COP1-Mediated Degradation of BBX22/LZF1 Optimizes Seedling Development in Arabidopsis    

Abstract

Light regulates multiple aspects of growth and development in plants. Transcriptomic changes govern the expression of signaling molecules with the perception of light. Also, the 26S proteasome regulates the accumulation of positive and negative regulators for optimal growth of Arabidopsis (Arabidopsis thaliana) in the dark, light, or light/dark cycles. BBX22, whose induction is both light regulated and HY5 dependent, is a positive regulator of deetiolation in Arabidopsis. We found that during skotomorphogenesis, the expression of BBX22 needs to be tightly regulated at both transcriptional and posttranslational levels. During photomorphogenesis, the expression of BBX22 transiently accumulates to execute its roles as a positive regulator. BBX22 protein accumulates to a higher level under short-day conditions and functions to inhibit hypocotyl elongation. The proteasome-dependent degradation of BBX22 protein is tightly controlled even in plants overexpressing BBX22. An analysis of BBX22 degradation kinetics shows that the protein has a short half-life under both dark and light conditions. COP1 mediates the degradation of BBX22 in the dark. Although dispensable in the dark, HY5 contributes to the degradation of BBX22 in the light. The constitutive photomorphogenic development of the cop1 mutant is enhanced in cop1BBX22ox plants, which show a short hypocotyl, high anthocyanin accumulation, and expression of light-responsive genes. Exaggerated light responsiveness is also observed in cop1BBX22ox seedlings grown under short-day conditions. Therefore, the proper accumulation of BBX22 is crucial for plants to maintain optimal growth when grown in the dark as well as to respond to seasonal changes in daylength.

The BBX subfamily IV

“…BBX22-GFP Protein analyses in the hy5-1 and cop1-4 mutants revealed that COP1 is required for protein degradation of BBX22 in the dark, while HY5 has a role in the degradation of BBX22 in light. 68 Consistent with these observations, phenotypic analyses of 35S:BBX22-GFPox-cop1 lines proved that the overexpression of BBX22-GFP enhanced the cop1 phenotype during skotomorphogenesis and photomorphogenesis. In addition, growth rate experiments for the bbx22 line showed that under short days the mutant displayed an increased growth rate compared with WT during the dark phase in day 4 after germination.…”

“…59,60 Experiments with cop1 alleles demonstrated that COP1 is required for correct protein turnover of BBX24 and BBX22. 60,68 …”

“…Model of BBX24 and BBX sub-family IV proteins mode of action and regulation. As potential transcriptional activators, 60,68 BBX24 and its close relatives probably require a DNA binding protein, such as HY5 to fulfill its function. 44,45 Once bound to the transcriptional complex, these BBX proteins will activate transcription in order to regulate diverse processes, such as photomorphogenesis, shade avoidance and light-dependent hypocotyl elongation.…”

“…This suggests a more prominent role for BBX22 in SD adaptive processes, by possibly transmitting the signal for attenuated hypocotyl elongation during the night. 68 A recent report links the BBX protein sub-family IV to the shade avoidance syndrome (SAS). 69 A T-DNA mutant screening for seedlings with longer hypocotyls under simulated shade, but with WT phenotype under white light and dark conditions, isolated a new bbx21-101 allele which displayed a longer hypocotyl than the WT when grown under shade.…”

HUA ENHANCER1 Is Involved in Posttranscriptional Regulation of Positive and Negative Regulators in Arabidopsis Photomorphogenesis

“…Total protein was extracted from 4-d-old etiolated seedlings as described (Chang et al, 2011) at different times of W light treatment. HY5 protein was detected by polyclonal anti-HY5 antibody (Liu et al, 2012).…”