Integrated Physiological, Transcriptomic, and Metabolomic Analyses Revealed Molecular Mechanism for Salt Resistance in Soybean Roots

Salinity stress is a threat to yield in many crops, including soybean (Glycine max L.). In this study, three soybean cultivars (JD19, LH3, and LD2) with different salt resistance were used to analyze salt tolerance mechanisms using physiology, transcriptomic, metabolomic, and bioinformatic methods. Physiological studies showed that salt-tolerant cultivars JD19 and LH3 had less root growth inhibition, higher antioxidant enzyme activities, lower ROS accumulation, and lower Na+ and Cl- contents than salt-susceptible cultivar LD2 under 100 mM NaCl treatment. Comparative transcriptome analysis showed that compared with LD2, salt stress increased the expression of antioxidant metabolism, stress response metabolism, glycine, serine and threonine metabolism, auxin response protein, transcription, and translation-related genes in JD19 and LH3. The comparison of metabolite profiles indicated that amino acid metabolism and the TCA cycle were important metabolic pathways of soybean in response to salt stress. In the further validation analysis of the above two pathways, it was found that compared with LD2, JD19, and LH3 had higher nitrogen absorption and assimilation rate, more amino acid accumulation, and faster TCA cycle activity under salt stress, which helped them better adapt to salt stress. Taken together, this study provides valuable information for better understanding the molecular mechanism underlying salt tolerance of soybean and also proposes new ideas and methods for cultivating stress-tolerant soybean.

Salt-induced growth promotion in rice varieties during nursery

Climate change will increase the occurrence of salinity in agricultural land along with the coastal areas. One of the technologies to reduce salinity is NaCl pretreatment. This study aimed to evaluate salinity treatment's effect during nurseries on the growth of lowland rice seedlings. There were three separate experiments, and all the experiments used Randomized Complete Block Design. In the first experiment, local black rice seeds (var. Jelitheng) was used. The nursery was carried out at three salinity levels, i.e. 0.2, 3 and 5 dS/m. The second experiment was conducted using salt-resistant rice seeds (var. Dendang) and salt susceptible rice seeds (var. IR 64). The salinity levels applied were non-saline (0.2 dS/m) and saline (5 dS/m). The third experiment used rice seedling var. IR 64, with the first factor being the salinity level (0.2 and 5 dS/m) and the second factor was a wet nursery and dry nursery. In general, the results from the three experiments showed that giving salinity levels of 3-5 dS/m in several rice varieties improved seedling performance. Although salinity during nursery could increase the concentration of Na+ and decrease the concentration of K+ in leaves, salinity during nursery increased the seedlings fresh weight, and dry weight increased the number of seedlings leaves and increased the concentration of leaf chlorophyll. The better seedlings growth variable in the saline nursery will help the plants cope with salinity in the later growth stage in the field.

Physiological and biochemical responses of seedlings of six contrasting barley (Hordeum vulgare L.) cultivars grown under salt-stressed conditions

Salinity stress affects plant growth and development and underlying metabolisms. To mitigate the effects of the stress, plants responded by changing their physiological and biochemical activities and withstand the stress. The present study aimed to determine barley's (Hordeum vulgare L.) physiological and biochemical response to salinity stress conditions for 7 days and 14 days. Six barley cultivars (Alfa93, DWRB73, DL88, NB1, NB3, NDB1173) were grown under controlled conditions, and different level of salinity stress was applied. In addition, seedling growth, physiological and biochemical parameters, plant leaves RWC, and electrolyte leakage were analyzed. The overall seedling growth, RWC, and electrolyte leakage in salt susceptible lines Alfa93 and DWRB73 were low than the salt-tolerant barley lines (DL88, NB1, NB3, and NDB1173). Electrolyte leakage was 26.0 and 20.6% in Alfa93 and DWRB73, whereas it was 17.6, 14.6, 15.3, and 10.4% in DL88, NB1, NB3, and NDB1173, respectively at 300 mM salinity stress. The loss of photosynthetic pigments under salt stress was high in susceptible lines, salinity treated (300 mM NaCl) Alfa93 plants exhibit 49.5% and 59.5% of Chl-a than control plants after 7 and 14 days of treatment, respectively. However, at 300 mM stress level, NB1 (ST) showed less Chl-a loss after 7 days, whereas NDB1173 showed less reduction in Chl-a after 14 days. Antioxidant enzymes such as SOD, POX, CAT, and APX activities in susceptible line Alfa93 and DWRB73 were lower than tolerant lines. PCA analysis demonstrated a positive correlation between antioxidant enzyme activities and genotypes under salinity stress. PCA analysis described DL88 as the most tolerant, and DWRB73 was the most salt susceptible genotype among the studied barley genotypes. The present findings suggest that barley cultivars' physiological and biochemical activities under salinity stress conditions may be used to screen salt-tolerant crops.

Comparative ribosome profiling reveals distinct translational landscapes of salt-sensitive and -tolerant rice

Background Soil salinization represents a serious threat to global rice production. Although significant research has been conducted to understand salt stress at the genomic, transcriptomic and proteomic levels, few studies have focused on the translatomic responses to this stress. Recent studies have suggested that transcriptional and translational responses to salt stress can often operate independently. Results We sequenced RNA and ribosome-protected fragments (RPFs) from the salt-sensitive rice (O. sativa L.) cultivar ‘Nipponbare’ (NB) and the salt-tolerant cultivar ‘Sea Rice 86’ (SR86) under normal and salt stress conditions. A large discordance between salt-induced transcriptomic and translatomic alterations was found in both cultivars, with more translationally regulated genes being observed in SR86 in comparison to NB. A biased ribosome occupancy, wherein RPF depth gradually increased from the 5′ ends to the 3′ ends of coding regions, was revealed in NB and SR86. This pattern was strengthened by salt stress, particularly in SR86. On the contrary, the strength of ribosome stalling was accelerated in salt-stressed NB but decreased in SR86. Conclusions This study revealed that translational reprogramming represents an important layer of salt stress responses in rice, and the salt-tolerant cultivar SR86 adopts a more flexible translationally adaptive strategy to cope with salt stress compared to the salt susceptible cultivar NB. The differences in translational dynamics between NB and SR86 may derive from their differing levels of ribosome stalling under salt stress.

