Abstract Two kinds of sigma factorscapable of inducing Ayrobaorerium vir gene expression were puri— fled and identified from leaf extrat. s of panicle-differentiating to flowering stage of rice(Oryza sativa L. IR 72)detected by Agrobarteriurn ur：lacZ fusion genes. The induction was similar to that observed with 5urm actosyringone (AS). Based on the eomprehemive analysis of the data by UV，IR，NMR，MS，HMQC and HMBC，the structures of these two signal factors are identified as 5，7，4'-trih3droxy-3'5'-dimethoxy-flamne (named tricin)and 5，4-dihydroxy•3'，5'-dimethoxy-7-(B-D-glumsyloxy)-flavone，respectively. These results dernomtrate that monocotyledonous plants do contain highly efficient v/r gene inducing factors of Ayrobacterium，and the reason why monoootyledonous plants are difficult to transform by. 4yrobacterium is not due to absence of vir gene inducing factors. but due to the signal factors only produced in specific stage and tissue of monocotyledonous plants