Unraveling the mechanisms of multiple resistance across glyphosate and glufosinate in Eleusine indica

The herbicides glyphosate and glufosinate are commonly used in citrus and sugarcane orchards in Guangxi Province, China, wherein the C₄ plant Eleusine indica (L.) Gaertn. is known to be a dominant weed species. However, high selection pressure has resulted in failure of control. In the present study, experiments were conducted to clarify resistance levels for the suspected populations and elucidate the mechanisms for multiple resistance. The resistance index to glyphosate was calculated for eight populations and ranged from 5.4 to 21.3, with a low-level shikimate content of 0.24–0.50 μg g⁻¹. In addition, three populations showed low-level resistance to glufosinate, with a resistance index ranging from 2.6 to 3.9. The amplification of the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene with a double-mutation T102I + P106S (TIPS) or a single-mutation (P106S and P106A) was observed in most populations. The target genes of glufosinate (GS1–1, GS1–2, and GS1–3) showed high-level expression, namely 145.5-fold that of susceptible plants. The content of EPSPS and glutamine synthetase (GS) protein in resistant plants can reach to 3.6 and 2.1 times higher than those in susceptible plants, respectively. The overexpression of the EPSPS gene with double (T102I + P106S) or single (P106S and P106A) mutations, plus the overexpression of GS1–1, GS1–2, and GS1–3, responded to multiple resistance mechanisms. Altogether, these results demonstrate that overexpression of GS1 is a novel form of resistant mechanism to glufosinate in weeds.