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)/

The plant hormone indoleacetic acid (IAA) rapidly induces transcriptio n of two genes, PS-IAA4/5 and PS-IAA6, in pea that encode nuclear prot eins. The proteins were expressed in Escherichia coil and polyclonal a ntibodies were raised. The proteins can neither be detected on immunob lots of pea extracts from IAA-treated epicotyls nor subcellularly loca lized by immunofluorescence, suggesting that they are of low abundance . However, they can be immunoprecipitated as S-35-methionine-labeled p roteins synthesized in vivo from control and IAA-treated tissue segmen ts. Short-term time-course experiments indicate that the amounts of PS -IAA4/5 and PS-IAA6 proteins decrease dramatically in non-IAA-treated tissue. However, the hormone slightly increases the PS-IAA4/5 and sign ificantly enhances the PS-IAA6 proteins compared with the initial amou nts 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 obser ved after a lag period of 30 min after addition of IAA. Little or no P S-IAA4/5 or PS-IAA6 protein is detected after 6 h of induction, even t hough PS-IAA4/5 and PS-IAA6 mRNAs remain detectable. Immunoprecipitati on of in vitro translated polypeptides with mRNAs from various auxin-t reated and untreated mono- and dicotyledonous plants reveals that simi lar proteins are encoded by constitutive or IAA-induced mRNAs. Phyloge netic analysis of 10 PS-IAA4-like proteins from various plant species reveals that the PS-IAA4 and PS-IAA6 proteins belong to different line ages; suggesting that they may have distinct functions. The data sugge st 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.