Nishiyama Kiyomi, Guis Monique, Rose Jocelyn K C, Kubo Yasutaka, Bennett Kristen A, Wangjin Lu, Kato Kenji, Ushijima Koichiro, Nakano Ryohei, Inaba Akitsugu, Bouzayen Mondher, Latche Alain, Pech Jean-Claude, Bennett Alan B
Graduate School of Natural Science and Technology, Okayama University, Okayama, 700-8530 Japan.
J Exp Bot. 2007;58(6):1281-90. doi: 10.1093/jxb/erl283. Epub 2007 Feb 17.
Cell wall disassembly in ripening fruit is highly complex, involving the dismantling of multiple polysaccharide networks by diverse families of wall-modifying proteins. While it has been reported in several species that multiple members of each such family are expressed in the same fruit tissue, it is not clear whether this reflects functional redundancy, with protein isozymes from a single enzyme class performing similar roles and contributing equally to wall degradation, or whether they have discrete functions, with some isoforms playing a predominant role. Experiments reported here sought to distinguish between cell wall-related processes in ripening melon that were softening-associated and softening-independent. Cell wall polysaccharide depolymerization and the expression of wall metabolism-related genes were examined in transgenic melon (Cucumis melo var. cantalupensis Naud.) fruit with suppressed expression of the 1-aminocyclopropane-1-carboxylate oxidase (ACO) gene and fruits treated with ethylene and 1-methylcyclopropene (1-MCP). Softening was completely inhibited in the transgenic fruit but was restored by treatment with exogenous ethylene. Moreover, post-harvest application of 1-MCP after the onset of ripening completely halted subsequent softening, suggesting that melon fruit softening is ethylene-dependent. Size exclusion chromatography of cell wall polysaccharides, from the transgenic fruits, with or without exogenous ethylene, indicated that the depolymerization of both pectins and xyloglucans was also ethylene dependent. However, northern analyses of a diverse range of cell wall-related genes, including those for polygalacturonases, xyloglucan endotransglucosylase/hydrolases, expansin, and beta-galactosidases, identified specific genes within single families that could be categorized as ethylene-dependent, ethylene-independent, or partially ethylene-dependent. These results support the hypothesis that while individual cell wall-modifying proteins from each family contribute to cell wall disassembly that accompanies fruit softening, other closely related family members are regulated in an ethylene-independent manner and apparently do not directly participate in fruit softening.
成熟果实中的细胞壁分解过程极为复杂，涉及多种细胞壁修饰蛋白家族对多个多糖网络的拆解。虽然在多个物种中都有报道称，每个此类家族的多个成员在同一果实组织中表达，但尚不清楚这是反映了功能冗余，即来自单一酶类的蛋白质同工酶发挥相似作用并对细胞壁降解做出同等贡献，还是它们具有不同的功能，其中一些同工型起主要作用。本文报道的实验旨在区分成熟甜瓜中与细胞壁相关的、与软化相关和与软化无关的过程。在1-氨基环丙烷-1-羧酸氧化酶（ACO）基因表达受抑制的转基因甜瓜（Cucumis melo var. cantalupensis Naud.）果实以及用乙烯和1-甲基环丙烯（1-MCP）处理的果实中，检测了细胞壁多糖解聚以及细胞壁代谢相关基因的表达。转基因果实的软化完全受到抑制，但用外源乙烯处理后可恢复。此外，在成熟开始后采后施用1-MCP完全阻止了随后的软化，这表明甜瓜果实软化是乙烯依赖性的。对有无外源乙烯的转基因果实的细胞壁多糖进行尺寸排阻色谱分析表明，果胶和木葡聚糖的解聚也是乙烯依赖性的。然而，对多种细胞壁相关基因的Northern分析，包括多聚半乳糖醛酸酶、木葡聚糖内转糖基酶/水解酶、扩展蛋白和β-半乳糖苷酶的基因，确定了单个家族内可归类为乙烯依赖性、乙烯非依赖性或部分乙烯依赖性的特定基因。这些结果支持了这样一种假设，即虽然每个家族的单个细胞壁修饰蛋白有助于果实软化过程中伴随的细胞壁分解，但其他密切相关的家族成员以乙烯非依赖性方式受到调控，显然不直接参与果实软化。
