Title : Plant tissue culture as a tool in Papaya breeding
Abstract:
The increases in world food demand due to population grow and climate change necessitates the continuous genetic improvement of crop species. Moreover, for maximal effect in perennial species, this genetic improvement must be integrated with the latest high throughput in vitro propagation techniques. Mexico is the third largest producer and the number one exporter of papaya worldwide. We report herein the application of latest genetic and in vitro propagation technologies to facilitate both short- and long-term genetic improvement of this crop. Traditionally, papaya has been propagated by seeds. However, there are three drawbacks to seed propagation: 1) genetic heterogeneity (unless the varieties have been inbred for many generations, which is time consuming). 2) segregation of sex chromosomes in seed propagated papayas resulting in different morphological forms: males, females, and hermaphrodites. This is problematic in that only the hermaphroditic plants produce fruit of the size, shape, weight and quality desired by consumers. Consequently, all non-hermaphrodites must be manually removed from commercial plantings. 3) papaya seeds have relatively short viability, requiring frequent regeneration of seed inventories. Working with Agromod-Frescos, NSIP has developed and deployed a genetic improvement strategy for papaya for which in vitro propagation plays a central role. Specifically, we have pursued three goals. (1) Development and testing of a reliable micropropagation protocol for commercial production. In a pilot project we have produced more than 70,000 clonal plants of elite genotypes via shoot tip organogenesis that, when trialed, produced 100 % hermaphroditic, homogeneous, virus free plants (2) Development of an embryo rescue technique to produce interspecific hybrids from embryos harvested from 30 to 90 days after-pollination resulting in F1 clones via somatic embryogenesis or in vitro germination for field evaluation of morphology traits, resistance or tolerance to virus and fertility. (3) Development of cryopreservation protocols allowing the long-term preservation of genetic diversity in papaya. In this regard, we have made extensive collections of both wild and cultivated papaya germplasm worldwide, DNA sequenced (GBS) the materials, and used NSIP’s optimization algorithms to define Optimized Core Collections that capture the majority of both allelic and haplotype diversity in papaya worldwide. Cryopreservation methods were developed and tested for both shoot tips and embryogenic calli, which were tested to determine their regeneration and normal development in response to different cryopreservation techniques: vitrification and slow freezing methodologies. By combining new breeding methods with the in vitro methods described herein we anticipate both an increase in the rate and efficiency of genetic improvement in papaya. Keywords: Carica papaya L., micropropagation, organogenesis, somatic embryogenesis, embryo recue, cryopreservation.
