Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/22/2008
Publication Date: 1/10/2009
Citation: Callahan, A.M., Scorza, R., Dardick, C.D. 2009. Characterization of 'Stoneless': A naturally occurring, partially stoneless plum cultivar. Journal of the American Society for Horticultural Science. 134:120-125. Interpretive Summary: Plums and all other stone fruit, such as cherries, peaches and apricots, contain a hard stone that surrounds the seed collectively known as the pit. This pit is removed from fruit that are processed as dried, frozen or canned at a large expense to the industry and hence, the consumer. Over 100 years ago, several cultivars of plum were produced by Luther Burbank that were stoneless, though at the present time, they are partially stoneless. As a preliminary step in producing 'pitless' stonefruit, germplasm from Burbank's program was analyzed to determine the reason for the stoneless phenotype. Two hypotheses were explored (the stone tissue did not harden or the stone tissue was not present). Fruit from 'Stoneless' and normal control plums were collected during the normal stone hardening time. The fruit were cut open and photographed with and without a staining solution that stained lignin, the material that stone tissue makes that becomes hard. There appeared to be less of the lignin staining material in the 'Stoneless' than other normal cultivars. This was confirmed by measuring the weight of that tissue relative to the control tissue, with 'Stoneless' containing approximately half the amount of lignin producing tissue. Gene activity of the genes that make lignin was measured in the stone tissue of both the 'Stoneless' and normal plum, and appeared to be normal in the 'Stoneless'. This lead to the conclusion that less stone tissue is present in the fruit resulting in a nearly stoneless fruit. This information will be used to identify the gene(s) responsible for determining how much stone tissue is made with the goal of eliminating all the stone tissue to make a completely stoneless fruit.
Technical Abstract: A plum (Prunus domestica L.) variety called 'Stoneless' was characterized to determine if the lack of stone was the result of reduced endocarp development or a decrease in lignification, the process that renders stones hard. Fruit were sampled from 24 days after 50% bloom (DAB) until stone hardening occurred and compared with plum fruit that had normal stones. By 24 DAB and throughout stone hardening (~50 DAB), there was less endocarp tissue and reduced lignin staining in the 'Stoneless' plum fruit. The tissue that did stain was associated with a layer that appeared to be under-represented in the 'Stoneless' plum. The presumed stone tissue was concentrated at the suture side and at the blossom end, as well as, the stem end. The lignin stain was concentrated in these regions beginning at 38 DAB through stone hardening, while the normal plums had a progression of staining beginning at the blossom end, suture side at 38 DAB and radiating up to the stem end and through-out the presumptive stone tissue just prior to stone hardening at 50 DAB. Comparison of wet weight and dry weight for dissected tissues agreed with the specific lack of stone tissue in the 'Stoneless' plum. Gene activity for the lignin pathway was analyzed using four known genes required for lignification. Expression was decreased in the 'Stoneless' whole fruit but was present in the remaining stone tissue. These results confirm that 'Stoneless' plum contains a unique alteration in a gene(s) that controls endocarp formation rather than affecting the lignification of the stone tissue.