We previously reported that the mechanism of quinclorac resistance in Echinochloa crus-galli var. zelayensis may be closely related to ethylene biosynthesis and the detoxification of cyanide. Differences in EcCAS gene sequences and expression levels may result in higher capacity to detoxify cyanide in resistant biotypes, which may avoid cyanide accumulation and avoid more ethylene and cyanide production and then avoid damage. In the present study, we focused on the mechanism of resistance related to ethylene biosynthesis in E. crus-galli var. zelayensis. The fresh weight of susceptible and moderately resistant biotypes were significantly reduced after treatment with quinclorac. However, AOA, an ethylene biosynthesis inhibitor, reduced the impact of quinclorac. On pretreatment with AOA, ethylene production was significantly reduced in the three biotypes. The highly resistant biotype produced less ethylene compared to the other two biotypes. Three ACS and seven ACO genes, which are the key genes in ethylene biosynthesis, were obtained. The expression levels of EcACS-like, EcACS7, and EcACO1 varied in the three biotypes upon treatment with quinclorac, which could be manipulated by AOA. In summary, it is inferred that the expression of EcACS-like, EcACS7, and EcACO1 can be stimulated to varying extent after quinclorac treatment in three E. crus-galli var. zelayensis biotypes, which consequently results in varying levels of ethylene production. Lower expression of these three genes results in more resistance to quinclorac, which may also be related to quinclorac resistance in E. crus-galli var. zelayensis.

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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 indole acetic acid (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.