Marker-Assisted Selection for Biotic Stress Resistance in Peanut

Peanut is the second-most important legume grown worldwide. Cultivated peanut is a disomic tetraploid, 2n—4x—40, with limited genetic diversity due to a genetic bottleneck in formation of the polyploid from ancestors A. duranensis and A. ipaensis. Consequently, resistance_to biotic stresses is limited in the cultigen; however, wild species possess strong resistances. Transfer o f these resistances is hindered by differences o f ploidy, but production o f synthetic amphidiploids, coupled with use o f molecular markers, enables efficient gene transfer. Marker maps have been made from interspecific crosses, and SSR-based maps from cultivated parents have been developed recently. At least 410 resistance gene analogues have been identified. The first markers for biotic stress tolerance were for root-knot nematode resistance and introgressed from one A. cardenasii chromosome. These and improved markers have been used for marker-assisted backcrossing, contributing to release of three cultivars. Additional QTLs have been identified since. Early and late leafspots cause significant yield losses worldwide, and resistance depends on multiple genes. Using interspecific populations, five resistance QTLs for early leafspot were identified using greenhouse inoculations, and five QTLs for late leafspot were identified using detached leaf assays. Using cultivated species populations, 28 QTLs were identified for LLS resistance; all but one were minor QTLs; the major QTL was donated by an interspecific introgression line parent. Rust often occurs alongside leafspots, and rust resistance was characterized as one major QTL, plus several smaller QTLs. Marker-assisted backcrossing o f this major QTL has been performed into different populations. QTLs for resistance to other biotic stresses have been identified, namely to groundnut rosette virus, Sclerotinia blight, afiatoxin contamination, aphids, and tomato spotted wilt virus. Marker-assisted breeding is still in early stages, and development o f more rapid and inexpensive markers from transcriptome and genome sequencing is expected to accelerate progress

Genetic Management of Virus Diseases in Peanut

Peanut, also known as groundnut (Arachis hypogaea L.) is a major oilseed crop in the world. About 31 viruses representing 14 genera are reported to naturally infe.ct peanut in different parts of the world, although only a few of these are of economic importance. These include groundnutrosette disease in Africa, tomato spotted wilt-disease in the United States, peanut bud necrosis disease in south Asia, and peanut stripe virus disease in east and southeast Asia. Cucumber mosaic virus disease in China and Argentina and peanut stem necrosis disease in certain -pockets in southern India are also economically important. Host plant resistance provides the most effective and economic option to manage virus diseases. However, for many virus diseases, effective resistance gene(s) in cultivated peanut have not been identified. With a few exceptions, the virus resistance breeding work has received little attention in peanut improvement programs. Transgenic resistance offers another option in virus resistance breeding. This review focuses on the status of genetic resistance to various economically important groundnut viruses and'use of transgenic-technology for the improvement of virus resistance