A genetic approach to the identification of new components regulating development in Arabidopsis thaliana

Sammanfattning: Two new genes involved in important processes of plant development were identified in the model plant Arabidopsis thaliana. The genes were isolated from mutants generated through insertional mutagenesis based on a transposon tagging approach.The first gene, ALB3, was isolated through the identification of the mutant albino3 (alb3), displaying severe defects in pigmentation and chloroplast biogenesis. The ALB3 protein shows sequence similarity to a yeast protein, OXA1, which is required in the mitochondria for proper assembly of the cytochrome oxidase complex. As ALB3 is localised in thylakoid membranes, we suggest that the ALB3 protein acts in the assembly of thylakoid membrane protein complexes and thereby is crucial for proper chloroplast development and function.The second gene, SHI, was identified through the short internodes (shi) mutation, a dwarfing mutation conferring a phenotype similar to mutants defective in the biosynthesis of the plant hormone gibberellin (GA). However, the shi mutant is unable to elongate following treatment with exogenous GA, which indicates that shi is defective in the response to GA. The level of active GA is elevated in the shi mutant, which is the expected result of reduced feedback control of GA biosynthesis. As the shi mutant phenotype is the result of overexpression of the SHI gene, we suggest that the SHI protein is a component of the GA signalling pathway, possibly acting as a repressor of GA-induced cell elongation.Sequence similarity database searches revealed that the SHI gene belongs to a new Arabidopsis gene family comprising at least eight members (SHI, LRP1, and SRS1 to SRS6). These genes encode regulatory proteins containing a putative zinc-binding RING finger-like domain. We have cloned SRS1 and SRS2, and have shown by overexpression of these genes in transgenic Arabidopsis that their gene products might function in similar processes as SHI.