摘要
磷(P)是植物生长发育最为重要的生命元素之一,广泛参与植物体内的生化合成、能量转移、信号转导等代谢过程。由于化学和微生物的强烈固定,大多数自然土壤中的有效磷很低,限制着植物的生长发育。植物为了适应低磷胁迫已经进化出了包括改变根系形态结构、与土壤中的菌根(Mycorrhiza)真菌形成共生体系、改善根际土壤化学性质、诱导高亲和磷转运蛋白(PT)的表达等形态结构和生理生化上的响应机制。其中植物通过在根系细胞膜上表达多种不同亲和力的磷转运蛋白来吸收和转运土壤中的有效磷。迄今为止,大多数被分离和鉴定的磷转运蛋白都属于高亲和的Pht1家族。水稻Pht1家族中共有13个成员,但它们中间只有部分基因的功能得到了阐明。由于水稻是我国最重要的粮食作物和模式作物,因此开展水稻高亲和磷转运蛋白基因功能研究,对揭示植物高效调控吸收和转运磷营养的机理具有重要意义。
本试验以水稻磷转运蛋白Pht1家族基因成员之一OsPht1;8(以下简称OsPT8)为研究对象,以粳稻品种日本晴和Dongjing为试验材料,通过RT-PCR、Q-PCR、酵母和蛙卵异源表达体系以及水稻转基因技术,研究了OsPT8的表达特征和功能,所获得的主要结果如下：
1.通过生物信息学分析表明整个水稻Pht1家族基因同源性很高,OsPT8位于水稻基因组10号染体上,没有内含子,在水稻基因组中以单拷贝形式存在。亚细胞定位结果显示OsPT8是一个质膜蛋白。
2.通过对OsPT8基因表达模式分析和异源系统中的功能研究表明：OsPT8近乎组成型表达,在根、茎、叶、胚乳、种壳和花药等部位都有强烈表达,但在根部和地上部仍然受低磷诱导,表达增强。OsPT8所编码的蛋白能够与缺失磷转运蛋白PH084的酵母突变体实现互补,酵母中的pH生长试验表明OsPT8所编码的蛋白是一个与质子相偶联的磷运输蛋白,33P吸收试验计算得出OsPT8在酵母中的Km=21μM;蛙卵异源表达试验也表明其具有吸收外源磷酸盐的功能,33P吸收试验计算得出OsPT8在蛙卵中的Km=27μM。两个异源系统结果共同显示OsPT8是一个高亲和磷转运蛋白。
3.通过野生型及超表达和沉默OsPT8基因的转基因水稻在高磷和低磷条件下的水培试验研究结果表明：在正常供磷条件下,OsPT8基因的增强表达增加了水稻根系对磷酸盐的吸收和转运,根部、茎、老叶、新叶中的有效磷含量达到野生型的2倍,阻碍了水稻的正常生长。超表达株系表现出生长迟缓,植株矮小,叶片发黄等典型的磷中毒症状；OsPT8的RNAi沉默株系显著降低了水稻对磷酸盐的吸收和转运,造成根部、茎、老叶、新叶各部位有效磷含量只有野生型的70%。在低磷条件下,OsPT8超表达株系和野生型在表型上无明显差异,同时植株内各个部位的有效磷含量也未达到显著性差异,但是OsPT8的RNAi沉默株系生长却显著受阻,生物量只有野生型的60%,表明OsPT8可能是高亲和磷转运蛋白。水培试验结果表明OsPT8在水稻磷酸盐的吸收和转运过程中起着非常重要的作用。
4.通过对OsPT8基因超表达株系在不同磷肥用量的盆栽试验研究表明,在四个磷肥用量条件下(低、偏低、中和高)超表达材料瘪壳中全磷含量均达到了野生型的2-3倍,籽粒中全磷含量较野生型提高了20%；在中高磷条件下,穗柄中全磷含量分别达到野生型的2.2和3.2倍；通过检测OsPT8基因的RNAi沉默株系和T-DNA插入突变体在正常供磷大田试验中穗柄、瘪壳、种子和种壳中的全磷含量,发现OsPT8基因沉默材料中穗柄全磷含量比野生型提高了30%,而瘪壳中全磷含量仅有野生型的70%。盆栽和大田试验结果表明OsPT8基因参与了磷素从营养器官向生殖器官的转运,沉默该基因造成水稻结实率很低甚至不育。
5.通过对高磷和低磷水培条件下OsPT8基因超表达材料的根系形态的研究表明,OsPT8参与了低磷胁迫对根系构型的调控。在正常供磷条件下,根部产生大量的根毛,这与OsPHR2超表达后在根部的形态变化结果一致,表明OsPT8可能参与OsPHR2调控根毛生长的途径。
6.通过对正常供磷条件下OsPT8超表达和敲除转基因株系中其余家族成员表达情况的分析,表明Pht1家族成员在部分功能上存在重叠。在OsPHR2超表达和RNAi沉默株系中检测OsPT8的表达情况,结果表明OsPT8可能是OsPHR2下游的一个靶基因。
总之,通过对OsPT8基因的功能进行分析和验证,我们发现OsPT8参与了水稻整个生育期磷酸盐的吸收和转运,尤其在发育后期通过对磷酸盐运输的调控会影响水稻的灌浆。由此,OsPT8基因在响应磷酸盐从营养器官向生殖器官转运的重要作用,可以为培养磷素高效利用的水稻新品种提供理论依据。
Phosphorus (P) is essential for plant growth and development due to its involvement in the processes of energy metabolism and synthesis of nucleic acids and membranes. However, the low availability of soil P is a major constraint for crop production in many agricultural systems worldwide. Higher plants thus alter their architecture and metabolism to acquire sparingly soluble P from soil. It was reported that plants are able to mobilize P by acidification of the rhizosphere by release of H+from the roots to balance excess intake of cations over anions. Pi uptake into the root symplasm involves transport from the apoplast where Pi concentration is less than2μM, across the plasma membrane (PM), and to the cytosol where Pi concentration ranges from5-17mM. Rice is one of the most important crops, feeding about one half of the world's population. In the genome of rice, there are a total of13genes encoding proteins that belong to the Phtl high affinity PT family. Consequently, research on functional characterization of rice PM phtl family genes with regard to rice nutrients uptake and translocation is of extreme importance for understanding of the mechanism and germplasm enhancement.
