Glutathione S-transferases (GSTs) are a critical superfamily of multifunctional enzymes in plants. As a ligand or binding protein, GSTs regulate plant growth and development and detoxification. Foxtail millet (Setaria italica (L.) P. Beauv) could respond to abiotic stresses through a highly complex multi-gene regulatory network in which the GST family is also involved. However, GST genes have been scarcely studied in foxtail millet. Genome-wide identification and expression characteristics analysis of the foxtail millet GST gene family were conducted by biological information technology. The results showed that 73 GST genes (SiGSTs) were identified in the foxtail millet genome and were divided into seven classes. The chromosome localization results showed uneven distribution of GSTs on the seven chromosomes. There were 30 tandem duplication gene pairs belonging to 11 clusters. Only one pair of SiGSTU1 and SiGSTU23 were identified as fragment duplication genes. A total of ten conserved motifs were identified in the GST family of foxtail millet. The gene structure of SiGSTs is relatively conservative, but the number and length of exons of each gene are still different. The cis-acting elements in the promoter region of 73 SiGST genes showed that 94.5% of SiGST genes possessed defense and stress-responsive elements. The expression profiles of 37 SiGST genes covering 21 tissues suggested that most SiGST genes were expressed in multiple organs and were highly expressed in roots and leaves. By qPCR analysis, we found that 21 SiGST genes were responsive to abiotic stresses and abscisic acid (ABA). Taken together, this study provides a theoretical basis for identifying foxtail millet GST family information and improving their responses to different stresses.

Drought is a major limiting factor affecting grain production. Drought-tolerant crop varieties are required to ensure future grain production. Here, 5597 DEGs were identified using transcriptome data before and after drought stress in foxtail millet (Setaria italica) hybrid Zhangza 19 and its parents. A total of 607 drought-tolerant genes were screened through WGCNA, and 286 heterotic genes were screened according to the expression level. Among them, 18 genes overlapped. One gene, Seita.9G321800, encoded MYBS3 transcription factor and showed upregulated expression after drought stress. It is highly homologous with MYBS3 in maize, rice, and sorghum and was named SiMYBS3. Subcellular localization analysis showed that the SiMYBS3 protein was located in the nucleus and cytoplasm, and transactivation assay showed SiMYBS3 had transcriptional activation activity in yeast cells. Overexpression of SiMYBS3 in Arabidopsis thaliana conferred drought tolerance, insensitivity to ABA, and earlier flowering. Our results demonstrate that SiMYBS3 is a drought-related heterotic gene and it can be used for enhancing drought resistance in agricultural crop breeding.

To explore the role of Brassinolide (BR) in improving the tolerance of Sigma Broad in foxtail millet (Setaria italica L.), effects of 0.1 mg/L of BR foliar application 24 h before 3.37 g/ha of Sigma Broad treatment at five-leaf stage of foxtail millet on growth parameters, antioxidant enzymes, malondialdehyde (MDA), chlorophyll, net photosynthetic rate (P_N), chlorophyll fluorescence and P₇₀₀ parameters were studied 7 and 15 d after herbicide treatment, respectively. Results showed that Sigma Broad significantly decreased plant height, activities of superoxide dismutase (SOD), chlorophyll content, P_N, PS II effective quantum yield (Y (II)), PS II electron transport rate (ETR (II)), photochemical quantum yield of PSI(Y (I)) and PS I electron transport rate ETR (I), but significantly increased MDA. Compared to herbicide treatment, BR dramatically increased plant height, activities of SOD, Y (II), ETR (II), Y (I) and ETR (I). This study showed BR pretreatment could improve the tolerance of Sigma Broad in foxtail millet through improving the activity of antioxidant enzymes, keeping electron transport smooth, and enhancing actual photochemical efficiency of PS II and PSI.

