Induction kinetics of the nuclear proteins encoded by the early indoleacetic acid‐inducible genes, PS‐IAA4/5 and PS‐IAA6 , in pea ( Pisum sativum L.)

Abstract

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 35 S‐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.