Species-specific responses to drought, salinity and their interactions in Populus euphratica and P. pruinosa seedlings

Aims Drought and salinity are severe abiotic stress factors, which limit plant growth and productivity, particularly in desert regions. In this study, we employed two desert poplars, Populus euphratica Oliver and Populus pruinosa Schrenk seedlings, to compare their tolerance to drought, salinity and combined stress. Methods We investigated species-specific responses of P. euphratica and P. pruinosa in growth, photosynthetic capacity and pigment contents, nonstructural carbohydrate concentrations, Cl− allocation, osmotic regulation and the accumulation of reactive oxygen species (ROS) under drought, salinity and the combined stress. Important Findings Populus pruinosa exhibited greater growth inhibitory effects, photosynthesis decline, stomatal closure and ROS accumulation, and lower antioxidant enzyme activities and osmotic regulation compared with P. euphratica under drought, salinity and especially under their combined stress. On the other hand, salt-stressed P. euphratica plants restricted salt transportation from roots to leaves, and allocated more Cl− to coarse roots and less to leaves, whereas salt-stressed P. pruinosa allocated more Cl− to leaves. It was shown that there is species-specific variation in these two desert poplars, and P. pruinosa suffers greater negative effects compared with P. euphratica under drought, salinity and especially under the combined stress. Therefore, in ecological restoration and afforestation efforts, species-specific responses and tolerances of these two poplar species to drought and salinity should be considered under climate change with increasing drought and soil salinity developing.

De novo transcriptome sequencing and gene expression profiling of Pinus sibirica under different cold stresses

Background: Pinus sibirica is an evergreen conifer tree species with strong cold stress. However, the transcriptional regulation patterns in response to cold stress have not been reported for P. Sibirica. To gain deeper insights into its regulation process of cold tolerance, transcriptome profiling analyses and 12 physiological indices measurement were performed under cold stress (-20 ℃) over time. Results: More than 54.1 million clean reads were produced, which were assembled into 97,376 unigenes. Among them, 56,994 unigenes had homology with known genes, 36,836 were assigned to 51 GO (gene ontology) categories and 46,972 were assigned to 24 COG (clusters of orthologous group) categories. P. sibirica showed the highest similarity with sequences from Picea sitchensis. In total, 871, 1397 and 872 DEGs (differentially expressed genes) were identified upon exposure to cold for 6 h, 24 h and 48 h at -20 ℃, respectively. Nine physiological indices increased significantly (P<0.05) under cold stress, including membrane permeability, relative conductivity, reactive oxygen species, malonaldehyde, peroxidase activity, catalase activity, soluble sugar, soluble protein and proline content. With extension of the cold stress time, 9 physiological indices generally showed a trend toward first an increase and then a decrease. The net photosynthetic rate, stomatal conductance and transpiration rate in P. sibirica dropped sharply (P<0.05) in response to cold stress, and they were also decreased significantly (P<0.05) with extension of the stress time at -20 ℃.Conclusions: There were two cold signal transduction pathways in P. sibirica, the Ca2+ and ABA (abscisic acid) pathways. The AP2 (ethylene-responsive transcription factor) family and some other transcription factors played an important role in transcriptional regulation. P. sibirica underwent antioxidant and osmotic regulation with changes in the expression of genes related to cold resistance. Photosynthesis was inhibited, and more DEGs associated with photosynthesis were downregulated under cold stress. The DEGs identified in cold signal sensing and transduction and transcriptional, antioxidant and osmotic regulation can provide genetic resources for the improvement of cold-tolerant characters in other conifer tree species and facilitate understanding of molecular control mechanism related to cold responses.

iTRAQ-Based Proteomic Analysis Reveals Several Strategies to Cope with Drought Stress in Maize Seedlings

Drought stress, especially during the seedling stage, seriously limits the growth of maize and reduces production in the northeast of China. To investigate the molecular mechanisms of drought response in maize seedlings, proteome changes were analyzed. Using an isotopic tagging relative quantitation (iTRAQ) based method, a total of 207 differentially accumulated protein species (DAPS) were identified under drought stress in maize seedlings. The DAPS were classified into ten essential groups and analyzed thoroughly, which involved in signaling, osmotic regulation, protein synthesis and turnover, reactive oxygen species (ROS) scavenging, membrane trafficking, transcription related, cell structure and cell cycle, fatty acid metabolism, carbohydrate and energy metabolism, as well as photosynthesis and photorespiration. The enhancements of ROS scavenging, osmotic regulation, protein turnover, membrane trafficking, and photosynthesis may play important roles in improving drought tolerance of maize seedlings. Besides, the inhibitions of some protein synthesis and slowdown of cell division could reduce the growth rate and avoid excessive water loss, which is possible to be the main reasons for enhancing drought avoidance of maize seedlings. The incongruence between protein and transcript levels was expectedly observed in the process of confirming iTRAQ data by quantitative real-time polymerase chain reaction (qRT-PCR) analysis, which further indicated that the multiplex post-transcriptional regulation and post-translational modification occurred in drought-stressed maize seedlings. Finally, a hypothetical strategy was proposed that maize seedlings coped with drought stress by improving drought tolerance (via. promoting osmotic adjustment and antioxidant capacity) and enhancing drought avoidance (via. reducing water loss). Our study provides valuable insight to mechanisms underlying drought response in maize seedlings.

Mechanistic basis for loss of water balance in green tree frogs infected with a fungal pathogen

Chytridiomycosis, a lethal skin disease caused by the fungal pathogen Batrachochytrium dendrobatidis ( Bd), disrupts skin function of amphibians, interfering with ionic and osmotic regulation. To regulate fungal loads, amphibians increase their rate of skin sloughing. However, sloughing also causes a temporary loss of ionic and osmotic homeostasis due to disruption of the skin, a key osmoregulatory organ. The combined effects of increased sloughing frequency and chytridiomycosis contribute to the high rates of mortality from Bd infections. However, the mechanisms responsible for the loss of cutaneous osmotic regulation remain unknown. We measured the changes in whole animal water uptake rates, in vitro transcutaneous water fluxes across the ventral skin, and the mRNA expression of epithelial water transport proteins (aquaporins, AQPs) and junctional proteins in Bd-infected and uninfected Litoria caerulea skin. We hypothesize that infected frogs would show reduction/inhibition in cutaneous water transporters responsible for regulating water balance, and sloughing would exacerbate cutaneous water fluxes. We found that infected, nonsloughing frogs had an impaired rate of water uptake and showed increased rates of in vitro water efflux across the ventral skin. In uninfected frogs, the expression of AQPs and junction genes increased significantly with sloughing, which may assist in regulating cutaneous water movements and barrier function in the newly exposed skin. In contrast, infected frogs did not show this postsloughing increase in AQP gene expression. The combination of increased sloughing frequency, impaired water uptake rates, and increased rates of water loss likely contributes to the loss of osmotic homeostasis in frogs infected with Bd.