利用RNAi提高植物营养价值

1、Using RNAi to improve plant nutritional valueApplicationContentsIntroductionMechanismEvaluation1234IntroductionIn 1998,Craig Mello and Andrew Fire found that traces of dsRNA(double-stranded RNA) in Caenorhabditis elegans（秀丽新小杆线虫） triggered a marked silencing of genes containing sequences identical t

2、o the dsRAN.They called this unconventional gene silencing RNA interference(RNAi).Some major human diseases,such as heart disease and cancer ,can be prevented by diatary supplements of specific nutrients.It can improve the nutritional value of plants for human and animal nurtition through RNAi. Mech

3、anism1. Small interfering RNA (siRNA) In 2000, Zamore etc.empirically showed that small RNAs of 21-23 nucleotides(核苷酸核苷酸),named small interfering RNAs (siRNA),are the key factors in mediating specific RNA degradation in an in vitro Drosophila（果蝇）（果蝇） system.siRNAs were predicted and confirmed to be

4、the direct products of dsRNA cleavage by the multidomain RNase enzyme Dicer(核酸内切酶核酸内切酶). siRNAs are extremely similar in length to those discovered previously in virus-induced gene silencing(VIGS,病毒诱导的基因沉默病毒诱导的基因沉默)in plants ,and indeed Dicer activity is readily detected in wheat germ （小麦种子）（小麦种子）an

5、d cauliflower extracts（花棷菜提取（花棷菜提取物）物）.siRNAs are subsequently assembled first into a multiprotein complex（多蛋白复合体）（多蛋白复合体） called an siRNP,and then into an active RNA-induced silencing complex(RISC,蛋白复合物蛋白复合物);these complexes seek out and cleave target mRNAs that are complementary to the siRNA.2.mic

6、roRNA(miRNA)pathwayAn endogenous negative gene（消极的内源性基（消极的内源性基因）因） regulatory pathway, known as the microRNA(miRNA)pathway,also uses small RNAs of about 22 nucleotides.miRNAs are universally derived from a hairpinRNA（hpRNA，短发夹结构），短发夹结构） with signifiant ,but imperfect,double-stranded character. Dicer

7、 is responsible for cleaing the miRNA precursor to produce the mature miRNA.Both siRNAs and miRNAs are thought to be assembled into similar RISC structures that regulate complementary RNA (cRNA) targets by targeting them either for cleavage or for translation repression.In plants ,most identified mi

8、RNAs show extensive or complete complementarity to their presumed target mRNAs ,and many of these mRNAs have been shown to be subject to miRNA-mediated cleavage .3.RNA -dependent RNA polymerase (RdRP)The endogenous plant RNA silencing machinery also involves an RNA-dependent RNA polymerase(RNA聚聚合酶合酶

9、).This polymerase uses RNA templates(模板模板) to synthesize（合成）（合成） cRNAs in either a manner;the cRNAs anneal（退火）（退火） to form dsRNAs,which are then processed by Dicer to generate siRNAs.Transgenic plants designed to overexpress exogenous(内源的内源的) or extra copies of endogenous（外源的）（外源的） genes produce abe

10、rrant（异常的）（异常的） mRNAs with incomplete coding regions（编码（编码区）区）.These aberrant RNA species have been proposed（提出）（提出） to function as templates for RdRP,which might mediate their clearance（清除）（清除） via the activation（激活）（激活）of RNAi pathway. RANi-based technologies for improving the nutritional qualitie

11、s of plantsApplicationIn many crops,the tissue that is consumed as food(mainly seeds) is diffierent from the tissues that control plant growth and productivity(mainly roots and shoots) .In many cases, genes controlling specific feature do not operate in a tissue-specific manner ,but function in all

12、or most plant organs.Therefore ,a given gene that is beneficial for improving seed quality is often hurmful for the growth of other plant organs .Now plant RNAi technologies show their advantages .In plants ,dsRNA-triggered RNAi is apparently the most powerful:it is the most efficient in terms of ex

13、tent of gene silencing ,and the resulting silencing is almost as complete as what achieved in a gene knockout approach.Another approach to silence genes in plants is VIGS.In this approach,target genes can be transiently inactivated by infecting the plants with a recombinant virus that expresses frag

14、ments of the endogenous plant gene transripts. VIGS is very useful for gene functional studies, but it can do nothing to change genetic information for plant improvement.RNAi technology has been used in several plants to improve their nutritional quality.For example, caffeine content in coffee plant

15、s has been markedly reduced by RNAi-mediated suppression of the caffeine synthase gene.In another study, RNAi has been successfully used to generate a high-lysine (高赖氨高赖氨酸酸)maize by knocking out the expression of the 22-kD maize zein storage protein ,a protein that is poor in lysine content.RNAi tec

16、hnology has also been successful in genetic modification of the fatty acid（脂肪酸）（脂肪酸） composition of oil.RNAi mediated by a hairpin RNA（短发夹结构（短发夹结构RNA） has been used in cotton to downregulate two key fatty acid desaturase（脱氢酶）（脱氢酶） genes ,which leads to an increase in nutritionally improved high-olei

17、c(高油酸的高油酸的) and high-stearic（高硬脂酸的）（高硬脂酸的） cotton-seed oils that are important fatty acids for health of the human heart.EvaluationApplication of RNAi to improving the nutritional value of plants for human and animal nutrition,and developement of the related RNAi technologies are still in their infa

18、ncy.We can predict that there are a great deal of opportunities and challenges we are faced with about it.Becuse I borrowed a English magazine from our library,I spent much time translating certain knowledge and I lookup new words one by one sometimes,especially the part of mechanism.However, I think my understanding to RNAi technologies is still