Tissue-specific and Organ-specific Expression of Soybean Auxin-responsive Transcripts GH3 and SAURs

We used in situ hybridization to localize two classes of auxin-regulated transcripts, GH3 and SAURs, within organs and tissues of soybean seedlings and flowers. GH3 transcripts occurred in the inner cortex and protoxylem ridges of roots and were expressed transiently during flower and pod development. SAUR transcripts were expressed in the epidermis, cortex, and starch sheath of epicotyls and immature hypocotyls. SAUR transcripts became more abundant on the bottom side of hypocotyls that were undergoing gravitropic curvature. SAURs were also expressed in developing xylem elements of the hypocotyl hook. When soybean organ sections were treated with 50 micromolar 2,4-dichlorophenoxyacetic acid (2,4-D), GH3 transcripts became more abundant in the vascular regions of all organs analyzed. High levels of GH3 transcripts were also found in developing palisade mesophyll cells of leaves, cotyledons, and flowers treated with 2,4-D. SAUR transcripts became more abundant in the epidermis, cortex, starch sheath, and pith of epicotyls and hypocotyls after 2,4-D treatment. Our results showed that a variety of tissues and cell types express auxin-responsive transcripts and that different tissues respond rapidly to exogenous auxin by expressing different hormone-responsive genes.