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Background Plant biostimulants are an emerging class of agricultural inputs known to enhance plant growth and improve their tolerance to abiotic stress. While biostimulants are widely used, their mechanisms of action remain poorly understood. This study investigates the effects of an Ascophyllum nodosum -based biostimulant (ANE) on the rhizosphere bacterial communities of corn ( Zea mays ). Results Root exudates from ANE root-treated plants promoted the swarming motility of Pseudomonas protegens (CHA0), a plant growth-promoting rhizobacterium. Gene expression analysis showed that root exudates from 0.01% ANE-treated plants up-regulated P. protegens CHA0 genes associated with chemotaxis ( cheW , cheV ), pyoverdine ( pvdS ), pyrrolnitrin ( prnD ), and hydrogen cyanide ( hcnA ) biosynthesis compared to controls. ANE also significantly altered rhizosphere microbiome composition, increasing the abundance of genera such as Chryseolinea , Pseudoxanthomonas , Novosphingobium , Quadrisphaera , Turneriella , and Kitasatospora . Liquid chromatography-high resolution mass spectrometry (LC–HRMS) and partial least squares-discriminant analysis (PLS-DA) revealed distinct chemical profiles in the root extracts of ANE-treated plants. Specifically, ANE increased the concentrations of benzoxazinoids, including 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) and 6-methoxybenzoxazolinone (MBOA) in maize roots by approximately 1.4-fold and 1.76-fold, respectively. Conclusion Overall, these findings suggest that ANE modifies the rhizosphere microbiome by influencing the chemical composition of both root tissue and root exudates. Graphical abstract

Key message Editing ZmGA20ox3 can achieve the effect similar to applying Cycocel, which can reduce maize plant height and enhance stress resistance. Drought stress, a major plant abiotic stress, is capable of suppressing crop yield performance severely. However, the trade-off between crop drought tolerance and yield performance turns out to be significantly challenging in drought-resistant crop breeding. Several phytohormones [e.g., gibberellin (GA)] have been reported to play a certain role in plant drought response, which also take on critical significance in plant growth and development. In this study, the loss-of-function mutations of GA biosynthesis enzyme ZmGA20ox3 were produced using the CRISPR–Cas9 system in maize. As indicated by the result of 2-year field trials, the above-mentioned mutants displayed semi-dwarfing phenotype with the decrease of GA 1 , and almost no yield loss was generated compared with wild-type (WT) plants. Interestingly, as revealed by the transcriptome analysis, differential expressed genes (DEGs) were notably enriched in abiotic stress progresses, and biochemical tests indicated the significantly increased ABA, JA, and DIMBOA levels in mutants, suggesting that ZmGA20ox3 may take on vital significance in stress response in maize. The in-depth analysis suggested that the loss function of ZmGA20ox3 can enhance drought tolerance in maize seedling, reduce Anthesis-Silking Interval (ASI) delay while decreasing the yield loss significantly in the field under drought conditions. The results of this study supported that regulating ZmGA20ox3 can improve plant height while enhancing drought resistance in maize, thus serving as a novel method for drought-resistant genetic improvement in maize.

• 2,4-Dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) and DIMBOA-glucoside (DIMBOA-Glc) are anti-insect benzoxazinoids in maize, yet very little information is known about how they are regulated. • Reverse genetics, kinase activity analysis, phytohormone and DIMBOA/DIMBOA-Glc quantification, bioassays and transcriptome analysis were employed to study the function of ZmMPK6, a mitogen-activated protein kinase, in maize response to herbivory. • ZmMPK6 was rapidly activated by wounding and simulated herbivory. Silencing ZmMPK6 in maize A188 compromised simulated herbivory-induced ethylene levels but not those of jasmonic acid or salicylic acid, and the ZmMPK6-silenced plants exhibited elevated DIMBOA/DIMBOA-Glc and insect resistance. An ethylene complementation experiment revealed that ZmMPK6 repressed the accumulation of DIMBOA/DIMBOA-Glc in an ethylene-dependent manner. Transcriptome analysis revealed that ZmMPK6 might meditate the transcription of BX1 by controlling a MYB transcription factor that is likely to be located in the ethylene signalling pathway and, furthermore, ZmMPK6 and ethylene signalling also specifically and commonly regulate the transcription of other benzoxazinoid biosynthetic genes. We also show that different maize lines have very different responses to simulated herbivory in terms of ZmMPK6 activation, ethylene emission and benzoxazinoid levels. • These results uncover that ZmMPK6 and ethylene pathway are novel repressors of DIMBOA/DIMBOA-Glc and provide new insight into the regulatory mechanisms underlying these two pathways.

