Evidence For Hybrid Rice Involved In The Origin Of Weedy Rice And The Ecological Risk Study Of Herbicide-resistant Transgenic Rice

Weedy rice,which is the same species with cultivated rice,is one of the most hurmful weeds in paddy fields and seriously impacts the rice production worldwide.Weedy rice infests worldwide at an alarmingly increasing rate recently,but its origin is attract concerned and still exist indefinite hypothesis.There is no effective chemical herbicide could be used for weedy rice control,given that weedy rice share similar morphological and physiological characteristics with rice.The herbicide-resistant transgenic rice was ever considered as an alternative approach to solve the weedy rice problem.However,the potential ecological safety of herbicide-resistant transgenic rice cultivation should be taken into a consideration.Firstly,the gene flow between weedy rice and transgenic rice may cause the ecological risk of transgene escape,the evolution of herbicide-resistant weedy rice will be becoming more difficult to control.Secondly,the commercial cultivation of herbicide-resistant transgenic rice will aggravate the dependence of non-selective herbicide glyphosate and glufosinate in paddy fields.Long-term use of a single herbicide will change the weed community dynamics.Furthermore,the herbicide-resistant/tolerant weed problem may aggravate,and bring new challenges for weeds control in paddy fields.Therefore,the origin of weedy rice,the herbicide-resistance gene escape risk and weed population dynamics under the long-term use of single herbicide are emphasized in this study.1.There is substantial evidence indicating that many weedy rice forms originated from or are closely related to cultivated rice.People increasingly convinced that weedy rice in numerous rice growing regions of China originated from or have close relationship with cultivated rice.Hybrid rice which use cytoplasmic male sterility rapidly became popular in China in the 1970s,and then accounted for approximately half of the area of cultivated rice rapidly.Characteristics of hybrid rice progeny often strongly segregate owing to the great genetic diversity of parental lines.Coincidentally,the spread of weedy rice in China is also relatively recent.Therefore,there is suspicion that the outbreak of weedy rice in China may be related to widely grown hybrid rice due to its heterosis and the diversity of its progeny.The cytoplasmic male sterility gene which contains by hybrid rice could be used to study the relationship between hybrid rice and weedy rice due to the cytoplasm gene is primarily maternally inherited.Thirty six weedy rice accessions initially collected in Liaoning,Jiangsu and Guangdong provinces were first selected for cytoplasmic male sterility gene detection by genetic method.Four weedy rice from Guangdong province were identified contain Wild abortive type cytoplasmic male sterility gene,and no materials contain Boro type cytoplasmic male sterility gene.Further,an additional 322 weedy rice accessions from 12 provinces where hybrid rice was or is widely planted were included in the molecular tests.Twelve more weedy rice were determined contain Wild abortive type cytoplasmic male sterility gene.This molecular tests result also be confirmed by genetic method.A total of sixteen weedy rice accessions were determined contain Wild abortive type cytoplasmic male sterility gene,and the hybrid rice which contain the same cytoplasm gene should be the initial maternal parent.The character segregation of hybrid rice progeny and the gene flow with other materials should play important roles in the origin and evolution process.Our results provide first direct evidence that hybrid rice can be involved in the evolution of some weedy rice accessions.However,these sixteen weedy rice only accounted for less than 5%of the total 358 accessions,low proportion of these weedy rice would rule out the notion that the outbreak of weedy rice in China can be attributed to the ample cultivation of hybrid rice One could surmise that most weedy rice accessions evolved from conventional cultivated rice much earlier.The sixteen recent evolution weedy rice accessions also suggest that weedy rice is continuously evolves with the cultivated rice it infests.2.Gene flow between transgenic herbicide-resistant crops to their wild and weedy relatives is one of the major concerns related to their possible ecological risks.Transgene flow from transgenic herbicide-resistant rice to weedy rice can transfer the herbicide-resistance trait to weedy rice has been proved by multiple studies.However,gene flow between transgenic rice and weedy rice is bidirectional.In transgene escape,transgenic rice can serve both as pollen donor and receptor,providing pollen with the transformed trait or