Effects Of Auxin On The Secondary Wall Deposition Of Cotton Fibers

Cotton is the most important crop that provides natural fiber source for textile industry.In China,cotton production takes a great role in domestic economy.In order to elevate value of cotton fibers in the market,fiber quality improvement is of the same importance as fiber yield.Cotton fiber is derived from epidermal cells of the ovule.The development of cotton fiber consists of four overlapping developmental stages: initiation,cell elongation,secondary cell wall synthesis,and maturation.In the stage of secondary wall synthesis,cellulose was synthesized in large quantities and deposited on secondary walls.This biological process has a great impact on fiber traits,particularly,fiber strength and fiber finess.Auxin,as a vital plant hormone,is involved in various processes of plant growth and development.In study of metabolism of indole-3-acetic acid(IAA),the major auxin in plants,it was suggested that indole-3-acetic acid(IAA)may be involved in ending fiber primary cell wall expansion and starting secondary cell wall thickening,and the transition between two processes.Using LC-MS/MS,our research group found that IAA was highly accumulated during secondary cell wall deposition,suggesting that IAA may play an important role in the regulation of secondary cell wall thickening.However,the effects of auxin on the secondary wall deposition of cotton fibers need to be investigated.To address this question,in this paper,by observation of cellulose synthesis in transgenic cotton in which the auxin level was down regulated,and by the exogenous application of IAA on tobacco and cotton,the relationship between auxin and secondary wall deposition was studied.The main results are as followings:1.IAA application promotes cellulose synthesisCotton ovules were cultured in vitro in the presence of different concentrations of IAA.With increase of IAA concentration,the cellulose content in fibers on cultured ovules was higher than in the control,indicating that auxin can promote cellulose synthesis in a dosage-dependent manner.Treating cotton seedlings with 25?M IAA for 48 hours,the cellulose content in the hypocotyls was 19% higher than that in the control.When the concentration of IAA was increased to 50?M,the enhancement of the cellulose content in hypocotyls was 1.23-fold higher than that of wild type.In addition,treating IAA on the stem of tobacco,same increase in cellulose content was observed.These results indicated that exogenous application of auxin could promote cellulose synthesis in plants.2.Down-regulation of IAA content in transgenic cotton decreases the secondary wall thickening of fibersEndogenous auxin level was successfully down-regulated in the transgenic cotton plants expressing BAN::PLA2?RNAi and E6::iaaL respectively.Using LC-MS/MS,IAA content in fibers of elongation and cellulose synthesis stages was determined.The result indicated that compared with wild-type control IAA level in fibers of BAN::PLA2?RNAi transgenic cottons had been decreased.In the same time,three secondary cell wall synthesis associated genes(CesA1,CesA2 and ?-1.3 glucanase)in BAN::PLA2?RNAi fibers was obviously decreased.Similar result was obtained in the fiber of E6::iaaL.These data suggested that the decreased cellulose content(in fibers),thickness(of secondary fiber cell wall)and micronaire value in fibers of the transgenic cotton may result from the down regulation of the expression of these secondary cell wall synthesis associated genes.To endogenously decrease auxin level through enhancing metabolism of auxin,transgenic tobacco expressing 35S::OsDAO was generated.As expected,IAA content in leaves and stems of the transgenic tobacco was decreased.Meanwhile,cellulose content in both leaves and stems were also decreased.These data comfirmed that genetically decreased IAA inhibited cellulose synthesis can lead to a decrease of cellulose synthesis.3.Construction of ethanol-inducible system in tobaccoAuxin is crucial plant growth and development.OsDAO decreases auxin level through degradation pathway.Over expression of OsDAO may produce auxin-deficiency phenotype that impair the normal growth and development of plants.In our experiment,we failed to obtain the transgenic cotton,in which even OsDAO was under the control of a fiber-specific promoter Fb L2 A.The failure may be due to the over-degradation of auxin by OsDAO.To overcome this problem,an ethanol-inducible expression system was used to control the expression of GUS report gene and OsDAO respectively,and transgenic tobacco was generated.Treating with ethanol GUS activity was induced in transgenic tobacco demonstrating that the system can successfully work in tobacco.The optimal condition for the induction was 0.1% ethanol for 12 h at 37?.Ethanol-induce system provides a promising tool for investigation of the role of auxin in the secondary wall deposition of cotton fibers.