Benzoxazinoids (BXs), such as 2,4-dihydroxy-7-methoxy-2H-1, 4-benzoxazin-3(4H)-one (DIMBOA), are secondary metabolites in grasses. The first step in BX biosynthesis converts indole-3-glycerol phosphate into indole. In maize (Zea mays), this reaction is catalyzed by either BENZOXAZINELESS1 (BX1) or INDOLE GLYCEROL PHOSPHATE LYASE (IGL). The Bx1 gene is under developmental control and is mainly responsible for BX production, whereas the Igl gene is inducible by stress signals, such as wounding, herbivory, or jasmona tes. To determine the role of BXs in defense against aphids and fungi, we compared basal resistance between Bx1 wild-type and bx1 mutant lines in the igl mutant background, thereby preventing BX production from IGL. Compared to Bxl wild-type plants, BX-deficient bx1 mutant plants allowed better development of the cereal aphid Rhopalosiphum padi, and were affected in penetration resistance against the fungus Setosphaeria turtica. At stages preceding major tissue disruption, R. padi and S. turtica elicited increased accumulation of DIMBOA-glucoside, DIMBOA, and 2-hydroxy-4,7-dimethoxy-1, 4-benzoxazin-3-one-glucoside (HDMBOA-glc), which was most pronounced in apoplastic leaf extracts. Treatment with the defense elicitor chitosan similarly enhanced apoplastic accumulation of DIMBOA and HDMBOA-glc, but repressed transcription of genes controlling BX biosynthesis downstream of BX1. This repression was also obtained after treatment with the BX precursor indole and DIMBOA, but not with HDMBOA-glc. Furthermore, BX-deficient bx1 mutant lines deposited less chitosan-induced callose than Bx1 wild-type lines, whereas apoplast infiltration with DIMBOA, but not HDMBOA-glc, mimicked chitosan-induced callose. Hence, DIMBOA functions as a defense regulatory signal in maize innate immunity, which acts in addition to its well-characterized activity as a biocidal defense metabolite.

Summary Maize is one of the most important crops for food, cattle feed and energy production. However, maize is frequently attacked by various pathogens and pests, which pose a significant threat to maize yield and quality. Identification of quantitative trait loci and genes for resistance to pests will provide the basis for resistance breeding in maize. Here, a β‐glucosidase ZmBGLU17 was identified as a resistance gene against Pythium aphanidermatum, one of the causal agents of corn stalk rot, by genome‐wide association analysis. Genetic analysis showed that both structural variations at the promoter and a single nucleotide polymorphism at the fifth intron distinguish the two ZmBGLU17 alleles. The causative polymorphism near the GT‐AG splice site activates cryptic alternative splicing and intron retention of ZmBGLU17 mRNA, leading to the downregulation of functional ZmBGLU17 transcripts. ZmBGLU17 localizes in both the extracellular matrix and vacuole and contribute to the accumulation of two defence metabolites lignin and DIMBOA. Silencing of ZmBGLU17 reduces maize resistance against P. aphanidermatum, while overexpression significantly enhances resistance of maize against both the oomycete pathogen P. aphanidermatum and the Asian corn borer Ostrinia furnacalis. Notably, ZmBGLU17 overexpression lines exhibited normal growth and yield phenotype in the field. Taken together, our findings reveal that the apoplastic and vacuolar localized ZmBGLU17 confers resistance to both pathogens and insect pests in maize without a yield penalty, by fine‐tuning the accumulation of lignin and DIMBOA.

