Regulation of Arabidopsis seed germination by SOM

Abstract

Since plant growth is dependent on ambient environmental conditions, seeds need to monitor the environ-ment in order to determine the proper timing of germination. Among these conditions, light and temperature have been shown to play critical roles in regulating seed germination. In Arabidopsis, light promotes seed germination, whereas high temperature inhibits seed germination. Previous studies have shown that these signals regulate seed germination via regulation of endogenous plant hormones such as gibberellic acid (GA) and abscisic acid (ABA), which have positive and negative roles in seed germination, respectively.Among the photoreceptors, phytochromes mediate light signaling to promote seed germination. The key negative regulator of light-dependent seed germination is PHYTOCHROME-INTERACTING FACTOR1 (PIF1, also known as PHYTOCHROME-INTERACTING FACTOR3-LIKE5 [PIL5]), a basic helix-loop-helix transcription factor. Among the direct targets of PIL5, we previously reported that SOMNUS (SOM), a seed specific tandem CCCH zinc finger protein, is a novel negative regulator of light-dependent seed germination. In dark, SOM inhibits seed germination via activating ABA biosynthesis and inhibiting GA biosynthesis. Light promotes seed germination via repressing the PIL5-mediated SOM expression. To date, the molecular function of SOM is unknown.We first studied about the molecular characteristics of SOM. SOM protein is consisted by a single exon which contains three tandem zinc finger motifs in the middle region. We found that SOM protein forms a homodimer through its C-terminal region, which contains a putative nuclear export signal (NES) at the end of exon. Consistent with this, protoplast assay showed that SOM localized predominantly to cytoplasm. Muta-genesis of zinc finger motif or exchange of NES to nuclear localization signal (NLS) showed that CCCH zinc finger motif and/or cytoplasmic localization of SOM are important to inhibit seed germination under high tempe...