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Abscisic acid (ABA) is a plant hormone found in all higher plants; it plays an important role in seed dormancy, embryo development, and adaptation to environmental stresses, most notably drought. The regulatory step in ABA synthesis is the cleavage reaction of a 9-cis-epoxy-carotenoid catalyzed by the 9-cis-epoxy-carotenoid dioxygenases (NCEDs). The parasitic angiosperm Cuscuta reflexa lacks neoxanthin, one of the common precursors of ABA in all higher plants. Thus, is C. reflexa capable of synthesizing ABA, or does it acquire ABA from its host plants? Stem tips of C. reflexa were cultured in vitro and found to accumulate ABA in the absence of host plants. This demonstrates that this parasitic plant is capable of synthesizing ABA. Dehydration of detached stem tips caused a big rise in ABA content. During dehydration, ^sup 18^O was incorporated into ABA from ^sup 18^O^sub 2^, indicating that ABA was synthesized de novo in C. reflexa. Two NCED genes, CrNCED1 and CrNCED2, were cloned from C. reflexa. Expression of CrNCEDs was up-regulated significantly by dehydration. In vitro enzyme assays with recombinant CrNCED1 protein showed that the protein is able to cleave both 9-cis-violaxanthin and 9'-cis-neoxanthin to give xanthoxin. Thus, despite the absence of neoxanthin in C. reflexa, the biochemical activity of CrNCED1 is similar to that of NCEDs from other higher plants. These results provide evidence for conservation of the ABA biosynthesis pathway among members of the plant kingdom.
Abscisic acid (ABA) is found in all higher plants and algae and is also produced by some fungi (Oritani and Kiyota, 2003; Schwartz and Zeevaart, 2004; Nambara and Marion-Poll, 2005). In higher plants, ABA is involved in seed dormancy, embryo development, and adaptation to various abiotic stresses. ABA is a sesquiterpenoid (C^sub 15^). In some fungi, there is a direct pathway from isopentenyl pyrophosphate (C^sub 5^) via farnesyl pyrophosphate (C^sub 15^; Oritani and Kiyota, 2003). Higher plants synthesize ABA via the C^sub 40^ indirect pathway. A C^sub 40^ carotenoid is oxidatively cleaved to form a C^sub 25^ byproduct and the C^sub 15^ precursor of ABA, xanthoxin. Biochemical and molecular evidence has shown that the cleavage reaction is the rate-limiting step in the ABA biosynthetic pathway.
Although the pathway ofABAbiosynthesis in higher plants has been well established, there are still a few...