Associate Professor The University of Arizona TUCSON, Arizona
Body of Abstract: 24-nt small interfering (si)RNAs establish DNA methylation at thousands of genomic loci in a process called RNA-directed DNA Methylation (RdDM). During reproduction, 24-nt siRNAs are particularly abundant in maternal sporophytic tissue such as the seed coat, largely due to overwhelming expression from a small number of “siren” loci that account for over 90% of siRNAs in ovules and developing seeds. Abundant siRNAs are produced from gene fragments embedded in siren loci and these siRNAs trigger methylation at homologous protein-coding genes despite mismatches between the siRNA and target gene. In a Brassica rapa mutant lacking siren siRNAs, the embryo and endosperm abort if RdDM is disrupted in the maternal sporophyte, implying communication between maternal and filial tissues. Siren siRNAs are maternally biased in the endosperm, suggesting that siRNAs might be produced in the maternal sporophyte and transferred to endosperm to influence seed development. Because siren siRNAs do not require perfect complementarity with their targets, we predict that maternal siRNAs would methylate both maternally- and paternally-derived alleles in the endosperm, possibly as a form of parental conflict.
Although RdDM is required for seed production in B. rapa and many other plant species, it is dispensable for growth and development in Arabidopsis. To investigate factors that influence RdDM’s reproductive function, we created mutations in RDR2 and NRPE1 in three Brassicaceae species that differ in mating system and history of polyploidy: Camelina sativa, Capsella grandiflora, and Capsella rubella. Combined with previous observations in B. rapa and Arabidopsis, we conclude that mating system is a key determinant in whether RdDM is required for sexual reproduction in Brassicaceae.