O'Donoghue Erin M, Somerfield Sheryl D, Watson Lyn M, Brummell David A, Hunter Donald A
New Zealand Institute for Plant and Food Research Limited, New Zealand.
Planta. 2009 Feb;229(3):709-21. doi: 10.1007/s00425-008-0862-6. Epub 2008 Dec 11.
Galactose was the major non-cellulosic neutral sugar present in the cell walls of 'Mitchell' petunia (Petunia axillaris x P. axillaris x P. hybrida) flower petals. Over the 24 h period associated with flower opening, there was a doubling of the galactose content of polymers strongly associated with cellulose and insoluble in strong alkali ('residual' fraction). By two days after flower opening, the galactose content of both the residual fraction and a Na(2)CO(3)-soluble pectin-rich cell wall fraction had sharply decreased, and continued to decline as flowers began to wilt. In contrast, amounts of other neutral sugars showed little change over this time, and depolymerisation of pectins and hemicelluloses was barely detectable throughout petal development. Size exclusion chromatography of Na(2)CO(3)-soluble pectins showed that there was a loss of neutral sugar relative to uronic acid content, consistent with a substantial loss of galactose from rhamnogalacturonan-I-type pectin. beta-Galactosidase activity (EC 3.2.1.23) increased at bud opening, and remained high through to petal senescence. Two cDNAs encoding beta-galactosidase were isolated from a mixed stage petal library. Both deduced proteins are beta-galactosidases of Glycosyl Hydrolase Family 35, possessing lectin-like sugar-binding domains at their carboxyl terminus. PhBGAL1 was expressed at relatively high levels only during flower opening, while PhBGAL2 mRNA accumulation occurred at lower levels in mature and senescent petals. The data suggest that metabolism of cell wall-associated polymeric galactose is the major feature of both the opening and senescence of 'Mitchell' petunia flower petals.
半乳糖是“米切尔”矮牵牛（腋花矮牵牛×腋花矮牵牛×杂交矮牵牛）花瓣细胞壁中主要的非纤维素中性糖。在与花朵开放相关的24小时内，与纤维素紧密结合且不溶于强碱的聚合物（“残留”部分）中的半乳糖含量增加了一倍。到花朵开放两天后，残留部分和富含碳酸钠可溶性果胶的细胞壁部分中的半乳糖含量急剧下降，并随着花朵开始枯萎而持续下降。相比之下，其他中性糖的含量在这段时间内变化不大，并且在花瓣发育过程中几乎检测不到果胶和半纤维素的解聚。对碳酸钠可溶性果胶进行尺寸排阻色谱分析表明，相对于糖醛酸含量，中性糖有所损失，这与鼠李糖半乳糖醛酸聚糖-I型果胶中半乳糖的大量损失一致。β-半乳糖苷酶活性（EC 3.2.1.23）在花蕾开放时增加，并一直保持高水平直至花瓣衰老。从混合阶段的花瓣文库中分离出两个编码β-半乳糖苷酶的cDNA。两个推导的蛋白质都是糖基水解酶家族35的β-半乳糖苷酶，在其羧基末端具有凝集素样糖结合结构域。PhBGAL1仅在花朵开放期间以相对较高的水平表达，而PhBGAL2 mRNA在成熟和衰老花瓣中的积累水平较低。数据表明，细胞壁相关聚合物半乳糖的代谢是“米切尔”矮牵牛花瓣开放和衰老的主要特征。
