Abstract
Strigolactones (SLs) are carotenoid-derived plant hormones that regulate shoot branching, secondary growth, root development, and responses to soil phosphate. In Arabidopsis (Arabidopsis thaliana), SL biosynthesis requires the sequential action of two carotenoid cleavage dioxygenases, MORE AXILLARY GROWTH3 (MAX3) and MAX4, followed by a cytochrome P450, MAX1. In rice (Oryza sativa), the plastid-localized protein DWARF27 (OsD27) is also necessary for SL biosynthesis, but the equivalent gene in Arabidopsis has not been identified. Here, we use phylogenetic analysis of D27-like sequences from photosynthetic organisms to identify AtD27, the likely Arabidopsis ortholog of OsD27. Using reverse genetics, we show that AtD27 is required for the inhibition of secondary bud outgrowth and that exogenous application of the synthetic SL GR24 can rescue the increased branching phenotype of an Atd27 mutant. Furthermore, we use grafting to demonstrate that AtD27 operates on a nonmobile precursor upstream of MAX1 in the SL biosynthesis pathway. Consistent with the plastid localization of OsD27, we also show that AtD27 possesses a functional plastid transit peptide. We demonstrate that AtD27 transcripts are subject to both local feedback and auxin-dependent signals, albeit to a lesser extent than MAX3 and MAX4, suggesting that early steps in SL biosynthesis are coregulated at the transcriptional level. By identifying an additional component of the canonical SL biosynthesis pathway in Arabidopsis, we provide a new tool to investigate the regulation of shoot branching and other SL-dependent developmental processes.
Highlights
Strigolactones (SLs) are carotenoid-derived plant hormones that regulate shoot branching, secondary growth, root development, and responses to soil phosphate
Only one of these two alleles resulted in a clear mutant phenotype: in plants homozygous for the GK-134E08 insertion, adult plants exhibited a substantial increase in axillary rosette branches relative to wild-type controls, while GK
From a genetic point of view, we identified the sequence with greatest similarity to OsD27 (Fig. 1; Supplemental Fig. S1), obtained homozygous T-DNA insertion Atd27 mutant lines (Fig. 2), and complemented this mutant phenotype to the wild type with overexpression of AtD27 (Fig. 3; Supplemental Fig. S2)
Summary
Strigolactones (SLs) are carotenoid-derived plant hormones that regulate shoot branching, secondary growth, root development, and responses to soil phosphate. Recent in vitro evidence suggests that D27 is a b-carotene isomerase that converts all-trans-b-carotene into 9-cis-b-carotene, which is further processed by CCD7 and CCD8 into a SL precursor termed carlactone (Alder et al, 2012) If this biochemical activity is the case in planta, it is likely that D27 acts upstream of MAX1 in the control of the biosynthesis of a mobile shoot-branching signal. We identify the Arabidopsis ortholog of OsD27 and characterize it with respect to its SL response, expression profile, and regulation and use grafting to place it within the SL biosynthesis pathway
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