The plant hormone indoleacetic acid (IAA) rapidly induces transcription of two genes, PS-IAA4/5 and PS-IAA6, in pea that encode nuclear proteins. The proteins were expressed in Escherichia coli and polyclonal antibodies were raised. The proteins can neither be detected on immunoblots of pea extracts from IAA-treated epicotyls nor subcellularly localized by immunofluorescence, suggesting that they are of low abundance. However, they can be immunoprecipitated as 35S-methionine-labeled proteins synthesized in vivo from control and IAA-treated tissue segments. Short-term time-course experiments indicate that the amounts of PS-IAA4/5 and PS-IAA6 proteins decrease dramatically in non-lAA-treated tissue. However, the hormone slightly increases the PS-IAA4/5 and significantly enhances the PS-IAA6 proteins compared with the initial amounts present in the tissue, despite a large induction of both mRNAs. A net increase in the amount of the in vivo synthesized PS-IAA6 is observed after a lag period of 30 min after addition of IAA. Little or no PS-IAA4/5 or PS-IAA6 protein is detected after 6 h of induction, even though PS-IAA4/5 and PS-IAA6 mRNAs remain detectable. Immunoprecipitation of in vitro translated polypeptides with mRNAs from various auxin-treated and untreated mono- and dicotyledonous plants reveals that similar proteins are encoded by constitutive or IAA-induced mRNAs. Phylogenetic analysis of 10 PS-IAA4-like proteins from various plant species reveals that the PS-IAA4 and PS-IAA6 proteins belong to different lineages, suggesting that they may have distinct functions. The data suggest that as a primary response to IAA plant tissues produce short-lived nuclear proteins whose synthesis is regulated at the transcriptional and post-transcriptional levels.