IAA8 expression during vascular cell differentiation.

We report the characterization of a member of the auxin-induced IAA gene family from zinnia, designated zIAA8, which is expressed by mesophyll cells differentiating as tracheary elements in vitro. Transcription of zIAA8 is up-regulated within 3 h after cell isolation in inductive medium, indicating that cells perceive and respond to growth factor stimulus early in culture. Transcript levels of zIAA8 remain high through 72 h of culture in medium containing auxin and cytokinin or auxin alone, but low in medium containing only cytokinin or control medium lacking growth factors, demonstrating auxin-specific induction and consistent with lack of desensitization to prolonged auxin stimulation. In situ localization shows zIAA8 is localized to primary vasculature, root tips, and nascent leaves in zinnia seedlings. The observation that zIAA8 is expressed during vascular development in planta supports the hypothesis that expression early in culture reflects early events during normal vascular differentiation. The promoter of Arabidopsis IAA8 drives expression of the GUS reporter in a pattern in Arabidopsis similar to that for zIAA8 in zinnia, suggesting conservation of cis regulatory elements between the species and confirming the results from in situ localization. The vascular expression pattern of the IAA8 promoter in leaves mirrors the developmentally regulated auxin gradient in expanding leaf blades. The expression patterns of zIAA8 and IAA8 yield new insight into vascular development in vitro and in planta, and provide much needed markers for early vascular differentiation.