果实硬度是苹果果实质地的重要品质性状，采收前成熟期是苹果果实质地形成的关键时期。扩展蛋白是一个细胞壁松弛蛋白家族，有四个亚家族：α-扩展蛋白（EXPA）、β-扩展蛋白（EXPB）、类扩展蛋白A（EXLA）和类扩展蛋白B（EXLB）。在本研究中，我们从果实纵径、横径、硬度、组织结构、呼吸强度、乙烯释放速率和扩展蛋白活性等方面研究了金冠苹果采前质地形成的关键时期。在收获前10天内，发现果实达到成熟。半定量RT-PCR显示，大多数扩展蛋白在收获前的成熟阶段表达。进一步鉴定了EXLB亚家族基因MdEXLB1的生物学功能，并通过瞬时转化实验证实了其亚细胞定位于细胞壁上。与野生型（WT）相比，过表达MdEXLB1的转基因番茄株高较低，结果期较早，果实成熟所需天数较少，果实成熟度较高，果实硬度较低，果实扩展蛋白活性较高，果肉细胞结构更离散，果实成熟过程加快。总的来说，这是首次提出苹果EXLB亚家族基因MdEXLB1具有生物学功能，并在促进果实成熟和软化方面发挥重要作用。

Fruit firmness is an important quality trait of apple fruit texture, and the pre-harvest ripening period is the key period for the formation of apple fruit texture.  Expansin is a cell wall loosing protein family that has four subfamilies: α-expansin (EXPA), β-expansin (EXPB), expansin-like A (EXLA), and expansin-like B (EXLB).  In this study, we investigated the key period of pre-harvest texture formation in ‘Golden Delicious’ apples based on fruit longitudinal diameter, transverse diameter, firmness, tissue structure, respiration intensity, ethylene release rate, and expansin activity.  Within the 10 days before harvest, the fruit was found to reach maturity.  Semi-quantitative RT-PCR revealed that most of the expansins were expressed at the ripening stage before harvest.  The biological function of the EXLB subfamily gene, MdEXLB1, was further identified, and its subcellular localization on the cell wall was confirmed by transient transformation experiments.  Compared with the wild type (WT), the transgenic tomato lines overexpressing MdEXLB1 had lower plant height, earlier fruiting period, fewer days for fruit ripening, higher fruit maturity, lower fruit firmness, higher fruit expansin activity, more discrete flesh cell structure, and accelerated fruit ripening process.  Overall, this is the first study to propose that the apple EXLB subfamily gene, MdEXLB1, has biological functions and plays an important role in promoting fruit ripening and softening.