Maize/soybean relay strip intercropping has been widely practiced in Southwest China due to its high productivity and effective application of agricultural resources; however, several seedborne diseases such as seedling blight, pod and seed decay are frequently observed causing severe yield loss and low seed quality. So far, the population and pathogenicity of the seedborne fungi associated with intercropped soybean remain unexplored. In this study, seeds of 12 soybean cultivars screened for intercropping were collected from three growing regions in Sichuan Province of Southwest China, and the seedborne fungi were isolated from the surface-sterilized seeds. Based on sequence analysis of ribosomal DNA internal transcribed spacer (rDNA ITS), 148 isolates were identified into 13 fungal genera, among which Fusarium covered 55.0% as the biggest population followed by Colletotrichum. Furthermore, Fusarium isolates were classified into five distinct species comprising F. fujikuroi, F. proliferatum, F. verticillioides, F. asiaticum and F. incarnatum through sequence analysis of translation elongation factor 1 alpha (EF-1α) and DNA-directed RNA ploymerase II second largest subunit (RPB2). Among them, F. fujikuroi accounted for 51.22% (42/82) and was isolated from 91.7% (11/12) soybean varieties. Pathogenicity assay showed that five Fusarium species were able to infect the seeds of soybean cultivar “Nandou12” and caused water-soaked or rot symptoms, while F. fujikuroi and F. asiaticum had much higher aggressiveness than other species with significant reductions of seed fresh weight and germination percentage. Accordingly, this study indicates that Fusarium species are the dominant seedborne fungi in the intercropped soybean in Sichuan, China, and this provides some useful references for the effective management of seedborne fungal diseases as well as soybean resistance breeding in maize/soybean relay strip intercropping.

The dynamic of soil-borne disease is closely related to the rhizosphere microbial communities. Maize–soybean relay strip intercropping has been shown to significantly control the type of soybean root rot that tends to occur in monoculture. However, it is still unknown whether the rhizosphere microbial community participates in the regulation of intercropped soybean root rot. In this study, rhizosphere Fusarium and Trichoderma communities were compared in either healthy or root-rotted rhizosphere soil from monocultured and intercropped soybean, and our results showed the abundance of rhizosphere Fusarium in intercropping was remarkably different from monoculture. Of four species identified, F. oxysporum was the most aggressive and more frequently isolated in diseased soil of monoculture. In contrast, Trichoderma was largely accumulated in healthy rhizosphere soil of intercropping rather than monoculture. T. harzianum dramatically increased in the rhizosphere of intercropping, while T. virens and T. afroharzianum also exhibited distinct isolation frequency. For the antagonism test in vitro, Trichoderma strains had antagonistic effects on F. oxysporum with the percentage of mycelial inhibition ranging from 50.59–92.94%, and they displayed good mycoparasitic abilities against F. oxysporum through coiling around and entering into the hyphae, expanding along the cell–cell lumen and even dissolving cell walls of the target fungus. These results indicate maize–soybean relay strip intercropping significantly increases the density and composition proportion of beneficial Trichoderma to antagonize the pathogenic Fusarium species in rhizosphere, thus potentially contributing to the suppression of soybean root rot under the intercropping.

Soybean (Glycine max L.) is an important food and oil crop widely planted by intercropping in southwest China. The shade caused by intercropping changes plant growth traits, such as soybean leaf and dry mass, thereby reducing yields. To improve the yield and elucidate the genetic mechanism of the leaf-related traits in intercropped soybeans, we measured the F_6:7–8 recombinant inbred lines (RILs) derived from the cross of ‘Nandou 12’ and ‘Jiuyuehuang’ for six leaf-related traits under monoculture and relay intercropping in 2015 and 2016. We found 6366 single-nucleotide polymorphisms (SNPs) markers that covered the whole genome of soybean distributed in 20 linkage groups, which spanned 2818.67 cM with an average interval of 0.44 cM between adjacent markers. Nineteen quantitative trait loci (QTLs) were detected in two environments in 2 years. Three candidate genes associated to leaf-related traits were found according to gene expression and GO enrichment analyses. These results revealed the susceptibility of leaf phenotype to shading and helped elucidate the mechanisms that control leaf-related traits.

Fusarium graminearum is the most devastating pathogen of Fusarium head blight of cereals, stalk and ear of maize, and it has recently become a potential threat for soybean as maize-soybean strip relay intercropping is widely practiced in China. To elucidate the pathogenesis mechanism of F. graminearum on intercropped soybean which causes root rot, transcriptional profiling of F. graminearum at 12, 24, and 48 h post-inoculation (hpi) on soybean hypocotyl tissues was conducted. In total, 2313 differentially expressed genes (DEGs) of F. graminearum were annotated by both KEGG pathway and Gene Ontology (GO) analysis. Among them, 128 DEGs were commonly expressed at three inoculation time points while the maximum DEGs were induced at 24 hpi. In addition, DEGs were also rich in carbon metabolism, ribosome and peroxisome pathways which might contribute to carbon source utilization, sexual reproduction, virulence and survival of F. graminearum when infected on soybean. Hence, this study will provide some basis for the deep understanding the pathogenesis mechanism of F. graminearum on different hosts and its effective control in maize-soybean strip relay intercropping systems.

Soybean (Glycine max L.) is one of the important oilseed and vegetable crop worldwide and provides the main source of vegetable oil and proteins for human and livestock (Hartman et al. 2011). In Oc...

The relative contents of glycinin and β-conglycinin from storage protein are closely related to the quality and function of soybean seeds. However, it is not clear whether or how glycinin and β-conglycinin contents change in maize-soybean strip intercropping system. The glycinin and β-conglycinin relative contents of the recombinant inbred lines (RILs) derived from the cross of ‘Nandou 12’ and ‘Jiuyuehuang’ were measured under different environments (E1: 2017, Renshou; E2: 2017, Ya’an; E3: 2016, Renshou) and planting patterns (M: monoculture; RI: relay intercropping), among which the differences were significant or extremely significant in parents and RILs. Based on a genetic linkage map with 6366 SNP markers, we detected ten QTLs for glycinin and β-conglycinin relative contents which were distributed in nine linkage groups with the phenotypic variation of 5.63%–9.68%. According to the soybean genomic information, 65 candidate genes were screened in the region of above-mentioned QTLs. These results lay a theoretical foundation for soybean quality breeding.