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Environmental Signals Modulate the Expression of an Indole-3-Acetic Acid Biosynthetic Gene in Erwinia herbicola

The ipdC gene of Erwinia herbicola strain 299R encodes an indolepyruvate decarboxylase involved in the biosynthesis of indole-3-acetic acid (IAA). Transcriptional fusions of ipdC to an ice nucleation reporter gene (inaZ) were used to study the expression of ipdC in vitro and in situ on plants. ipdC was expressed only at low levels in liquid media and independently of factors such as richness of the medium, pH, nitrogen availability, the presence of l-tryptophan or oxygen, and growth phase of the culture. However, the transcriptional activity of ipdC increased approximately 18-fold under low solute and matric potentials in culture. ipdC was also induced 32-fold on leaves of bean and tobacco and 1,000-fold on pear flowers. This is the first report of the plant-inducible transcription of a bacterial IAA biosynthetic gene. It strongly supports the role of ipdC, and thus that of the indole-3-pyruvic acid pathway, in IAA biosynthesis by strain 299R in situ. The plant induction and apparent regulation of ipdC by low water availability indicate that this gene, and presumably IAA synthesis, are involved in a response to conditions encountered by E. herbicola in its natural habitat on leaves.