Light exposure of arabidopsis seedlings causes rapid de-stabilization as well as selective post-translational inactivation of the repressor of photomorphogenesis SPA2

Martin Balcerowicz, Kirsten Fittinghoff, Lennart Wirthmueller, Alexander Maier, Petra Fackendahl, Gabriele Fiene, Csaba Koncz, Ute Hoecker*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

42 Scopus citations


Summary The COP1/SPA complex acts as an E3 ubiquitin ligase to repress photomorphogenesis by targeting activators of the light response for degradation. Genetic analysis has shown that the four members of the SPA gene family (SPA1-SPA4) have overlapping but distinct functions. In particular, SPA1 and SPA2 differ in that SPA1 encodes a potent repressor in light- and dark-grown seedlings, but SPA2 fully loses its function when seedlings are exposed to light, indicating that SPA2 function is hyper-inactivated by light. Here, we have used chimeric SPA1/SPA2 constructs to show that the distinct functions of SPA1 and SPA2 genes in light-grown seedlings are due to the SPA protein sequences and independent of the SPA promoter sequences. Biochemical analysis of SPA1 and SPA2 protein levels shows that light exposure leads to rapid proteasomal degradation of SPA2, and, more weakly, of SPA1, but not of COP1. This suggests that light inactivates the COP1/SPA complex partly by reducing SPA protein levels. Although SPA2 was more strongly degraded than SPA1, this was not the sole reason for the lack of SPA2 function in the light. We found that the SPA2 protein is inherently incapable of repressing photomorphogenesis in light-grown seedlings. The data therefore indicate that light inactivates the function of SPA2 through a post-translational mechanism that eliminates the activity of the remaining SPA2 protein in the cell.

Original languageEnglish (US)
Pages (from-to)712-723
Number of pages12
JournalPlant Journal
Issue number5
StatePublished - Mar 2011


  • Arabidopsis
  • SPA1
  • SPA2
  • photomorphogenesis
  • protein degradation
  • ubiquitin ligase

ASJC Scopus subject areas

  • Genetics
  • Plant Science
  • Cell Biology

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