A review for the molecular research of russet/semi-russet of sand pear exocarp and their genetic characters

Exocarp protects the fruit against external stresses by means of its special physical and biochemical properties. It is also a commercially important quality for fruits characterized with certain color and structure. The color of sand pear exocarp can be divided into three types of russet, semi-russet and green. The green of exocarp is formed by accumulation of chlorophyll in epidermal cells. Russeting is a disorder of the fruit skin that results from microscopic cracks caused by growth stresses and several additional factors and the subsequent formation of higher plasticity periderm membranes by the accumulation of suberin on the inner part of the cell wall of the outer epidermal cell layers. Genes and pathways that are specific to exocarp russet formation have been identified in sand pear and its genetically related apple. The cuticle biosynthetic genes were repressed while stress response genes and suberin deposition genes were enhanced underlying the exocarp russeting. One major 'QTL' associated with russet of exocarp was identified at the top of LG 8. However, despite these advances, important aspects of russet and semi russet inheritance remain obscure. Central questions include whether russet and semi-russet are belong to one type of quantitative trait or not, and their genetic characters. These issues are reviewed. Greater emphasis on gene mapping and cloning, biochemical characterization of metabolic intermediates and putative enzymes identified to put together correct and detailed pathways will be required to solve these unknowns of the mechanisms underlying the exocarp russet and semi-russet formation. (C) 2016 Elsevier B.V. All rights reserved.