Plants have evolved a complex regulatory network to perceive and transmit light signals. In Arabidopsis, the COP1/SPA complex acts as a central repressor within this network. It forms part of a ubiquitin ligase that targets activators of the light response for degradation and thereby regulates processes such as seedling development, stomata differentiation, vegetative plant growth and the induction of flowering. But while light signal transduction has been extensively studied over the past decades, light regulation of the COP1/SPA complex is still not fully understood and in some aspects of plant development, additional regulators of the light response are yet to be identified.
In the first part of this study, I investigated the role of the SPA proteins within the COP1/SPA complex and their regulation by light. Light controls COP1 nucleocytoplasmic partitioning, but monitoring COP1 subcellular localisation in a spa quadruple mutant showed that its nuclear accumulation in darkness is not changed by the absence of the SPA proteins. However, analysis of protein levels revealed that SPA1 and SPA2 are themselves regulated by rapid, light-induced proteasomal degradation, suggesting that light inactivates COP1/SPA complexes in part by reducing SPA protein levels. SPA2 is more strongly degraded than SPA1, which correlates with the fact that SPA2, but not SPA1, loses its repressor function when seedlings are exposed to light. However, degradation is not the sole reason for the lack of SPA2 function in light-grown seedlings, implying that an additional post-translational mechanism must inactivate the remaining SPA2 protein
in the cell.
In the second part of this study, I characterised the Aux/IAA protein AXR3, a repressor of auxin signalling, as a novel regulator of light-dependent stomatal development. The axr3-1 gain-of-function mutant displays enhanced stomata formation in darkness, which results from increased cell divisions in the stomatal lineage. Epistasis analysis demonstrated that AXR3 acts genetically upstream of the YDA MAP kinase cascade, but in parallel with COP1, TMM and members of the ER family to regulate stomatal development.
Furthermore, this study showed that auxin is required for the suppression of stomata formation in darkness while light appears to counteract its effect. Taken together, these results imply that AXR3 regulates stomatal development in response to light and auxin signals although the mechanism of this regulation remains elusive
