Plant osmoprotectants protect against abiotic stresses. Introgression of osmoprotectant genes into crop plants via genetic engineering is an important strategy in developing more productive plants. Plants employ adaptive mechanisms to survive various abiotic stresses. One mechanism, the osmoprotection system, utilizes various groups of low molecular weight compounds, collectively known as osmoprotectants, to mitigate the negative effect of abiotic stresses. Osmoprotectants may include amino acids, polyamines, quaternary ammonium compounds and sugars. These nontoxic compounds stabilize cellular structures and enzymes, act as metabolic signals, and scavenge reactive oxygen species produced under stressful conditions. The advent of recent drastic fluctuations in the global climate necessitates the development of plants better adapted to abiotic stresses. The introgression of genes related to osmoprotectant biosynthesis from one plant to another by genetic engineering is a unique strategy bypassing laborious conventional and classical breeding programs. Herein, we review recent literature related to osmoprotectants and transgenic plants engineered with specific osmoprotectant properties.

Osmoprotection in plants under abiotic stresses: new insights into a classical phenomenon

Trehalose is a natural non-reducing sugar that is found in the vast majority of organisms such as bacteria, yeasts, invertebrates and even in plants. Regarding its features, it is considered as a unique compound. It plays a key role as a carbon source in lower organisms and as an osmoprotectant or a stabilizing molecule in higher animals and plants. Although in plants it is present in a minor quantity, its levels rise upon exposure to abiotic stresses. Trehalose is believed to play a protective role against different abiotic stressful cues such as temperature extremes, salinity, desiccation. Moreover, it regulates water use efficiency and stomatal movement in most plants. Detectable endogenous trehalose levels are vital for sustaining growth under stressful cues. Exogenously applied trehalose in low amounts mitigates physiological and biochemical disorders induced by various abiotic stresses, delays leaf abscission and stimulates flowering in crops. External application of trehalose also up-regulates the stress responsive genes in plants exposed to environmental cues. The genetically modified plants with trehalose biosynthesis genes exhibit improved tolerance against stressful conditions. An increased level of trehalose has been observed in transgenic plants over-expressing genes of microbial trehalose biosynthesis. However, these transgenic plants display enhanced tolerance to heat, cold, salinity, and drought tolerance. Due to multiple bio-functions of this sugar, it has gained considerable ground in various fields. However, exogenous use of this bio-safe sugar would only be possible under field conditions upon adopting strategies of low-cost production of trehalose. In short, trehalose is a unique chemical that preserves vitality of plant life under harsh ecological conditions. Certainly, the new findings of this disaccharide will revolutionize a wide array of new avenues.

Trehalose: A Key Organic Osmolyte Effectively Involved in Plant Abiotic Stress Tolerance

Osmoprotective functions conferred to soybean plants via inoculation with Sphingomonas sp. LK11 and exogenous trehalose.

Hydrogen sulfide (H2S) is a novel type signaling molecule in plants Seed germination is a key stage of life cycle of plants, which is vulnerable to environmental stress including high temperature However, under high temperature stress, whether pre-soaking of maize seeds with NaHS (a H2S donor) could improve seed germination and seedling growth and the possible mechanisms are not completely clear In this study, maize seeds pre-soaked with NaHS enhanced germination percentage, sprout length, root length, and fresh weight compared with the control without NaHS treatment, illustrating that H2S could improve maize seed germination and seedling growth under high temperature In addition, in comparison to the control, NaHS pre-soaking stimulated antioxidant enzymes [ascorbate peroxidase (APX), glutathione reductase (GR), guaiacol peroxidase (GPX), superoxide dismutase (SOD), and catalase (CAT)] activities and the contents of water soluble non-enzymatic antioxidants [ascorbic acid (AsA) and glutathione (GSH)], as well as the ratio of reduced antioxidant to oxidized antioxidant Moreover, pre-soaking with NaHS activated Δ1-pyrroline-5-carboxylate synthetase (P5CS) and ornithine aminotransferase [OAT; both are rate-limiting enzymes in proline (Pro) synthesis], betaine aldehyde dehydrogenase [BADH; a key enzyme in glycine betaine (GB)], and trehalose (Tre)-6-phosphate phosphatase (a key step in Tre synthesis), which in turn accumulated Pro, GB, and Tre in germinating seeds compared with the control Also, an improved germination by NaHS under high temperature was reinforced by the above osmotic adjustment substances (osmolytes) alone, while deteriorated by the inhibitors of osmolyte biosynthesis [gabaculine (GAB), disulfiram (DSF), and sodium citrate (SC)] These results imply that H2S could improve maize seed germination and seedling growth under high temperature by inducing antioxidant system and osmolyte biosynthesis

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Signaling Molecule Hydrogen Sulfide Improves Seed Germination and Seedling Growth of Maize (Zea mays L.) Under High Temperature by Inducing Antioxidant System and Osmolyte Biosynthesis.

