COI1 affects myrosinase activity and controls the expression of two flower-specific myrosinase-binding protein homologues in Arabidopsis

Two cDNA clones homologous to myrosinase-binding proteins (MBPs) were ident ified by differential display in Arabidopsis thaliana. (L.) Heynh. The cDNA s (MBP1 and MBP2) correspond to two open-reading frames found in a gene clu ster of seven putative MBP genes located on chromosome 1. The predicted pro teins MBP1 and MBP2 are similar to lectins and plant aggregating factors. I n addition, MBP2 contains a region of high content of proline and alanine r esidues, commonly found in arabinogalactan proteins and hydroxyproline-rich glycoproteins. Transcripts corresponding to MBP1 and MBP2 genes are exclus ively and abundantly expressed in flowers but are not detected in male-ster ile flowers of coil plants, insensitive to jasmonic acid. Northern analysis and in situ hybridization revealed that MBP mRNAs are present in higher le vels in immature flowers and are localized in several floral organs, includ ing the ovary, ovules, style, anthers and filament. Transcripts of the Arab idopsis myrosinase gene TGG1 show a pattern of expression similar to that o bserved for the MBP genes during flower development; however, they are also abundant in green tissues and are only partially affected by COI1. Crude p reparations of soluble proteins from leaf and flower extracts of wild-type Arabidopsis showed myrosinase activity when sinigrin was used as substrate. In contrast, coil plants showed significantly reduced myrosinase activitie s in both leaves and flowers. The results show that COI1 controls MBP expre ssion in flowers and significantly affects the expression and activity of m yrosinase in Arabidopsis.