Study On The Mode Of Action Of Halauxifen-methyl Against Broadleaved Grass In Wheat Field

Halauxifen-Methyl,a post-shoot herbicide using for controlling broadleaf weeds in wheat field,is a new auxin herbicide developed by Dow Agro Sciences as the first herbicide that be classified as arylpyridinecarboxylic acid.In this study,7 species of broadleaf weeds which are dominant or regional dominant in wheat fields in China,were collected in our laboratory to explore the laboratory toxicity of halauxifen-methyl on common broadleaf weeds in wheat fields and we selected Galium spurium as the material for the later research.And then,the changes of plant hormones such as IAA,ABA and ethylene,as well as the contents of reactive oxygen species（ROS）in G.were studied,to further reveal the molecular mechanism of halauxifen-methyl.It provides valuable insights into expand its utility and helping to address issues associated with its use,extending its useful life well into the future.The main results are as follows:1.The inhibitory spectra of weeds,inhibition of growth and effects on IAA level of halauxifen-methyl.The whole plant bioassay method was used to determine the herbicidal activity of halauxifen-methyl on 7 common broadleaf weeds in wheat fields,and the results showed that halauxifen-methyl remarkable inhibitory effect on Galium spurium,Descurainia sophia,Vicia gigantea,Capsella bursa-pastoris,and a certain inhibitory effect on Geranium carolinianum,Malachium aquaticum,Raphanus raphanistrum.And the effects of different concentrations of halauxifen-methyl on the growth of Galium spurium seedlings were studied by using hydroponic method.The results showed that the treatment with halauxifen-methyl can cause growth inhibition of root and shoot,which is consistent with the inhibiting effect of 2,4-D,IAA and ethephon.The fresh weight and root length of shoot were significantly decreased,and exhibited a positive correlation with the concentration of the compounds in general.The IAA content in the above-ground and underground parts after treated with halauxifen-methyl were evaluated by ELISA respectively.The results showed that halauxifen-methyl could induce the content of IAA in the aboveground and underground parts of the sensitive Galium spurium to increase obviously,which disturbed the auxin balance in the plant,thus occurred symptoms of IAA overdose,eventually lead to growth inhibition and plant death.2.The effects of halauxifen-methyl on the expression of genes related to weed auxin homeostasis were studied by real-time quantitative PCR（q RT-CR）.Eight genes involved in plant endogenous auxin content were identified,in which two genes（Gayucca6,Gayucca8）are responsible for auxin biosynthesis,three genes（Ga GH3.1,Ga GH3.6,Ga GH3.9）are involved in amino acid conjugation for IAA,three genes（Ga ABCB1,Ga ABCB4,Ga ABCB19）are involved in auxin transport.The expression profile of these 8 genes in both aboveground and underground parts of G.were analyzed by q RT-PCR.The results show that the expression of Gayucca6 and Gayucca9 in both aboveground and underground parts of G.declined significantly after treatment with halauxifen-methyl and 2,4-D。On the contrary,the expression of Ga GH3.1 and Ga GH3.6 in both aboveground and underground parts of G.was induced significantly after treatment.Meanwhile,the expression of Ga GH3.9 in both aboveground and underground parts of of Galium spurium was decreased significantly.At the Ga ABCBs’transcriptional level,Ga ABCB1 in the in both aboveground and underground parts and Ga ABCB19 in the underground parts of the treated G.was significantly lower than the control.Nevertheless,the expression of Ga ABCB4 in both aboveground and underground parts of G.was significantly increased after treatment.In conclusion,the mode of action of halauxifen-methyl acts as synthetic mimics of IAA at high concentration and disturb the auxin balance in the plant.It is possible to disrupt auxin balance in plants through multiple pathways,however the specific pathways and their priorities,require further study.3.The effect on the ethylene and ABA biosynthetic pathway after treatment with halauxifen-methyl were measured by Gas chromatography and ELISA respectively.The result show that compared with the control,ethylene synthesis,ACC content,ACC synthase activity and ACC oxidase activity increased in a period of time after the treatment.In addition,the ABA content showed a trend of increasing after treatment with halauxifen-methyl.And then,quantitative analysis of ethylene synthesis related genes（Ga ACS4,Ga ACS7）and ABA synthesis related genes（Ga NCED1）was measured by q RT-PCR.The result show that the expression of Ga ACS4,Ga ACS7 and Ga NCED1 was significantly up-regulated after treatment.In conclusion,the inhibition of plant growth by halauxifen-methyl is related to the increase of ethylene synthesis and the accumulation of ABA.4.The content of H₂O₂,.O?-and MDA was measured after treatment with halauxifen-methyl and 2,4-D,and their contents show showed an upward trend,suggested that the treatment of halauxifen-methyl and 2,4-D caused damage to the plant and made it suffer oxidative stress.Based on the above results,it can be concluded that the mode of action of halauxifen-methyl is disturbing the auxin balance in the plant,and then thylene synthesis,ABA content and ROS content increased,eventually leads to plant senescence and death.It is possible to disrupt auxin balance in plants through multiple pathways,such as IAA synthesis,IAA metabolism and IAA transport,however the specific pathways and their priorities,require further study.