Effect Of Paenibacillus Polymyxa SQR-21 On Watermelon Root Exudates And Root Proteins

Fusarium wilt of watermelon is a destructive disease,which can restrict the production gravely across the whole world.The pathogen of watermelon wilt,Fusarium oxysporum f.sp.niveum(FON),has been identified and the pathogenic mechanism has since been investigated.A plant growth-promoting rhizobacterium,P.polymyxa SQR-21(SQR-21)was found to be of great effects on disease-suppressing.However,little was known about the interaction between watermelon and microbes in the protein aspect,which will help study the mechanisms of pathogenesis and resistence at the cellular level.In this study,we aimed to discover the proteome difference of watermelon roots after inoculation of SQR-21 or FON,in order to explore the mechanism of pathogenicity of FON,and growth-promoting and disease-suppressing capability of SQR-21.The main results were exhibited as follows:1.The growth conditions of watermelon were found to be better in plant nutrition solution in an artificial chamber.Compared to sand cultivation and agar cultivation,hydroponic cultivation allowed watermelon cultivars to grow stronger.The best time to collect the root exudates for proteomic analysis is 4 days post SQR-21 inoculation.The leaf number,main stem length and plant weight increased by 16.7%,23.4%and 43.4%,respectively,when watermelon roots were inoculated with SQR-21 for 15 days,which indicated SQR-21 can promote the growth of watermelon under hydroponic conditions.Though the treatment of 4-day inoculation made no difference,it showed an increasing trend and was suitable for researching the effect of SQR-21 on watermelon roots.2.The diversity of watermelon roots secreted proteins increased after inoculation of SQR-21.UHPLC-MS/MS results displayed some proteins different between the control and treated samples.There were three kinds of unique proteins found in the control,of which two were identified as ribosomal proteins and one Acyl-[acyl-carrier-protein]desaturase,while nine kinds of unique proteins in inoculation treatments were classified as plasma intrinsic protein,ATP synthase subunit alpha,alpha-tubulin,heat shock protein,protochlorophyllide reductase and so on,which were associated with cytoskeleton reorganization,energy providing,signal transduction or metabolism and so on.3.The less volatile organic components with high boiling point in exudates of watermelon root composited differently,and esters and benzene-esters account for a large proportion in each treatments.The highest proportion in the control,benzene-esters,occupied 21.5%,followed by esters,as 20.4%.The treatment which inoculated SQR-21(SQR)contained phenolics,benzene-esters and esters,as 27.5%,20.5%and 16.3%,respectively.The less volatile organic components in treatments inoculated FON(FON)and inoculated both SQR-21 and FON(F21)were similar,in which the most component was esters,accounting for 66.2%and 58.9%,and benzene-esters accounted for 16.6%and 20.6%,respectively.4.Root proteins expression differed among treatments.2D gel electrophoresis showed 41 kinds of differential proteins,and SQR,FON and F21 had 7,17 and 14 kinds of proteins up-or down-regulated,respectively,compared to CK.Label free quantitative proteomic analysis identified 472,532,511 and 491 reliable proteins in CK,SQR,FON and F21,respectively.Compared to CK,there were 39,26 and 44 proteins up-or down-regulated in SQR,FON and F21,respectively.In addition,compared with CK,SQR had 50 kinds of proteins new and 13 kinds deleted,FON newly increased 33 kinds and lost 14 kinds,and F21 newly increased 36 kinds and lost 31 kinds.5.The differentially expressed proteins exhibited different function trends in different treatments.Gene Ontology(GO)annotation analysis showed that in terms of biological processes,more proteins involved in metabolic processes,the proportion of new and up-regulated groups was 49%,lost and down-regulated groups was 63%.In terms of molecular function,proteins of catalytic activity were maximal,in which new and up-regulated groups accounted for 52%,and lost and down-regulated groups accounted for 66%.Cluster of Orthologous Groups of proteins(COG)annotation analysis showed that many of the new proteins in the inoculated treatments had functions related to metabolism,cellular process and signaling.The deleted proteins were mainly presented in CK-F21 group,and most were related to cellular process and signaling.The down-regulated proteins in treatments was mostly associated with metabolism.6.The clustering result of root proteins was different from that of the organic matters in the secretion.In terms of the organic components,treatments with FON inoculation(FON,F21)were similar,while treatments without FON inoculation(CK,SQR)were similar.In terms of root proteins,CK and FON clustered together,while SQR and F21 clustered together,indicating that root proteins expression in treatments with SQR-21 inoculation were similar and that treatments without SQR-21 inoculation were similar.In summary,the effects of FON and SQR-21 on watermelon root exudates and root proteins were obvious.Although exudates outside roots was strongly influenced by FON,SQR-21 could greatly regulate the root proteins.We speculate that,SQR-21 will stimulate the activity of root itself,which helps plant to promote growth and enhance resistibility to stress.