Induction and superinduction of auxin-responsive mRNAs with auxin and protein synthesis inhibitors.

We have identified a class of small mRNAs (approximately 0.5 kilobases), referred to as small auxin-up RNAs (SAURs), that increases in abundance within minutes after auxin application to excised elongating hypocotyl sections of soybean. In this study, we present evidence that SAURs accumulate in the absence of auxin when protein synthesis is inhibited. Superinduction of SAURs occurs if the synthetic auxin 2,4-dichlorophenoxyacetic acid is added under conditions where protein synthesis is inhibited. Transcription run-on experiments with isolated nuclei show that, unlike 2,4-dichlorophenoxyacetic acid, protein synthesis inhibitors do not activate transcription of the SAUR genes. These results suggest that protein synthesis inhibitors act by stabilizing SAURs and that some labile protein(s) are involved in the rapid turnover of SAURs. This stabilization is not observed with GH3, another auxin-inducible mRNA. Whether induced with 2,4-dichlorophenoxyacetic acid or cycloheximide, SAURs are primarily expressed in epidermal and cortical cells of elongating hypocotyl sections, with little or no expression in vascular tissue. Unlike 2,4-dichlorophenoxyacetic acid-induced SAUR accumulation, the increase in abundance of SAURs induced by cycloheximide is transient, with a peak approximately 1 h after inhibitor addition. Complete inhibition of protein synthesis is not required for SAUR accumulation in the presence of protein synthesis inhibitors.