COP1-mediated degradation of BBX22/LZF1 optimizes seedling development in Arabidopsis.

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.

LZF1, a HY5-regulated transcriptional factor, functions in Arabidopsis de-etiolation.

We surveyed differential gene expression patterns during early photomorphogenesis in both wild-type and mutant Arabidopsis defective in HY5, an influential positive regulator of the responses of gene expression to a light stimulus, to identify light-responsive genes whose expression was HY5 dependent. These gene-expression data identified light-regulated zinc finger protein 1 (LZF1), a gene encoding a previously uncharacterized C2C2-CO B-box transcriptional regulator. HY5 has positive trans-activating activity toward LZF1 and binding affinity to LZF1 promoter in vivo. HY5 is needed but not sufficient for the induction of LZF1 expression. Anthocyanin content is significantly diminished in lzf1 under far red, which is the most efficient light for the induction of LZF1. The expression of PAP1/MYB75 is elevated in plants overexpressing LZF1, which leads to the hyperaccumulation of anthocyanin in transgenic Arabidopsis. The transition from etioplast to chloroplast and the accumulation of chlorophyll were notably compromised in the lzf1 mutant. We provide molecular evidence that LZF1 influences chloroplast biogenesis and function via regulating genes encoding chloroplast proteins. In the absence of HY5, mutation of LZF1 leads to further reduced light sensitivity for light-regulated inhibition of hypocotyl elongation and anthocyanin and chlorophyll accumulation. Our data indicate that LZF1 is a positive regulator functioning in Arabidopsis de-etiolation.