The Arabidopsis GAMYB-like genes, MYB33 and MYB65, are microRNA-regulated genes that redundantly facilitate anther development

The functions of the vast majority of genes encoding R2R3 MYB domain proteins remain unknown. The closely related MYB33 and MYB65 genes of Arabidopsis thaliana have high sequence similarity to the barley (Hordeum vulgare) GAMYB gene. T-DNA insertional mutants were isolated for both genes, and a myb33 myb65 double mutant was defective in anther development. In myb33 myb65 anthers, the tapetum undergoes hypertrophy at the pollen mother cell stage, resulting in premeiotic abortion of pollen development. However, myb33 myb65 sterility was conditional, where fertility increased both under higher light or lower temperature conditions. Thus, MYB33/MYB65 facilitate, but are not essential for, anther development. Neither single mutant displayed a phenotype, implying that MYB33 and MYB65 are functionally redundant. Consistent with functional redundancy, promoter-beta-glucuronidase (GUS) fusions of MYB33 and MYB65 gave identical expression patterns in flowers (sepals, style, receptacle, anther filaments, and connective but not in anthers themselves), shoot apices, and root tips. By contrast, expression of a MYB33:GUS translational fusion in flowers was solely in young anthers (consistent with the male sterile phenotype), and no staining was seen in shoot meristems or root tips. A microRNA target sequence is present in the MYB genes, and mutating this sequence in the MYB33:GUS fusion results in an expanded expression pattern, in tissues similar to that observed in the promoter-GUS lines, implying that the microRNA target sequence is restricting MYB33 expression. Arabidopsis transformed with MYB33 containing the mutated microRNA target had dramatic pleiotrophic developmental defects, suggesting that restricting MYB33 expression, especially in the shoot apices, is essential for proper plant development.     

Myb Genes in Ciliates: A Common Origin with the myb Protooncogene?

In animals, the protooncogene myb family is characterized by a DNA-binding domain (so-called MYB domain), which consists of 3 imperfect tandem repeats of a helix-turn-helix motif. Homologous genes have been characterized in plants and also in Dictyostelium discoideum. However, in plants, the myb family is more diverse and displays 2 types of MYB domains: the animal-like 3 repeats (MYB-3R) and the 2 repeats (MYB-2R) domains. The question is therefore raised as to the putative existence of genes with MYB-3R and/or MYB-2R domains in their last common unicellular ancestor. Here, we present evidence that in ciliates like in plants, both types of domain exist. A gene having a MYB-3R domain has been identified in the oxytrichid Sterkiella histriomuscorum and a gene having a MYB-2R domain has been identified in the euplotid Euplotes aediculatus. Both genes are expressed during the vegetative growth of the cells. A conserved intron exists in the gene of Sterkiella and phylogenetical analyses show that the 2 ciliate genes belong to the myb protooncogene family as deeply split lineages. This is the first report of a myb homolog in a ciliated protist, thus, confirming its origin in strict unicellular eukaryotes.