Unfolding the Mysteries of Strigolactone Signaling

Phytohormones integrate plant growth and developmental programs with extrinsic biotic and abiotic stimuli, thereby ensuring plant architecture is optimized for the local environment. Strigolactones (SLs), newly identified carotenoid-derived plant hormones, play non-redundant and multifaceted roles throughout the plant life cycle (de Saint Germain et al., 2013), similarly to other plant hormones. Although SLs were originally identified as germination stimulants of the seeds of parasitic plants (e.g. Striga spp.), they were later shown to play a role in symbiotic plant–arbuscular mycorrhizal (AM) fungi interactions. SLs were officially classified as plant hormones in 2008 for their functions in inhibiting shoot branching. Since then, their versatility in modulating seed germination, seedling photomorphogenesis, shoot morphology, and root development has been widely explored. Elucidation of the biosynthetic and signaling pathways of SL has been greatly facilitated by identification of defective genes from branching mutants of several plant species including Arabidopsis max (more axillary growth) mutants, rice d (dwarf) mutants, pea r.m.s. (ramosus) mutants, and petunia dad (decreased apical dominance) mutants (de Saint Germain et al., 2013). D14/AtD14/DAD2, a member of the α/β-hydrolase family, may function as an SL receptor, and D3/MAX2/RMS4, an F-box protein in a Skp1–Cullin–F-box (SCF) ubiquitin E3 ligase complex, is required for SL responses, likely because SL signaling involves the regulated turnover of one of the signaling components. However, the target(s) of SCF(D3/MAX2) complex has remained elusive. Now, two new studies provide strong evidence that D53, a protein with homology to Clp ATPase and heat shock protein HSP101, is a target of MAX2-mediated degradation in the SL signaling pathway (Jiang et al., 2013; Zhou et al., 2013). A semi-dominant mutation in the rice D53 gene resulted in dwarf plants with increased tiller numbers, mimicking loss-of-function mutations in D14, the proposed receptor, as well as D3, a predicted F-box protein. Reduced expression of D53 can partially suppress the dwarf and increased tillering phenotypes of either d14 or d3 mutant, suggesting that D53 functions downstream of D14 and D3, and is a negative regulator of the SL signaling pathway. While D53 physically interacts with D3 in the presence or absence of SL, it binds D14 in a SL-dependent manner. Importantly, D53 is degraded through the proteasome-mediated pathway shortly following SL treatment; and its degradation requires functional D14 and D3 proteins. While it is unclear whether the three proteins, D14, D3, and D53, form a complex, the data show that, upon perception of SL by the receptor D14, the D53 repressor is targeted by SCFD3(MAX2) ubiquitin ligase complex and degraded by the proteasome-dependent pathway, allowing SL signaling to occur (Jiang et al., 2013; Zhou et al., 2013). D53 also interacts with the well-known transcriptional co-repressor TOPLESS (TPL) and its paralogs (TPR) (Causier et al., 2012; Jiang et al., 2013), suggesting that a D53–TPL complex may repress the activities of downstream transcription factors involved in SL signaling. This mechanism is analogous to what has been observed for the AUX/IAA–TPL complex in inhibiting the ARF transcription factors in auxin signaling and the JAZ–NINJA–TPL complex in inhibiting the MYC transcription factor in jasmonate signaling (Causier et al., 2012). A TCP transcription factor FC1 in rice (TB1 in maize, or BRC1 in Arabidopsis) negatively regulates the SL signaling pathway (de Saint Germain et al., 2013). Thus, it is very likely that the SL-triggered degradation of D53 may release the inhibitory effect of the D53–TPL complex on the transcriptional activity of FC1. The resulting active FC1 could then inhibit branching (Figure 1). Figure 1. Proposed Model for Strigolactone Signaling Pathways in Inhibiting Shoot Branching. A complication in this simple scenario is that SCF(D3/MAX2) has other functions as well. An exciting paper from Xuelu Wang and colleagues (2013b) demonstrated that MAX2 can recruit BES1, a transcription factor known from the brassinosteroid (BR) signaling pathway, for proteasome-mediated degradation (Figure 1). In the presence of exogenously added SL, BES1 interacts directly with MAX2, which induces BES1’s degradation via the proteasome-mediated pathway in a MAX2-dependent manner. This leads to inhibition of shoot branching (Wang et al., 2013b). SL also triggers a physical interaction between D14 and SLR1 (Nakamura et al., 2013), a rice DELLA protein initially characterized as the transcriptional repressor of gibberellin (GA) signaling. The biological significance of this interaction in SL signaling pathways remains to be determined. However, DELLA interacts with BES1 (Li et al., 2012), making it intriguing to speculate that perception of SL by D14 may facilitate the formation of a D14–SLR1(DELLA)–BES1 complex, which in turn inhibits the DNA binding activities of BES1 and represses shoot branching (Figure 1). Together, these new studies suggest more plausible mechanisms for how inputs from different hormone signaling pathways are integrated. First, it has been shown that TPL/TPR may directly interact with BES1 (Wang et al., 2013a). Thus, one possible scenario is that SL-promoted, D14-, and MAX2-dependent degradation of D53 may in turn enhance the formation of the TPL/TPR–BES1 complex, and the resulting repressed BES1 activity may contribute to inhibition of shoot branching (Figure 1). Second, it has long been acknowledged that the activated auxin signaling pathways can increase apical dominance, although the underlying molecular mechanisms are largely unknown. Here, we believe it is reasonable to conjecture that, in the absence of SL, the TPL/TPR–AUX/IAA co-repressor complex is favored to promote branching through either repressing SL biosynthesis or regulating the expression of downstream genes directly involved in branching; then, sensing SL allows plants to relocate more TPL/TPR into other complexes (e.g. TPL/TPR–BES1 complex) that eventually lead to inhibition of shoot branching (Figure 1). A better understanding of the signal transduction pathways of SL may also shed fresh light on those of Karrikins (KARs), a group of germination-stimulating compounds derived from the smoke of burning plants (Guo et al., 2013). KARs are sensed by a α/β-hydrolase KAI2, a homolog of D14, and also rely on MAX2 for signaling. SMAX1, an Arabidopsis homolog of D53, was identified from a max2 suppressor screen (Stanga et al., 2013). Thus, by analogy, the perception of KARs by KAI2 may induce the formation of a KAI2–MAX2 complex that targets different repressors for degradation, leading to distinct responses. A key question for future studies will be to learn how signaling specificity is achieved. The latest studies, described here, implicate several known plant hormones in the repression of shoot branching (Figure 1). One assumption of our model is that SL signaling prioritizes all those components in one direction—to inhibit shoot branching. Because plants are constantly being bombarded by diverse and simultaneous biotic and abiotic signals, it will be interesting to know how plants allocate the activities of versatile transcriptional regulators, such as DELLA, TPL/TPR, and BES1, to specific hormone signaling pathways. As sessile organisms, plants may have found a unique solution to coordinate their growth and development by fine-tuning the activities of shared key transcriptional regulators among various hormone signaling pathways.