Medicago SPX1 and SPX3 recruit nitrogen-fixing microbes to promote growth through controlling flavonoid biosynthesis

Forming symbiotic associations with beneficial microbes are important strategies for sessile plants to acquire nitrogen and phosphorus nutrients from the soil. Root exudates play key roles on set-up of the rhizosphere microbiome. According to the needs for nitrogen or phosphorus, plants can adjust the root exudates composition to attract proper microbes. Flavonoids are a group of secondary metabolites that are well studied in shaping the root microbiome, especially the root nodule symbiosis in legumes. Here, we show the medicago truncatula phosphate sensors SPX1 and SPX3 regulate flavonoids biosynthesis to recruit nitrogen-fixing microbes for nitrogen acquisition. Nitrogen-fixing microbes were less recruited in spx1spx3 double mutant root rhizosphere. This was caused by lower flavonoids biosynthesis related genes expression, which resulted in lower flavonoids levels in the root exudates compared to wild type plant R108. Further analysis indicates the regulation of flavonoids biosynthesis is through the SPX1 and SPX3 interaction transcription factor PHR2. We propose the SPX-PHR phosphate homeostasis regulation network also control microbe-dependent nitrogen acquisition according to phosphate levels. Thus, SPX1 and SPX3 play important roles to keep a microbe-dependent nitrogen and phosphorus absorption balance for optimal growth. R108 wild type and spx1spx3 mutant roots grew in low Pi conditions

与有益微生物形成共生关联，是固着植物从土壤中获取氮、磷养分的重要策略。根系分泌物在根际微生物组的构建过程中发挥关键调控作用。植物可根据自身对氮或磷的需求，调整根系分泌物的组成以招募适配的微生物。类黄酮是一类在塑造根系微生物组方面被广泛研究的次生代谢物，尤其在豆科植物根瘤共生研究中备受关注。本研究发现，蒺藜苜蓿（Medicago truncatula）的磷感应蛋白SPX1与SPX3可通过调控类黄酮的生物合成，招募固氮微生物以获取氮素。spx1spx3双突变体的根际固氮微生物招募量显著低于野生型植株R108，该现象源于类黄酮生物合成相关基因的表达水平下调，致使其根系分泌物中的类黄酮含量较野生型R108更低。进一步分析表明，SPX1与SPX3通过与转录因子PHR2互作，实现对类黄酮生物合成的调控。我们提出，SPX-PHR磷稳态调控网络可根据土壤磷水平，同步调控依赖于微生物的氮素获取过程。综上，SPX1与SPX3在维持依赖于微生物的氮、磷吸收平衡以实现最优生长方面发挥关键作用。本实验中，R108野生型与spx1spx3突变体的根系均培养于低磷（Pi）条件下。