Decreased expression of fructosyltransferase genes in asparagus roots may contribute to efficient fructan degradation during asparagus spear harvesting
Decreased expression of fructosyltransferase genes in asparagus roots may contribute to efficient fructan degradation during asparagus spear harvesting
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DOI：
10.1016/j.plaphy.2020.09.007
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发表时间：
2020-11-01
影响因子：
6.5
通讯作者：
Onodera, Shuichi
中科院分区：
文献类型：
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作者：
Ueno, Keiji;Sonoda, Takahiro;Onodera, Shuichi
芦笋（Asparagus officinalis L.）在根部积累菊糖和菊糖新系列型果聚糖，它们是由三种果糖转移酶合成的：蔗糖：蔗糖1-果糖基转移酶（1-SST, EC 2.4.1.99）、果聚糖：果聚糖1-果糖基转移酶（1-FFT, EC 2.4.1.100）和果聚糖：果聚糖6(G)-果糖基转移酶（6G-FFT, EC 2.4.1.243）。根中的果聚糖被认为是矛生长的能量来源，研究表明，在矛收获季节（出芽和拔节期），根中的果聚糖含量逐渐减少。然而，某些三种果糖基转移酶在收获季节的作用尚未阐明。在这里，我们研究了三种果糖转移酶的酶活性和基因表达水平的变化，并检测了矛收获前和收获期间根系中的糖含量。从根cDNA文库中分离到aoft2和aoft3两个cDNA。分别由毕赤酵母产生的重组蛋白（rAoFT2和rAoFT3）进行了表征：rAoFT2具有1-FFT活性（从1-酮糖产生nystose），而rAoFT3具有1-SST活性（从蔗糖产生1-酮糖）。这些重组蛋白的反应谱与先前纯化的天然酶相似。这些结果表明编码1-FFT和1-SST的aoft2和aoft3参与了根果聚糖的合成。果糖转移酶基因和相应酶的活性在收获期间逐渐下调，这也与低聚果糖的减少和果糖的增加相一致，这是由于果聚糖外水解酶的活性。这些发现表明，在收获期间，果糖转移酶基因的下调可能有助于有效降解果聚糖，而果聚糖是颖果生长所必需的。
Asparagus (Asparagus officinalis L.) accumulates inulin and inulin neoseries-type fructans in root, which are synthesized by three fructosyltransferases-sucrose:sucrose 1-fructosyltransferase (1-SST, EC 2.4.1.99), fructan: fructan 1-fructosyltransferase (1-FFT, EC 2.4.1.100), and fructan:fructan 6(G)-fructosyltransferase (6G-FFT, EC 2.4.1.243). Fructans in roots are considered as energy sources for emerging of spears, and it has been demonstrated that a gradual decrease in root fructan content occurs during the spear harvesting season (budding and shooting up period). However, the roles of certain three fructosyltransferases during the harvest season have not yet been elucidated. Here, we investigated the variation in enzymatic activities and gene expression levels of three fructosyltransferases and examined sugar contents in roots before and during the spear harvest period. Two cDNAs, aoft2 and aoft3, were isolated from the cDNA library of roots. The respective recombinant proteins (rAoFT2 and rAoFT3), produced by Pichia pastoris, were characterized: rAoFT2 showed 1-FFT activity (producing nystose from 1-kestose), whereas rAoFT3 showed 1-SST activity (producing 1-kestose from sucrose). These reaction profiles of recombinant proteins were similar to those of native enzymes purified previously. These results indicate that aoft2 and aoft3 encoding 1-FFT and 1-SST are involved in fructan synthesis in roots. A gradual downregulation of fructosyltransferase genes and activity of respective enzymes was observed in roots during the harvest period, which also coincided with the decrease in fructooligosaccharides and increase in fructose due to fructan exohydrolase activity. These findings suggest that downregulation of fructosyltransferases genes during harvest time may contribute to efficient degradation of fructan required for the emergence of spears.