Impact of F. pseudograminearum stress on wheat growth and development
The results showed that F. pseudograminearum stress significantly affected the growth and development of wheat seedlings, especially the root system (Figure 1a–1h). At 3 days post-inoculation (dpi), the light brown symptoms of disease initially appeared at the stem bases of susceptible cultivar UC1110, which indicated that the incubation period was over. (Figure 1a, 1b). According to our observations, the average root diameters of UC1110 and PI610750 seedlings were both significantly decreased by 12.5% and 18.5% at 3 dpi compared with 0 dpi, respectively (Figure 1f), whereas total root length, total root surface area, total root volume, and forks were significantly increased (Figure 1c–1e, 1g, 1h). The average number of root tips of UC1110 and PI610750 seedlings were both significantly increased by 70.2% and 123.5% at 3 dpi compared with 0 dpi, respectively.
The results of these physiological experiments showed that compared with those of 0 dpi seedlings, the root vigor, leaf total chlorophyll content, root soluble protein content, and root superoxide dismutase (SOD) activity of 3 dpi UC1110 wheat seedlings were significantly increased by 43.9%, 27.0%, 74.1%, and 91.9%, respectively, as well as in PI610750, where these values were increased by 102.4%, 33.3%, 123.2%, and 42.4%, respectively (Figure 2a, 2c–2e). The extent of these increases was higher in PI610750 than in UC1110. The activity of root peroxidase (POD) in UC1110 and PI610750 wheat seedlings, however, was significantly decreased at 3 dpi (Figure 2f). In addition, the root soluble sugars and malondialdehyde (MDA) content of PI610750 wheat seedlings were significantly decreased by 17.3% and 18.0% at 3 dpi, respectively (Figure 2b, 2h), whereas the root catalase (CAT) activity of UC1110 wheat seedlings was significantly increased by 79.8% (Figure 2g). Together, these results indicated that there were certain differences in the morphological, physiological, and biochemical responses of disease-tolerant cultivar PI610750 and disease-susceptible cultivar UC1110 under F. pseudograminearum stress.
Identification of DEPs in response to F. pseudograminearum infection
We comprehensively examined and identified defense-related proteins of stem bases of two wheat cultivars, UC1110 and PI610750, under F. pseudograminearum stress using TMT quantitative proteomics technology. We selected the stem bases of wheat for proteomic analysis in this experiment because they represent the first obstacle to the invasion of the crown rot pathogen F. pseudograminearum.
To investigate the mechanisms of the differences in resistance of UC1110 and PI610750 at the protein level after F. pseudograminearum inoculation, we compared two groups, that is, S1-3dpi/S1-0dpi and S2-3dpi/S2-0dpi, using TMT quantitative proteomics study. Compared with the S1-0dpi treatment, we identified 163 differentially expressed proteins (DEPs) in the S1-3dpi treatment, including 75 up-regulated and 88 down-regulated protein species, of which 100 protein species were specifically expressed in this group (Figure 3 and Figure 4). In S2-3dpi/S2-0dpi, 203 protein species were differentially expressed, containing 133 up-regulated and 70 down-regulated proteins, of which 140 proteins were specifically expressed in this group. A total of 63 proteins were common in S1-3dpi/S1-0dpi and S2-3dpi/S2-0dpi, including 23 up-regulated and 40 down-regulated proteins in S1-3dpi/S1-0dpi and 22 up-regulated and 41 down-regulated proteins in S2-3dpi/S2-0dpi (Figure 4).
Cluster analysis of DEPs based on GO enrichment
To determine their potential functions, we annotated 366 DEPs by Gene Ontology (GO). GO functional enrichment analysis showed that 186, 126, and 161 protein species were enriched in biological processes, cellular components, and molecular function, respectively (Supplementary Table S1).
