ABA Is Synthesized from a Carotenoid Intermediate ABA biosynthesis takes place in chloroplasts and other plastids via the pathway depicted in Figure 23.2. Several ABA-deficient mutants have been identified with lesions at specific steps of the pathway. These mutants exhibit abnormal phenotypes that can be corrected by the appli- cation of exogenous ABA. For example, flacca (flc) and sitiens (sit) are “wilty mutants” of tomato in which the ten- dency of the leaves to wilt (due to an inability to close their stomata) can be prevented by the application of exogenous ABA. The aba mutants of Arabidopsis also exhibit a wilty phenotype. These and other mutants have been useful in elucidating the details of the pathway (Milborrow 2001).
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