IAA synthesis and root induction with iaa genes under heat shock promoter control.

We have devised a heat shock-inducible indole-3-acetic acid (IAA) synthesis system for plant cells, which is based on the iaa genes of the Agrobacterium tumefaciens T-DNA and the heat shock promoter hsp70 of Drosophila melanogaster. Two DNA constructs were tested: one contains the iaaM gene linked to the hsp70 promoter (hsp70-iaaM) and encodes the production of indoleacetamide (IAM), the other contains hsp70-iaaM and the wild-type iaaH gene which codes for the conversion of IAM into IAA (hsp70-iaaM/iaaH). Heat shock-controlled IAM and IAA synthesis was tested on two levels: biochemically by measuring IAM and IAA levels in Kalanchoe stem segments infected with the two constructs, and morphologically by IAA-dependent root formation on Kalanchoe plants, on carrot discs and on tobacco leaf fragments. At both levels the responses were found to be controlled by the heat shock promoter. IAM levels of segments infected with hsp70-iaaM increased 6-fold upon heat shock induction to 240 pmol IAM per stem segment. The accumulation of IAA in segments infected with hsp70-iaaM/iaaH and heat-shocked was found to be more variable, possibly due to IAA transport and metabolism. Heat shock treatment of Kalanchoe plants and tobacco leaf fragments infected with hsp70-iaaM/iaaH led to a strong increase in root formation. On carrot discs, heat shock-specific root induction was also demonstrated, but the responses differed between individual carrots.