Involvement of indole-3-acetic acid in fascicular and interfascicular cambial growth and interfascicular extraxylary fiber differentiation in Arabidopsis thaliana inflorescence stems

The effect of altering the auxin supply on the development and function of the vascular cambium and the differentiation of interfascicular extraxylary fibers was investigated in the inflorescence stem of Arabidopsis thaliana Columbia wild-type plants. Young, ca. 7-cm-tall inflorescence stems were either left untreated (control), decapitated 2 cm above their base and the stump apex treated with indole-3-acetic acid (IAA), or ringed 1 cm above their base with N-1-naphthylphthalamic acid (NPA), an auxin-transport inhibitor. At the beginning of the 4-wk experimental period, the stem base contained vascular bundles with a functioning fascicular cambium; the bundles were separated laterally by a band of immature, interfascicular extraxylary fibers. After ca. 1 wk, stem radial development, as manifested in xylem production by the fascicular cambium and in wall thickening and lignification of interfascicular extraxylary fibers, was finished in control inflorescence stems. Decapitating the inflorescence stem inhibited subsequent stem radial development in the remaining stump. Applying IAA to the stump apex markedly stimulated fascicular xylem production and completed interfascicular extraxylary fiber differentiation. Moreover, IAA application induced the production of interfascicular xylem by initiating interfascicular cambium. Ringing with NPA inhibited stem radial development below the ringing site, while it promoted interfascicular xylem production above. The results indicate that, in the Arabidopsis inflorescence stem, IAA (1) is required for wall thickening and lignification in interfascicular extraxylary fibers and for the initiation of interfascicular cambium and (2) stimulates both fascicular and interfascicular cambial growth.