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 various developmental stages. Results show that free and bound IAA content in the single seed parts is quite different, and changes with different patterns during seed growth. The highest concentration of total IAA was found in early-heart stage embryos, while the total IAA concentration in the suspensor at early-heart stage is much lower than in the embryos, and remains almost constant in the later stages. Integuments have an intermediate content at the first two developmental stages, while showing the highest concentration of total IAA at the cotyledonary stage. Concerning the percentage of free IAA in relation to the total pool, we found that in the embryos free IAA accounts for 26-28% in the first two stages then increases up to 44% at the cotyledonary stage. In suspensors, the percentage of free IAA is much higher (90%) in all developmental stages. We discuss the idea that the different level of auxin between embryos and the suspensor might play an important role in the establishment of embryo polarity.