by being pollinated by foreign pollen with weediness traits.But the ecological risk caused by reverse gene flow from weedy rice to transgenic herbicide-resistant rice still lack of enough attention.Considering our weedy rice origin study conclusion that the self weediness of cultivated rice is also a possible evolutionary approach.In order to evaluate the ecological risk caused by reverse gene flow,in this study we choose field without weedy rice for transgenic rice cultivate,adjacent field was grown weedy rice for potential pollen donor.Two transgenic glufosinate-resistant rice varieties 86B,Bar68-1;two transgenic glufosinate-resistant hybrid rice II You 86B,Xiang 125s/Bar68-1 and their F2 generation were planted in first year.Harvested rice seed(succeeding generations)were planted the following year in the same plots.The experiment continued for a third year with weeds being controlled in all years by glufosinate.Weedy-rice-like plants,both with red pericarp and resistant to glufosinate,were initially found only among the F3 generations of the two transgenic glufosinate-resistant hybrid rice plots in the second year.And the third year weedy-rice-like plants also was found in plots planting the F3 generations of the same two transgenic glufosinate-resistant hybrid rice.None weedy-rice-like plant was found in plots where two transgenic glufosinate-resistant rice varieties was planted.The molecular markers evidence proved that weedy-rice-like plants were caused by reverse gene flow from weedy rice to transgenic glufosinate-resistant hybrid rice progeny.The weedy-rice-like progeny had significantly higher relative fitness than its parents under common garden conditions.This suggests that weedy-rice-like progeny has the potential evolving into weedy rice.The weedy-rice-like plant first be found from the F3 generation of hybrid rice,this mean the reverse gene flow event were happen in F2 generation.We further use a particular gene flow experiment to explore the outcrossing rate different between hybrid rice generations.Two hybrid rice 6 Liang You 86B,II You 98 and their F2 generation were used as pollen receptor,Three transgenic rice Bar68-1,TIC-19 and 86B were used as pollen donor.The result shows that,combinations no matter with which pollen donor,the hybrid rice F2 generations had significantly higher outcrossing rates(0.96-1.65%)than the hybrid rice cultivars(0.07-0.98%).The reverse gene flow from weedy rice to transgenic herbicide-resistant rice will rapidly result in the evolution of herbicide-resistant weedy individuals,even in fields where weedy rice is well controlled by glufosinate but that are adjacent to infested fields.The segregating progeny of transgenic hybrid rice shows higher reverse gene flow ecological risk than transgenic conventional rice.Therefore,transgenic herbicide-resistant hybrid rice requires more stringent safety management practices both in their breeding and field production.3.The non-selective herbicide could use for paddy field weeds control benefited from the herbicide-resistant transgenic rice technology.But the planting of transgenic herbicide-resistant rice requires a continuous application of unique targeting herbicide,this may certainly lead to the dynamic change of weed community,even further worse the herbicide-resistant weeds problem.In this study,the weed community structure under the treatment of non-selective herbicide glufosinate was observed continuously in glufosinate-resistant transgenic rice Bar68-1 direct seeding field from 2011 to 2013,compared with the treatment of conventional selective herbicide Bensulfuron-methyl+retilachlo(BMP).The results showed that both weed species richness and total weed density decreased year by year after continuous glufosinate and BMP treatments.But the non-selective herbicide glufosinate show better weed control effect than the selective herbicide BMP in the long-term use.Weed control effect increased continuously and maintianed better level with the increase year of glufosinate treatment.However,under the long-term treatment of conventional selective herbicides BMP,the total weed density had an increasing trend and show significantly lower rice yields than glufosinate treatment from the second year.The perennial weed Paspalum paspaloides which is not sensitive to BMP becoming the dominate weed is the main reason cause the total weed density increased year by year.Besides,there was no glufosinate-resistant weed be found in this study.It is suggested that cultivation of herbicide-resistant transgenic rice under continuously using single non-selective herbicide would not rapidly lead to aggravation of weed community structure before the evolution of herbicide-resistant weeds.But long-term weed community succession still needs further to be observed.