Abstract:
Priming is a natural phenomenon that pre-conditions plants for enhanced defence against
a wide range of pathogens. It represents a complementary strategy, or sustainable alternative that
can provide protection against disease. However, a comprehensive functional and mechanistic
understanding of the various layers of priming events is still limited. A non-targeted metabolomics
approach was used to investigate metabolic changes in plant growth-promoting rhizobacteria
(PGPR)-primed Sorghum bicolor seedlings infected with the anthracnose-causing fungal pathogen,
Colletotrichum sublineolum, with a focus on the post-challenge primed state phase. At the 4-leaf growth
stage, the plants were treated with a strain of Paenibacillus alvei at 108 cfu mL1. Following a 24 h
PGPR application, the plants were inoculated with a C. sublineolum spore suspension (106 spores
mL1), and the infection monitored over time: 1, 3, 5, 7 and 9 days post-inoculation. Non-infected
plants served as negative controls. Intracellular metabolites from both inoculated and non-inoculated
plants were extracted with 80% methanol-water. The extracts were chromatographically and
spectrometrically analysed on an ultra-high performance liquid chromatography (UHPLC) system
coupled to high-definition mass spectrometry. The acquired multidimensional data were processed
to create data matrices for chemometric modelling. The computed models indicated time-related
metabolic perturbations that reflect primed responses to the fungal infection. Evaluation of orthogonal
projection to latent structure-discriminant analysis (OPLS-DA) loading shared and unique structures
(SUS)-plots uncovered the di erential stronger defence responses against the fungal infection
observed in primed plants. These involved enhanced levels of amino acids (tyrosine, tryptophan),
phytohormones (jasmonic acid and salicylic acid conjugates, and zeatin), and defence-related components of the lipidome. Furthermore, other defence responses in both naïve and primed plants
were characterised by a complex mobilisation of phenolic compounds and de novo biosynthesis of
the flavones, apigenin and luteolin and the 3-deoxyanthocyanidin phytoalexins, apigeninidin and
luteolinidin, as well as some related conjugates.
Description:
Supplementary Material: Figure S1: Evaluation of disease symptoms in Colletotrichum sublineolum infected sorghum plants; Figure S2:
Representative BPI MS chromatograms of ESI(+) data (3 d.p.i.); Figure S3: Unsupervised chemometric modelling
of ESI(-) data; Figure S4: OPLS-DA modelling and variable/feature selection. Table S1: Annotated (MSI-level 2)
metabolites reported in Table 1, with fragmentation information.