Radix Isatidis (Isatis indigotica Fort.) is one of the most important traditional Chinese medicine plants. However, there is no suitable herbicide used for weed control in Radix Isatidis field during postemergence stage. To explore the safety of sulfonylurea herbicide nicosulfuron on Radix Isatidis (Isatis indigotica Fort.) seedlings and the photosynthetic physiological response of the plant to the herbicide, biological mass, leaf area, photosynthetic pigment content, photosynthetic rate, chlorophyll fluorescence characteristics, and P₇₀₀ parameters of Radix Isatidis seedlings were analyzed 10 d after nicosulfuron treatment at 5th leaf stage in this greenhouse research. The results showed that biological mass, total chlorophyll, chlorophyll a, and carotenoids content, photosynthetic rate, stomatal conductance, PS II maximum quantum yield, PS II effective quantum yield, PS II electron transport rate, photochemical quenching, maximal P₇₀₀ change, photochemical quantum yield of PS I, and PS I electron transport rate decreased with increasing herbicide concentrations, whereas initial fluorescence, quantum yield of non-regulated energy dissipation in PS II and quantum yield of non-photochemical energy dissipation due to acceptor side limitation in PS I increased. It suggests that nicosulfuron ≥1 mg L−1 causes the damage of chloroplast, PS II and PS I structure. Electron transport limitations in PS I receptor side, and blocked dark reaction process may be the main cause of the significantly inhibited growth and decreased photosynthetic rate of Radix Isatidis seedlings.

Photosynthetic carbohydrate metabolism is an important biochemical process related to crop production and quality traits. Potassium (K) critically contributes to the process of photosynthetic carbon assimilation and carbohydrate metabolism. We explored the effects of potassium fertilization on physiological mechanisms including carbohydrate metabolism in foxtail millet and its yield. Field experiments were performed using two foxtail millet (Setaria italica L.) cultivars: 1) Jingu 21 (low-K sensitive); and 2) Zhangza 10 (low-K tolerant). Effect of five different potassium fertilizer (K₂O) rates (0, 60, 120, 180, and 240 kg·hm⁻²) were tested in two consecutive years, 2020 and 2021. We found that potassium application significantly increases the K content, dry matter accumulation and yield. Jingu 21 and Zhangza 10 had maximum yields at 180 kg·hm⁻² K application, which were 29.91% and 31.51% larger than without K application, respectively. Excessive K application (K₂₄₀) did not further improve their yields. The suitable K fertilizer application of Jingu 21 and Zhangza 10 are 195.25–204.27 and 173.95–175.87 kg·K2O·hm⁻², respectively. The net photosynthetic rate (P_n), ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), phosphoenolpyruvate carboxylase (PEPC), and fructose-1,6-bisphosphatase (FBPase) were positively correlated with the potassium content. Potassium application improved the availability of carbon sources for carbohydrate synthesis. Compared with the K₀ treatment, variations in the activities of sucrose phosphate synthase (SPS) and sucrose synthase (SuSy) in potassium-treated Jingu 21 (K₆₀, K₁₂₀, K₁₈₀, and K₂₄₀) were 17.94%–89.93% and 22.48%–182.10%, respectively, which were greater than those of Zhangza 10 (11.34%–71.12% and 16.18%–109.13%, respectively) and indicate that Jingu 21 is more sensitive to potassium application. The sucrose contents in the Jingu 21 and Zhangza 10 grains were 0.97%–1.15% and 1.04%–1.23%, respectively. The starch contents were 28.99%–37.75% and 24.81%–34.62%, respectively. The sucrose: ratio of Jingu 21 was smaller than that of Zhangza 10, indicating that Jingu 21 utilized nutrients better than Zhangza 10. Stepwise regression and path analysis showed that leaf and grain SuSy activity, by coordinating the source-sink relationship, have the greatest direct effect on Jingu 21 yield, whereas leaf SuSy activity, by promoting the generation of photosynthates at the source leaf, plays a leading role in Zhangza 10 yield increase. In conclusion, optimized K application can increase foxtail millet grain yield by improving photosynthesis and promoting carbohydrate accumulation and distribution.