Estrogenicity of soybeans (Glycine max) varies by plant organ and developmental stage

Phytoestrogens are a broad group of environmentally labile plant molecules that can influence development and reproduction of animals by binding estrogen and ecdysone receptors. In plants, phytoestrogens regulate physiological mechanisms such as auxin transport and mediate ecological interactions including attraction of symbionts. Most analytical phytoestrogen studies focus on quantifying particular phytoestrogen compounds (e.g., genistein) rather than measuring the accompanying biological estrogenic activity, and studies that evaluate temporal or spatial estrogenic variation in plants are rare. Therefore, in the present study, variation in estrogenic activity of field-grown soybeans (Glycine max cv Shirofumi) was examined across plant parts and life stages. Ethanolic organ extracts were analyzed for estrogenicity in a yeast reporter assay expressing human estrogen receptors α (ERα) or β (ERβ). All tissues were significantly more active with ERβ than ERα. The estrogenicity of roots, leaves, and shoots varied significantly across growth stages. Roots were the most estrogenic organ, being one order of magnitude more estrogenic than leaves, and up to two orders of magnitude higher than shoots, immature seeds, stems, flowers, and pods. We propose that estrogenic molecules are abundant where they participate in a plant’s outreach activities: recruitment of mutualists, allelopathy, defense, or synthesis for systemic distribution. Estrogenic activity is lower in plant organs that use phytoestrogens mainly for local regulation. We interpret our results in the context of plant chemical ecology to explain how and why plants vary their estrogenic activity and what this means for plant–animal co-evolution.