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Ann M. Callahan, Chris Dardick and Ralph Scorza

The plum (Prunus domestica) cultivar Stoneless was characterized to determine if the lack of stone was the result of reduced endocarp development or a decrease in lignification. Fruit were sampled at several times from 37 days before stone hardening (DBH) until the stone was too hard to cut with a knife and were compared with plum fruit that had normal stones. At all sampling times there was less endocarp tissue and reduced lignin staining in the ‘Stoneless’ plum fruit. The tissue that did stain appeared to be small endocarp remnants present in the ‘Stoneless’ plum, and was concentrated at the suture side and at the blossom end as well as the stem end. The lignin stain was detected in these regions beginning at 19 DBH, while the normal plums had a progression of staining beginning at the blossom end, suture side at 23 DBH and radiating up to the stem end and throughout the presumptive stone tissue at 8 DBH. Comparison of dry weight for dissected tissues agreed with the specific lack of endocarp tissue in the ‘Stoneless’ plum. Gene activity for the lignin pathway was analyzed using four known genes required for lignification. All four genes showed endocarp-specific expression in ‘Stoneless’ similar to that observed for the control. These results support the idea that the phenotype of ‘Stoneless’ plum fruit is due to a decrease in endocarp formation rather than a decrease in endocarp lignification.

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Ann Callahan, Chris Dardick, Roberta Tosetti, Donna Lalli and Ralph Scorza

The theme running through many of Luther Burbank’s breeding programs was to make plants more tailored to human uses. Mr. Burbank thought that the stone in plum fruits was unessential to a tree that was propagated vegetatively, so he chose stoneless plums as a breeding goal. He made two releases, ‘Miracle’ in 1903 and his final and almost perfect ‘Conquest’ in 1916, which he considered one of his best accomplishments in plum breeding. ‘Conquest’ had only a grain of stone and flavor and size comparable to the best French types of the time but was not commercially successful. In view of the current desire for convenience food such as seedless fruit (citrus, grapes, watermelon) and advanced knowledge of genetics and breeding technologies, we have taken up where Mr. Burbank left off in the production of a better than “almost perfect” stoneless plum. We began by locating what were most likely remnants from Mr. Burbank’s breeding program and we are now using 21st century technology to achieve a completely stoneless, high-quality plum fruit. These technologies include molecular markers, genetic engineering, and accelerated breeding cycles (FasTrack). Initial experiments had characterized the stoneless trait as a decrease in the number of endocarp cells that form the stone. We defined the time critical to the formation of endocarp by analyzing gene expression of a number of transcription factors involved with determining endocarp cells. We identified genes that were expressed differently during this period between normal stone cultivars and one of the stoneless cultivars. In addition, we targeted genes for genetic engineering to reduce the lignification in endocarp and to reduce or convert endocarp cells to non-lignifying cells. A system, FasTrack, using a flowering gene from poplar, has been incorporated to reduce the juvenility period and eliminate the seasonal aspect of fruiting to see the results of the breeding as well as the genetic engineering approach much faster. The combination of these approaches is now in place to attempt to improve on Mr. Burbank’s stoneless plum.

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Ralph Scorza, Michel Ravelonandro, Ann Callahan, Ioan Zagrai, Jaroslav Polak, Tadeuz Malinowski, Mariano Cambra, Laurene Levy, Vern Damsteegt, Boris Krška, John Cordts, Dennis Gonsalves and Chris Dardick