Antagonistic Action of Strigolactone and Cytokinin in Bud Outgrowth Control  

Abstract

Cytokinin (CK) has long been implicated as a promoter of bud outgrowth in plants, but exactly how this is achieved in coordination with other plant hormones is unclear. The recent discovery of strigolactones (SLs) as the long-sought branchinhibiting hormone allowed us to test how CK and SL coordinately regulate bud outgrowth in pea (Pisum sativum). We found that SL-deficient plants are more sensitive to stimulation of bud growth by low concentrations of locally applied CK than wildtype plants. Furthermore, in contrast with SL mutant plants, buds of wild-type plants are almost completely resistant to stimulation by CK supplied to the vasculature. Regardless of whether the exogenous hormones were supplied locally or to the xylem stream, SL and CK acted antagonistically on bud outgrowth. These data suggest that SLs do not affect the delivery of CK to axillary buds and vice versa. Rather, these data combined with dose-response experiments suggest that SLs and CK can act directly in buds to control their outgrowth. These hormones may converge at a common point in the bud outgrowth regulatory pathway. The expression of pea BRANCHED1, a TCP transcription factor expressed strongly in buds and thought to act downstream of SLs in shoot branching, is regulated by CK and SL without a requirement for protein synthesis and in a manner that correlates with observed bud growth responses.Shoot branching is a major determinant of plant shoot architecture. Many factors contribute to the ability of an axillary bud to grow out to form a branch, including developmental, positional, genetic, hormonal, and environmental factors. Auxin, cytokinin (CK), and strigolactones (SLs) are implicated in the hormonal regulation of bud outgrowth; auxin and SLs as inhibitors of bud outgrowth and CK as a promoter of bud outgrowth (Dun et al., 2009a;Leyser, 2009;Beveridge and Kyozuka, 2010). Many studies over a number of decades have investigated the antagonistic action of auxin and CK in bud outgrowth control (ShimizuSato et al., 2009) and, more recently, the relationships between auxin and SL (Brewer et al., 2009;Crawford et al., 2010;Liang et al., 2010), but how SL and CK integrate to antagonistically control bud outgrowth remains unclear.Prior to their identification as hormones involved in shoot branching, certain properties of SLs were characterized based on studies of the long-distance branchinhibiting signal in a series of increased branching mutants. These mutants include ramosus (rms) in pea (Pisum sativum), more axillary growth (max) in Arabidopsis (Arabidopsis thaliana), decreased apical dominance (dad) in Petunia hybrida, and dwarf (d) and high tillering dwarf (htd) in rice (Oryza sativa; for review, see Dun et al., 2009a;Beveridge and Kyozuka, 2010;Domagalska and Leyser, 2011). Grafting studies demonstrated that the branch-inhibiting signal can be synthesized in root or shoot tissue, moves upward to inhibit bud outgrowth, and that a subset of the branching mutants are unable to synthesize the signal (now named SL synthesis mutants; rms1...

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

“…Together with auxin and cytokinin, the SLs and their derivatives mediate the remarkable plasticity of plant architecture that results from outgrowth of axillary branches Dun et al, 2012). In addition to having a hormonal role in plants, SLs stimulate development of arbuscular mycorrhizae and are essential for establishment of these symbioses in maize (Zea mays; GomezRoldan et al, 2007).…”

Using Arabidopsis to Study Shoot Branching in Biomass Willow

“…bud was treated with SL alone, it activated slightly earlier than untreated buds. This does not support a direct role for SL acting locally in the bud to inhibit growth (Brewer et al, 2009;Dun et al, 2012); rather, it argues for the auxin transport canalization model for the regulation of bud outgrowth and its modulation by SLs (Shinohara et al, 2013). Crawford et al (2010) proposed that SLs were acting to set the global context by which buds compete for auxin export into the main stem.…”

“…This could be achieved by influencing the transcription of growth-regulating genes in the bud (Aguilar-Martínez et al, 2007;Brewer et al, 2009;Braun et al, 2012;Dun et al, 2012) and/or by modulating auxin transport properties, both locally and systemically, thereby affecting the ability of buds to establish auxin transport canalization into the main stem (Bennett et al, 2006;Lazar and Goodman, 2006;Lin et al, 2009;Prusinkiewicz et al, 2009;Crawford et al, 2010;Marhavý et al, 2011;Shinohara et al, 2013). Activation of buds will in turn affect the amount of auxin transported out into the main stem.…”

Strigolactone Involvement in Root Development, Response to Abiotic Stress, and Interactions with the Biotic Soil Environment

“…This activity was shown by both computational model and experimental data, and it was correlated to the level of shoot branching observed in various mutant combinations and strigolactone treatments (Shinohara et al, 2013). In pea, strigolactones were shown to induce the expression of the budspecific target gene BRANCHED1, which encodes a transcription factor repressing bud outgrowth (Dun et al, 2012), and be an auxin-promoted secondary messenger (Brewer et al, 2009;Ferguson and Beveridge, 2009;Dun et al, 2012Dun et al, , 2013. Other activities of strigolactone include repression of adventitious root formation (Rasmussen et al, 2012) and plant height .…”