The Genetic Basis of Adaptation to Environmental Stress in Two Grass Genomic Model Systems

Plants are exposed to a wide variety of environmental stress in the wild and have developed an equally diverse set of adaptations to tolerate them. The evolutionary processes that have led to this functional diversification, and the specific genes and physiological mechanisms involved, are of immense interest to both evolutionary biologists and crop breeders. In this dissertation I investigate adaptation to different types of environmental stress in two economically important grass species, seashore paspalum (Paspalum vaginatum Sw.) and rice (Oryza sativa L.). Seashore paspalum is a halophytic turfgrass that occupies habitats which can dramatically differ in salt concentration. Populations may be locally adapted to these divergent environments, but the genetic and physiological basis for such adaptations are poorly understood. Chapter 2 of this dissertation describes the relationship between genetic diversity and local ocean salinity in populations of P. vaginatum. I show that the majority of wild P. vaginatum plants in the southeastern US are previously-undescribed diploid and triploid hybrids, which are derived from P. vaginatum and an unidentified Paspalum species. Furthermore, the subgenome copy number varies among hybrid genotypes, with AAB, AB and ABB plants all represented in the wild. Local ocean salinity increases with the relative count of the P. vaginatum subgenome, indicating that there are whole-genome dosage effects on salt tolerance. Additionally, clonal propagation strategies differ between hybrid and non-hybrid genotypes, with clonally propagating hybrids having much wider ranges than non-hybrid clones. In Chapter 3 I show that genetic populations of non-hybrid P. vaginatum differ in their salt tolerance ability and elemental accumulation traits. Shoot accumulation of K, Fe, and Ca in particular are shown to be associated with salt tolerance in the sampled genotypes, indicating their potential role in local adaptation to different salt concentrations. Rice is one of the most important food crops in the world. Accordingly, there is much interest in improving its stress tolerance. Sheath blight disease, caused by the fungal pathogen Rhizoctonia solani, is one of the main causes of reduced rice yields worldwide. De-domesticated forms of rice, known as weedy or red rice, appear to be more resistant to sheath blight than cultivated varieties and may harbor favorable disease resistance alleles. In Chapter 4, I used two recombinant inbred line populations developed from crosses between weedy and cultivated rice to identify novel quantitative trait loci associated with resistance to sheath blight disease, and which are independent of the confounding traits of plant height and heading date. These markers may be used to breed more pathogen-resistant rice varieties