Proline, a multifaceted signalling molecule in plant responses to abiotic stress: understanding the physiological mechanisms

Ghosh U.K., Islam M.N., Siddiqui M.N., Khan M.A.

Abiotic stresses are significant environmental issues that restrict plant growth, productivity, and survival while also posing a threat to global food production and security. Plants produce compatible solutes known as osmolytes to adapt themselves in such changing environment. Osmolytes contribute to homeostasis maintenance, provide the driving gradient for water uptake, maintain cell turgor by osmotic adjustment, and redox metabolism to remove excess level of reactive oxygen species (ROS) and reestablish the cellular redox balance as well as protect cellular machinery from osmotic stress and oxidative damage. Perceiving the mechanisms how plants interpret environmental signals and transmit them to cellular machinery to activate adaptive responses is important for crop improvement programs to get stress-tolerant varieties. A large number of studies conducted in the last few decades have shown that osmolytes accumulate in plants and have strong associations with abiotic stress tolerance. Production of abundant osmolytes is needed for tolerance in many plant species. In addition, transgenic plants overexpressing genes for different osmolytes showed enhanced tolerance to various abiotic stresses. Many important aspects of their mechanisms of action are yet to be largely identified, especially regarding the relevance and relative contribution of specific osmolytes to the stress tolerance of a given species. Therefore, more efforts and resources should be invested in the study of the abiotic stress responses of plants in their natural habitats. The present review focuses on the possible roles and mechanisms of osmolytes and their association toward abiotic stress tolerance in plants. This review would help the readers in learning more about osmolytes and how they behave in changing environments as well as getting an idea of how this knowledge could be applied to develop stress tolerance in plants.

Ramos-Cormenzana A.

Furlan A.L., Bianucci E., Giordano W., Castro S., Becker D.F.

Proline accumulation and metabolism are associated with mechanisms of abiotic stress avoidance in plants. Proline accumulation generally improves osmotic stress tolerance whereas proline metabolism can have varying effects from ATP generation to the formation of reactive oxygen species. To further understand the roles of proline in stress protection, two peanut cultivars with contrasting tolerance to drought were examined by transcriptional and biochemical analyses during water stress. Plants exposed to polyethylene glycol had diminished relative water content and increased proline content; while, only the drought sensitive plants, cultivar Granoleico, showed lipid oxidative damage (measured as thiobarbituric acid reactive substances). The expression of proline biosynthesis genes (P5CS1, P5CS2a, P5CS2b, P5CR) was increased in both cultivars upon exposure to water stress. However, the relative expression of proline catabolism genes (ProDH1, ProDH2) was increased only in the sensitive cultivar during stress. Exogenous addition of proline and the proline analogue thiazolidine-4-carboxylic acid (T4C), both substrates of proline dehydrogenase, was also used to exacerbate and identify plant responses. Pretreatment of plants with T4C induced unique changes in the drought tolerant EC-98 cultivar such as higher mRNA levels of proline biosynthetic and catabolic ProDH genes, even in the absence of water stress. The increased levels of ProDH gene expression, potentially associated with higher T4C conversion to cysteine, may contribute to the tolerant phenotype.

Kerchev P., van der Meer T., Sujeeth N., Verlee A., Stevens C.V., Van Breusegem F., Gechev T.

Abiotic stresses, including drought, salinity, extreme temperature, and pollutants, are the main cause of crop losses worldwide. Novel climate-adapted crops and stress tolerance-enhancing compounds are increasingly needed to counteract the negative effects of unfavorable stressful environments. A number of natural products and synthetic chemicals can protect model and crop plants against abiotic stresses through induction of molecular and physiological defense mechanisms, a process known as molecular priming. In addition to their stress-protective effect, some of these compounds can also stimulate plant growth. Here, we provide an overview of the known physiological and molecular mechanisms that induce molecular priming, together with a survey of the approaches aimed to discover and functionally study new stress-alleviating chemicals.

van Zelm E., Zhang Y., Testerink C.

Crop loss due to soil salinization is an increasing threat to agriculture worldwide. This review provides an overview of cellular and physiological mechanisms in plant responses to salt. We place cellular responses in a time- and tissue-dependent context in order to link them to observed phases in growth rate that occur in response to stress. Recent advances in phenotyping can now functionally or genetically link cellular signaling responses, ion transport, water management, and gene expression to growth, development, and survival. Halophytes, which are naturally salt-tolerant plants, are highlighted as success stories to learn from. We emphasize that ( a) filling the major knowledge gaps in salt-induced signaling pathways, ( b) increasing the spatial and temporal resolution of our knowledge of salt stress responses, ( c) discovering and considering crop-specific responses, and ( d) including halophytes in our comparative studies are all essential in order to take our approaches to increasing crop yields in saline soils to the next level.

