Role of defense enzymes, proteins, solutes and Δ1-pyrroline-5-carboxylate synthase in wheat tolerance to drought

Introgression of Δ1-pyrroline-5-carboxylate synthetase (PgP5CS) confers enhanced resistance to abiotic stresses in transgenic tobacco

Δ1-pyrroline-5-carboxylate synthetase (P5CS) is one of the key regulatory enzymes involved in the proline biosynthetic pathway. Proline acts as an osmoprotectant, molecular chaperone, antioxidant, and regulator of redox homeostasis. The accumulation of proline during stress is believed to confer tolerance in plants. In this study, we cloned the complete CDS of the P5CS from pearl millet (Pennisetum glaucum (L.) R.Br. and transformed into tobacco. Three transgenic tobacco plants with single-copy insertion were analyzed for drought and heat stress tolerance. No difference was observed between transgenic and wild-type (WT) plants when both were grown in normal conditions. However, under heat and drought, transgenic plants have been found to have higher chlorophyll, relative water, and proline content, and lower malondialdehyde (MDA) levels than WT plants. The photosynthetic parameters (stomatal conductance, intracellular CO2 concentration, and transpiration rate) were also observed to be high in transgenic plants under abiotic stress conditions. qRT-PCR analysis revealed that the expression of the transgene in drought and heat conditions was 2-10 and 2-7.5 fold higher than in normal conditions, respectively. Surprisingly, only P5CS was increased under heat stress conditions, indicating the possibility of feedback inhibition. Our results demonstrate the positive role of PgP5CS in enhancing abiotic stress tolerance in tobacco, suggesting its possible use to increase abiotic stress-tolerance in crops for sustained yield under adverse climatic conditions.

Changes in the Platycodin Content and Physiological Characteristics during the Fruiting Stage of Platycodon grandiflorum under Drought Stress

Medicinal plants are affected by drought stress, mainly reflected in the growth process and secondary metabolite synthesis. Platycodon grandiflorum (Jacq.) A. DC. is a traditional Chinese herbal medicine. The yield of Platycodon grandiflorum cannot meet the market demand, while its yield and quality are limited by the plant growth conditions. We assessed relevant indicators of growth during the fruiting stage of Platycodon grandiflorum under drought stress. The results showed that the fresh root weight (FW), photosynthesis, and chlorophyll fluorescence parameters were significantly reduced after withholding water (AW), but total superoxide dismutase (T-SOD), peroxidase (POD), and catalase (CAT) activities and the contents of soluble protein (SP), proline (PRO), and malondialdehyde (MDA) were significantly increased. The contents of platycodin D (PD) and platycodin D3 (PD3) did not change obviously after withholding water (AW), but in the autumn period, the values increased by 8.95% and 11.67%, respectively. The content of total platycodin increased significantly under drought stress, during the after rewatering (AR) and in the autumn period. The different physiological stress indicators exhibited strong correlations, had synergistic effects of mutual promotion and restriction, and responded to changes in the soil water content. These results suggest that during the fruiting stage, Platycodon grandiflorum encounters drought stress and may resist oxidative damage by increasing protective enzyme activity and osmoregulatory materials to ensure normal plant growth. According to the effect of drought stress on dry weight, the yield of Platycodon grandiflorum was not affected by drought stress, but the total platycodin content in Platycodon grandiflorum roots increased significantly. Therefore, in agricultural production, short-term drought stress should be conducted in the fruiting stage of Platycodon grandiflorum, which can both guarantee the yield and improve the quality of medicinal materials.

Role of exogenous-applied salicylic acid, zinc and glycine betaine to improve drought-tolerance in wheat during reproductive growth stages

BACKGROUND

Drought has become a dangerous threat to reduce crop productivity throughout the world. Exogenous applications of regulators, micronutrients, and/or osmoprotectants for inducing drought-tolerance in field crops have been effectively adopted. A controlled pot study was performed to investigate the relative efficacy of salicylic acid (SA), zinc (Zn), and glycine betaine (GB) as foliar applications on the growth, tissues pigments content, relative water content (RWC), leaf gas-exchange, antioxidant enzymes activity, reactive oxygen species (ROS) accumulation, osmolytes contents, and the yield parameters of wheat plants subjected to two soil water conditions (85% field capacity: well-watered, 50% field capacity: water-deficient) during reproductive growth stages.

RESULTS

Water deficient conditions significantly decreased the growth, yield parameters, RWC, photosynthesis pigment, and gas-exchange attributes except for intercellular CO₂ concentration. However, foliar applications remarkably improved the growth and yield parameters under water deficit conditions. Under drought condition, exogenous applications of SA, Zn, and GB increased the grain yield pot^- 1 by 27.99, 15.23 and 37.36%, respectively, as compared to the control treatment. Drought stress statistically increased the contents of hydrogen peroxide (H₂O₂), superoxide anion radical (O₂ ^•-), and malonaldehyde (MDA), and elevated the harmful oxidation to cell lipids in plants, however, they were considerably reduced by foliar applications. Foliar applications of SA, Zn, and GB decreased MDA content by 29.09, 16.64 and 26.51% under drought stress, respectively, as compared to the control treatment. Activities of all antioxidant enzymes, proline content, and soluble sugar were increased in response to foliar applications under water deficit conditions.

