Shape-Shifters: How Strigolactone Signaling Helps Shape the Shoot[OPEN]

When a deer eats the primary shoot of a plant, this can activate a nearby dormant axillary bud, causing it to formasecondary shoot. Genetic and environmental factors also affect shoot architecture, which strongly influences crop productivity. Changes in shoot architecture are mediated by long-distance signals, including phytohormones. For example, strigolactones (SLs) help regulate shoot branching, as well as branching angle, plant height, stem thickness, and leaf blade elongation (reviewed in Borghi et al., 2016). In general, little is known about how downstream targets of SLs regulate shoot architecture, including whether different responses occur via different pathways. A bit more is known about the proximal events in SL signaling. The SL receptor, the a/b-fold hydrolase DWARF14 (D14), appears to have an SL-dependent association with the F-box protein MAX2, likely forming part of a ubiquitin ligase complex that functions in SL signaling using a classic tactic: regulating target protein degradation by the 26S proteasome. The main (and perhaps only) proteolytic targets of SL signaling are SUPPRESSOR OF MORE AXILLARY GROWTH2-LIKE (SMXL) family members, such as SMXL7 in Arabidopsis thaliana. SMXL proteins contain evolutionarily conserved ETHYLENE-RESPONSE FACTOR Amphiphilic Repression (EAR) motifs, whichmight allow them to interact with their targets, although theunderlyingmechanism is unclear (reviewed in Bennett and Leyser, 2014). Using a powerful mechanistic approach, Liang et al. (2016) provide direct, in planta evidence in support of this D14-mediated SL signaling pathway. First, they assessed the subcellular localization of SMXL7 by fusing it to a fluorescent reporter gene and expressing it inNicotiana benthamiana epidermal cells, finding that it strongly localized to thenucleus. Thenuclear localization signal in SMXL7 is required for its function; transforming smxl max2mutants with wild-type SMXL7, but not a version lacking this signal, restored their branching patterns to wild-type levels. Similar analyses revealed that both D14 andMAX2 primarily localize to the nucleus. The interactions between these components were then explored in N. benthamiana leaf epidermal cells harboring a suite of reporter-protein constructs using Förster resonance energy transfer with fluorescence lifetime imaging microscopy. This technique can reveal whether two fluorophores are within several Effect of SMXL7 dose on shoot morphology in various mutants. Adult shoot morphology in smxl max2 (left) andmax2 (right) mutants untransformed (upper panels) and overexpressing SMXL7 (lower panels). (Reprinted from Liang et al. [2016], Figures 5I to 5L.)