Arabidopsis mutants affected in the transcriptional control of allene oxide synthase, the enzyme catalyzing the entrance step in octadecanoid biosynthesis.

Allene oxide synthase (AOS) catalyzes the entrance reaction in the biosynthesis of the octadecanoids 12-oxophytodienoic acid (OPDA) and jasmonic acid (JA). The enzyme is feedback-regulated by JA and thus a target of the JA-signalling pathway. A fusion genetic approach was used to isolate mutants in this signalling pathway. Seeds from transgenic Arabidopsis thaliana plants expressing the Escherichia coli uidA gene encoding beta-glucuronidase (GUS) under the control of the AOS promoter were mutagenized with ethylmethane sulfonate and the progeny was screened for individuals exhibiting constitutive expression of uidA in the absence of an added octadecanoid. From 21,000 mutagenized plants, 8 lines showing constitutive AOS expression were obtained. The mutant lines were characterized further and fell into four classes, I to IV. All showed signs of growth inhibition encompassing both shoot and root systems, and accumulated higher than normal levels of OPDA. Mutants belonging to classes I and IV failed to set seeds due to defects in flower development which prevented self-pollination. One mutant, designated cas1, was characterized in more detail and showed, in addition to elevated levels of AOS mRNA, AOS polypeptide, OPDA, and JA, constitutive expression of JA-responsive genes ( VSP2, PDF1.2). The cas1 mutation is recessive and affects a single locus. Using cleaved amplified polymorphic sequences (CAPS) and simple sequence length polymorphisms (SSLP), the mutated gene was mapped to chromosome IV next to the SSLP marker CIW7.