Leaf metabolites play a key role in host defensive responses against viral (Manasseh et al.) and fungal diseases (Chang et al.; Mashabela et al.). Manasseh et al. used untargeted metabolomics to investigate the responses of potato virus Y (PVY)-resistant cv. Premier Russet and susceptible cv. Russet Burbank to the inoculation with different PVY strains including PVY^NTN, PVY^N-Wi, and PVY°. Generally, more than 45 common and/or strain-specific metabolites were detected in fully expanded leaves after the extraction by a solvent containing methanol, 2-propanol, and water (5:2:2; v/v/v) and derivatization with O-methoxylamine hydrochloride dissolved in pyridine and N-methyl-N-(trimethylsilyl)-trifluoroacetamide (MSTFA) containing 1% trimethylchlorosilane (TMCS). Briefly, the GC-MS-based analysis revealed PVY infection in potatoes leads to marked changes in amino acid, energy, and fatty acid metabolism, with a focus on 20 amino acids, 11 organic acids, and 14 sugar derivatives. It is worth noting that the accumulation of L-allothreonine, mannose, and fructose was stimulated by all three strains of PVY.

In the PVY-resistant cv. Premier Russet only two differentially accumulated metabolites (DAMs) were detected under PVY^NTN inoculation compared to six and 13 DAMs upon the infection with PVY° and PVY^N-Wi, respectively (Manasseh et al.). On the other hand, in the PVY-susceptible cv. Russet Burbank, PVY^NTN, and PVY° strains pronounced higher metabolic changes (13 and 12 DAMs, respectively), compared to the PVY^N-Wi strain which altered only four DAMs. Moreover, although 14 metabolic pathways occurred solely in PVY^N-Wi-infected potato plants, a major overlap was noticed between PVY^N-Wi and PVY° in Premier Russet, and eight DAMs were detected specifically in PVY^N-Wi-infected leaves, whereas DAMs of PVY^NTN and PVY° overlapped in Russet Burbank. Taken together, the authors suggest that five metabolites included; maltose, D-glyceric acid, L-malic acid, lysine, and oxoproline, were unique to Premier Russet, and another five metabolites included; L-allothreonine, 5-methoxytryptamine, spermidine, fructose, and aconitic acid, were cultivar-specific for Russet Burbank which could be used as potential biomarkers/indicators of PVY infection and susceptibility/resistance of potato genotypes (Manasseh et al.).

In another study, Chang et al. investigated the physiological and metabolic responses of soybean seed of the moderately susceptible cultivar “Nandou 12” after soaking in spore suspension of the seed-borne phytopathogen Fusarium fujikuroi - isolate No. S88, the causal agent of seed decay, at 0, 3, and 5 days post-inoculation (dpi). Although both mycelial coverage percentage and disease severity index increased rapidly upon the inoculation with F. fujikuroi, the germination rate and the levels of soluble sugars and soluble protein of treated seeds were markedly altered over time. Moreover, the enzymatic activities of cell wall–degrading enzymes such as chitinase and β-1,3-glucanase of soybean seeds were promptly stimulated after the infection with F. fujikuroi. Collectively, these findings suggest that the seeds of the moderately susceptible cultivar “Nandou 12” might accumulate more soluble protein and activate the cell wall–degrading enzymes as early defense strategies against F. fujikuroi, but reduce the soluble sugar content to improve the respiration of infected seeds.

To better understand the potential role(s) of the key defense-related metabolites and their corresponding metabolic pathways during the soybean-F. fujikuroi interactions, the authors examined the metabolic responses of soybean seed after the extraction with 80% methanol and 0.1% formic acid and analyzed using Ultra Performance Liquid Chromatography Tandem Mass Spectrometry (UPLC-MS/MS). Metabolic profiling revealed that soybean seed metabolites were altered by F. fujikuroi including secondary metabolites, amino acids, carbohydrates, cofactors & vitamins, and lipid metabolism. Briefly, the metabolism of glycine, serine, threonine, and tryptophan was noticeably provoked by F. fujikuroi. Moreover, infection with F. fujikuroi significantly activated the isoflavonoid metabolites in soybean seeds. For instance, the upstream metabolites such as naringenin and liquiritigenin in the isoflavone biosynthesis pathway were rapidly accumulated at 3 dpi but diminished at 5 dpi. On the other hand, downstream metabolites such as daidzin, daidzein, genistin, and genistein decreased, while liquiritigenin, glycitein, and glycitin notably increased at 3 dpi and then rapidly decreased. Taken together, the authors suggest that the accumulation of isoflavonoid metabolites in soybean seeds might play a key role in response to F. fujikuroi infection by acting as a signal of early defense response.

In a previous study, Mashabela et al. used untargeted metabolomics to explore the metabolic profile of the rhizosphere and the above-ground tissues of susceptible and resistant wheat after the application of two PGPR; namely Paenibacillus alvei - T22 and Bacillus subtilis under neither biotic nor abiotic stress (Mashabela et al., 2022). Briefly, several metabolic pathways/groups were differentially accumulated in the rhizosphere and leaves of PGPR-treated plants including phenylpropanoids, organic acids, lipids, organoheterocyclic compounds, and benzenoids. These metabolites are implicated in chemotaxis, biocontrol, plant growth promotion, and host–pathogen interactions (Mashabela et al., 2022). In the current Research Topic, they added another crucial piece to the puzzle to evaluate the capabilities of PGPR in stimulating plant growth and inducing systemic resistance to protect wheat plants against the obligate biotrophic phytopathogen, Puccinia striiformis f. sp. tritici (Pst), the causal agent of stripe rust (Mashabela et al.). In addition, Mashabela et al. tried to identify signatory biomarkers of the Pst-susceptible Gariep cultivar against Pst after bio-priming with two PGPR strains namely B. subtilis and P. alvei - T22 under greenhouse conditions using untargeted Ultra-High Performance Liquid Chromatography-High-Definition Mass Spectrometry (UHPLC-HDMS).

Mashabela et al. reported that seed bio-priming using B. subtilis and P. alvei showed potential differential metabolic responses between treated and non-treated plants. These metabolic responses included but were not limited to, time-dependent reprogramming of primary metabolites (amino acids and organic acids), as well as secondary metabolites such as phenolic compounds (flavonoids, hydroxycinnamic acid, and its amides). It is worth mentioning that these metabolic responses were further reflected by a reduction in the development of disease symptoms as suggested by visual inspection of the plant phenotypes. PGPR-treated plants develop chlorosis and necrosis at the site of infection with Pst, and a reduced infection rate is recognized by developing smaller and fewer spores compared to the non-primed Gariep plants. These findings correspond to the observations from the Pst-resistant wheat cultivar Koonap in their previous study(Mashabela et al., 2022) which suggest that bio-priming using B. subtilis and P. alvei alters the metabolic profile linked to induced resistance against Pst in the treated plants. The current study of Mashabela et al. is an important contribution to the field of metabolomics-based semi-quantitative and qualitative analyses to clarify wheat-Pst interactions and to identify primary and secondary metabolic biomarkers associated with these interactions.