The Arabidopsis D27‐LIKE1 is a cis / cis / trans ‐β‐carotene isomerase that contributes to Strigolactone biosynthesis and negatively impacts ABA level

The enzyme DWARF27 (D27) catalyzes the reversible isomerization of all-trans- into 9-cis-β-carotene, initiating strigolactone (SL) biosynthesis. Genomes of higher plants encode two D27-homologs, D27-like1 and -like2, with unknown functions. Here, we investigated the enzymatic activity and biological function of the Arabidopsis D27-like1. In vitro enzymatic assays and expression in Synechocystis sp. PCC6803 revealed a yet not reported 13-cis/15-cis/9-cis- and a 9-cis/all-trans-β-carotene isomerization. Although disruption of AtD27-like1 did not cause SL deficiency phenotypes, overexpression of AtD27-like1 in the d27 mutant restored the more-branching phenotype, indicating a contribution of AtD27-like1 to SL biosynthesis. Accordingly, generated d27 d27like1 double mutants showed more pronounced branching phenotype, compared to d27. The contribution of AtD27-like1 to SL biosynthesis is likely due to its formation of 9-cis-β-carotene that was present at higher levels in AtD27-like1 overexpressing lines. In contrast, AtD27-like1 expression correlated negatively with the content of 9-cis-violaxanthin, a precursor of abscisic acid (ABA), in shoots. Consistently, ABA levels were higher in shoots and also in dry seeds of the d27like1 and d27 d27like1 mutants. Transgenic lines expressing β-glucuronidase (GUS) driven by the AtD27LIKE1 promoter and transcript analysis of hormone-treated Arabidopsis seedlings unraveled that AtD27LIKE1 is expressed in different tissues and regulated ABA and auxin. Taken together, our work revealed a cis/cis-β-carotene isomerase that affects the content of both cis-carotenoid derived plant hormones ABA and SLs.