21st Century Approach to Improving Burbank’s ‘Stoneless’ Plum

Luther Burbank considered the stoneless plum, ‘Conquest’ (1911–12 catalog), one of his best accomplishments.  The plum had only a grain of stone and a complete seed, but was not commercially successful.  In view of the current desire for seedless fruit such as citrus, grape, and  watermelon, we have utilized a nearly stoneless plum, most likely a remnant from Burbank’s breeding program, and are applying 21^st century technology to achieve a pitless plum.  These technologies include molecular markers, accelerated breeding cycles (‘FasTrack’), genetic engineering and next generation sequencing. One aspect of this project is to define the genes that are responsible for the trait in order to create markers for breeding and targets for genetic engineering.   Initial experiments have characterized the stoneless trait as a decrease in the number of endocarp cells that form the stone.  Cool temperatures during pollination of ‘Stoneless’ result in very little stone formed while warm temperatures result in a more normal stone.  During the period flanking pollination, gene expression of a number of transcription factors involved with determining endocarp cells was compared in ‘Stoneless’ and two cultivars that produce normal stones.  Next generation sequencing of the RNA was done to evaluate all genes expressed during that time.  A number of genes have been identified that are altered in their expression in ‘Stoneless’ including one associated with embryo development.