Target-Site and Metabolic Resistance Mechanisms to Penoxsulam in Late Watergrass (Echinochloa phyllopogon) in China

Echinochloa phyllopogon, a malignant weed in Northeast China's paddy fields, is currently presenting escalating resistance concerns. Our study centered on the HJHL-715 E. phyllopogon population, which showed heightened resistance to penoxsulam, through a whole-plant bioassay. Pretreatment with a P450 inhibitor malathion significantly increased penoxsulam sensitivity in resistant plants. In order to determine the resistance mechanism of the resistant population, we purified the resistant population from individual plants and isolated target-site resistance (TSR) and nontarget-site resistance (NTSR) materials. Pro-197-Thr and Trp-574-Leu mutations in acetolactate synthase (ALS) 1 and ALS2 of the resistant population drove reduced sensitivity of penoxsulam to the target-site ALS, the primary resistance mechanisms. To fully understand the NTSR mechanism, NTSR materials were investigated by using RNA-sequencing (RNA-seq) combined with a reference genome. High-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) analysis further supported the enhanced penoxsulam metabolism in NTSR materials. Gene expression data and quantitative reverse transcription polymerase chain reaction (qRT-PCR) validation confirmed 29 overexpressed genes under penoxsulam treatment, with 16 genes concurrently upregulated with quinclorac and metamifop treatment. Overall, our study confirmed coexisting TSR and NTSR mechanisms in E. phyllopogon's resistance to ALS inhibitors.

Functional characterization of cytochrome P450 CYP81A subfamily to disclose the pattern of cross-resistance in Echinochloa phyllopogon

CYP81A P450s armor Echinochloa phyllopogon against diverse and several herbicide chemistries. CYP81A substrate preferences can be a basis for cross-resistance prediction and management in E. phyllopogon and other related species. Metabolism-based herbicide resistance is a major threat to agriculture, as it is unpredictable and could extend resistance to different chemical groups and modes of action, encompassing existing, novel and to-be-discovered herbicides. Limited information on the enzymes involved in herbicide metabolism has hindered the prediction of cross-resistance in weeds. Members of CYP81A subfamily in multiple herbicide resistant (MHR) Echinochloa phyllopogon were previously identified for conferring cross-resistance to six unrelated herbicide classes. This suggests a critical role of CYP81As in endowing unpredictable cross-resistances in E. phyllopogon, thus the functions of all its nine putative functional CYP81A genes to 33 herbicides from 24 chemical groups were characterized. Ectopic expression in Arabidopsis thaliana identified the CYP81As that can confer resistance to multiple and diverse herbicides. The CYP81As were further characterized for their enzymatic functions in Escherichia coli. CYP81A expression in E. coli was optimized via modification of the N-terminus, co-expression with HemA gene and culture at optimal temperature. CYP81As metabolized its herbicide substrates into hydroxylated, N-/O-demethylated or both products. The cross-resistance pattern conferred by CYP81As is geared towards all chemical groups of acetolactate synthase inhibitors and is expanded to herbicides inhibiting photosystem II, phytoene desaturase, protoporphyrinogen oxidase, 4-hydroxyphenylpyruvate dioxygenase, and 1-deoxy-D-xylulose 5-phosphate synthase. Cross-resistance to herbicides pyrimisulfan, propyrisulfuron, and mesotrione was predicted and confirmed in MHR E. phyllopogon. This study demonstrated that the functional characterization of the key enzymes for herbicide metabolism could disclose the cross-resistance pattern and identify appropriate chemical options to manage the existing and unexpected cross-resistances in E. phyllopogon.

Identification of massive molecular markers in Echinochloa phyllopogon using a restriction-site associated DNA approach

Echinochloa phyllopogon proliferation seriously threatens rice production worldwide. We combined a restriction-site associated DNA (RAD) approach with Illumina DNA sequencing for rapid and mass discovery of simple sequence repeat (SSR) and single nucleotide polymorphism (SNP) markers for E. phyllopogon. RAD tags were generated from the genomic DNA of two E. phyllopogon plants, and sequenced to produce 5197.7 Mb and 5242.9 Mb high quality sequences, respectively. The GC content of E. phyllopogon was 45.8%, which is high for monocots. In total, 4710 putative SSRs were identified in 4132 contigs, which permitted the design of PCR primers for E. phyllopogon. Most repeat motifs among the SSRs identified were dinucleotide (>82%), and most of these SSRs were four motif-repeats (>75%). The most frequent motif was AT, accounting for 36.3%-37.2%, followed by AG and AC. In total, 78 putative polymorphic SSR loci were found. A total of 49,179 SNPs were discovered between the two samples of E. phyllopogon, 67.1% of which were transversions and 32.9% were transitions. We used eight SSRs to study the genetic diversity of four E. phyllopogon populations collected from rice fields in China and all eight loci tested were polymorphic.