Diverse Roles of Strigolactone Signaling in Maize Architecture and the Uncoupling of a Branching-Specific Subnetwork      

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.

Strigolactones fine-tune the root system

“…2) (Arite et al 2012;Guan et al 2012;Kretzschmar et al 2012). In support of the strigolactone-promoting primary root growth, Ruyter-Spira et al (2011) demonstrated that when grown without sucrose, GR24 applications trigger root growth in a MAX2-dependent manner.…”

Strigolactone biosynthesis and signaling in plant development

“…This transcription factor acts as a key integrator of several other pathways, such as the cytokinin pathway and the recently proposed sucrose signaling pathway in pea (Mason et al, 2014;Rameau et al, 2015). Interestingly, the maize ortholog (TB1) of the gene encoding this transcription factor seems to act independently of SLs to repress shoot branching (Guan et al, 2012). In rice, other transcription factors, such as MADS57 and IPA1/OsSPL14, that are involved in shoot branching have also been connected to key components of the SL signaling pathway, but whether these various transcription factors mediate SL signaling, and if or how they lie downstream of the D14-D3-D53 axis, are still not clear (Guo et al, 2013a;Lu et al, 2013).…”

Ecological relevance of strigolactones in nutrient uptake and other abiotic stresses, and in plant-microbe interactions below-ground

“…Accordingly, SL-deficient mutants have a higher lateral root density . They also have a shorter primary root, not only in Arabidopsis, but also in rice and maize (Arite et al 2012;Guan et al 2012;Ruyter-Spira et al 2011). These phenotypes could only be rescued by the application of the synthetic SL analogue GR24 to the SL biosynthesis mutants, but not in those affected in signalling, indicating that SLs regulate root architecture in a MAX2-dependent manner Koltai et al 2010;MayzlishGati et al 2012;Ruyter-Spira et al 2011).…”