Sugar beet is among the most salt-tolerant crops. This study aimed to investigate the metabolic adaptation of sugar beet to salt stress at the cellular and subcellular levels. Seedlings were grown hydroponically and subjected to stepwise increases in salt stress up to 300 mM NaCl. Highly enriched fractions of chloroplasts were obtained by non-aqueous fractionation using organic solvents. Total leaf metabolites and metabolites in chloroplasts were profiled at 3 h and 14 d after reaching the maximum salinity stress of 300 mM NaCl. Metabolite profiling by gas chromatography-mass spectrometry (GC-MS) resulted in the identification of a total of 83 metabolites in leaves and chloroplasts under control and stress conditions. There was a lower abundance of Calvin cycle metabolites under salinity whereas there was a higher abundance of oxidative pentose phosphate cycle metabolites such as 6-phosphogluconate. Accumulation of ribose-5-phosphate and ribulose-5-phosphate coincided with limitation of carbon fixation by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Increases in glycolate and serine levels indicated that photorespiratory metabolism was stimulated in salt-stressed sugar beet. Compatible solutes such as proline, mannitol, and putrescine accumulated mostly outside the chloroplasts. Within the chloroplast, putrescine had the highest relative level and probably assisted in the acclimation of sugar beet to high salinity stress. The results provide new information on the contribution of chloroplasts and the extra-chloroplast space to salinity tolerance via metabolic adjustment in sugar beet.

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Metabolite profiling at the cellular and subcellular level reveals metabolites associated with salinity tolerance in sugar beet

Trehalose is a nonreducing disaccharide composed of two glucose residues bound by an α-α linkage. First reported in 1832 in rye following ergot infection, trehalose has since been detected in a wide range of organisms, including bacteria, fungi, invertebrates, and plants. It has role in plant response to various environmental stresses such as cold and salinity, and in regulation of stomatal conductance and water-use efficiency. Trehalose is a potential signal metabolite in plant interactions with pathogenic or symbiotic microorganisms and herbivorous insects. Recently it has been found that trehalose metabolism is also crucial for normal plant growth and development. In plants, as in other eukaryotes and many prokaryotes, trehalose is synthesized via a phosphorylated intermediate, trehalose 6-phosphate (T6P). In plants, trehalose 6-phosphate (T6P), the intermediate of trehalose biosynthesis, is thought to be a signal of sucrose status and current studies point towards the role of T6P as a regulatory molecule, especially in sugar influx and metabolism. A meta-analysis reveals that the levels of T6P changes are in parallel with sucrose, which is the major product of photosynthesis and the main transport sugar in plants. The intermediate T6P is now confirmed to act as a sensor for available sucrose, hereby directly influencing the type of response to the changing environmental conditions. This is possible because T6P and/or trehalose or their biosynthetic enzymes are part of complex interaction networks with other crucial hormone and sugar-induced signaling pathways, which may function at different developmental stages.

Trehalose: Metabolism and Role in Stress Signaling in Plants

Exogenously supplied BR and JA help KS101 and KBS3 genotypes of Brassica rapa to alleviate drought stress by modifying osmolyte concentration, levels of antioxidant enzymes and photosynthetic system. Oilseed plants are susceptible to drought stress and a significant loss in yield has been reported during recent decades. Thus, it is imperative to understand the various underlying drought response mechanisms in Brassica oilseed plants to formulate the sustainable strategies to protect the crop yield under water-limiting conditions. Phytohormones play a key role in fine-tuning various regulatory mechanisms for drought stress adaptation in plants, and the present study explores the response of several physiological stress markers by exogenous supplementation of 24-epibrassinolide (EBL) and jasmonic acid (JA) on two genotypes of Brassica rapa, KS101 and KBS3 under drought stress conditions. The exogenous application of BR and JA, separately or in combination, significantly alleviated the drought stress by improving photosynthetic rate, photosynthetic pigments, stomatal conductance, transpiration rate and antioxidant defence. We observed that concentration of different osmolytes increased and membrane damage significantly reduced by the application of BR and JA. The overall activity of antioxidant enzymes POD, CAT, GR, APX and CAT elevated during all the treatments, be it stress alone or in combination with BR and JA, compared to the control. However, we observed that the BR was much better in mitigating the drought stress compared to JA. Thus, the present study suggests that BR and JA supplementation improves the performance of B. rapa on exposure to drought stress, which hints at the critical role of BR and JA in improving crop productivity in drought-prone areas.

