The seed is the most important plant reproductive unit responsible for the evolutionary success of flowering plants. Aside from its essential function in the sexual reproduction of plants, the seed also represents the most economically important agricultural product worldwide, providing energy, nutrients, and raw materials for human nutrition, livestock feed, and countless manufactured goods. Hence, improvements in seed quality or size are highly valuable, due to their economic potential in agriculture. Recently, the importance of indolic compounds in regulating these traits has been reported for Arabidopsis thaliana. The transcriptional and physiological mechanisms involved, however, remain largely undisclosed. Potassium transporters have been suggested as possible mediators of embryo cell size, controlling turgor pressure during seed maturation. In addition, it has been demonstrated that the expression of K+ transporters is effectively regulated by auxin. Here, we provide evidence for the identification of two Arabidopsis K+ transporters, HAK/KT12 (At1g60160) and KUP4 (At4g23640), that are likely to be implicated in determining seed size during seed maturation and, at the same time, show a differential regulation by indole-3-acetic acid and indole-3-acetamide.