Chapter 9 - Papaya ringspot virus-P: overcoming limitations of resistance breeding in Carica papaya L.

Abstract

Papaya ringspot virus strain papaya (PRSV-P) is the main constraint in all papaya (Carica papaya L.) cultivating areas of the world. The only successful option of managing PRSV-P infection via transgenic resistance is confined to Hawaii due to some technical and environmental issues; other viral management strategies have had limited success. Therefore, in the present scenario, introgression of the PRSV-P resistance gene into C. papaya from its wild relatives (species of Vasconcellea) is the most optimistic option available with researchers. The earlier attempts at crossing C. papaya with species of Vasconcellea were not successful because the species chosen as the PRSV-P-resistant parent (V. cauliflora) was genetically distant from C. papaya, which often led to embryo abortion. In the case of seed formation, or when this problem of incompatibility was overcome with an embryo rescue technique, the resultant F1 hybrids were infertile. The approach was refined with the advancement of knowledge of genetic mapping of C. papaya vis-à-vis other species of the genus Vasconcellea with the help of molecular taxonomy. As a result, other PRSV-P-resistant species of Vasconcellea, which were genetically closer to C. papaya, were used in the breeding program. Adequate success was achieved in getting fertile F1 hybrids by crossing C. papaya with V. quercifolia. These hybrids showed varying degrees of viral resistance, which was neither uniform nor stable. To overcome this problem, recent emphasis has shifted toward the use of PRSV-P-immune V. pubescens. The problem of incompatibility between C. papaya and V. pubescens was resolved by using V. parviflora as a bridge species. The screening of hybrids was accelerated with the development of molecular markers to establish hybridity of crosses and for PRSV-P resistance in V. pubescens. The putative resistance genes for PRSV-P in V. pubescens have been further sequenced and characterized. These efforts, and recently the advancement in papaya genomics, have raised hope for development of PRSV-P-resistant hybrids with commercially acceptable fruit quality in the near future.