Related to Ubiquitin 1 and 2 Are Redundant and Essential and Regulate Vegetative Growth, Auxin Signaling, and Ethylene Production in Arabidopsis

Related to Ubiquitin (RUB)/Nedd8 is a ubiquitin-like protein that covalently attaches to cullins, a subunit of the SCF (for Skp, Cdc53p/Cul1, and F-box protein) complex, an E3 ubiquitin ligase, and has been shown to be required for robust function of the complex. The effects of reducing protein levels for two Rub proteins, RUB1 and RUB2, were characterized in Arabidopsis thaliana. T-DNA insertional null lines homozygous at a single RUB-encoding locus were analyzed and found to have a wild-type phenotype. A double mutant was never recovered. More than one-quarter of the progeny from the self-fertilization of plants with a single functional RUB-encoding gene died as embryos at the two-cell stage. Outcrosses demonstrated reduced inheritance of the null allele from both the male and female parent. Hemigglutinin-tagged forms of RUB1 and RUB2 conjugate to the same cullin protein, CUL1, and produce the same conjugation pattern. To further understand the function of the RUB proteins, a construct designed to produce a double-stranded RUB1 mRNA was introduced into plants, and three lines with reduced levels of RUB1- and RUB2-encoding mRNA and RUB1/2 protein content were analyzed in detail. Mature plants were severely dwarfed, seedlings were insensitive to auxin in root assays, and dark-grown seedlings had a partial triple-response phenotype that was suppressed when seedlings were grown on ethylene perception or synthesis inhibitors. The dsrub lines produced threefold to fivefold more ethylene than the wild type. This study illustrates that RUB1 and RUB2 are genetically and biochemically redundant and demonstrates that RUB1/2 proteins are essential for early embryonic cell divisions and that they regulate diverse processes.