In this thesis, we report the tissue localization, expression pattern and the in planta function of one member of rice Phtl family. In addition, we provide electrophysiological and yeast evidence specifically demonstrating plant PT's function in a heterologous expression system. The main results are summarized as follows:
1. Sequence alignment between genomic and cDNA sequences showed that OsPT8does not have any intron. OsPT8is localized on chromosome10, and has a single copy in the genome of Nipponbare. The deduced amino acid sequence of OsPT8suggests that OsPT8is a typical intergrating membrane protein and contains consensus site for N-glycosylation. Moreover, subcelular localization analysis by OsPT8::GFP fusion proteins in onion epidermal cells confirmed that it is localized in the cell plasma membrane.
2. Both Real-time RT-PCR and semi-quantitative RT-PCR indicate that the expression of OsPT8increased in roots upon Pi starvation. The expression level of OsPT8in leaves is lower than that in root, and did not show dramatic change in response to varying Pi supply. We tested the transport activity of OsPT8in two heterologous expression systems, yeast and Xenopus oocytes. OsPT8exhibited an apparent Km of21μM and was able to complement Pi uptake mutant yeast in the high affinity concentration range. In oocyte expression system the Km is27μM. The pH experiments in yeast heterologous expression systems suggested that the activity of OsPT8is dependent on electrochemical membrane potential mediates by H+-ATPase. Thus, we concluded that OsPT8might be a high affinity phosphate transporter.
3. By transforming the construct of overexpression and knock-down of OsPT8into Nipponbare cultivar, we successfully obtained the transgenic lines with overexpression and knock-down of OsPT8. We found that the transgenic lines with overexpressed OsPT8displayed Pi toxicity phenotype in the shoots under normal Pi supply condition. Significant reduction in plant growth rate, necrosis in old leaves and dead old leaves were observed in the transgenic plants. The Pi concentration of root, clum, young leaf and old leaf in overexpression lines were all2-fold as high as that of wild type plants. In contrast, the Pi concentration of root, clum, young leaf and old leaf in knock-down lines were all only70%of that in wild type plants. All the data suggest that OsPT8plays very important roles in plants, although the underlying mechanism for Pi over-accumlation in OsPT8overexpressed plants still needs to be further investigated.
4. We also carried out pot experiment with four levels of fertilizer P. The total P concentration in unfilled rice hull is2-3folds than that in wild type under all Pi supply, while the total P concentration in brown rice enhance20%than wild type. Under high Pi conditions, the total P concentration in panicle axis are2.2-folds and3.2-folds than wild type, respectively. We also measured the total P concentration of panicle axis, unfilled rice hull, brown rice in RNAi transgenic plants and T-DNA insertion mutant. The data acquired by analysis of RNAi plants showed that the total P in panicle axis enhance30%than WT, but the total P in unfilled rice was only70%of that in WT. All these data showed that OsPT8is involved in the Pi translocation from vegetative organs to reproductive organs in rice.
5. The transgenic lines with overexpressed OsPT8also show higher sensitivity to Pi starvation with enhanced induction rate of primary root and adventitious roots than the WT. Interestingly, the overexpression lines have abundant long root hair in normal solution culture comparing with WT palnts. This phenotype is similar to that observed in the OsPHR2overexpressing plants. Considering that the expression of OsPT8was up-regulated in OsPHR2overexpressing plants, OsPT8might function downstream of OsPHR2.
6. We analyzed the relative expression of the other Phtl family members in the OsPT8overexpression and knock-down plants by the Real-time PCR, the data shown that there might be partial functional redundancy between the Phtl family members with regarding to Pi uptake and translocation.
Taken together, all these results indicated that OsPT8played very important roles in Pi uptake and translaction at each stage of rice development. Notably, OsPT8was also found to be involved in Pi translocation from vegetative organs to reproductive organs in rice. Our results pave the way for making rice plants with high P use efficiency.
引文
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