The role of the ribosome in plant development

Life is maintained through a central chain of events: DNA-mRNA-protein. For any organism to divide, grow, develop and differentiate the chain of events leading to protein synthesis needs to be regulated. Multiple mechanisms exist to help achieve when, where and how much of each protein in an individual cell is produced. One way to regulate protein production is to control the machine that reads mRNA and builds proteins from the basic amino acid building blocks. This remarkable machine is the ribosome. The ribosome is composed of a small and a large subunit, together consisting of 3 RNA molecules and nearly 80 proteins. As part of a screen to find genes controlling leaf development we have isolated mutations in large and small subunit ribosomal proteins. These mutants are called piggyback (pgy) and, surprisingly, they produce a very specific developmental defect when combined with the leaf development mutant asymmetric leaves1 (as1). as1 pgy double mutants have a dramatic phenotype where leaves grow out from the surface of other leaves. However, by themselves pgy mutants have very little effect on plant development. The developmental phenotype of as1 pgy double mutants indicates only selected target transcripts are more sensitive to loss of ribosomal protein function. What might these targets be and why are they sensitive to ribosomal protein loss? We have several testable models to address these important questions. One possibility is that a subset of ribosomes interacts with other proteins and it is this subset that is affected by pgy mutations. Another possibility is that sequences in some transcripts are sensitive to pgy mutations. Arabidopsis has several genes for each ribosomal protein so a third possibility is that ribosomal proteins within a family are not all doing the same thing. In this case ribosomes carrying different family members are functionally different. We are using genetics and biochemistry to test these models and find out why mutations in PGY genes produce a specific phenotype and how PGY genes might work. For instance one goal is to identify direct target genes. Finding a direct target will enable us to manipulate sequence features of the target that may be sensitive to ribosome function. We can also use genetics to compare the function of ribosomal proteins within a gene family to see if they are the same or different. With the results of these experiments we can start to address the mechanism by which the ribosome controls gene expression.

We have isolated a number of modifiers of the leaf patterning gene asymmetric leaves1. These modifiers, called piggyback (pgy), all condition ectopic outgrowths on the upper side of as1 leaves. Six PGY genes have been isolated and all encode ribosomal proteins. The specific developmental phenotype of as1 pgy mutants suggests that the ribosome and translation can function in patterning events during leaf development. More generally, the ribosome serves as a control point in expression of patterning genes. We propose several testable hypotheses for ribosome regulatory function. Firstly, ribosome target specificity may be mediated by interaction with extra-ribosomal proteins. Secondly, specific sequence features of mRNA may determine sensitive to changes in ribosomes. For instance our genetic data indicates PGY sensitive transcripts may be microRNA targets. In this case the mechanism of ribosome directed control of translation might be via gene silencing or the RNA interference (RNAi) machinery. Thirdly, ribosomal proteins within a gene family may be distinct such that individual ribosomes, each carrying a different family member, are functionally distinct. We will use combined biochemical and genetic approaches to address each of these possibilities. Through these studies we anticipate a greater understanding of ribosome function and translation as control points in plant development.