Se is important for human health, deficiency of Se in dietary causes a serious of diseases, and between 0.5 and 1 billion people have insufficient Se intake in the world [17]. Supplement of Se into the human dietary is one of most useful and common methods to solve Se deficient [3]. Se is also important for plants. It was reported that Se in low doses protects plants from abiotic and biotic stresses, while high concentration of Se in plants induces oxidative stresses on the contrary [16]. Bread wheat is one of the principal cereal grains produced and consumed globally, and the Se also affects bread wheat growth, development and biotic and abiotic resistance, so the Se response in bread wheat were detected by transcriptional analysis and proteomic analysis in this research. The molecular mechanism of Se response in bread wheat was uncovered.
Proteins related with Se accumulation were accelerated after Se treatment
In plants, the selenate is absorbed through sulfate transporters and the selenite is taken by phosphate transporters in an active process [1]. The absorbed selenate is converted to selenite by two enzymes ATP sulfurylase (APS) and APS reductase (APR). The APS catalyzes the hydrolysis of ATP to form adenosine phosphoselenate, which is then reduced to selenite by APR [28]. The selenite is then converted to selenide by glutathione or glutaredoxins (GRXs) in plants [29]. Selenide is converted to SeCys by enzyme cysteine synthase (CS). SeCys is then converted to elemental Se by Cys lyase, or is methylated to methyl-SeCys (Me-SeCys) by selenocysteine methyltransferase, or is converted to selenomethionine (SeMet) by a series of enzymes in different plant species and different environmental conditions.
In this research, the proteins related with Se accumulation were detected significantly changed in the bread wheat seedlings (Supplementary material 3–5). In details, 13 sulfate transporters were detected playing important roles in Se transportation. The expressions of sulfate transporter coding genes TraesCS7A02G088700, TraesCS4A02G029100, and TraesCS4B02G264100 were increased sharply in first 3 hours, and then decreased quickly. As a result, their expression changes were not significant any more after 12 h of Se treatment. The expression of TraesCS4D02G264200 was increased sharply in first 3 hours, and then decreased quickly. However, its expression changes were still significant after 12 h of Se treatment. These results indicated these 4 sulfate transporter coding genes probably function in the early hours of Se treatment. However, the expression of sulfate transporters coding genes TraesCS3A02G288800, TraesCS4A02G043400, TraesCS4B02G263900, TraesCS4D02G264100, TraesCS4A02G043500, TraesCS5D02G237800, TraesCS5A02G229700, TraesCS5B02G163700, and TraesCS2A02G508200 were increased slowly in first 3 hours, and their expression changes were finally detected significantly changed after 12 h of continuing increase, indicating these 9 sulfate transporter coding genes playing important roles in late hours of Se treatment.
The APS coding gene TraesCS2A02G032500 were detected changed significantly after 12 h of Se treatment in both protein and RNA level (Supplementary material 3–5), which meant the assimilation of selenite was significantly improved in 12 h after Se treatment and the APS was important in Se accumulation of bread wheat. This result was consistent with previous reports. Such as, the APS genes have been detected by Se treatment in Astragalus chrysochlorus by RNA-Seq [30]. Overexpression of Arabidopsis thaliana AtAPS in Brassica juncea resulted in significantly improved Se accumulation [20, 21]. It was reported that the expression of CS gene is related with Se accumulation in leaves of plants [25, 31, 32]. The CS enzyme coding gene novel.8735 was also significantly changed only in RNA level after 12 h of Se treatment in bread wheat in this research (Supplementary material 3–5). The SeCys lyase coding gene TraesCS5B02G407300 was significantly changed only in protein level after 12 h of Se treatment in this research (Supplementary material 3–5). These genes were also proved playing important roles in Se assimilation in this research.
ROS scavenging enzymes functioned in Se response
It was reported that Se in low doses protects the plants from variety of abiotic stresses by decreasing ROS concentration [11–13]. GSTs protect cells from oxidative damages by combining excess toxin with glutathione and forming, transferring to and separating S-glutathione conjugates in the vacuole [33]. SODs catalyze the dismutation of superoxide radicals to produce hydrogen peroxide (H2O2), which is decomposed into oxygen and water by CAT in plants.
In this research, the expression of 20 GSTs (TraesCS2A02G578400, TraesCS2B02G244100, TraesCS3A02G437400, TraesCS3B02G539100, TraesCS3D02G486100, etc.), 1 GSH-Px (TraesCS2D02G407700), 1 GRX (TraesCS6B02G361200), 3 SODs (TraesCS2D02G538300, TraesCS7D02G290700, and TraesCS7A02G292100), and 3 CATs (TraesCS6B02G462300, TraesCS7B02G140600, and TraesCS5A02G113100) were significantly changed in protein level after Se treatment (Supplementary material 3). There are more DEPs about ROS scavenging enzymes coding genes were detected in the transcription level in this research (Supplementary material 5). Transcriptional analysis of tea plant also showed that ROS scavenging enzyme coding genes of GSTs, glutathione synthetases, GSH-Px, glutathione reductases, GRXs, and CATs were detected significantly changed after selenite treatment [25]. Antioxidant genes coding GSTs and CATs were proved in the Se response pathway in Stanleya pinnata and Arabidopsis thaliana [32, 34].
