Distribution of transcripts of thetub B1 β-tubulin gene in developing soybean (Glycine max [L.] Merr.) seedling organs

Summary

A previously cloned and sequenced soybean (Glycine max [L.] Merr.) β-tubulin gene, designatedtub B1, encodes a rather divergent β-tubulin protein and appears to be transcribed in a developmentally regulated manner. The steady state level oftub B1 gene transcripts in seedling tissues and organs was examined using a sensitive RNase protection assay and atub B1-gene-specific probe. A soybean ubiquitin gene probe was used as a control to insure that equivalent amounts of RNA were assayed from each sample. Consistent with a previous report of ours, we observed high levels oftub B1 gene transcripts only in the hypocotyls of etiolated seedlings. Illumination of the etiolated seedlings at any time between day three and six halted further hypocotyl elongation and reduced thetub B1 gene transcripts levels to near the detection limit of the RNase protection assay. Here we show further thattub B1 transcripts are confined to the apical-most region of the etiolated hypocotyl, including the hypocotyl hook and the subjacent region of cell elongation, and are absent in the lower half of the hypocotyl throughout its development. However, transcripts of thetub B1 gene also were detected in the cotyledons at low levels. Transcript levels did not change significantly during cotyledon development and the transcript levels were not significantly affected by environmental conditions. Illumination of the seedlings did not decreasetub B1 transcript levels in cotyledons, as it did in the hypocotyls, and may even have increased these slightly, as compared to transcript levels present in the cotyledons of etiolated seedlings. Low levels oftub B1 gene transcripts also were detected in the terminal 2 cm of the root, but the appearance oftub B1 gene transcripts in these tissues was transitory. Transcripts were most abundant in the root within the first few days of germination. They declined subsequently and were not detectable after the sixth day of germination. Transcripts of an additional five β-tubulin genes were detected in etiolated hypocotyls using a PCR technique. Transcripts of another two β-tubulin genes were found in light-grown hypocotyls, but not in etiolated hypocotyls. Thus, thetub B1 gene exhibits a complex, and apparently unique, pattern of expression. It exhibits apparently constitutive expression in cotyledons, temporally regulated expression in the root, and strong, organspecific expression in the hypocotyl, but only in dark-grown seedlings, and this hypocotyl expression is down-regulated dramatically by light.