Hadfield K A, Rose J K, Yaver D S, Berka R M, Bennett A B
Department of Vegetable Crops, University of California, Davis, CA 95616, USA.
Plant Physiol. 1998 Jun;117(2):363-73. doi: 10.1104/pp.117.2.363.
Ripening-associated pectin disassembly in melon is characterized by a decrease in molecular mass and an increase in the solubilization of polyuronide, modifications that in other fruit have been attributed to the activity of polygalacturonase (PG). Although it has been reported that PG activity is absent during melon fruit ripening, a mechanism for PG-independent pectin disassembly has not been positively identified. Here we provide evidence that pectin disassembly in melon (Cucumis melo) may be PG mediated. Three melon cDNA clones with significant homology to other cloned PGs were isolated from the rapidly ripening cultivar Charentais (C. melo cv Reticulatus F1 Alpha) and were expressed at high levels during fruit ripening. The expression pattern correlated temporally with an increase in pectin-degrading activity and a decrease in the molecular mass of cell wall pectins, suggesting that these genes encode functional PGs. MPG1 and MPG2 were closely related to peach fruit and tomato abscission zone PGs, and MPG3 was closely related to tomato fruit PG. MPG1, the most abundant melon PG mRNA, was expressed in Aspergillus oryzae. The culture filtrate exponentially decreased the viscosity of a pectin solution and catalyzed the linear release of reducing groups, suggesting that MPG1 encodes an endo-PG with the potential to depolymerize melon fruit cell wall pectin. Because MPG1 belongs to a group of PGs divergent from the well-characterized tomato fruit PG, this supports the involvement of a second class of PGs in fruit ripening-associated pectin disassembly.
甜瓜中与成熟相关的果胶分解的特征是分子量降低和聚半乳糖醛酸苷的溶解度增加，在其他果实中这些变化被认为是由多聚半乳糖醛酸酶（PG）的活性引起的。尽管有报道称甜瓜果实成熟过程中不存在PG活性，但尚未明确鉴定出一种不依赖PG的果胶分解机制。在这里，我们提供证据表明甜瓜（黄瓜）中的果胶分解可能是由PG介导的。从快速成熟的品种Charentais（甜瓜栽培品种Reticulatus F1 Alpha）中分离出三个与其他克隆的PG具有显著同源性的甜瓜cDNA克隆，它们在果实成熟期间高水平表达。这种表达模式在时间上与果胶降解活性的增加和细胞壁果胶分子量的降低相关，表明这些基因编码功能性PG。MPG1和MPG2与桃果实和番茄脱落区PG密切相关，MPG3与番茄果实PG密切相关。MPG1是甜瓜中最丰富的PG mRNA，在米曲霉中表达。培养滤液使果胶溶液的粘度呈指数下降，并催化还原基团的线性释放，表明MPG1编码一种具有使甜瓜果实细胞壁果胶解聚潜力的内切PG。由于MPG1属于一类与特征明确的番茄果实PG不同的PG，这支持了第二类PG参与果实成熟相关果胶分解的观点。
