Dissection Of Factors Affecting Race-specific And Age-related Disease Resistance Of Leucine-rich Repeat Receptor Kinase-type R Genes In Rice

Bacterial blight(BB),a vascular disease caused by Xanthomonas oryzae pv Oryza (Xoo),is one of the most serious disease of rice production.Rice is not only a staple in Asia,but also a model for monocots and cereal plant.With increasing resistance gene of BB and publication of Xoo genome sequence,study of BB has been a model of research on plant-microbe interactions.Hence,it is of important theoretical and practical significance to comprehensively and in-depth study of BB and elucidate the genetic and molecular mechanism of resistance.Xa21 and Xa3/Xa26 are two broad-spectrum resistance genes to Xoo.They both encode receptor like protein kinase and belong to a family.However,they are different in race-specificity.Xa21 is resisitant to PXO99 while Xa3/Xa26 is susceptible.In addition, function of Xa21 was affected by developmental stages of rice.The resistance to PXO99 gradually increased from susceptible 2-leaf stage to complete resistant 9/10-leaf stage with growth of IRBB21.Xa3/Xa26 expressing in Mudanjiang 8 is resistant to PXO61 at whole life.We constructed chimeric genes encoding diverse part of domain of XA21 and XA3/XA26 and transfer them to the same rice variety Mudanjiang 8.In order to understand the relationship of structure and function of each domain,the resistance, spectrum of resistance and resistance at different stages of chimeric genes and Xa21 as well as Xa3/Xa26 were then investigated.Additionally,we screen proteins interacting with kinase domain of XA3/XA26 by using yeast two hybrid technology.Chimeric gene D56S.which encodes product containing leucine-rich repeat domain with transmembrane domain of XA3/XA26 and protein kinase domain of XA21,and D57S encodes product containing eucine-rich repeat domain with transmembrane domain of XA21 and protein kinase domain of XA3/XA26.Resistance-spectrum analysis showed that resistance spectrums of D56S or D57S were similar to that of Xa3/Xa26 or Xa21 respectively.However,D52S and D92S,which were different with D57S or D56S only at encoding sequence of cytoplamtic juxtamembrane region,confered no resistance to PXO61.Above results suggested that leucine-rich repeat domain provided the race specificity of XA21 and XA3/XA26 and the important function of juxtamembrane region of XA21 and XA3/XA26.In addition,juxtamembrane region also affect the race specificity of XA21 and XA3/XA26.Transgenic lines and other rice materials carrying Xa21 or Xa3/Xa26 at 2-leaf stage, 4-leaf stage,maximum-tillering stage and heading stage were inoculated with Xoo strain PXO61,PXO99 and PXO341.Results show that in Mudanjiang 8,D56S,D57S and Xa21 confered resistance to PXO61 at all stages as well as Xa3/Xa26.While in background of IR24 or Minghui 63,Xa21 and Xa3/Xa26 provided resistance to PXO61 only at adult stage.Same results were achieved in experiment when inoculated with PXO99 and PXO341,which indicated that genetic background affected the performance of Xa21 and Xa3/Xa26 at diverse developmental stages.Relative expression level of Xa21 in transgenic Mudanjiang 8 was notable increased compared with in IRBB21.Additionally, at maximum-tillering stage,the expression level of Xa21 reach its top,this was consistent with its resistance.We concluded that gene dosage-effect is an important fact affecting function of Xa21.Three proteins interact with protein kinase domain of XA3/XA26 were obtained by yeast two hybrid screen.D4 is an mRNA splicing factor,C5 is a autophagy-related protein and C17 is a subunit of G protein.We speculated these proteins involved in Xa3/Xa2 6-mediated resistance.