Functional Characterization Of Major Disease Resistant Gene Xa4 Against Xanthomonas Oryzae Pv. Oryzae And Identification Of Candidates Involved In Xa3/Xa26-mediated Defense Signaling Pathway Against Xanthomonas Oryzae Pv. Oryzae

Bacterial blight of rice,caused by Xanthomonas oryzae pv.oryzae（Xoo）,jeopardized the stability of rice production.Deployment of host-plant resistance into rice breeding programs would benefit sustainable control of this disease in the light of its economical,effective,and environmentally protective superiority.Featured for its high strength,qualitative resistance or complete resistance conferred by major disease resistance（MR）genes is easily manipulated during cultural practice.Rice MR gene Xa4 against Xoo is a completely dominant gene,mediating a relative broad-spectrum resistance to Xoo at all stages of plant growth.It is one of the most widely exploited resistance gene in breeding program in many Asian countries.Using a candidiate gene approach,we have shown a gene encoding a wall-associated kinase-type protein functions as Xa4.In addition to conferring durable resistance,Xa4 also regulates plant height and mechanical strength via modifying cell wall related features.Xa4 mediates race-specific resistance against Xoo.Located on the cell wall,it could recognize specific“ligands”from imcompatible strain to set up defense response.It is proposed that Xa4 mediates disease resistance towards Xoo via transcriptional reprogramming of secondary cell wall（SCW）Ces A and expansin genens,thereby maintaining the cell wall integrity.First,a decrease in cellulose content was found in Xa4-RNAi plants,along with an aberrant microfibirls arrangement.Such defect in cell wall not only leads to a decline in mechanical force,but also might cause a leak of the intracellular nutrients,generating a buffer beneficial for Xoo growth.Upon attack,Xa4 could be rapidly upregulated,stimulating the SCW synthesis.Genes（Ces A4,Ces A7 and Ces A9）involved in SCW synthesis are induced.Impairment in SCW Ces A aggravates the susceptibility towards Xoo,suggesting they are positive determinants in rice immune system.Second,Xa4-progressively inhibits Xoo-induced expression of genes（Expansins）involved in expansion of the cellulose matrix,which could weaken the non-covalent binding between cellulose and non-cellulosic polysaccharides.Composed ofβ-1,4 glucan,,cellulose could be a potential source of carbon for Xoo,but is difficult to access as it embeds in the non-cellulosic polysaccharides matrix.The effect brought by expansins would make cellulose more vulnerable to certain enzymes secreted by Xoo,which might release more oligo-cellulose to promote Xoo growth and development.Apart from conferring disease resistance,Xa4 has synergistic effects by additively improving plant height and mechanical strength.Thus,cloning of Xa4 would lead to smarter use of it in controlling disease spread.In parallel,breeders could use Xa4 to reduce internodes length and enhance mechanical strength,which would allow rice plant to transform more photosynthesis to total biological yield.Together,these traits conferred by Xa4 would attribute to a better yield potential.Another rice MR gene,Xa3/Xa26,encodes a leucine-rich repeat（LRR）kinase-type protein,conferring durable resistance to Xoo.The enhanced resistance mediated by Xa3/Xa26 is accompanied by a rise in expression level,known as dosage effect.To further unveil its molecular basis,a suppressor screen of Xa3/Xa26 was performed.Both components of Xa3/Xa26 signaling pathway or other genes regulate Xa3/Xa26-independent resistance might be identified.At lease three mutants exhibiting inherited fully defect in Xoo-defense reaction were nailed,among which two were undergoing map-based cloning by construction of F₂populations.Quantitative resistance controlled by quantitative trait loci（QTL）is another valuable source for broad-spectrum and durable disease resistance.Because of the complex genetic control and unknown of the genes underlying resistance QTL,quantitative resistance is not effectively employed in rice improvement.To isolate disease resistance QTL,we established a candidate gene strategy integrating linkagemap,expression profile,and functional complementation analysis.The strategy has been proven to be applicable for identifying the genes underlying minor resistance QTL in rice-Xoo and rice-M.oryzae pathosystems and it may also help to shed light on disease resistance QTL of other cereals.