Abstract
Strigolactones (SLs) control lateral branching in diverse species by regulating transcription factors orthologous to Teosinte branched1 (Tb1). In maize (Zea mays), however, selection for a strong central stalk during domestication is attributed primarily to the Tb1 locus, leaving the architectural roles of SLs unclear. To determine how this signaling network is altered in maize, we first examined effects of a knockout mutation in an essential SL biosynthetic gene that encodes CAROTENOID CLEAVAGE DIOXYGENASE8 (CCD8), then tested interactions between SL signaling and Tb1. Comparative genome analysis revealed that maize depends on a single CCD8 gene (ZmCCD8), unlike other panicoid grasses that have multiple CCD8 paralogs. Function of ZmCCD8 was confirmed by transgenic complementation of Arabidopsis (Arabidopsis thaliana) max4 (ccd8) and by phenotypic rescue of the maize mutant (zmccd8::Ds) using a synthetic SL (GR24). Analysis of the zmccd8 mutant revealed a modest increase in branching that contrasted with prominent pleiotropic changes that include (1) marked reduction in stem diameter, (2) reduced elongation of internodes (independent of carbon supply), and (3) a pronounced delay in development of the centrally important, nodal system of adventitious roots. Analysis of the tb1 zmccd8 double mutant revealed that Tb1 functions in an SL-independent subnetwork that is not required for the other diverse roles of SL in development. Our findings indicate that in maize, uncoupling of the Tb1 subnetwork from SL signaling has profoundly altered the balance between conserved roles of SLs in branching and diverse aspects of plant architecture.
Highlights
Strigolactones (SLs) control lateral branching in diverse species by regulating transcription factors orthologous to Teosinte branched1 (Tb1)
To facilitate genetic analysis of SL signaling in maize, we focused on identifying steps in the maize SL pathway that were encoded by single genes
Mutants in SL biosynthesis and signaling increase branching in diverse plant species, including Arabidopsis, pea, and petunia, as well as tomato (Solanum lycopersicum; Vogel et al, 2010), kiwifruit (Actinidia deliciosa; Ledger et al, 2010), and rice (Wang and Li, 2011)
Summary
Strigolactones (SLs) control lateral branching in diverse species by regulating transcription factors orthologous to Teosinte branched (Tb1). SLs induce seed germination in a group of destructive, parasitic, flowering plants, Key constituents of the SL biosynthetic pathway and signaling network have emerged through analysis of branching mutants in multiple plant species (Goulet and Klee, 2010) These include the more axillary growth (max) mutants of Arabidopsis (Arabidopsis thaliana; Sorefan et al, 2003; Booker et al, 2004), ramosus (rms) mutants of pea (Pisum sativum; Foo et al, 2005), decreased apical dominance (dad) mutants of petunia (Petunia hybrida; Snowden et al, 2005; Drummond et al, 2009), and the dwarf (d) and high tillering dwarf (htd) mutants of rice (Oryza sativa; Zou et al, 2006; Arite et al, 2007). The role of SL signaling in maize architecture remains unclear
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