FUNCTIONAL MARKERS AS GENETIC APPROACH TO STUDY ETHYLENE PRODUCTION AND FRUIT SOFTENING IN APPLE (MALUS X DOMESTICA BORKH.)

Role of MdERF3 and MdERF118 natural variations in apple flesh firmness/crispness retainability and development of QTL-based genomics-assisted prediction

Retention of flesh texture attributes during cold storage is critical for the long-term maintenance of fruit quality. The genetic variations determining flesh firmness and crispness retainability are not well understood. The objectives of this study are to identify gene markers based on quantitative trait loci (QTLs) and to develop genomics-assisted prediction (GAP) models for apple flesh firmness and crispness retainability. Phenotype data of 2664 hybrids derived from three Malus domestica cultivars and a M. asiatica cultivar were collected in 2016 and 2017. The phenotype segregated considerably with high broad-sense heritability of 83.85% and 83.64% for flesh firmness and crispness retainability, respectively. Fifty-six candidate genes were predicted from the 62 QTLs identified using bulked segregant analysis and RNA-seq. The genotype effects of the markers designed on each candidate gene were estimated. The genomics-predicted values were obtained using pyramiding marker genotype effects and overall mean phenotype values. Fivefold cross-validation revealed that the prediction accuracy was 0.5541 and 0.6018 for retainability of flesh firmness and crispness, respectively. An 8-bp deletion in the MdERF3 promoter disrupted MdDOF5.3 binding, reduced MdERF3 expression, relieved the inhibition on MdPGLR3, MdPME2, and MdACO4 expression, and ultimately decreased flesh firmness and crispness retainability. A 3-bp deletion in the MdERF118 promoter decreased its expression by disrupting the binding of MdRAVL1, which increased MdPGLR3 and MdACO4 expression and reduced flesh firmness and crispness retainability. These results provide insights regarding the genetic variation network regulating flesh firmness and crispness retainability, and the GAP models can assist in apple breeding.

Apple hypanthium firmness: new insights from comparative proteomics

Fruit firmness constitutes an important textural property and is one of the key parameters for estimating ripening and shelf life, which has a major impact on commercialization. In order to decipher the mechanisms related to firmness of apples (Malus × domestica Borkh.), two-dimensional gel electrophoresis (2-DE) was used to compare the total proteome of high and low firmness phenotypes from apple hypanthia of a 'Golden Delicious' × 'Dietrich' population. A total of 36 differentially regulated protein spots were positively identified by matrix assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry (MS) and then validated against the Malus expressed sequence tags (EST) database. The findings of this study indicated a lower expression of ethylene biosynthesis related proteins in the high firmness phenotype, which could be linked to the slowing down of the ripening and softening processes. The reduced accumulation of proteins involved in ethylene biosynthesis juxtaposed to the upregulation of a transposase and a GTP-binding protein in the high firmness phenotype. The results also showed higher expression of cytoskeleton proteins in the high firmness phenotype compared to the low firmness phenotype, which play a role in maintaining cell structure and possibly fruit integrity. Finally, a number of proteins involved in detoxification and defense were expressed in fruit hypanthium. This proteomic study provides a contribution towards a better understanding of regulatory networks involved in fruit hypanthium firmness and/or softening, which could be instrumental in the development of improved fruit quality.