Abstract：Echinochloa crus-galli is one of the most noxious grass weeds in Chinese rice paddies, which has evolved resistance to quinclorac after continuous application since 1990. But little is known about its resistance mechanism. In order to explore the mechanism involved, we described isolation and mRNA expression of a quinclorac-responsive gene MYB in E. crus-galli. Transcriptal factor MYBs played important roles in regulating plant growth and development and withstanding environmental stresses. But, little is known about expression profile of MYBs mRNA in herbicide-resistant barnyardgrass (E. crus-galli) under quinclorac stress. A homologue sequence of MYB was obtained from quinclora-resistant-identified E.crus-galli using RACE(rapid amplification of cDNA ends). The gene, designated EcMYB1 (GenBank accession number: JX518599) with whole length 1 844 bp, had a 1 527 bp open reading frame predicted to encode a protein of 508 amino acids with molecular weight 55.4 kD and isoelectric point 8.6. The predicted EcMYB1 contained three conserved MYB domains that make the EcMYB1 belong to the R1R2R3 gene family which was not common in plants. Based on phylogenic analysis of protein sequence between EcMYB1 and others MYB, EcMYB1 had the closest relationship with the MYBs of gramineous plants, such as Sorghum bicolor, Zea mays. The result of whole-plant dose-reponse experiments demonstrated that the near iso-genicline material used in this study was susceptible or resistant to quinclorac. The GR50 (for 50% growth reduction) was 158.3 g·AI/hm2 for quinclorac-susceptible (S) biotypes or 858.0 g·AI/hm2 for quinclorac-resistant (R) biotypes. Ratios (R/S) of GR50 value was 5.4. EcMYB1 transcriptional expression was monitored in seedlings (leaves and roots) and adult plants (leaves, roots, stems and seeds) of R and S biotypes of E.crus-galli through Real-time PCR experiment with EcActin (GenBank accession number: HQ395760) as the reference sequence. The relative expression of EcMYB1 in transcriptional level was 26.22~52.50 in R biotypes and 6.85~34.62 in S biotypes, such as 52.50 in leaves of seedling of R biotypes and 6.85 in leaves of S adult biotypes, and 1.22~4.78 times greater in the R plants than that in the S plants. After exposure to quinclorac, its increased relative expression was 189.66~395.45 in R biotypes and 61.91~144.29 in S biotypes，such as 395.45 in roots of seedling of R biotypes and 61.91 in stems of S adult biotypes, and 2.46~4.06 times greater in the R plants than that in the S plants. Its expression was higher in all tissues tested of R biotypes than in that of S plants before or after quinclorac treatment. The difference expression of EcMYB1 mRNA in R and S biotypes suggested that EcMYB1 maybe involve in resistance to quinclorac in E. crus-galli. The results of this study provide basic information for the further research of function of the EcMYB1 in resistance to quinclorac in barnyardgrass.