Mohammadi H., Hazrati S., Ghorbanpour M.

There is no doubt that rapid or gradual environmental changes are the direct threat to agricultural sustainability and food security. Plants have developed different mechanisms to respond to environmental stresses. Short-term changes in environmental factors can lead to cumulative reactions, while gradual changes can lead to adaptation in plants. Individual’s response to environmental stressors depends on many factors such as type and duration of stress and plant species. The exposure of plants to adverse environmental conditions such as extremely high or low temperature, light stress and ultraviolet (UV), heavy metals, water shortage, air pollutants, nutrients deficiency, and salt stress results in the generation of reactive oxygen species including superoxide, hydrogen peroxide, and hydroxyl radical. Plants have developed a set of different mechanisms for adaptation and survival under severe environmental conditions. The adaptation and/or survival of plants grown under harsh environments cause(s) remarkable variations at the cellular and molecular levels. Among the most critical environmental factors, both temperature and water significantly affect plants geographical distribution and consequently agricultural sustainability in different regions of the world.

El-Beltagi H.S., Mohamed H.I., Sofy M.R.

In recent years, the harmful effects of drought stress have been be mitigated by using bioactive compounds such as antioxidants and osmolytes. In this research, pot experiments were carried out to investigate the effects of ascorbic acid, glutathione and proline on alleviating the harmful effect of drought stress in chickpea plants during season 2017. Chickpea plant seeds were soaked in ascorbic acid (0.75 mM), glutathione (0.75 mM), proline (0.75 mM) singly and/or in sequence combinations for 4 h and then planted in pots. The pots were irrigated with water after seven days (to serve as control), after 14 days (moderate drought stress) and after 28 days (severe drought stress). The sequence combination of antioxidants and proline under drought stress has not been studied yet. The results showed significantly decreased in plant growth, yielding characteristics, photosynthetic pigments and soluble protein content in response to moderate and severe drought stress. Moreover, treatment with antioxidants caused increment the antioxidant enzyme activity, non-enzymatic antioxidant (ascorbic acid and glutathione) contents and endogenous proline in stressed and unstressed plants. In conclusion, The sequence combination of antioxidants and proline caused improvement in plant growth under drought stress by up-regulating the antioxidant defense system and osmolyte synthesis.

Guan C., Cui X., Liu H., Li X., Li M., Zhang Y.

Understanding the regulation of proline metabolism necessitates the suppression of two Δ1-pyrroline-5-carboxylate synthetase enzyme (P5CS) genes performed in switchgrass (Panicum virgatum L.). The results reveal that overexpressing PvP5CS1 and PvP5CS2 increased salt tolerance. Additionally, transcript levels of spermidine (Spd) and spermine (Spm) synthesis and metabolism related genes were upregulated in PvP5CS OE-transgenic plants and downregulated in the PvP5CS RNAi transformants. According to salt stress assay and the measurement of transcript levels of Polyamines (PAs) metabolism-related genes, P5CS enzyme may not only be the key regulator of proline biosynthesis in switchgrass, but it may also indirectly affect the entire subset of pathway for ornithine to proline or to putrescine (Put). Furthermore, application of proline prompted expression levels of Spd and Spm synthesis and metabolism-related genes in both PvP5CS-RNAi and WT plants, but transcript levels were even lower in PvP5CS-RNAi compared to WT plants under salt stress condition. These results suggested that exogenous proline could accelerate polyamines metabolisms under salt stress. Nevertheless, the enzymes involved in this process and the potential functions remain poorly understood. Thus, the aim of this study is to reveal how proline functions with PAs metabolism under salt stress in switchgrass.

Ali S., Eum H., Cho J., Dan L., Khan F., Dairaku K., Shrestha M.L., Hwang S., Nasim W., Khan I.A., Fahad S.

