Gossypol biosynthesis in cotton revealed through

21 Gossypol plays an important role in defense mechanism of Gossypium species and the 22 presence of gossypol also limits the utilization of cottonseeds. However, little is 23 known about the metabolism of gossypol in cotton plant. Here, Detection on the 24 dynamic tendency of gossypol content illustrated that at the germination stage, the 25 main source of gossypol was cotyledon, and at the later stages, gossypol mainly came 26 from root system. Plant grafting between cottons and sunflower proved that gossypol 27 was mainly synthesized in the root systems of cotton plants and both of the glanded 28 and glandless cottons had the ability of gossypol biosynthesis. Besides, the pigment 29 glands expression was uncoupled with gossypol biosynthesis. Root tip and rootless 30 seedling organ culture in vitro further revealed other parts of the seedlings also got the 31 ability to synthesize gossypol except root system. Moreover, root system produced the 32 racemic gossypol and plant synthesized the optically active gossypol. The expression 33 profiling of key genes in the gossypol biosynthetic pathway suggested that 34 downstream key genes had relatively high expression levels in root systems which 35 confirmed that gossypol was mainly synthesized in the root systems. Taken together, 36 our results helped to clarify the complex mechanism of gossypol metabolism. 37