Different regulation of auxin homeostasis would be a possible mechanism conferring quinclorac resistance in Echinochloa crusgalli var. zelayensis

Abstract

Differences in ethylene biosynthesis, and cyanide detoxification have been reported to be mechanisms of quinclorac resistance in Echinochloa crusgalli var. zelayensis. Resistant phenotypes could be a consequence of the altered endogenous IAA homeostasis induced by the herbicide. In this study we determined the IAA content and expression levels of auxin homeostasis-related genes in susceptible and resistant biotypes of E. crusgalli var. zelayensis after quinclorac treatment. The results showed that the IAA content of JNNX-S (susceptible biotype) was significantly higher than that of SSXB-R (resistant biotype) after treatment with 50 μM quinclorac. To better understand this rise in IAA, the expression profiles of seven genes (one for auxin synthesis, five for IAA conjugation, and one for IAA oxidation) and the biochemical activities of two oxidases involved in IAA homeostasis were measured. The expression of EcYUCCA10 was significantly higher in JNNX-S than in SSXB-R. The expression levels of the EcGH3s were significantly lower in JNNX-S than in SSXB-R. These expression profiles were consistent with the elevation of IAA levels in the susceptible biotype. In contrast, EcUGT and EcDAO were induced in each biotype, but a smaller increase was observed in SSXB-R than in JNNX-S. The enzymatic activities of IAA oxidases and peroxidases were higher in SSXB-R than in JNNX-S 24 h after treatment. It was inferred that altered expression of specific genes involved in IAA synthesis, conjugation, and oxidation resulted in less IAA being induced in the resistant biotype, resulting in a lower ethylene burst and the associated quinclorac resistance. These results suggest novel layers of complexity in the mechanism of quinclorac resistance