Understanding the Role of Cytoplasmic Genomes in Stress Tolerance of Wheat

During the evolution of both animals and plants, numerous cytoplasmic (i.e. mitochondria and chloroplast) genes migrated to the nuclear genome necessitating tightly coordinated expression of many organellar- and nuclear-encoded genes. In wheat (Triticum aestivum), the effects of disrupting these native interactions by substituting the cytoplasm of domesticated wheat with that of wild relatives (alloplasmic lines) can range from negative to positive phenotypes (e.g. cytoplasmic male sterility versus yield heterosis and improved stress tolerance, respectively). Pilot studies suggest that sequence changes, increased levels of heteroplasmy (genome variants), and altered organellar gene expression in alloplasmic lines may lead to increased biotic and abiotic stress tolerance, however, the molecular mechanisms responsible for these improvements remain elusive. To gain a deeper understanding of how the alloplasmic condition influences organellar genomes and impacts phenotype a series of experiments were performed. Next-generation sequencing and de novo genome assembly of organellar DNA isolated from alloplasmic lines (backcross generation 7 or higher) revealed the mitochondria and chloroplast genomic diversity across divergent species. In parallel, the homogeneity of the nuclear genome after backcrossing was verified utilizing a high-density 90,000 single nucleotide polymorphism genotyping array. Screening with biotic (leaf and stem rust caused by Puccinia species) and abiotic (salt tolerance) stressors indicated that cytoplasmic genomes from specific wild relatives in the Aegliops and Triticum genera could confer enhanced tolerance. Ongoing experiments are investigating the mechanisms by which alien cytoplasm can modify stress tolerance. Our results suggest that organellar genomes provide a unique resource to rapidly improve the stress tolerance of existing cultivars.