Enrichment of DEPs related to biological processes
Under biological processes, the common DEPs (C1) in the two comparison groups of S1-3dpi/S1-0dpi and S2-3dpi/S2-0dpi were significantly enriched in the terms of organic acid catabolism, cell wall polysaccharide metabolism, and cell wall macromolecule metabolism (Figure 5a). The specific DEPs (C2) of S1-3dpi/S1-0dpi were significantly enriched in the terms of negative regulation of hydrolase activity, dephosphorylation, regulation of proteolysis, negative regulation of protein metabolism, negative regulation of cellular metabolism, organic acid biosynthesis, carboxylic acid metabolism, oxoacid metabolism, and negative regulation of macromolecule metabolism (Figure 5a). The specific DEPs (C3) of S2-3dpi/S2-0dpi were significantly enriched in the terms of cellular protein metabolism, photosynthesis (dark reaction), carbohydrate biosynthesis, cellular macromolecule biosynthesis, the photosynthetic electron transport chain, polysaccharide biosynthesis, lipid transport, cellular carbohydrate biosynthesis, hexose metabolism, cellular polysaccharide metabolism, photosynthesis (light harvesting), amide biosynthesis, peptide biosynthesis, and peptide metabolism (Figure 5a). This analysis showed that the DEPs related to organic acid catabolism and cell wall metabolism responded to F. pseudograminearum stress in the seedling stem bases of both susceptible and tolerant cultivars. The disease-susceptible cultivar UC1110 also responded to stress through the DEPs related to dephosphorylation and carboxylic acid metabolism, and the disease-tolerant cultivar PI610750 mainly through the DEPs related to photosynthesis and sugar metabolism.
Enrichment of DEPs related to cellular components
In the cellular components category, the common DEPs (C1) in the two comparison groups of S1-3dpi/S1-0dpi and S2-3dpi/S2-0dpi were significantly enriched in the terms of cell walls, external encapsulating structures, the cell periphery, and extracellular regions (Figure 5b). The specific DEPs (C2) of S1-3dpi/S1-0dpi were significantly enriched in apoplasts (Figure 5b). The specific DEPs (C3) of S2-3dpi/S2-0dpi were significantly enriched in the terms of plastid thylakoids, chloroplast thylakoids, the photosystem, photosynthetic membranes, cytoplasmic parts, thylakoid membranes, ribosomes, the ribonucleoprotein complex, cytoplasm, the membrane protein complex, and organelles (Figure 5b). This analysis showed that the DEPs related to the cell wall first responded to F. pseudograminearum stress in the seedling stem bases of both susceptible and tolerant cultivars. The disease-susceptible cultivar UC1110 also responded to stress through the DEPs related to apoplasts, and the disease-tolerant cultivar PI610750 mainly through the DEPs related to chloroplasts.
Enrichment of DEPs related to molecular function
In terms of molecular function, the common DEPs (C1) in the two comparison groups of S1-3dpi/S1-0dpi and S2-3dpi/S2-0dpi were significantly enriched in the terms of glucosidase activity, hydrolase activity, phosphatidylcholine1-acylhydrolase activity, cysteine-type peptidase activity, phospholipase activity, lipase activity, and carboxylic ester hydrolase activity (Figure 5c). The specific DEPs (C2) of S1-3dpi/S1-0dpi were significantly enriched in the terms of peptidase regulator activity, endopeptidase regulator activity, phosphatase activity, oxidoreductase activity, nitronate monooxygenase activity, flavin mononucleotide binding, hydrolase activity, enzyme inhibitor activity, transferase activity, dioxygenase activity, and acid phosphatase activity (Figure 5c). The specific DEPs (C3) of S2-3dpi/S2-0dpi were significantly enriched in the terms of electron transporter activity, electron carrier activity, ribulose-bisphosphate carboxylase activity, cellulose synthase activity, glucosyltransferase activity, metal cluster binding, protein heterodimerization activity, oxidoreductase activity, lipid binding, tetrapyrrole binding, structural constituents of ribosomes, structural molecule activity, and chlorophyll binding (Figure 5c). This analysis showed that the disease-tolerant cultivar PI610750 mainly responded to stress through the DEPs related to electron transporter activity, electron carrier activity, cellulose synthase activity, and oxidoreductase activity in the process of plant-pathogen interaction.
Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis of DEPs
Further analysis using Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment showed that all DEPs of the two comparison groups were significantly enriched in in the terms of ribosomes (20%), phenylpropanoid biosynthesis (14%), photosynthesis (11%), glutathione metabolism (11%), carbon fixation in photosynthetic organisms (7%), alpha-linolenic acid metabolism (7%), glyoxylate and dicarboxylate metabolism (7%), linoleic acid metabolism (6%), cyanoamino acid metabolism (6%), photosynthesis-antenna proteins (6%), and flavone and flavonol biosynthesis (5%) (Supplementary Figure S1). The 63 common DEPs (C1) in the two comparison groups of S1-3dpi/S1-0dpi and S2-3dpi/S2-0dpi were significantly enriched in the terms of biosynthesis of secondary metabolites (osa01110), phenylpropanoid biosynthesis (osa00940), protein processing in endoplasmic reticulum (osa04141), starch and sucrose metabolism (osa00500), and cyanoamino acid metabolism (osa00460) (Figure 6, Supplementary Table S2).
The specific DEPs (C2) of S1-3dpi/S1-0dpi were significantly enriched in the terms of linoleic acid metabolism (osa00591), glutathione metabolism (osa00480), alpha-linolenic acid metabolism (osa00592), MAPK signaling pathway-plant (osa04016), flavone and flavonol biosynthesis (osa00944), and ubiquinone and other terpenoid-quinone biosynthesis (osa00130) (Figure 6, Supplementary Table S3). The specific DEPs (C3) of S2-3dpi/S2-0dpi were significantly enriched in the terms of ribosomes (osa03010), photosynthesis (osa00195), photosynthesis-antenna proteins (osa00196), flavone and flavonol biosynthesis (osa00944), carbon fixation in photosynthetic organisms (osa00710), and glyoxylate and dicarboxylate metabolism (osa00630) (Figure 6, Supplementary Table S4). This analysis showed that the DEPs related to secondary metabolites, protein processing, and energy metabolism pathways responded to stress in the seedling stem bases of both susceptible and tolerant cultivars. The susceptible cultivar UC1110 responded to stress mainly through the DEPs related to linoleic acid metabolism and glutathione metabolism, and the disease-tolerant cultivar PI610750 mainly through the DEPs related to photosynthesis and glyoxylic acid and dicarboxylate metabolism.
Interaction network analysis of F. pseudograminearum-responsive proteins in wheat
The present study used the online STRING database and Cytoscape software to construct a protein-protein interaction network for all DEPs of the two comparison groups in response to F. pseudograminearum. This network showed that 76 of the possible DEPs interacted. With the MCODE plug-in toolkit, three enriched interaction clusters were associated with ribosomes, photosynthesis, and sugar metabolism (Figure 7). Sixteen interaction proteins belonged to the ribosome network. These proteins included 15 up-regulated proteins and one down-regulated protein in the comparison group S2-3dpi/S2-0dpi. Seven interaction proteins belonged to the carbohydrate metabolic process network, including four down-regulated proteins in the two comparison groups of S1-3dpi/S1-0dpi and S2-3dpi/S2-0dpi, and one down-regulated and two up-regulated proteins in the comparison group of S2-3dpi/S2-0dpi. Three interaction proteins belonged to the photosynthesis network, including three up-regulated proteins in S2-3dpi/S2-0dpi. Other information about proteins is shown in Table S5.
Correlation between mRNA and protein abundance
To further validate the reliability of the proteomics data, we selected 16 genes for quantitative real time–polymerase chain reaction (qRT-PCR) analysis. Three common genes in the two comparison groups showed similar tendencies as those for protein expression, including NMT1, GLU1B, and XIPI. In the comparison group of S1_3dpi/sS1_0dpi, NMT1 and GLU1B were up-regulated and down-regulated, respectively. In the comparison group of S2_3dpi/S2_0dpi, however, both XIPI and GLU1B were down-regulated (Table 1). In the comparison group of S1_3dpi/S1_0dpi, five specific DEPs were up-regulated at both the transcription and translation levels. Similarly, in the comparison group of S2_3dpi/S2_0dpi, eight specific DEPs were up-regulated at both the transcription level and translation level. The primer sequences for 16 genes are listed in Supplementary Table S6.