The seed protein IAP1 from bean (PvIAP1; Phaseolus vulgaris L.) that is modified by the phytohormone indole-3-acetic acid (IAA) was heterologously expressed in the two reference plant species Arabidopsis thaliana and Medicago truncatula. For the transformation of Medicago we devised a novel protocol using seedling infiltration. When PvIAP1 was overexpressed under the control of the constitutive 35SCaMV promoter in Arabidopsis, the plants showed signs of earlier bolting and enhanced branching. Expression of a fusion protein of PvIAP1 with both a green fluorescence protein (GFP) as reporter and 6x histidine (His) tag under the control of the native bean IAP1 promoter resulted in the accumulation of the protein in both plant species exclusively in seeds as shown by immunoblotting and by fluorescence microscopy. During seed development, PvIAP1 was first expressed in the vascular bundle of Arabidopsis, whereas in later stages GFP fluorescence was visible essentially in all tissues of the seed. Fluorescence decreased rapidly after imbibition in the seeds for both Arabidopsis and Medicago, although the fluorescence persisted longer in Arabidopsis. GFP fluorescence was distributed evenly between an organelle fraction, the microsomal membrane fraction, and the cytosol. This was also confirmed by immunoblot analysis. Clusters of higher GFP fluorescence were observed by confocal microscopy. Although PvIAP1 protein accumulated in seeds of both Arabidopsis and Medicago, neither species post-translationally modified the protein with an indoleacyl moiety as shown by quantitative GC-MS analysis after alkaline hydrolysis. These results indicate an apparent specificity for IAA attachment in different plant species.