Plants have evolved a variety of mechanisms to adapt growth and development to the ambient light environment. To monitor the light, plants use several classes of photoreceptors (e.g. phytochromes, cryptochromes). Downstream of these photoreceptors, an important negative regulator, the COP1/SPA complex, suppresses light signaling in dark-grown Arabidopsis seedlings. This complex is an E3 ubiquitin ligase, i.e. an enzyme that attaches ubiquitin to other proteins which are subsequently recognized by the proteasome and degraded.  The COP1/SPA ubiquitin ligase inhibits light signaling in darkness by ubiquitinating activators of the light response, such as the transcription factors HY5, LAF1 and HFR1. cop1 and spa mutants fail to degrade these transcription factors in darkness and, therefore, undergo constitutive photomorphogenesis showing features of light-grown plants even in complete darkness.  In the light, active photoreceptors are thought to inhibit COP1/SPA function, thus HY5, LAF1 and HFR1 transcription factors are stabilized and regulate transcription of light-responsive genes (for review see Hoecker 2005).

One Ph.D. project focuses on the functional specificity provided by the four different SPA proteins (SPA1-SPA4).  We found that SPA1 and SPA2 primarily regulate seedling growth, while SPA3 and SPA4 control leaf expansion.  To unravel the molecular mechanism underlying functional specificity, the Ph.D. student will:

• conduct domain swaps between different SPA proteins and express these hybrid SPA proteins in transgenic plants
• identify and characterize proteins interacting with the different SPA proteins
• perform protein-protein interaction assays among components constituting an active E3 ubiquitin ligase

A second Ph.D. project addresses cell-cell communication and long-distance signaling in light-regulated plant development.  We found that expression of light signaling intermediates in phloem cells is sufficient to control seedling development and leaf expansion.  This project will investigate the involvement of candidate hormones in the control of light signaling.