CONCLUSIONS

Overall, foliar application of GB, SA, and Zn compounds improved the drought-tolerance in wheat by decreasing the ROS accumulation, promoting enzymatic antioxidants, and increasing osmolytes accumulation. Finally, GB treatment was most effective in thoroughly assessed parameters of wheat followed by SA and Zn applications to alleviate the adverse effects of drought stress.

Functional Characterization of the Stipa purpurea P5CS Gene under Drought Stress Conditions

Free proline has multiple functions in plant cells, such as regulating osmotic potential and protecting both proteins and cell membranes. The expression of Δ1-Pyrroline-5-carboxylate synthase (P5CS), a key enzyme in the proline biosynthetic pathway, increases under drought, salt and cold stress conditions, causing plant cells to accumulate large amounts of proline. In this study, we cloned and identified the P5CS gene from Stipa purpurea, which has a full-length of 2196 bp and encodes 731 amino acids. A subcellular localization analysis indicated that SpP5CS localized to the cytoplasm. The ectopic overexpression of SpP5CS in Arabidopsis thaliana resulted in higher proline contents, longer roots, higher survival rates and less membrane damage under drought stress conditions compared with wild-type controls. SpP5CS-overexpressing A. thaliana was more resistant to drought stress than the wild type, whereas the deletion mutant sp5cs was less resistant to drought stress. Thus, SpP5CS may be a potential candidate target gene for increasing plant resistance to drought stress.

Exogenous application of spermine and putrescine mitigate adversities of drought stress in wheat by protecting membranes and chloroplast ultra-structure

Polyamines (PAs) are positively charged molecules known to mitigate drought stress; however, little is known about their mechanism of alleviating drought stress. We investigated the effects of PAs exogenously applied as a seed primer and as a foliar spray on the growth, membrane stability (MS), electrolyte leakage (EL), Na+ and K+ cations, reactive oxygen species (ROS), catalase (CAT; EC 1.11.1.6) and guaiacol peroxidase (GPX; EC 1.11.1.7) activity and chloroplast ultra-structure in wheat (Triticum aestivum L.; cv. Sakha-94) under drought stress. Three PA solutions, namely, putrescine, spermine and a mixture of the two (Mix), were each applied at a concentration of 100 µM. Our study demonstrated that the retardation of chlorophyll loss and elevation of Rubisco levels were involved in PA-enhanced growth under drought stress. These relationships were mainly reflected in elevated fresh weight and dry weight in response to foliar spraying with all PA solutions and seed priming with the Mix solution. The elevated growth seemed to be due to increased photosynthetic pigments, protein and Rubisco. In contrast, drought decreased growth, photosynthetic pigments, protein and Rubisco. MS was enhanced by PAs applied as a seed primer or foliar spray, as shown by clear reductions in EL %, malondialdehyde (MDA) content and the Na+/K+ ratio as well as reduced ROS markers and elevated CAT (but not GPX) activity. Further study showed that the Mix solution of PAs, applied either during seed priming or as a foliar spray, improved chloroplast ultra-structure, suggesting that improvements in Rubisco and photosynthetic pigments were involved in PA maintenance of chloroplast stability. Therefore, the present study showed that elevated CAT activity is the main mechanism through which PAs reduce ROS and MDA, thereby improving MS and protecting mesophyll cells structurally and functionally under drought stress in wheat.

Functional activation of a novel R2R3-MYB protein gene, GmMYB68, confers salt-alkali resistance in soybean (Glycine max L.)

Soil salinity significantly reduces soybean (Glycine max L.) production worldwide. Plants resistance to stress conditions is a complex characteristic regulated by multiple signaling pathways. The v-Myb avian myeloblastosis viral oncogene homolog (MYB) transcription factor (TF) plays a crucial role in plant development, secondary metabolism, and abiotic stress responses. GmMYB68-overexpression and RNA interference (RNAi) lines were established for examining the function of G. max GmMYB68 in plant responses to abiotic stresses. The predicted amino acid sequence of GmMYB68 was similar to that of R2R3-MYB proteins. Quantitative real-time PCR analysis revealed that GmMYB68 expression varied in response to abiotic stresses. GmMYB68-overexpression lines showed enhanced resistance to salt and alkali stresses and their osmotic adjustment and photosynthetic rates were also stronger than that of GmMYB68-RNAi and wild type plants. Although wild type and transgenic plants showed no significant differences in agronomic traits under normal conditions, the overexpression of GmMYB68 increased grain number and 100-grain weights under salt stress. Our study identified a valuable TF associated with stress response in soybean, as its overexpression might help improve salt and alkali tolerance in soybean and other crops.