient mineral nutrients such as phosphate (Pi) from the soil, most plants engage in symbiosis with arbuscular mycorrhizal(AM) fungi. Attracted by plant-secreted strigolactones, the fungi colonize the roots and form highly-branched hyphal structures called arbuscules inside inner cortex cells. The host plant must control the different steps of this interaction to maintain its symbiotic nature. 

提出问题：However, how plants sense the amount of Pi obtained from the fungus, and how this determines the arbuscule lifespan, are far from understood.

主要发现：Here, we show that Medicago truncatula SPX-domain containing proteins SPX1 and SPX3 regulate root Pi starvation responses, in part by interacting with PHOSPHATE RESPONSE REGULATOR2, as well as fungal colonization and arbuscule degradation. 

结果1-表达模式：SPX1 and SPX3 are induced upon Pi starvation but become more restricted to arbuscule-containing cells upon the establishment of symbiosis. This induction in arbuscule-containing cells is associated with the presence of cis-regulatory AW-boxes and transcriptional regulation by the WRINKLED1-like transcription factor WRI5a. 

结果2-作用机制：Under Pi-limiting conditions, SPX1 and SPX3 facilitate the expression of the strigolactone biosynthesis gene DWARF27, which could help explain the increased fungal branching in response to root exudates. Later, in arbuscule-containing cells, SPX1 and SPX3 redundantly control arbuscule degradation. 

结论：Thus, SPX proteins play important roles as phosphate sensors to maintain a beneficial AM symbiosis.

 摘 要 

为了从土壤中获得充分的诸如磷酸盐（Pi）等矿质营养，大多数植物会与丛枝菌根（AM）真菌共生。真菌在受到由植物分泌的独脚金内酯的吸引，在植物的根上定殖，并在根的内皮层细胞内部形成一种称之为丛枝的高度分枝的菌丝结构。寄主植物必须控制这种相互作用的不同阶段，以维持其共生性质。然而，植物如何感知从真菌获得的Pi总量，以及这如何决定丛枝的持续年限，这些都还不是十分清楚。本文中，作者发现蒺藜苜蓿中含有SPX结构域的蛋白SPX1和SPX3，部分通过与PHR2的互作调控根的磷饥饿响应，以及真菌定殖和丛枝根降。Pi饥饿会SPX1和SPX3的表达，但在共生关系建立后，SPX1和SPX3蛋白仅在含有丛枝的细胞中表达。这种在含有丛枝细胞中的诱导，与顺式调控元件AW盒以及转录因子WRI5a的转录调控有关。在Pi限制条件下，SPX1和SPX3会促进独脚金内酯生物合成基因DWARF27的表达，这可能能够解释真菌分枝响应于根分泌物所引起的分枝增加表型。接着，在含有丛枝的细胞中，SPX1和SPX3控制丛枝的降解，并且存在功能冗余。因此，SPX蛋白作为磷酸盐传感器发挥重要作用，以维持有益的丛枝菌根共生。