Endogenous auxins and embryogenesis in Phaseolus coccineus

The occurrence and the dynamics of free and bound indole-3-acetic acid (IAA ) in different parts of Phaseolus coccineus L. seeds were investigated at v arious developmental stages. Results show that free and bound IAA content i n the single seed parts is quite different, and changes with different patt erns during seed growth. The highest concentration of total IAA was found i n early-heart stage embryos, while the total IAA concentration in the suspe nsor at early-heart stage is much lower than in the embryos, and remains al most constant in the later stages. Integuments have an intermediate content at the first two developmental stages, while showing the highest concentra tion of total IAA at the cotyledonary stage. Concerning the percentage of f ree IAA in relation to the total pool, we found that in the embryos free IA A accounts for 26-28% in the first two stages then increases up to 44% at t he cotyledonary stage. In suspensors, the percentage of free IAA is much hi gher (90%) in all developmental stages. We discuss the idea that the differ ent level of auxin between embryos and the suspensor might play an importan t role in the establishment of embryo polarity.