1. OsSCE1a的图位克隆与功能分析
研究前期利用元江野生稻和籼稻材料93-11构建了一套渗入系(9YJ系列),并从中筛选出叶绿素含量提高且抽穗期提前的渗入系9YJ30(图1a,b,c),该系在长日照条件下的抽穗期比93-11提前约5-7天,在短日照条件下提前约3天。随后,利用93-11和9YJ30构建F2分离群体,并鉴定出调控叶绿素含量和抽穗期的QTL qHSCH3,进一步筛选出杂合单株,构建了包含7864个单株的群体进行精细定位。通过筛选重组单株并种成纯合系,以提高表型评估的准确性。结果表明叶绿素含量和抽穗期性状共分离,调控这两个性状的基因被锁定在14.4-kb区域内。该区间包含4个ORF,其中ORF1(不表达)和ORF2(转座子)不予考虑,ORF4被功能验证排除,ORF3(LOC_Os03g03130)编码SUMO E2结合酶OsSCE1a。测序结果表明OsSCE1a 编码区没有序列变异,但在93-11和9YJ30的启动子之间存在差异,9YJ30的启动子包含一个367-bp的缺失,这是一个微型反向重复转座元件(MITE)。
图1 OsSCE1a的定位、克隆、功能验证与基因表达
2. OsSCE1a负向调控功能持绿和提早抽穗
为了研究OsSCE1a在持绿性状中的作用,研究者从拔节期(种植后约80天)到收获期(种植后约150天),每隔7天测定93-11,渗入系9YJ30以及敲除突变体OsSCE1a-KO93-11中叶绿素含量(图2a-g)。结果表明抽穗期(种植后约101~108天)叶绿素含量最高,随后降低。与93-11相比,渗入系9YJ30以及敲除突变体OsSCE1a-KO93-11中叶绿素含量在第101-129天较高,且与93-11相比,渗入系9YJ30以及敲除突变体OsSCE1a-KO93-11中叶绿素下降较慢(图2g)。然而,由于渗入系9YJ30以及敲除突变体OsSCE1a-KO93-11早熟,在收获阶段的叶绿素含量明显低于93-11(图2c,f,g)。除收获期外,渗入系9YJ30以及敲除突变体OsSCE1a-KO93-11中光合效率均高于93-11(图2h-i),由此证明OsSCE1a调控功能持绿表型。此外研究表明9YJ30和OsSCE1a-KO93-11突变体的抽穗期在北京分别提前了5-7天和6-8天,这使得其生育期也相应缩短(图2j-k)。
图2 OsSCE1a负向调控功能持绿和提早抽穗
3. OsSCE1a负向调控氮利用效率
为了探索叶片衰老的生理基础,研究人员在苗期用去离子水中培养93-11和9YJ30,以诱导营养饥饿。饥饿条件下,9YJ30的叶绿素含量显著高于93-11(图3a,b),表明叶片衰老是由营养缺乏引起的。随后,研究人员在田间对两亲本和OsSCE1a-KO93-11突变体进行了低氮处理(图3c)。与对照相比,9YJ30和OsSCE1a-KO93-11的抽穗期提前7~9天,且叶绿素含量提高。9YJ30和OsSCE1a-KO93-11在抽穗期低氮(LN)和正常氮(NN)的相对叶绿素含量(图3d),相对分蘖数(图3e),功能上三叶的相对氮含量(图3f)均高于93-11。此外,与93-11相比,9YJ30和OsSCE1a-KO93-11突变体的生育期缩短,在LN条件下产量相关性状显著增加,而在北京正常条件下无显著差异(图3g)。93-11、9YJ30和OsSCE1a-KO93-11突变体的收获指数(籽粒产量与地上部生物量之比)差异不显著(图S6l),说明氮肥利用效率提高,但收获指数没有因为生育期提前而受到损害。与93-11相比,9YJ30和OsSCE1a-KO93-11突变体在NN和LN条件下的C含量和N含量均较高,但C/N比较低。LN条件下93-11的C/N比增加,而9YJ30和OsSCE1a-KO93-11突变体的C/N比在LN和NN条件下无显著差异。此外,种子质量测定结果显示,OsSCE1a-KO93-11突变体的蛋白质含量为11.1%,与蛋白质含量为10.7%的93-11相比略有增加,但OsSCE1a-KO93-11突变体的白垩度较低,抽穗率较高,其他品质相关参数无显著差异。这些结果表明,较低的C/N比有利于延缓叶片衰老(Martin et al., 2002),OsSCE1a不仅可以调节光合作用,还可以调节N利用效率(NUE),影响C、N的适当平衡,从而影响功能持绿。
图3 OsSCE1a负向调控氮利用效率
4. OsSCE1a启动子自然变异影响功能持绿和抽穗期
