Photoreceptor Specificity in the Light-Induced and COP1-Mediated Rapid Degradation of the Repressor of Photomorphogenesis SPA2 in Arabidopsis

Abstract

The Arabidopsis COP1/SPA E3 ubiquitin ligase is a key negative regulator that represses light signaling in darkness by targeting transcription factors involved in the light response for degradation. The COP1/SPA complex consists of COP1 and members of the four-member SPA protein family (SPA1-SPA4). Genetic analysis indicated that COP1/SPA2 function is particularly strongly repressed by light when compared to complexes carrying the other three SPAs, thereby promoting a light response after exposure of plants to extremely low light. Here, we show that the SPA2 protein is degraded within 5–15 min after exposure of dark-grown seedlings to a pulse of light. Phytochrome photoreceptors are required for the rapid degradation of SPA2 in red, far-red and also in blue light, whereas cryptochromes are not involved in the rapid, blue light-induced reduction in SPA2 protein levels. These results uncover a photoreceptor-specific mechanism of light-induced inhibition of COP1/SPA2 function. Phytochrome A (phyA) is required for the severe blue light responsiveness of spa triple mutants expressing only SPA2, thus confirming the important role of phyA in downregulating SPA2 function in blue light. In blue light, SPA2 forms a complex with cryptochrome 1 (cry1), but not with cryptochrome 2 (cry2) in vivo, indicating that the lack of a rapid blue light response of the SPA2 protein is only in part caused by a failure to interact with cryptochromes. Since SPA1 interacts with both cry1 and cry2, these results provide first molecular evidence that the light-regulation of different SPA proteins diverged during evolution. SPA2 degradation in the light requires COP1 and the COP1-interacting coiled-coil domain of SPA2, supporting that SPA2 is ubiquitinated by COP1. We propose that light perceived by phytochromes causes a switch in the ubiquitination activity of COP1/SPA2 from ubiquitinating downstream substrates to ubiquitinating SPA2, which subsequently causes a repression of COP1/SPA2 function.

SINAT E3 Ligases Control the Light-Mediated Stability of the Brassinosteroid-Activated Transcription Factor BES1 in Arabidopsis

“…Interestingly, dark treatment reduced the levels of SINATs (Figures 5H and S5N–S5P), indicating that SINATs are regulated by light signals to control BES1 degradation and BR signaling. As was previously reported, autodegradation of E3 ligases induced by specific signals is a general regulatory mechanism existing in both animals and plants (Chen et al, 2015; David et al, 2006; Yu et al, 2015). Our results indicated that light promotes the accumulation of SINATs, with the exception of SINAT5 (Figures 5H and S5M–S5P), which is catalytically inactive, suggesting that darkness may induce the autodegradation of SINAT2, −4, −3, and −1.…”

Light Controls Cytokinin Signaling via Transcriptional Regulation of Constitutively Active Sensor Histidine Kinase CKI1

“…Based on the differences in the rates of root growth between phyA phyB and wild type in the darkness, the authors suggested a role for the inactive (Pr) form of phytochromes in the root elongation. Nevertheless, these changes were not observed in other phyA mutants like phyA-201 (Ler ecotype), phyA-101, and phyA-105 (both RLD ecotype; Reed et al, 1994;Hoecker et al, 1998;Chen et al, 2015). Thus, we cannot exclude that the changes observed during skotomorphogenesis of phyA-211 could be allele/ecotype specific.…”

Signaling mechanisms of plant cryptochromes in Arabidopsis thaliana

“…The exact mechanism of how the E3 ubiquitin ligase activity of COP1 is suppressed by strengthened interaction between CRY2 and COP1 remains unclear. CRY1 and CRY2 also bind with the other SPA proteins (SPA2-4), but binding affinities to other SPA proteins vary between CRY1 and CRY2 (Lian et al 2011; Liu et al 2011a, b; Zuo et al 2011; Chen et al 2015), although the results are inconsistent among the experiments. The difference in CRY1 and CRY2 binding affinity to SPA proteins may explain the functional specificity of CRY1 and CRY2.…”

“…Photoreceptor interactions and the convergence of photoreceptor signaling at the COP1/SPA complex provide strong clues for molecular mechanism. Additionally, it has been recently reported that phyA predominantly degrades SPA2 protein to inactivate the COP1/SPA2 complex not only in far-red light but also in blue light (Chen et al 2015). Both phyA and phytochrome B, another major phytochrome species, are required for red light induced SPA2 degradation.…”