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Just like humans, plants also have their own microbiome. Microorganisms that occur in plants are called endophytes. They increase the biomass, the photosynthesis rate and the nutrient supply, make the plant more stress-tolerant and produce useful compounds. Many known remedies or poisons are due to endophytes, e.g. the ergot poison and the cancer drug taxol.
Explanation for children: Microorganisms that occur in plants are called endophytes (endo = inside). Just like the bacteria in us, the bacteria in plants have many advantages: Nutrient uptake and photosynthesis are improved and sometimes endophytes make useful compounds.