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Plants sustain intimate relationships with diverse microbes. While it is well recognized that these plant-associated microbiota shape individual performance and fitness of host plants, how they exert their function and maintain their homeostasis are largely unexplored. Here, using the pink lady plant (Heterotis rotundifolia) as a model, we investigated the phenomenon of microbiota-mediated nitrogen fixa-tion as well as the proximal homeostasis mechanisms of this process in the aerial root mucilage. The aerial root mucilage is enriched in primary metabolites such as carbohydrates, and abundant diazo-trophic bacteria. Nitrogen-labeled experiments and gene expression analysis indicate that the aerial root-mucilage microhabitat fixes atmospheric nitrogen to support plant growth. The presence of a key "police" fungus in the mucilage helps the host plant to defend against environmental microbes rather than diazotrophs. The discovery of new biological function and “police” fungi in the aerial root-mucilage microhabitat provides insights into microbial homeostasis maintenance of microenvironmental function and rhizosphere ecology.

A potted experiment was conducted to study thegrowth and biomass of Rauvolfia vomitoria plant under different light intensities (15%,40% and 70% of full sunlight) and nitrogen levels (15g,30g and 60g per plant).The results show that thegrowth,individual biomass and biomass allocation are significantly influenced by light intensity and nitrogen level (P0.05).The plant height,ground diameter,relativegrowth rate of height andground diameter (RGR H,RGR D) and individual biomass under 70% light intensity are higher than those under 15% and 40% light intensity,and more biomass is allocated to underground plant part.The height,ground diameter,RGR H,RGR D and individual biomass decrease with increasing nitrogen level under 15% and 40% light intensity.Under 70% light intensity,R.vomitoria plantgrows best and individual biomass reaches the highest value of 559.6g at 30g nitrogen per plant.At the same light intensity,root mass ratio (RMR) and root to shoot ratio (R/S) decrease with increasing nitrogen level.On the other hand,specific leaf area (SLA) and leaf mass ratio (LMR) are highest at 15% light intensity and 60g nitrogen per plant.Analysis on individual biomass,RMR,R/S andgrowth characteristics including height,ground diameter,RGR H,RGR D reveals that 70% light intensity and 30g nitrogen per plant combination yields the optimum condition for R.vomitoria plants.