Detection of ustilaginoidins in rice samples by immunoassay based on monoclonal antibodies prepared from hemiustilaginoidin-derived haptens.

Ustilaginoidins are a class of bis-naphtho-γ-pyrone mycotoxins to threaten humans, animals and environment. Ustilaginoidins are produced by Villosiclava virens, the rice false smut pathogen. To prepare antibodies for quantitatively analyzing ustilaginoidins in rice samples, hemiustilaginoidins D and F from the laccase gene deficiency mutant of V. virens respectively reacted with diazonium 4-aminobenzoic acid to obtain haptens with a carboxyl group, which further reacted with bovine serum albumin or ovalbumin to get their complete antigens. Two monoclonal antibodies (mAbs) designated as 4A12C6 and 5F4F6 were developed by immunization. The relationships between mAb sensitivity and 20 ustilaginoidins were described. 4A12C6 was chosen for further analysis as it could recognize main ustilaginoidins and was more sensitive than 5F4F6. The achieved indirect competitive enzyme-linked immunosorbent assay (icELISA) based on 4A12C6 had a half maximal inhibitory concentration (IC50) of 0.76 ng/mL and working range of 0.2-2.8 ng/mL to ustilaginoidin A. The results of ustilaginoidins-contaminated rice samples by icELISA detection were consistent with those determined by HPLC‒DAD detection. Therefore, we developed a new strategy to get haptens from the biosynthetic precursors with half structures of ustilaginoidins. The achieved icELISA was demonstrated as a convenient method to monitor ustilaginoidin content in rice samples, and showed that the contents of total ustilaginoidins from the rice cultivars with low resistance to rice false smut were more than those of high resistance cultivars.

UvSorA and UvSorB Involved in Sorbicillinoid Biosynthesis Contribute to Fungal Development, Stress Response and Phytotoxicity in Ustilaginoidea virens.

Ustilaginoidea virens (teleomorph: Villosiclava virens) is an important fungal pathogen that causes a devastating rice disease. It can produce mycotoxins including sorbicillinoids. The biosynthesis and biological functions of sorbicillinoids have not been reported in U. virens. In this study, we identified a sorbicillinoid biosynthetic gene cluster in which two polyketide synthase genes UvSorA and UvSorB were responsible for sorbicillinoid biosynthesis in U. virens. In ∆UvSorA and ∆UvSorB mutants, the mycelial growth, sporulation and hyphal hydrophobicity were increased dramatically, while the resistances to osmotic pressure, metal cations, and fungicides were reduced. Both phytotoxic activity of rice germinated seeds and cell wall integrity were also reduced. Furthermore, mycelia and cell walls of ∆UvSorA and ∆UvSorB mutants showed alterations of microscopic and submicroscopic structures. In addition, feeding experiment showed that sorbicillinoids could restore mycelial growth, sporulation, and cell wall integrity in ∆UvSorA and ∆UvSorB mutants. The results demonstrated that both UvSorA and UvSorB were responsible for sorbicillinoid biosynthesis in U. virens, and contributed to development (mycelial growth, sporulation, and cell wall integrity), stress responses, and phytotoxicity through sorbicillinoid mediation. It provides an insight into further investigation of biological functions and biosynthesis of sorbicillinoids.

A Nanobody-Based Immunoassay for Detection of Ustilaginoidins in Rice Samples.

Ustilaginoidins are a class of bis-naphtho-γ-pyrone mycotoxins produced by the pathogen Villosiclava virens of rice false smut, which has recently become one of the most devastating diseases in rice-growing regions worldwide. In this research, the nanobody phage display library was established after an alpaca was immunized with the hemiustilaginoidin F-hapten coupled with bovine serum albumin (BSA). Heterologous antigen selection and combing trypsin with competition alternant elution methods were performed for nanobody screening. Two nanobodies, namely, Nb-B15 and Nb-C21, were selected for the establishment of indirect competitive enzyme-linked immunosorbent assays (ic-ELISAs). For Nb-B15 and Nb-C21, their IC50 values were 11.86 µg/mL and 11.22 µg/mL, and the detection ranges were at 3.41-19.98 µg/mL and 1.17-32.13 µg/mL, respectively. Two nanobodies had a broad spectrum to quantify the contents of total ustilaginoidins in rice samples according to cross-reactivity. The recognition mechanisms of Nb-B15 and Nb-C21 against ustilaginoidin A were elucidated by molecular modeling and docking. The key amino acid sites for the binding of Nb-B15 or Nb-C21 to ustilaginoidin A were mainly located in the FR1 and CDR1 regions. As Nb-B15 was superior to Nb-C21 in the aspects of protein expression, ELISA titer, and tolerance to organic solvents, it was selected for application in the detection of actual contaminated rice samples. The total ustilaginoidin contents of rice samples were analyzed by Nb-B15-based ic-ELISA and HPLC-DAD, between which the results were found to be consistent. The developed immunoassay based on the nanobody from the alpaca can be employed as a rapid and effective method for detection of total utilaginoidins in contaminated rice samples.