D27-like carotenoid isomerases: at the crossroads of Strigolactone and ABA biosynthesis.

Strigolactones and ABA are apocarotenoid-derived plant hormones. Their biosynthesis starts with the conversion of trans-carotenes into cis forms, which serve as direct precursors. Iron-containing DWARF27 isomerases were shown to catalyse or contribute to the trans/cis conversions of these precursor molecules. D27 converts trans-β-carotene into 9-cis-β-carotene which is the first committed step in strigolactone biosynthesis. Recent studies found that its paralog, D27-LIKE1 also catalyses this conversion. A crucial step in ABA biosynthesis is the oxidative cleavage of 9-cis-violaxanthin and/or 9-cis-neoxanthin which are formed from their trans isomers by unknown isomerases. Several lines of evidence point out that D27-like proteins directly, or indirectly contribute to 9-cis-violaxanthin conversion and eventually, ABA biosynthesis. Apparently, the diversity of D27-like enzymatic activity is essential for the optimization of cis/trans ratios, and hence, act to maintain apocarotenoid precursor pools. In this review we discuss the functional divergence and redundancy of D27 paralogs and their potential direct contribution to ABA precursor biosynthesis. We provide updates on their gene expression regulation and alleged Fe-S cluster binding feature. Finally, we conclude that the functional divergence of these paralogs is not fully understood and we give an outlook on potential directions in research.