对来源于39个国家和地区的115份核心种质进行测序分析,发现9种突变类型(图4a),Hap93-11主要包含籼稻类型(MITE插入),Hap9YJ30主要包含粳稻类型(MITE缺失)。且Hap9YJ30的叶绿素含量显著高于Hap93-11类型(图4b)。利用IRGCIS数据库结合2055份水稻资源的抽穗期表型进行关联分析,发现几乎所有的位点都与抽穗期显著相关。qRT-PCR结果表明Hap93-11的相对表达量要高于Hap9YJ30(图4d)。以上结果表明OsSCE1a的启动子结构变异通过控制OsSCE1a的表达进而调控功能持绿和抽穗期。水稻原生质体瞬时表达实验表明,MITE插入促进了OsSCE1a的表达。通过预测MITE的转录因子结合位点,同时结合酵母单杂交、EMSA和瞬时表达实验证实OsNAC2直接结合到OsSCE1a启动子中的MITE区域并激活其表达。研究人员在93-11背景下获得OsNAC2-KO突变体,该突变体表现出叶绿素含量显著提高、抽穗期提前以及光合速率和叶绿素荧光提高。因此,OsNAC2通过调节OsSCE1a的表达,进而影响水稻功能持绿和生育期。
图4 OsSCE1a启动子自然变异影响功能持绿和抽穗期
5. 对OsSCE1a的遗传操作揭示了该基因在水稻改良中的潜力
为进一步验证OsSCE1a在促进早抽和持绿特性中的作用,研究人员将OsSCE1a-KO转入优良籼稻品种黄华占(HHZ)和优质粳稻品种武运粳27(WYG)和稻花香(DHX),分别获得OsSCE1a-KOHHZ、OsSCE1a-KOWYG和OsSCE1a-KODHX突变体。与野生型相比,OsSCE1a-KO突变体的叶绿素含量、光合速率和叶绿素荧光显著提高,抽穗期提前。在北京,9YJ30、OsSCE1a-KO93-11突变体与籼稻93-11以及OsSCE1a-KOHHZ突变体与籼稻HHZ的单株产量没有显著差异。然而,OsSCE1a-KO突变体的生育期比对照短,这可能有助于将种植范围扩展到北方,并在南方多季种植。在北京,WYG几乎不结实,而OsSCE1a-KOWYG突变体的结实率约为60%。然而,OsSCE1a-KODHX突变体在北京的产量性状略有下降。因此,对于生育期过长(如WYG)或过短(如DHX)的水稻品种,敲除OsSCE1a可能不会导致在北京种植时产量足够高,但这些材料在适宜的纬度种植时可能会成功,这表明OsSCE1a具有应用潜力。
与93-11相比,通过93-11和9YJ30以及93-11和OsSCE1a-KO93-11突变体的杂交得到的F1, 其叶绿素水平增加,抽穗期提前,光合速率和叶绿素荧光也显著提高。当将93-11和OsSCE1a-KO93-11突变体分别与不育系Y58S杂交时,Y58S和OsSCE1a-KO93-11的叶绿素含量、光合速率和叶绿素荧光增加,其生育期缩短。总之,高光合效率和早熟表现出显性特征,这为水稻杂种优势的利用提供了巨大的潜力。
6. OsSCE1a介导OsGS2的苏木化并抑制GS酶的活性
OsSCE1a编码SUMO E2结合酶。IP-MS显示OsSUMO1与OsSCE1a的互作,并通过BiFC、Co-IP和酵母双杂实验证明其互作关系。IP-MS中还筛选到一个谷氨酰胺合成酶基因OsGS2(定位于叶绿体中参与氮同化的关键基因),BiFC和Co-IP实验结果表明OsSCE1a或OsSUMO1在叶绿体中与OsGS2互作。研究者使用了苏木化系统,将AtSCE1a替换为OsSCE1a进行苏木化分析。结果表明通过使用His和SUMO标签抗体,观察到由于AtSUMO1的结合,分子质量从89 KD转移到107 KD的变化,这一变化在含有AtSUMO(GG)的样本中观察到,但在含有AtSUMO(AA)的样本中未观察到,说明OsSCE1a可以苏木化OsGS2。而对9YJ30和OsSCE1a-KO93-11的谷氨酰胺酶活性测定发现均高于对照(图6f),说明OsSCE1a直接与OsGS2互作,介导OsGS2的OsSCE1a抑制GS酶活性。
图5 OsSCE1a介导OsGS2的苏木化并抑制GS酶的活性
7.OsSCE1a 靶向并苏木化转录因子如OsGBP1进而调控生育期
研究人员在抽穗期对93-11和OsSCE1a-KO93-11突变体的剑叶进行转录组分析。差异表达基因(DEGs)在多个生物学过程中富集,识别出许多与衰老相关的DEGs。同时,也识别出与抽穗期相关的DEGs,包括正向调控抽穗期的OsMADS1、OsMADS14、OsMADS18和负向调控抽穗期的Ghd7等。在先前的研究中,Ghd7表达上调延迟抽穗,而Ghd7过表达抑制了Ehd1-Hd3a/RFT1开花途径。RT-PCR分析证实,93-11中Ghd7的表达较高,但开花基因OsMADS1、OsMADS14、OsMADS18、Hd3a和RFT1的表达水平在93-11中低于9YJ30和OsSCE1a-KO93-11突变体。由此猜测OsSCE1a促进OsMADS1、OsMADS14、OsMADS18、Hd3a和RFT1的表达,抑制Ghd7的表达,表明OsSCE1a可能影响与抽穗期相关的基因表达,进而调控生育期。
论文链接:
http://doi.org/10.1111/pbi.14524
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