Cellular Mechanism Of Major Resistance Genes Against Bacterial Blight And Bacterial Leaf Streak In Rice

Bacterial blight(BB)and bacterial leaf streak(BLS)caused by Xanthomonas pv.oryzae(Xoo)and Xanthomonas oryzae pv.oryzicola(Xoc)respectively are both devasting disease of rice.The major resistance genes against BB and auntitative trait loci(QTL)against BLS are always exploited in rice production.Therefore,the researches on the mechnasim of rice resistance to BB and BLS will prevent the BB and BLS diseases and provide theoretical evidences to rice resistance breeding.The molecular mechnasim have been explored about the cloned major resistance(MR)genes against BB,but little cellular mechnasim have been detected in rice lines carry the MR genes.The ultrastructures of xylem parenchyma cells were analyzed by transmission electron microscopy in the dominant MR genes Xa1/Xa4/Xa21 and the recessive MR genes xa5/xa13.There were brown color hypersensitive response-programmed cell death(HR-PCD)on the inoculation site of all the resistant plants with MR genes.There were classical autophagosome-like bodies such as the double-membrane vesicles in cytoplasm and the single-membrane vesicles in vacuole of xylem parenchyma cells during the Xa1/Xa4/Xa21 dominant MR genes mediated-resistance against Xoo.There was destructive vacuole mediated cell death characterized by the tonoplast rupture during xa5 and xa13 recessive MR genes mediated-resistance against Xoo.However,the necrosis characterized by plasmolysis and plasma menbrane rupture happened in susceptible reaction to Xoo.The autophagy inhibitor 3-methyladenine and alkaline Na2HPO4 solution would partially reduce the resistances mediated by dominant and recessive MR genes,respectively.These evidences suggest the autophagy possibly keeps the cell alive to resistant against Xoo and destructive vacuole mediated cell death maybe provides low p H environment to inhibit the Xoo growth.The MR gene Xa3/Xa26 was widely exploited in china rice production.In Xa3/Xa26 mediated resistance against Xoo,the autophagy related genes ATG5,ATG6 a,ATG7 and ATG8 c were induced higher expression,the classic autophagosome-like bodies appeared in xylem parenchyma cells,the content of hydrogen peroxide localizing on the plasma membrane decreased and the calcium ion accumulated in vacuole.The autophagy inhibitor 3-methyladenine would increase the content of hydrogen peroxide,inhibit the calcium ion transportion from extracellular to vacuole and partially reduce Xa3/Xa26 mediated reistance.These evidences show that the autophagy could reduce the excessive hydrogen peroxide stress and activate calcium signals to participate the Xa3/Xa26 mediated resistance.The xa5 as a MR gene against Xoo,is also the biggest contribution gene in the largest effect major QTL q Blsr5 a against Xoc.In xa5 mediated resistance to Xoc,the cell wall of mesophyll cell contacting directly with bacteria kept tight surface structures,the cellulose microfibril arrangement and the cell wall linkage between mesophyll cells kept well.Meanwhile,xa5 could induce cellulos synthase 4/7/9 higher expression and reduce the number of bacteria in neighbor sites of infiltrating inoculation site.These evidences show the integrity cell wall mediated by xa5 would prevent the spread of Xoc in extracellular spaces of mesophyll cells.The lower expression of cellulos synthase 4/7/9 and the higher cell wall degradation ability of Xoc than that of Xoo could prove that the cell wall plays more important role in xa5 mediated resistance to Xoc than in xa5 mediated resistance to Xoo.Furthermore,the higher content of jamonic acid(JA)and JA precursor 12-oxo-phytodienoic acid and less starch granules in chloroplast suggest JA and saccharides for biogenesis of starch could possibly promote cell wall biogenesis to participate xa5 mediated resistance to Xoc.The XA4 proteins localization on cell wall was analyzed by immune-gold label method.Meanwhile,the calcofluor staining cellulose method was better applied in analysis the cellulose content and the cell length of rice stem influenced by Xa4.