Exogenous brassinosteroid and jasmonic acid improve drought tolerance in Brassica rapa L. genotypes by modulating osmolytes, antioxidants and photosynthetic system

Crosstalk between Brassinosteroid Signaling, ROS signaling and Phenylpropanoid Pathway During Abiotic Stress in Plants: Does it exist?

The nomadic pastoral indigenous communities of the Ladakhi people share roots with Tibetan culture in terms of food, clothing, religion, festivals, and habits, and rely widely on plant resources for survival and livelihood. This survey was conducted during 2019-2021 to document the indigenous knowledge about plant resources of the Balti, Beda, and Brokpa communities of the Ladakh region, trans-Himalayas. Open- and close-ended semi-structured interviews (N = 184) and group discussions (N = 17) were used to collect the data. Quantitative data was further analyzed using various statistical tools. A total of 105 plant species belonging to 82 genera and 39 families were used as medicine, fuel wood, fragrance, oil, food, flavor, fodder, decoration, and dye. Among these, medicinal use was most prevalent, with 70% of use reports, followed by fodder and fuel wood. Leaves (27%) were the most preferred plant part used, followed by roots and flowers. The principal component analysis revealed five clusters of ethnobotanical usage, i.e., food, medicine, fuel wood, fodder, and fragrance, oil, dye, and flavor. The maximum number of plant species used was reported by the Brokpa, while the Beda reported the minimum number of plant species uses. Delphinium brunonianum, Waldheimia tomentosa, and Juniperus indica played a significant role in the cultural and religious ritual aspects, whereas Allium przewalskianum, Waldheimia tomentosa, Juniperus indica, and Hippophae rhamnoides were commonly used as a livelihood source among Ladakhi communities. The local people collected most plants (65%) for self-consumption, while the rest (35%) were sold in markets as a source of income. The sustainable utilization and management of plant resources by local people is a strategy to boost livelihoods and food security and alleviate poverty.

Plant Resources Utilization among Different Ethnic Groups of Ladakh in Trans-Himalayan Region

Riverine ecosystem highly prone to invasion thereby provides pathway for efficient dispersal of propagules between isolated protected areas. Since invasive species have the potential to lead the loss of species diversity and ecosystems function at regional scale, it increases demands on the natural capitol being conserved. In the current study, we used the Floristic Quality Assessment Index (FQAI) which is a vegetation-based index having the property to assess the ecological health and coefficient of conservatism values for each species along Dagwan Stream in the Dachigam National Park of Kashmir Himalayas. The values of the Floristic Quality Assessment Index for the riparian zone are FQAI = 41.42. The results revealed that 33% species were non-native, 9% ranking 1–3; 32% ranking 4–6; 21% ranking 7–8; and 5% ranking 9–10. Among all (78 species) recorded, 67% were native and 33% were alien. Out of total alien species documented, six (6) invasive, seventeen (17) naturalized and three (3) casual species. Most invasive tree species growing were Quercus robur, Robinia pseudoacacia, Ailanthus altissima, shrubs like Rubus ulmifolius, Buddleja davidii and herbs such as Leucanthemum vulgare, Digitalis purpurea, Erigeron canadensis. The results of the present study provide a method to measure the response of vegetation communities to alien invasions and subsequently draft management strategies to protect the different ecosystems, particularly the protected areas from this biodiversity threat.

Umer Yaqoob

Papers

Trehalose: Metabolism and Role in Stress Signaling in Plants

Exogenous brassinosteroid and jasmonic acid improve drought tolerance in Brassica rapa L. genotypes by modulating osmolytes, antioxidants and photosynthetic system

Crosstalk between Brassinosteroid Signaling, ROS signaling and Phenylpropanoid Pathway During Abiotic Stress in Plants: Does it exist?

Plant Resources Utilization among Different Ethnic Groups of Ladakh in Trans-Himalayan Region

Floristic Quality Assessment Index of the Dagwan Stream in Dachigam National Park of Kashmir Himalaya