植物体内谷胱甘肽还原酶的功能研究

谷胱甘肽还原酶（GR，EC1.6.4.2）是一重要的抗氧化酶，许多生理学和遗传工程研究都证明GR酶在抗氧化中的重要作用。但改变GR酶怎样影响植物的抗氧化系统却不清楚。GR是抗坏血酸－谷胱甘肽循环途径中的重要组成部分，其功能必然与其密切相关。本文用RNAi技术获得具有较低GR酶活性的转基因烟草，系统测定了非胁迫条件和胁迫条件下抗坏血酸－谷胱甘肽循环的变化，得出以下主要结果： 1．选择一烟草叶绿体GR酶编码基因（X76293, gi: 431954）进行RNAi载体构建，构建好的双元载体转化根癌农杆菌LBA4404，然后侵染转化烟草叶圆片。获得的转基因烟草具有30－70％的GR酶活性。分子检测结果表明GR在RNA和蛋白水平上与GR酶活性的变化一致。文中我们第一次用2-D电泳对烟草中GR同工酶进行分析，并确定发生抑制的GR同工酶在细胞中的定位。2-D电泳后的Western杂交检测到烟草的10种GR同工酶，pI值分布在4.5-6.3，其中3种GR同工酶定位在叶绿体内，其蛋白量占据所有GR酶含量的大部分。RNAi发生在叶绿体内和叶绿体外，表明发生抑制的GR同工酶的基因序列具有很高的同源性。igr转基因烟草在表型上与野生型对照烟草无明显差异。 2．所有igr转基因植株和对照植株中的活性氧（O2-和H2O2）、MDA含量和光合作用都无明显差异，表明正常生长条件下GR酶活性的降低不会引起氧化胁迫。测定正常生长条件下igr转基因烟草中谷胱甘肽库的变化。结果表明与对照烟草相比，GR酶活性降低70％会引起转基因植株中GSH/GSSG比率明显降低，而GSH和GSSG的含量稍有增加；测定抗坏血酸－谷胱甘肽循环的变化，结果显示igr转基因烟草中DHAR和MDHAR的酶活性升高，表明非胁迫条件下较低的GR酶活性可能会诱导抗坏血酸－谷胱甘肽循环不能正常的运转。这一作用可能与改变的谷胱甘肽库有关。GR酶活性降低30％的转基因烟草中未检测到这些变化，表明70％的GR酶活对于非胁迫条件下igr转基因烟草可能是足够的。 3. MV处理结果显示，igr转基因烟草的离体叶圆片和活体植株在MV处理后都发生比对照烟草严重的光漂白作用。igr转基因烟草的活性氧和MDA含量明显高于对照烟草，igr转基因烟草的光合作用明显低于对照烟草。以上这些指标表明igr转基因烟草对MV处理更为敏感。MV处理条件下igr转基因烟草谷胱甘肽的含量明显高于对照烟草，但是GSH/GSSG的比率明显低于对照烟草，GR酶活性仍明显低于对照烟草，表明在MV胁迫条件下igr转基因烟草中较低的GR酶活性不能有效的将GSSG还原生成GSH。igr转基因烟草中较高的谷胱甘肽净含量说明其谷胱甘肽的合成能力提高，但这仍不能补偿胁迫条件下较低GR酶引起的GSH/GSSG比率降低。MV处理条件下igr转基因烟草和对照烟草相比ASC的含量大大降低，导致DHA/ASC明显升高。测定MDHAR和DHAR的结果表明，MV处理后igr转基因烟草的MDHAR酶活性明显降低，这表明较低的GR酶活性引起ASC再生循环受到抑制。MV处理后较低的GR酶还引起igr转基因烟草中APX的活性大大降低。以上这些结果表明MV处理条件下降低GR酶活性会削弱抗坏血酸－谷胱甘肽循环，从而引起活性氧的大量积累，造成严重的氧化伤害。 4．低温处理的结果和MV处理的结果稍有不同。在GR酶活性较高的i2转基因烟草中所有检测指标与对照烟草无明显差异。而GR酶活性较低的i21、i28和i42植株与对照烟草相比表现出明显差异。低温下生长的对照烟草叶绿素含量明显高于i21、i28和i42植株。i21、i28和i42中活性氧（O2-和H2O2）和MDA的含量都明显高于对照烟草，表明低温处理下i21、i28和i42受到更严重的胁迫伤害。与MV处理后的变化相似，低温处理后i21、i28和i42中较低的 GR酶活性导致GSH/GSSG大大降低，ASC再生循环受抑制，APX活性明显降低，从而使抗坏血酸－谷胱甘肽循环不能高效的清除活性氧，导致ROS和MDA的大量积累，造成严重的低温伤害。

Ramadan school holidays as a natural experiment : impacts of seasonality on school dropout in Bangladesh

In 2000, Ramadan school vacation coincided with the original annual exam period of December in Bangladesh. This forced schools to pre-pone their final exam schedules in November, which was the month before the harvest begins. 'Ramadan 2000' is a natural experiment that reduced the labor demand for children during the exam period. Using household level panel data of 2000 and 2003, and after controlling for various unobservable variations including individual fixed effects, aggregate year effects, and subdistrict-level year effects, this paper finds evidence of statistically significant impact of seasonal labor demand on school dropout in Bangladesh among the children from agricultural households.

Where does Philippine education go? : the "K to 12" program and reform of Philippine basic education

In 2012 the Philippines launched its "K to 12" Program, a comprehensive reform of its basic education. Through this reform, the Philippines is catching up with global standards in secondary education and is attaching a high value to kindergarten. The structure, curricula, and philosophy of the education system are undergoing reform and improvement. The key points of the new policy are "preparation" for higher education, "eligibility" for entering domestic and overseas higher educational institutions, and immediate "employability" on graduating, all leading toward a "holistically developed Filipino". This policy appears admirable and timely, but it faces some pedagogical and socioeconomic problems. The author wants to point out in particular that the policy needs to address gender problems and should be combined with demand-side approaches in order to promote poverty alleviation and human development in the Philippines.

The Effect of Private Management of Public Schools on Student Achievement in Bogotá, Colombia

During the last 15 years, the public school system in Bogotá, Colombia has maintained a concession system in which 25 schools are managed privately with exemptions to many of the rules required in the traditional schools -- This study uses the propensity score matching technique to examine whether students in the privately-managed schools have better scores on the Saber 11° examinations taken upon completion of secondary school -- The results for 251 schools indicates that students with comparable socioeconomic characteristics score considerably better on these tests in the privately-managed schools than in the traditional public schools -- Thus, there is evidence that the privately-managed public schools are a cost-effective alternative to the traditional public school