Ranjan, Aashish (2010) The role of COP1/SPA in light signaling: Growth control, cell-cell communication and functional conservation in plants. PhD thesis, Universität zu Köln.
Light is one of the most important environmental factors affecting almost all stages of plant growth and development. Arabidopsis SPA and COP1 proteins act as repressors of light signaling in darkness. Members of the SPA protein family (SPA1-SPA4) can physically interact with COP1 and, together, they constitute a functional E3-ubiquitin ligase complex. The COP1/SPA complex regulates seedling development, stomata differentiation, leaf size and photoperiodic flowering in Arabidopsis by targeting transcription factors such as HY5, HFR1, CO etc. for degradation. In the present study, I investigated in which tissues SPA1 needs to be expressed to regulate different plant developmental processes. To this end, I expressed a GUS-SPA1 fusion protein under the control of various tissue-specific promoters (phloem, leaf-mesophyll, epidermis, meristem and root) in a spa mutant background and analyzed the transgenic plants for complementation of the spa mutant phenotype. The results show that SPA1 functions exclusively in the phloem to regulate photoperiodic flowering suggesting that SPA1 acts cell-autonomously in the phloem to target its substrate CO for degradation. To regulate the leaf size, SPA1 acts in both the phloem and the leaf mesophyll, but not in the epidermis indicating non-cell autonomous effects in SPA1-dependent leaf size regulation. Moreover, phloem-specific expression of SPA1 has major effects on seedling development in both darkness and light. Eventually, stomata differentiation and epidermal pavement cell shape are also regulated by phloem-specific functions of SPA1. These results indicate that cell-cell communication plays a very important role in SPA1-regulated plant developmental processes. SPA proteins and, therefore, the COP1/SPA complexes are plant specific. However, the function of COP1 and SPA proteins are not known in plant species other than the dicot Arabidopsis. In a second project, I examined the functionality of the COP1 and SPA proteins from the moss Physcomitrella and the monocot rice in Arabidopsis. To this end, I expressed the open reading frames of rice and Physcomitrella COP1 and SPA homologs in Arabidopsis cop1 and spa mutant plants, respectively, and then analyzed the transgenic plants for complementation of the respective mutant phenotypes. Rice and Physcomitrella COP1 homologs were functional in Arabidopsis, whereas SPA homologs from these species were not functional, suggesting a conserved basic mechanism of action of COP1, but functional divergence of SPA proteins during plant evolution. Interestingly, Physcomitrella COP1 and SPA proteins interact in vitro suggesting the possibility of formation of a COP1/SPA complex early in evolution.
Actions (login required)