Maize seedlings contain high amounts of 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), and the effect of DIMBOA is directly associated with multiple insect-resistance against insect pests such as Asian corn borer and corn leaf aphids. Although numerous genetic loci for multiple insect-resistant traits have been identified, little is known about genetic controls regarding DIMBOA content. In this study, the best linear unbiased prediction (BLUP) values of DIMBOA content in two ecological environments across 310 maize inbred lines were calculated; and their phenotypic data and BLUP values were used for marker-trait association analysis. We identified nine SSRs that were significantly associated with DIMBOA content, which explained 4.30–20.04% of the phenotypic variation. Combined with 47 original genetic loci from previous studies, we detected 19 hot loci and approximately 11 hot loci (in Bin 1.04, Bin 2.00–2.01, Bin 2.03–2.04, Bin 4.00–4.03, Bin 5.03, Bin 5.05–5.07, Bin 8.01–8.03, Bin 8.04–8.05, Bin 8.06, Bin 9.01, and Bin 10.04 regions) supported pleiotropy for their association with two or more insect-resistant traits. Within the 19 hot loci, we identified 49 candidate genes, including 12 controlling DIMBOA biosynthesis, 6 involved in sugar metabolism/homeostasis, 2 regulating peroxidases activity, 21 associated with growth and development [(auxin-upregulated RNAs (SAUR) family member and v-myb avian myeloblastosis viral oncogene homolog (MYB)], and 7 involved in several key enzyme activities (lipoxygenase, cysteine protease, restriction endonuclease, and ubiquitin-conjugating enzyme). The synergy and antagonism interactions among these genes formed the complex defense mechanisms induced by multiple insect pests. Moreover, sufficient genetic variation was reported for DIMBOA performance and SSR markers in the 310 tested maize inbred lines, and 3 highly (DIMBOA content was 402.74–528.88 μg g−1 FW) and 15 moderate (DIMBOA content was 312.92–426.56 μg g−1 FW) insect-resistant genotypes were major enriched in the Reid group. These insect-resistant inbred lines can be used as parents in maize breeding programs to develop new varieties.

Plants show considerable within-species variation in their resistance to insect herbivores. In the case of Zea mays (cultivated maize), Rhopalosiphum maidis (corn leaf aphids) produce approximately twenty times more progeny on inbred line B73 than on inbred line Mo17. Genetic mapping of this difference in maize aphid resistance identified quantitative trait loci (QTL) on chromosomes 4 and 6, with the Mo17 allele reducing aphid reproduction in each case. The chromosome 4 QTL mapping interval includes several genes involved in the biosynthesis of DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one), a maize defensive metabolite that also is required for callose accumulation in response to aphid feeding. Consistent with the known association of callose with plant defence against aphids, R. maidis reproduction on B73×Mo17 recombinant inbred lines was negatively correlated with both DIMBOA content and callose formation. Further genetic mapping, as well as experiments with near-isogenic lines, confirmed that the Mo17 allele causes increased DIMBOA accumulation relative to the B73 allele. The chromosome 6 aphid resistance QTL functions independently of DIMBOA accumulation and has an effect that is additive to that of the chromosome 4 QTL. Thus, at least two separate defence mechanisms account for the higher level of R. maidis resistance in Mo17 compared with B73.

