Trade-offs between host growth rate and longevity in a plant-microbe mutualism

Correlations between microbiome composition and host physiology abound, yet experiments that show causal microbe-host relationships are relatively rare. Using the tiny aquatic plant duckweed, which grows by budding off new fronds, we compare the growth of duckweed plants that are reared germ-free (n=130 replicates), plants inoculated with their natural microbiome (n=130), and plants inoculated with microbial pathogens (n=60). These experiments were performed at high-throughput with automated duckweed growth chambers photographing each replicate three times per day. Tracking budding events (by hand and with machine learning) produces a distribution of division times between budding events as well as a distribution of the number of budding events per frond. These statistics, respectively, report on frond growth rate (divisions/time) and frond longevity (divisions/lifetime) and suggest a growth/longevity trade-off space that is modulated by microbiome composition. We find that germ-free duckweeds grow to lower total abundance than plants with their natural microbiome, and that pathogenic microbes strongly inhibit duckweed longevity.