Fackendahl, Petra Christina (2012) Functional Analysis of Arabidopsis SPA Proteins in Plant Growth Control. PhD thesis, Universität zu Köln.
For successful development, plants have to respond appropriately to environmental signals such as light quality and quantity. COP1/SPA complexes are important repressors of light signalling in darkness but also prevent overstimulation of the plant by light. In Arabidopsis thaliana, the four SPA proteins act in concert with COP1 as E3 ubiquitin ligase to regulate seedling development, photoperiodic flowering and leaf growth by promoting the degradation of transcription factors like HY5, HFR1 or CO via the 26S proteasome. SPA proteins share a typical domain structure including a C-terminal WD-repeat domain for substrate binding, a central coiled-coil domain, which is essential for SPA-SPA and SPA-COP1 interaction and an N-terminal part containing a kinase-like motif of unknown function. In this study it was shown that the dwarfed phenotype of spa triple and weak cop1 mutants is caused by a reduction in the number and the size of epidermal and mesopyll cells. Genetic interaction studies indicated that hyper-accumulation of COP1/SPA targets contributes to the reduced leaf size of cop1 and spa mutants. SPA4 was shown to be the key regulator of COP1/SPA-regulated plant growth. Here, a structure function analysis of SPA4 revealed that the N-terminal part of the protein is dispensable for SPA4 function in seedling de-etiolation and leaf size control, whereas its coiled-coil domain is essential. In contrast, the SPA4 N-terminus is seemed to be involved in SPA4 protein de-stabilization in adult plants. It is known that the SPA1 N-terminus is dispensable for seedling de-etiolation, but necessary for photoperiodic flowering control. In the present study, I could show that the SPA1 N-terminus does not contribute to plant growth, but that it is involved in the light-induced de-stabilization of SPA1. Moreover, it was revealed that the coiled-coil domain is important for SPA1 degradation, suggesting that SPA de-stabilization is dependent on SPA-COP1 interaction. cop1 null mutants are seedling lethal and therefore Arabidopsis development depends on COP1 function. Here, the SPA proteins were shown to be important, but not essential for the survival of the plants as spa quadruple null mutants were tiny, but viable. This suggests that COP1 is at least partially functional without the SPA proteins or acts in concert with other factors, but not vice versa. Besides, a spa1 enhancer mutant was mapped to a 46 kb region on chromosome 3. 16 gene loci are remaining as putative novel regulator of light signal transduction.
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