Abstract
Strigolactone (SL), auxin, and cytokinin (CK) are hormones that interact to regulate shoot branching. For example, several ramosus (rms) branching mutants in pea (Pisum sativum) have SL defects, perturbed xylem CK levels, and diminished responses to auxin in shoot decapitation assays. In contrast with the last of these characteristics, we discovered that buds on isolated nodes (explants) of rms plants instead respond normally to auxin. We hypothesized that the presence or absence of attached roots would result in transcriptional and hormonal differences in buds and subtending stem tissues, and might underlie the differential auxin response. However, decapitated plants and explants both showed similar up-regulation of CK biosynthesis genes, increased CK levels, and down-regulation of auxin transport genes. Moreover, auxin application counteracted these trends, regardless of the effectiveness of auxin at inhibiting bud growth. Multivariate analysis revealed that stem transcript and CK changes were largely associated with decapitation and/or root removal and auxin response, whereas bud transcript profiles related more to SL defects. CK clustering profiles were indicative of additional zeatin-type CKs in decapitated stems being supplied by roots and thus promoting bud growth in SL-deficient genotypes even in the presence of added auxin. This difference in CK content may explain why rms buds on explants respond better to auxin than those on decapitated plants. We further conclude that rapid changes in CK status in stems are auxin dependent but largely SL independent, suggesting a model in which auxin and CK are dominant regulators of decapitation-induced branching, whereas SLs are more important in intact plants.
Highlights
Strigolactone (SL), auxin, and cytokinin (CK) are hormones that interact to regulate shoot branching
In relation to bud growth responses, the pea (Pisum sativum) SL biosynthesis mutant ramosus1 is hypersensitive to exogenous CK and this hypersensitivity is counteracted by SL addition (Dun et al, 2012)
Increased shoot branching in loss of function brc1 mutants of Arabidopsis, rice, and pea is not rescued by SL addition (Brewer et al, 2009; Minakuchi et al, 2010; Braun et al, 2012), indicating that SL signaling is dependent on BRC1
Summary
Strigolactone (SL), auxin, and cytokinin (CK) are hormones that interact to regulate shoot branching. CK clustering profiles were indicative of additional zeatin-type CKs in decapitated stems being supplied by roots and promoting bud growth in SL-deficient genotypes even in the presence of added auxin. This difference in CK content may explain why rms buds on explants respond better to auxin than those on decapitated plants. In relation to bud growth responses, the pea (Pisum sativum) SL biosynthesis mutant ramosus (rms; orthologous to more axillary branches4 [max4] in Arabidopsis [Arabidopsis thaliana] and dwarf in rice [Oryza sativa]) is hypersensitive to exogenous CK and this hypersensitivity is counteracted by SL addition (Dun et al, 2012). The consensus view is that TB1/BRC1 and related genes may be conserved branching regulators
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