molecular analysis of mechanism of fruit ripening and softening in pears

为了更好地为延长果实货架期奠定采后生理基础，本研究从分子水平探讨了果实成熟软化的机理。我们克隆了乙烯生物合成基因(4个ACC合成酶的同源基因和2个ACC氧化酶的同源基因)和编码细胞壁修饰的基因(2个聚半乳糖醛酸酶的同源基因、2个纤维素酶的同源基因和8个Expansin的同源基因)。1-MCP是一种强的乙烯作用抑制剂，在果实开始跃变后用于果实时，随着ACC合成酶和ACC氧化酶转录本的减少，果实的成熟乙烯产量下降。结果表明，梨果实中成熟乙烯的生物合成是以正反馈方式调节的。除抑制乙烯生物合成外，成熟期喷施1-MCP显著延缓果实软化，货架期延长一倍以上。在1-MCP处理的果实中，3个膨胀素和2个多聚半乳糖醛酸酶的同源基因的表达受到抑制，而其他成员的膨胀素和2个纤维素酶基因的表达没有受到影响。这些结果表明，一些膨胀素和聚半乳糖醛酸酶基因的表达依赖于乙烯，在果实软化过程中起着重要作用。1-MCP和ACC合成酶、ACC氧化酶等乙烯生物合成基因代谢工程的应用为梨果实采后的运输和销售提供了先进的技术手段。

In order to establish physiological base of postharvest technology for extended shelf life with better quality, we investigated the mechanism of fruit ripening and softening at molecular level. We isolated ethylene biosynthetic gene (four isogenes for ACC synthase and two isogenes for ACC oxidase) and genes encoding cell wall modifying (two isogenes for polygalacturonase, two isogenes for cellulase and eight isogenes for expansin). The ripening ethylene production declined with decrease of transcripts for ACC synthase and ACC oxidase by 1-MCP, a strong inhibitor of ethylene action when it was applied to fruit after onset of climacteric ethylene. The results indicate that ripening ethylene biosynthesis in pear fruit is regulated in positive feedback manner. In addition of suppression of ethylene biosynthesis, 1-MCP application during ripening stage significantly delayed the subsequent fruit softening, resulting in extended shelf life more than twice of non-treated fruit. In the 1-MCP treated fruit the expression of three isogenes for expansin and two for polygalacturonase was suppressed while other members of expansin and two cellulase genes were not affected. These results suggest that expression of some expansin and polygalacturonase genes depends on ethylene and plays an important role in fruit softening. We concluded that application of 1-MCP and metabolic engineering of ethylene biosynthetic gene such as ACC synthase and ACC oxidase promise advanced postharvest technology in transportation and marketing in pear fruit.