Climate change and abiotic stress factors are the key players in crop losses around the globe. Among the different stresses, extreme temperatures (heat and cold) disturb plant growth and development, reduces productivity, and in severe cases, leads to plant death. The plants have developed numerous strategies to safeguard themselves against the detrimental impact of temperature stress. Exposure to stress evokes the accumulation of various metabolites like sugars, sugar alcohols, organic acids, and amino acids. Plants accumulate the amino acid proline (Pro) in response to several abiotic stresses, including temperature stress. Proline abundance may result from de novo synthesis, hydrolysis of proteins, and reduced utilization or degradation. Pro also imparts stress tolerance by maintaining the osmotic balance (still controversial), cell turgidity, and indirectly modulating the metabolism of reactive oxygen species (ROS). Furthermore, the crosstalk of Pro with other osmoprotectants and signaling molecules like glycine betaine, abscisic acid, nitric oxide, hydrogen sulfide, soluble sugars, etc., helps in strengthening the protective mechanism under stressful environments. The development of temperature-smart cultivars can be achieved by manipulating the biosynthesis of Pro through genetic engineering. Therefore, this review presented an overview of plant responses to extreme temperatures and an outline of the Pro metabolism under extreme temperatures. The exogenous application of Pro as a "shield molecule" under extreme temperatures is also presented. Proline crosstalk and interaction with other molecules have also been discussed. Finally, the potential of genetic engineering of Pro-related genes has been explained to develop temperature-smart plants. In short, we conclude that exogenous application of Pro and genetic engineering of Pro genes promise ways forward for developing temperature-smart future crop plants. It is proposed various approaches can be adopted to increase Pro levels in "temperature-smart plants" to combat the impending crisis of climate change.

Assessment of proline function in higher plants under extreme temperatures.

Salinity stress is one of the significant abiotic stresses that influence critical metabolic processes in the plant. Salinity stress limits plant growth and development by adversely affecting various physiological and biochemical processes. Enhanced generation of reactive oxygen species (ROS) induced via salinity stress subsequently alters macromolecules such as lipids, proteins, and nucleic acids, and thus constrains crop productivity. Due to which, a decreasing trend in cultivable land and a rising world population raises a question of global food security. In response to salt stress signals, plants adapt defensive mechanisms by orchestrating the synthesis, signaling, and regulation of various osmolytes and phytohormones. Under salinity stress, osmolytes have been investigated to stabilize the osmotic differences between the surrounding of cells and cytosol. They also help in the regulation of protein folding to facilitate protein functioning and stress signaling. Phytohormones play critical roles in eliciting a salinity stress adaptation response in plants. These responses enable the plants to acclimatize to adverse soil conditions. Phytohormones and osmolytes are helpful in minimizing salinity stress-related detrimental effects on plants. These phytohormones modulate the level of osmolytes through alteration in the gene expression pattern of key biosynthetic enzymes and antioxidative enzymes along with their role as signaling molecules. Thus, it becomes vital to understand the roles of these phytohormones on osmolyte accumulation and regulation to conclude the adaptive roles played by plants to avoid salinity stress.

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Salt stress resilience in plants mediated through osmolyte accumulation and its crosstalk mechanism with phytohormones

Seed priming with methyl jasmonate mitigates copper and cadmium toxicity by modifying biochemical attributes and antioxidants in Cajanus cajan

Drought limits crop development and yields. bHLH (basic helix-loop-helix) transcription factors play critical roles in regulating the drought response in many plants, but their roles in this process in sweet potato are unknown. Here, we report that two bHLH proteins, IbbHLH118 and IbbHLH66, play opposite roles in the abscisic acid (ABA)-mediated drought response in sweet potato. ABA treatment repressed IbbHLH118 expression but induced IbbHLH66 expression in the drought-tolerant sweet potato line Xushu55-2. Overexpressing IbbHLH118 reduced drought tolerance, whereas overexpressing IbbHLH66 enhanced drought tolerance, in sweet potato. IbbHLH118 directly binds to the E-boxes in the promoters of ABA-insensitive 5 (IbABI5), ABA-responsive element binding factor 2 (IbABF2), and tonoplast intrinsic protein 1 (IbTIP1) to suppress their transcription. IbbHLH118 forms homodimers with itself or heterodimers with IbbHLH66. Both of the IbbHLHs interact with the ABA receptor IbPYL8. ABA accumulates under drought stress, promoting the formation of the IbPYL8-IbbHLH66-IbbHLH118 complex. This complex interferes with IbbHLH118's repression of ABA-responsive genes, thereby activating ABA responses and enhancing drought tolerance. These findings shed light on the role of the IbPYL8-IbbHLH66-IbbHLH118 complex in the ABA-dependent drought response of sweet potato and identify candidate genes for developing elite crop varieties with enhanced drought tolerance.

The IbPYL8-IbbHLH66-IbbHLH118 complex mediates the abscisic acid-dependent drought response in sweet potato.

As a vital osmoticum, proline has an important role in enhancing the tolerance of plants to environmental stress. It is unclear whether the application of exogenous proline can improve the tolerance of Brassica juncea to cadmium (Cd).This study investigated the effects of different concentrations of proline (20, 40, 60, 80, and 100 mg/L) under Cd stress at different times (0 d, 2 d, and 7 d) on the growth and physiology of B. juncea. Treatment with exogenous proline not only increased the content of proline in B. juncea but also alleviated Cd-induced seedling growth inhibition via the maintenance of higher photosynthetic pigment content and cell viability and a decrease in the content of Cd. Moreover, it increased the activities of antioxidant enzymes and the glutathione/glutathione disulfide ratio to reduce the accumulation of reactive oxygen species. Compared with other concentrations, 60 mg/L of exogenous proline was the most effective at mitigating Cd toxicity in B. juncea.Exogenous proline treatment enhanced the tolerance to Cd via a decrease in Cd accumulation and reestablishment of the redox homeostasis in B. juncea. 

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Exogenous proline mitigates toxic effects of cadmium via the decrease of cadmium accumulation and reestablishment of redox homeostasis in Brassica juncea

Abiotic stresses have a detrimental impact on plant growth and productivity and are a major threat to sustainable crop production in rapidly changing environments. Proline, an important amino acid, plays an important role in maintaining the metabolism and growth of plants under abiotic stress conditions. Many insights indicate a positive relationship between proline accumulation and tolerance of plants to various abiotic stresses. Because of its metal chelator properties, it acts as a molecular chaperone, an antioxidative defence molecule that scavenges reactive oxygen species (ROS), as well as having signalling behaviour to activate specific gene functions that are crucial for plant recovery from stresses. It also acts as an osmoprotectant, a potential source to acquire nitrogen as well as carbon, and plays a significant role in the flowering and development of plants. Overproduction of proline in plant cells contributes to maintaining cellular homeostasis, water uptake, osmotic adjustment and redox balance to restore the cell structures and mitigate oxidative damage. Many reports reveal that transgenic plants, particularly those overexpressing genes tailored for proline accumulation, exhibit better adaptation to abiotic stresses. Therefore, this review aims to provide a comprehensive update on proline biosynthesis and accumulation in plants and its putative regulatory roles in mediating plant defence against abiotic stresses. Additionally, the current and future directions in research concerning manipulation of proline to induce gene functions that appear promising in genetics and genomics approaches to improve plant adaptive responses under changing climate conditions are also highlighted.

M. N. Islam

Papers

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