SNP DISCOVERY IN CANDIDATE GENES FOR DISEASE RESISTANCE (PITCH CANKER AND FUSIFORM RUST) AND DROUGHT TOLERANCE IN LOBLOLLY PINE.

¹ Environmental Horticulture, University of California -Davis, Davis, CA, 95616, US

² Center of Forest Research, INIA, PO Box 8111 28080 Madrid, Spain

³ School of Forest Resources and Conservation, University of Florida, Gainesville, FL 32611,US

⁴ Institute of Forest Genetics, Pacific Southwest Research Station, USDA Forest Service, Davis, CA, 95616, US

Loblolly pine is the most commercially important forest tree species in the US. The fungal disease Pitch Canker, caused by the necrotrophic fungus Fusarium circinatum, has been detected in loblolly pine plantations since 1974 where it causes extensive mortality associated with excessive pitch production of the host. Fusiform rust, caused by the biotrophic fungus Cronartium quercuum ssp. fusiforme, has been a major disease of southeastern conifer plantations since early 1920's. On the other hand, bad adaptation to drought is considered the cause of major growth losses in forest trees. Both pathogen attacks and drought cause millions of dollars in losses to growers annually and thus demand for resistant/tolerant varieties is high. The challenge in developing disease resistant varieties, however, is that the genetic basis of stable resistance to both diseases is quantitative, rather than simple, gene for gene resistance common in other plant-pathogen systems. Our objective is to identify single nucleotide polymorphisms in candidate genes conferring resistance to these pathogens as well as tolerance to drought, which will be tested for association with quantitative phenotypes. Candidate genes for disease resistance are categorized as: : Positional Candidates, referring to candidate genes that are near QTLs for cell wall chemistry like pheylpropanoid pathway genes and cellulose synthase genes, Expression Candidates, referring to genes that are being identified by expression analyses (chitinases and PR10), Functional Candidates, referring to genes and regulatory regions whose roles in disease resistance have been identified in other plant systems and related to loblolly pine system through sequence homology and bioinformatics(myb and wrky class transcription factors,peroxidases, pinene synthases, terpene synthases, etc.). A total of ~45 candidate genes and regulatory regions have been sequenced for identification of polymorphic locations that might lead to physiological changes in the disease resistance phenotype. In the case of drought, ~20 drought tolerance related genes identified by expressional studies were sequenced, including some extensively studied in other species, such as dhn-1. Progress to date on candidate gene and SNP discovery will be presented.