Auxin Enhancement of mRNAs in Epidermis and Internal Tissues of the Pea Stem and Its Significance for Control of Elongation

The epidermis has been considered the site of auxin action on elongation of stems and coleoptiles. To try to identify mRNAs that might mediate auxin stimulation of cell enlargement, we compared, using in vitro translation assays, mRNA enhancement by indoleacetic acid (IAA) in the epidermis, with that in the internal tissues, of pea (Pisum sativum L., cv Alaska) third internode segments. We used seedlings that had been grown under red light, which enables the epidermis to be peeled efficiently from the internode. Most of the `early' IAA enhancements previously reported using etiolated peas, plus several hitherto undescribed enhancements, occur in both the epidermis and the internal tissue of the light-grown plants after 4 hours of IAA treatment. These enhancements, therefore, do not fulfill the expectation of elongation-specific mRNAs localized to the epidermis. One epidermis-specific IAA enhancement does occur, but begins only subsequent to 1 hour (but before 4 hours) of auxin treatment. Similarly, the previously mentioned IAA enhancements common to epidermis and internal tissue do not begin, in the light-grown plants, within 1 hour of IAA treatment. Since IAA stimulates elongation in light-grown internodes within 15 minutes, it appears that none of these mRNAs can be responsible for auxin induction of elongation. We confirmed, with our methods, the previous reports that some of these mRNAs are enhanced by IAA within 0.5 hour in etiolated internodes. This indicates that we could have detected an early enhancement in light-grown tissue had it occurred.