Advances and Challenges in the Breeding of Salt-Tolerant Rice

Soil salinization and a degraded ecological environment are challenging agricultural productivity and food security. Rice (Oryza sativa), the staple food of much of the world’s population, is categorized as a salt-susceptible crop. Improving the salt tolerance of rice would increase the potential of saline-alkali land and ensure food security. Salt tolerance is a complex quantitative trait. Biotechnological efforts to improve the salt tolerance of rice hinge on a detailed understanding of the molecular mechanisms underlying salt stress tolerance. In this review, we summarize progress in the breeding of salt-tolerant rice and in the mapping and cloning of genes and quantitative trait loci (QTLs) associated with salt tolerance in rice. Furthermore, we describe biotechnological tools that can be used to cultivate salt-tolerant rice, providing a reference for efforts aimed at rapidly and precisely cultivating salt-tolerance rice varieties.

Grafting Tomato as a Tool to Improve Salt Tolerance

Salinity in soil or water is a serious threat to global agriculture; the expected acreage affected by salinity is about 20% of the global irrigated lands. Improving salt tolerance of plants through breeding is a complex undertaking due to the number of traits involved. Grafting, a surgical mean of joining a scion and rootstock of two different genotypes with the desired traits, offers an alternative to breeding and biotechnological approaches to salt tolerance. Grafting can also be used to circumvent other biotic and abiotic stresses. Increasing salinity tolerance in tomato (Solanum lycopresicum L.), a highly nutritious and economical vegetable, will have greater impact on the vegetable industry, especially in (semi) arid regions where salinity in soil and water are more prevalent. Besides, plants also experience salt stress when water in hydroponic system is recycled for tomato production. Grafting high yielding but salt-susceptible tomato cultivars onto salt-resistant/tolerant rootstocks is a sustainable strategy to overcome saline stress. Selection of salt-tolerant rootstocks though screening of available commercial and wild relatives of tomato under salt stress conditions is a pre-requisite for grafting. The positive response of grafting exerted by tolerant rootstocks or scion-rootstock interactions on yield and fruit characteristics of tomato under saline conditions is attributed to several physiological and biochemical changes. In this review, the importance of tomato grafting, strategies to select appropriate rootstocks, scion-rootstock interaction for growth, yield and quality characteristics, as well as the tolerance mechanisms that (grafted) plants deploy to circumvent or minimize the effects of salt stress in root zones are discussed. The future challenges of grafting tomato are also highlighted.

Comparative Water Relations of Two Contrasting Date Palm Genotypes under Salinity

Salinity is a global agricultural problem, resulting in a significant reduction in the plantation areas and the crop yields, especially in arid and semiarid regions. The date palm is relatively salt-tolerant plant species, although the nature of salt tolerance is poorly understood. In this study, the salt stress responses of a salt-tolerant “Umsila” was compared with salt-susceptible “Zabad” date palm cultivars. Various physiological parameters, plant-water relations, and anatomical characteristics were analyzed. The results revealed that although salinity has negatively affected both cultivars, Umsila exhibited more stable photosynthesis than Zabad as reflected by the quantum yield (Qy) and the stomatal conductance (GS). Similarly, Umsila showed a more dynamic root system and efficient water relations than Zabad as demonstrated by the leaf water potential (LWP) and relative water content (RWC) during salinity. Umsila also accumulated greater abundances of soluble sugars, potassium (K+), calcium (Ca+2), proline, glycine betaine, and lignin and formed extra layers of Casparian strips in the root tissues when the seedlings were grown under saline conditions. Together, the results obtained from this study have offered some insights into the salt tolerance mechanisms in the date palm.

Change in Chlorophyll Content Over Time Well-differentiated Salt-tolerant, Moderately Salt-tolerant, and Salt-susceptible Cowpea Genotypes

Previous investigations showed that accumulations of Na+ and Cl− in leaves resulted in reductions in chlorophyll content, thereby affecting photosynthesis. Understanding how chlorophyll content evolves over time will help plant breeders to select cowpea genotypes with better tolerance to salinity by allowing them to choose those with more stable chlorophyll content under salt stress. The objective of this study was to assess how the chlorophyll content of cowpea genotypes changed over the course of 24 d of salt stress at the seedling stage. A total of 24 cowpea genotypes with different salt responses were used in this study. The experiment used a split-plot design with salt treatment as the main plot and cowpea genotypes as the subplot. In the main plot, there were two salt treatments: 0 mm (ionized water) and 200 mm NaCl. In the subplot, the cowpea genotypes were arranged as a completely randomized design with three replicates per genotype. The results revealed that: a1) the time × genotype interaction was significant under conditions with and without salt; 2) chlorophyll content slowly decreased in the salt-tolerant genotypes; 3) chlorophyll content slightly increased on day 6 and day 9 of salt stress in both moderate and sensitive genotypes, but it decreased at a faster rate than in the salt-tolerant genotypes; and 4) salt-sensitive genotypes were completely dead on day 24 of salt stress, whereas the salt-tolerant genotypes were able to maintain a significant amount of chlorophyll content. These results can be used to advance breeding programs for salt tolerance in cowpea.