DWARF14, A Receptor Covalently Linked with the Active Form of Strigolactones, Undergoes Strigolactone-Dependent Degradation in Rice

Abstract

Strigolactones (SLs) are the latest confirmed phytohormones that regulate shoot branching by inhibiting bud outgrowth in higher plants. Perception of SLs depends on a novel mechanism employing an enzyme-receptor DWARF14 (D14) that hydrolyzes SLs and becomes covalently modified. This stimulates the interaction between D14 and D3, leading to the ubiquitination and degradation of the transcriptional repressor protein D53. However, the regulation of SL perception in rice remains elusive. In this study, we provide evidences that D14 is ubiquitinated after SL treatment and degraded through the 26S proteasome system. The Lys280 site of the D14 amino acid sequence was important for SL-induced D14 degradation, but did not change the subcellular localization of D14 nor disturbed the interaction between D14 and D3, nor D53 degradation. Biochemical and genetic analysis indicated that the key amino acids in the catalytic center of D14 were essential for D14 degradation. We further showed that D14 degradation is dependent on D3 and is tightly correlated with protein levels of D53. These findings revealed that D14 degradation takes place following D53 degradation and functions as an important feedback regulation mechanism of SL perception in rice.

An allelic series at the KARRIKIN INSENSITIVE 2 locus of Arabidopsis thaliana decouples ligand hydrolysis and receptor degradation from downstream signalling

“…Indeed, it would be of interest to know whether D14 exhibits the same apparent degree of hypersensitivity to amino acid substitutions as KAI2, as this might yield insight into how and why the mechanisms of receptor degradation differ. In this vein, a systematic series of conservative lysine‐to‐glutamic acid substitutions in D14 appeared to be well tolerated, at least when expressed transiently in rice protoplasts (Hu et al ., ).…”

“…Nevertheless, the propensity of KAI2 for proteolysis suggests that the process is biologically significant. The timing of events downstream of KAI2 is not well understood, at least in terms of the proposed degradation of SMAX1/SMXL2; however, strigolactone perception by D14 leads to degradation of D53/SMXL7 on a timescale of minutes (Jiang et al ., ; Zhou et al ., ; Wang et al ., ), whereas the degradation of D14 itself takes place over a period of hours (Chevalier et al ., ; Hu et al ., ). We therefore expect that receptor turnover provides a means to modulate signalling during development, rather than a requirement for signalling per se .…”

“…First, both KAI2 and D14 undergo protein degradation as part of the ligand perception process. In the case of D14, this process depends on the presence of MAX2 and on ubiquitination, but KAI2 degradation is MAX2‐independent and not related to ubiquitination (Chevalier et al ., ; Waters et al ., ; Hu et al ., ). It is also not clear for both KAI2 and D14 proteins whether receptor degradation is a direct consequence of signalling, whereby only functional, active receptors undergo degradation (perhaps simultaneously with SMXL proteins), or whether receptor degradation and signalling are distinct and separable processes.…”

Fellowship of the rings: a saga of strigolactones and other small signals

“…In addition to the subsequent recruitment and degradation of target proteins, the interactions with SCF MAX2 have also been implicated in D14 degradation. In rice, rac-GR24 treatment causes rapid (c. 4 h) D14 destabilisation in planta; D14 was polyubiquitinated and degraded following GR24 treatment, but this was impaired in d3 mutants (Hu et al, 2017). The same was observed in Arabidopsis, with rapid (c. 4 h) MAX2-and proteasome-dependent degradation of D14 induced by rac-GR24 treatment (Chevalier et al, 2014;Wang et al, 2015).…”

Rational design of novel fluorescent enzyme biosensors for direct detection of strigolactones

“…As an additional feedback mechanism, SL signalling also induces degradation of D14/DAD2 24 (Figure 1). D14 degradation is dependent on MAX2 and coupled to D53 degradation; however it remains unknown whether D14 is a direct ubiquitination target of the SL-induced SCF complex, or is the target of a ubiquitin ligase expressed in response to D53 degradation 25 . In the absence of SL, D53 (or SMXL6/7/8 orthologues) repress downstream SL signalling in complex with SPL transcription factor IPA1, and TPR corepressors.…”