Germination-Related Universal Stress Protein (GRUSP) Is Involved in Abscisic Acid Responses During Imbibition and Early Post-Germination Growth of Arabidopsis Seeds.

The universal stress protein (USP) family is a large but poorly studied group of plant proteins whose roles are considered primarily in the context of stress tolerance. This study demonstrates that the functions of USPs are not limited to the stress response but also influence plant developmental processes, namely, the regulation of seed germination and seedling establishment. Using reverse genetics, we found that disruption of the Arabidopsis thaliana At3g58450 gene, encoding germination-related USP (GRUSP), results in delayed germination and abscisic acid (ABA) hypersensitivity in the mutant the GABI-kat 115C08 (grusp-115) knockout line. This hypersensitivity was only partially rescued by the application of fluridone, an ABA biosynthesis inhibitor. Altered ABA content and expression patterns of genes involved in ABA metabolism and signaling, along with a decrease in the mRNA levels of gibberellin (GA) oxidases in dry and either Murashige and Skoog (MS)- or ABA-imbibed mutant seeds, indicate that GRUSP function is primarily associated with seed imbibition and early germination stages. Furthermore, the grusp-115 mutant showed upregulation of ABI5 transcripts in imbibed seeds and seedlings as well as an accumulation of ABI5 protein level under excess GA conditions, indicating a functional relationship between these proteins. Hence, GRUSP is a novel component of the GA and ABA pathways whose function is initiated during seed germination.