Chaperons played roles in Se response
Chaperon proteins including HSPs improve protein stability by regulating protein folding, localization, accumulation and degradation under multiple abiotic stresses treatments, such as heat, cold, salt, oxidative, and heavy metals in plants [35, 36].
In this research, 15 chaperon proteins were detected differently expressed in the Se response pathway, including 2 HSP90s (TraesCS7D02G241100 and TraesCS2B02G047400), 2 HSP70s (TraesCS1A02G133100 and TraesCS1A02G285000), 9 HSP20s (TraesCS1B02G471900, TraesCS3A02G113000, TraesCS4A02G092100, TraesCS4B02G212300, TraesCS4D02G212500, TraesCS6D02G322300, TraesCS6A02G181700, TraesCS2A02G312900, and TraesCS3A02G034500), and 2 other chaperon proteins (TraesCS2B02G320000 and TraesCS5D02G497200) (Supplementary material 3). In transcriptional analysis, 15 DEGs were identified including TraesCS4A02G098600, TraesCS5A02G268100, TraesCS1B02G151300, TraesCS2B02G374700, TraesCS5D02G492900, TraesCS5B02G492500, TraesCS5A02G479300, TraesCS5B02G267900, TraesCS1D02G284000, TraesCS1B02G294300, TraesCS6A02G342400, TraesCS4B02G142400, TraesCS6B02G058300, TraesCS4B02G206300, and TraesCS5A02G078000 (Supplementary material 5). Unexpectedly, there were no overlap between the proteomic analysis results and transcriptional analysis results. As a result, it was concluded that these 30 chaperon proteins probably functioned in Se response pathway.
Secondary metabolism was enhanced due to Se treatment
Secondary metabolisms produce a serious of small compounds called secondary metabolites, which include basic nutrients such as proteins, fats or carbohydrates, and other compounds such as taxoids, polysaccharides, flavones, etc. These secondary metabolites are dispensable for plant metabolism and growth, and tolerance to both biotic and abiotic stresses [37]. The transcriptional analysis of diploid wheat relative Aegilops tauschii (DD) after Se treatment indicated that DEGs involved in flavone and flavonol biosynthesis, flavonoid biosynthesis, and selenocompound metabolism were believed to be potentially related to selenium metabolism [3].
In this research, 14 UFGTs (TraesCS2B02G040500, TraesCS2B02G081400, TraesCS2D02G069100, TraesCS3D02G120200, TraesCS5B02G436300, TraesCS5D02G440900, TraesCS5D02G476400, TraesCS7A02G492800, TraesCS7B02G074700, TraesCS4B02G226700, TraesCS6D02G162700, TraesCSU02G009000, TraesCS1B02G062100, and TraesCS2A02G273800) were differently expressed in protein level after Se treatment of bread wheat (Supplementary material 3). In transcriptional analysis, 124 and 192 UFGTs were differently expressed after Se treatment of bread wheat (Supplementary material 5). UFGTs are the last enzyme in anthocyanin synthesis process, which catalyze unstable anthocyanins into stable anthocyanins, so the activity of UFGTs is positively correlated with anthocyanins synthesis. Together, it was concluded that anthocyanin synthesis was enhanced due to Se treatment.
Carbohydrate metabolism was changed after Se treatment
In this research, 10 Suc-6-PHs (TraesCS7D02G008800, TraesCS7D02G009400, TraesCS2A02G588300, TraesCS2B02G594900, TraesCS2D02G489200, TraesCS4A02G485600, TraesCS7A02G009200, TraesCS7A02G009800, TraesCS7D02G010000, and TraesCS2A02G488900), 4 archaeal phosphoglucose isomerase (APGI) (TraesCS6D02G367000, TraesCS4D02G031900, TraesCS6D02G367700, and TraesCSU02G137500), 2 MSs including TraesCS2A02G345500 and TraesCS2D02G344200, and 1 Xyn (TraesCS5D02G448800) were differently expressed in protein level (Supplementary material 3).
It was reported that Suc-6-PH hydrolyzes the terminal non-reducing beta-D-fructofuranoside residues in beta-D-fructofuranosides, which involves in sucrose metabolism and glycan biosynthesis. Phosphoglucose isomerases catalyze glucose 6- phosphate to form fructose 6-phosphate. The MSs combine glyoxylic acid with acetyl CoA to form malic acid in photosynthesis. The Xyn hydrolyzes 1,4-beta-D-xylosidic linkages in xylans, and is involved in the xylan degradation pathway and glycan degradation. In this research, 1 Xyn (TraesCS5D02G448800) decreased after Se treatment of bread wheat, indicating the Se treatment prevented the xylan degradation in bread wheat. Together, Se treatment affected the Carbohydrate metabolism in the bread wheat.