New discoveries of stone cell differentiation in fruitlets of 'Yali' pears (Pyrus bretschneideri Rehd.)

Abstract

The stone cell content in fruits is a key determinant affecting internal fruit quality of ‘Yali’ pears. In the present study, the microstructure, ultrastructure and lignin deposition process of stone cells in fruitlets during the early differentiating period were examined by using light microscopy, fluorescence microscopy and transmission electron microscopy (TEM). The results indicated that in fruitlets, most of the stone cells started to differentiate in the 7 th day after blooming although some stone cells formed more quickly than the others. Sections stained with phloroglucinol-HCl showed lignin in the stone cell secondary wall deposited in the 12 th day after blooming, and fluorescence microscopy demonstrated that lignin substrates were already synthesized before secondary wall thickening. Moreover, cytoplasmic autophagic vacuoles were clearly observed, implying that the dying stone cells actually originated from a result of autophagy, a type of programmed cell death (PCD), which was also confirmed by the increasing number of mitochondria and dilation of the endoplasmic reticulum. At the end of differentiation, the stone cell formed a thicker secondary wall attaching to the primary wall formed and lost their nuclei and cell endocyte, leaving a hollow lumen. This discovery might be the newest in studies on stone cell development of pear fruit in relation to PCD.