Climate change is a phenomenon that is unequivocally altering the natural systems in all parts of the world but the alteration in climate extremes may pose more severe and unexpected impacts on Pakistan. The current study provides a comprehensive outlook of observation (1976–2005) and changes in climate extremes between the reference (1976–2005) and future periods (2020s: 2006–2035, 2050s: 2036–2065 and 2080s: 2066–2095). The analysis was conducted across six sub-regions of Pakistan including North Pakistan (NP), Monsoon Region (MR), Khyber Pakhtunkhwa (KP), Southern Punjab (SP), Balochistan and Sindh for which Coupled Model Intercomparison Project Phase 5 (CMIP5) 14 General Circulation Models (GCMs) under Representative Concentration Pathways 4.5 (RCP4.5) and RCP8.5 were downscaled and bias corrected by three statistical downscaling methods. The spatial disaggregation and quantile delta mapping (SDQDM) method was used for future projections in this study. Changes in climate extremes were detected by Expert Team on Climate Change Detection and Indices (ETCCDI). In case of temperature, the results indicate a projected increase in frequencies and magnitudes for warm extremes, while it is decreasing for cold extremes in the 21st century. The corresponding trends of maximum and minimum temperature extremes are greater than the mean temperature trend; where the frequency and magnitude of minimum temperature extremes is higher than maximum temperature extremes over Pakistan particularly over North in last half of the 21st century for both RCPs. Also, the average of temperature extremes (TXx, TXn, TNx and TNn) are severe in the order of NP (+4.8 °C), KP (+4.6 °C) and MR (+4.5 °C). In the case of precipitation extremes, most of the sub-regions across Pakistan show a higher increase in total annual precipitation and intense precipitation events with the highest increase in MR, KP and NP and the least increase in Sindh. Despite the increase in total precipitation, numbers of consecutive dry days (CDD) are increasing while consecutive wet days (CWD) are decreasing which can give rise to drought conditions particularly in Sindh. The study provides complementary and consistent climate extremes information over Pakistan for local decision makers to incorporate into policy-making, disaster management, and infrastructure planning.

Barickman T.C., Simpson C.R., Sams C.E.

Waterlogging occurs because of poor soil drainage and/or excessive rainfall and is a serious abiotic stress affecting plant growth because of declining oxygen supplied to submerged tissues. Although cucumber (Cucumis sativus L.) is sensitive to waterlogging, its ability to generate adventitious roots facilitates gas diffusion and increases plant survival when oxygen concentrations are low. To understand the physiological responses to waterlogging, a 10-day waterlogging experiment was conducted. The objective of this study was to measure the photosynthetic and key metabolites of cucumber plants under waterlogging conditions for 10 days. Plants were also harvested at the end of 10 days and analyzed for plant height (ht), leaf number and area, fresh mass (FM), dry mass (DM), chlorophyll (Chl), carotenoid (CAR), proline, and soluble sugars. Results indicated that cucumber plants subjected to the 10-day waterlogging stress conditions were stunted, had fewer leaves, and decreased leaf area, FM, and DM. There were differences in physiological performance, Chl, CAR, proline, and soluble sugars. Overall, waterlogging stress decreased net photosynthesis (A), having a negative effect on biomass accumulation. However, these decreases were also dependent on other factors, such as plant size, morphology, and water use efficiency (WUE) that played a role in the overall metabolism of the plant.

Sharma P., Sharma P., Arora P., Verma V., Khanna K., Saini P., Bhardwaj R.

Environmental stresses such as flood, drought, high or low temperatures, excessive soil salinity, inadequate mineral nutrients, and excess or insufficient light have adverse effects and cause damage to crop plants. Overproduction of reactive oxygen species (ROS) under stress conditions is a hallmark, comprising both free radical, superoxide radicals, hydroxyl radical, perhydroxy radical, and alkoxy radicals and nonradical (molecular) forms, like hydrogen peroxide and singlet oxygen. Production of excess ROS is very reactive in nature and interact with numerous biomolecules such as DNA, proteins, RNA, lipids, pigments, and other vital cellular molecules, leading to serious damages. The antioxidant defense mechanism of plants protects them against damage caused by environmental stress. ROS also play a key role in the acclimation process of plants to several abiotic stresses. During stress response, ROS act as important molecules and play pivotal roles in activating downstream metabolic pathways. ROS also influence the expression of a number of genes and therefore control many processes. The present chapter focuses on the description of ROS, damage caused to macromolecules by ROS, the process of ROS signaling, and antioxidant defense system under abiotic stress in plants and its role in the removal of ROS.

Patel N., Gantait S., Panigrahi J.

