Genetic transformation and gene delivery strategies in Carica papaya L.

Papaya (Carica papaya L.)is a popular tropical fruit of high commercial value due to its nutritional, medicinal and industrial properties. However, conventional breeding methods for papaya have several limitations, including a scarcity of natural genetic resources in the Carica genus and sexual incompatibility among closely-related species. To overcome these challenges, particle bombardment, Agrobacterium-mediated transformation, and pollen tube-mediated transformation have been developed for generating genetically modified papaya with the desired traits over the past three decades. These transformation methods have been successfully employed to improve papaya's resistance to biotic and abiotic stress, fruit quality, and even to produce edible vaccines. Notably, papaya ringspot virus-resistant transgenic papaya is the first successful commercial application of genetic engineering technology in a fruit crop. Recently, CRISPR/Cas9-mediated genome editing technology and viral vector-based gene delivery system have emerged as promising tools for papaya genetic improvement and functional genomic studies in papaya. Genome editing will open a new era of precision breeding for papaya. This review aims to highlight past and recent gene transformation methods and their applications in papaya breeding, as well as future perspectives and challenges in improving papaya via genetic engineering.