Genetic analysis of β-carotene isomerase in Arabidopsis thaliana.

Abscisic acid (ABA) and strigolactones (SLs) are carotenoid-derived plant hormones with important roles in plant physiological responses. Abscisic acid is involved in drought stress responses and maintaining seed dormancy, whereas SLs are biosynthesized under nutrient-poor conditions and affect plant branching and germination of root parasitic plants. Both ABA and SLs are synthesized from β-carotene via pathways sharing common isomerization and oxidative cleavage reactions. The enzyme D27 catalyzes the isomerization reaction during SL biosynthesis. However, the enzyme involved in the isomerization reaction during ABA biosynthesis remains unclear. Arabidopsis thaliana has two D27 homologs, D27LIKE1 (D27L1) and D27LIKE2 (D27L2). While D27L1 is reported to partially overlap in function with D27, the function of D27L2 remains unknown. Here, we generated single, double, and triple mutants of these genes and examined their physiological functions and involvement in ABA/SL biosynthesis. The results suggested these genes contribute to SL biosynthesis, with functions that complement one another. This would allow each gene to be appropriately regulated in organs requiring SLs. In contrast, our data also indicated that these genes are not involved in ABA biosynthesis. These findings provide important insights into the understanding of carotenoid-derived hormone biosynthesis by demonstrating that D27 homologs are selectively involved in SL, but not ABA, biosynthesis.