To extend the postharvest shelf-life of green bell pepper (Capsicum annuum L.), an exogenous application of two polyamines, Spermidine and Putrescine (SPD-PUT), was tested in multiple combinations (10 μM plus 10 μM, 20 μM plus 20 μM, and 30 μM plus 30 μM) at 4 ± 1 °C for 40 days. The titratable acidity, protein content, activities of catalase and peroxidase, chlorophyll and capsaicin content gradually decreased for all the treated and untreated fruits throughout the storage period. On the other hand, proline content and antioxidant 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical scavenging activity continuously increased with the extension of time-span under storage. Among the three treatment combinations, the combination of 20 µM SPD and 20 µM PUT was found to be the optimum, based on the response of all the morphological and physicochemical traits assessed. The same combination was effective in sustaining the quality of green bell pepper following its harvest, for a period of at least 40 days.

Sadeghipour O.

Ghaffari H., Tadayon M.R., Nadeem M., Cheema M., Razmjoo J.

Drought stress (DS) is a major concern in the agricultural sector and, in particular, for sugar beet production and sugar content. As such several agricultural practices have been used to minimize yield losses from DS, and foliar application of proline is considered one such approach to improve drought tolerance in growing plants. Hence, the current study examined the proline-related improvements to induce drought tolerance in sugar beet plants. A field experiment was conducted at two locations (Shahrekord and Shalamzar) in Chaharmahal-Bakhtiari province, Iran. Experimental treatments comprised of three DS levels (well water: 100%; mild stress: 75%; severe stress: 50% water requirement of plant), and three proline applications (control: 0; low: 5 mM; high: 10 mM). DS caused a significant up-regulation in leaf proline content, malondialdehyde (MDA) content, hydrogen peroxide (H2O2) content, ascorbate peroxidase, catalase, and peroxidase enzymatic activities. This increase was more pronounced under proline application with concomitant down-regulation of MDA and H2O2 contents. DS also caused a decrease in leaf photosynthetic pigments, leaf relative water contents, membrane stability index and sugar beet root production; however, proline application mitigated these adverse DS effects. The study results suggest beneficial effects of proline applications, which is crucial to mitigation of the detrimental effects of DS in sugar beet by enhancing antioxidant enzymatic activities with concomitant reduction in MDA and H2O2 contents.

Hasanuzzaman M., Fujita M., Oku H., Islam M.T.

Ma X., Ouyang Z., Luo H., Shang W., Ma H., Zhu M., Dong H., Guo Z., Dong X., Piao F., Shen S., Li X., Wang Y., Zhang T.

Riaz M., Kamran M., Hussain S., Yan L.

Li J., Zhou P., Hu Z., Xiong A., Xing-Hui Li, Chen X., Zhuang J.

Dušica J., Jelena J., Ana M.J., Federica Z., Rossella M., Zorica N., Jelena O.

Zhang Y., Liu Y., Zhang C., Xie T., Xia J., Ma R., Wang J., You H., Ke L., Hua X.

Tang Y., Wang X., Wang Y., Xie J., Zhang R., Liu T., Jia S., Bao X.

Singh A., Choudhary K.K.

Chaudhary D., Aggarwal H., Kaur B., Aggarwal H., Das A., Kumar A., Mishra V., Singh P.K., Joshi N.C.

Chen W., Lian J., Hong C., Sun S., Hao J., Huang S., Wang J., Guan Y., Lu Z., Wang Z., Zhu S., Wei Z.

Cultivated lettuce (Lactuca sativa L.) is considered one of the most important economic vegetables worldwide; however, it is subjected to different stresses (salt stress, etc.) during its growth and development, resulting in yield reductions. In this study, we selected cultivated red lettuce and wild lettuce species (Lactuca serriola L.) to investigate the phenotypic and physiological changes in these lettuce under different salt treatment conditions. Functional annotation and enrichment analysis of the differentially expressed genes (DEGs) in the lettuce leaves and roots between the control and salt treatments were performed, identifying the key genes responding to salt stress. The results showed that the growth of the two types of lettuce was limited by salt stress, with decreased leaf area, main root length, biomass, and photosynthesis parameters noted. The cultivated red lettuce and the wild lettuce exhibited similar trends in terms of the variation in their antioxidant enzymatic activity and the content of osmoregulatory compounds in their leaves. The results of our transcriptomic analysis revealed that the mitogen-activated protein kinase (MAPK) signaling pathway, transporters, cytochrome P450, phenylpropanoid biosynthesis, and isoflavonoid biosynthesis were involved in the response to salt stress in the lettuce seedlings. The red lettuce cultivar showed a greater abundance of DEGs related to secondary metabolite biosynthesis and aquaporins under the salt treatment, resulting in a salinity-tolerant capacity comparable to that of the wild lettuce species. These results reveal important biosynthesis pathways that may play a key role in the salt tolerance of lettuce seedlings and provide key candidate genes that could be functionally characterized further and utilized to genetically improve new salt-tolerant varieties.