Background Weeds cause low crop productivity and increasing costs, and therefore, different solutions, such as manual weeding or synthetic herbicides, have been suggested to solve this problem. These methods involve high efforts and costs, in addition to being harmful to the environment in the case of herbicides, which also result in increasing resistance mechanisms in weeds. Therefore, this work addresses the use of in vivo allelopathic crops to control surrounding weeds. To carry out the experiments, co-cultivation of wheat, rice and barley with the monocot weed annual ryegrass ( Lolium rigidum Gaud.) and the dicot weed common purslane ( Portulaca oleracea L.) was conducted without physical contact among crop and weed plants. Germination and growth parameters of weeds, and growth parameters and chemical profile of crops, were analysed after the end of the experiment. Results The three crops tested caused inhibitory effects on the two target weeds, and significant concentrations of benzoxazinoids were found in the plant tissues and/or root exudates of the different crops in response to the presence of weeds. All the crops showed different responses to the treatments. While the growth of rice was stimulated, barley was not affected, and wheat growth experienced inhibition due to the presence of weeds. Conclusions This study demonstrates the capacity of wheat, rice and barley to inhibit both growth and germination of L. rigidum and P. oleracea . The effects observed could be due to the accumulation and/or exudation of benzoxazinoids such as DIMBOA, DIBOA, BOA or HBOA. Barley and rice are able to sustainably manage both target weeds without disrupting their development, while growth of wheat was affected by the presence of weeds. Based on our results, rice would be the most promising crop, since it has the ability to control weeds, while stimulating the development of rice plants. Nevertheless, more research should be carried out to fully confirm this fact, especially under non-controlled conditions. Graphical Abstract

Background Although there have been numerous studies describing plant growth systems for root exudate collection, a common limitation is that these systems require disruption of the plant root system to facilitate exudate collection. Here, we present a newly designed semi-hydroponic system that uses glass beads as solid support to simulate soil impedance, which combined with drip irrigation, facilitates growth of healthy maize plants, collection and analysis of root exudates, and phenotyping of the roots with minimal growth disturbance or root damage. Results This system was used to collect root exudates from seven maize genotypes using water or 1 mM CaCl 2 , and to measure root phenotype data using standard methods and the Digital imaging of root traits (DIRT) software. LC–MS/MS (Liquid Chromatography—Tandem Mass Spectrometry) and GC–MS (Gas Chromatography—Mass Spectrometry) targeted metabolomics platforms were used to detect and quantify metabolites in the root exudates. Phytohormones, some of which are reported in maize root exudates for the first time, the benzoxazinoid DIMBOA (2,4-Dihydroxy-7-methoxy-1,4-benzoxazin-3-one), amino acids, and sugars were detected and quantified. After validating the methodology using known concentrations of standards for the targeted compounds, we found that the choice of the exudate collection solution affected the exudation and analysis of a subset of analyzed metabolites. No differences between collection in water or CaCl 2 were found for phytohormones and sugars. In contrast, the amino acids were more concentrated when water was used as the exudate collection solution. The collection in CaCl 2 required a clean-up step before MS analysis which was found to interfere with the detection of a subset of the amino acids. Finally, using the phenotypic measurements and the metabolite data, significant differences between genotypes were found and correlations between metabolites and phenotypic traits were identified. Conclusions A new plant growth system combining glass beads supported hydroponics with semi-automated drip irrigation of sterile solutions was implemented to grow maize plants and collect root exudates without disturbing or damaging the roots. The validated targeted exudate metabolomics platform combined with root phenotyping provides a powerful tool to link plant root and exudate phenotypes to genotype and study the natural variation of plant populations.

Genetic transformation in barley is limited by low transformation efficiency and high genotype dependence. Callus is used to regenerate transgenic plants during transformation, so it is quite imperative to improve regeneration capacity of callus. Firstly, four independent regenerable calli (RE) and four independent non-regenerable calli (NO), which differed in the regeneration capacity significantly, were generated by subculture of callus. The metabolic profiles of RE and NO calli were compared. Overall, a total of 48 differential metabolites were identified. These metabolites were enriched in 13 KEGG pathways, and 7 differential metabolites were involved in these KEGG pathways. The metabolism of amino acid, purine, and arachidonic acid were closely associated with plant regeneration. The results also showed that trans-zeatin-riboside, (+)-dihydrojasmonic acid, DIMBOA glucoside, arachidonic acid and its related metabolites, and allantoin may be effective in the improvement of transformation efficiency. The current research identified the key metabolites affecting plant regeneration of callus in barley.