Cayci A., Akcin A., Yalcin E., Akcin T.A.

Li Z., Yu Q., Ma Y., Miao F., Ma L., Li S., Zhang H., Wang Z., Yang G., Su K.

IntroductionAlfalfa is the most widely cultivated high-quality perennial leguminous forage crop in the world. In China, saline-alkali land represents an important yet underutilized land resource. Cultivating salt-tolerant alfalfa varieties is crucial for the effective development and utilization of saline-alkali soils and for promoting the sustainable growth of grassland-livestock farming in these regions. The NAC (NAM, ATAF, and CUC) family of transcription factors plays a key role in regulating gene expression in response to various abiotic stresses, such as drought, salinity and extreme temperatures, thereby enhancing plant stress tolerance.MethodsThis study evaluated the structure and evolutionary relationship of the members of the NAC-like transcription factor family in alfalfa using bioinformatics. We identified 114 members of the NAC gene family in the Zhongmu No.1 genome and classified them into 13 subclasses ranging from I to XIII. The bioinformatics analysis showed that subfamily V might be related to the response to salt stress. Gene expression analysis was conducted using RNA-seq and qRT-PCR, and MsNAC40 from subfamily V was chosen for further investigation into salt tolerance.ResultsMsNAC40 gene had an open reading frame of 990 bp and encoded a protein containing 329 amino acids, with a molecular weight of 3.70 KDa and a conserved NAM structural domain. The protein was hydrophilic with no transmembrane structure.After treating both the MsNAC40 overexpressing plants and the control group with 150 mmol/L NaCl for 15 days, physiological and biochemical measurements revealed that these plants had significantly greater height, net photosynthetic rate, stomatal conductance, and transpiration rate compared to the control group, while their conductivity was significantly lower. Additionally, the levels of abscisic acid in the roots and leaves, along with the activities of peroxidase, superoxide dismutase, and catalase in the leaves, were significantly higher in the overexpressing plants, whereas the malondialdehyde content was significantly lower. Moreover, the Na+ content in the overexpressing plants was significantly reduced, while the K+/Na+ ratio was significantly increased compared to the control group.DiscussionThese results indicated that the MsNAC40 gene improved the salt tolerance of Pioneer Alfalfa SY4D, but its potential mechanism of action still needs to be further explored.

Ismail A., Gajjar P., Darwish A.G., Abuslima E., Islam T., Mohamed A.G., Tsolova V., Nick P., El Kayal W., El-Sharkawy I.

Xu T., Wei H., Yang P., Zhou X., Ma D., Luo C., Chen Y., Zhang J.

Giri S., Panwar H., Singh K., Kumar M.

Liu T., Wu B., Zhang Y., Li Z., Xue Y., Ding X., Yang Z., Zhu J., Han Y.

Peroxiredoxin (Prx) plays a role in maintaining the balance of intracellular reactive oxygen species. The peroxidase SiPrx gene from the Tianshan Snow Lotus (Saussurea involucrata) has been proved to significantly enhance the stress resistance of plants. In this study, the SiPrx gene was expressed heterogeneously in high-quality herbage Silphium perfoliatum L. (SP). After treatment with NaCl, the transgenic SP only exhibited partial leaf wilting, whereas the wild-type (WT) plants were on the brink of death. Simultaneously, physiological and biochemical assays indicated that under high-salt conditions, the content of malondialdehyde in the transgenic plants was significantly lower than that in the WT plants, while the activity of antioxidant enzymes was significantly higher than that in the WT plants. The expression of the SiPrx gene has been shown to significantly enhance the salt stress resistance of transgenic SP. Furthermore, after treatment at −10 °C for 48 h, the leaves of transgenic plants were able to maintain a certain morphological structure, whereas the WT plants were completely wilted. Physiological and biochemical index measurements indicated that all indicators in the transgenic plants were significantly better than those in the WT plants. Based on these findings, this study plans to overexpress the SiPrx gene extracted from Saussurea involucrata in Comfrey using the Agrobacterium-mediated method and then study its effects on the stress resistance of transgenic SP. The research results indicate that the SiPrx gene shows significant application potential in enhancing the cold resistance and salt tolerance of SP. This study provides a certain research basis and scientific evidence for the mining of stress resistance genes in Saussurea involucrata and the cultivation of new varieties of SP.