A potential seedling-stage evaluation method for heat tolerance in Indian mustard (Brassica juncea L. Czern and Coss)

Melatonin Reverses High-Temperature-Stress-Inhibited Photosynthesis in the Presence of Excess Sulfur by Modulating Ethylene Sensitivity in Mustard

Melatonin is a pleiotropic, nontoxic, regulatory biomolecule with various functions in abiotic stress tolerance. It reverses the adverse effect of heat stress on photosynthesis in plants and helps with sulfur (S) assimilation. Our research objective aimed to find the influence of melatonin, along with excess sulfur (2 mM SO42-), in reversing heat stress's impacts on the photosynthetic ability of the mustard (Brassica juncea L.) cultivar SS2, a cultivar with low ATP-sulfurylase activity and a low sulfate transport index (STI). Further, we aimed to substantiate that the effect was a result of ethylene modulation. Melatonin in the presence of excess-S (S) increased S-assimilation and the STI by increasing the ATP-sulfurylase (ATP-S) and serine acetyltransferase (SAT) activity of SS2, and it enhanced the content of cysteine (Cys) and methionine (Met). Under heat stress, melatonin increased S-assimilation and diverted Cys towards the synthesis of more reduced glutathione (GSH), utilizing excess-S at the expense of less methionine and ethylene and resulting in plants' reduced sensitivity to stress ethylene. The treatment with melatonin plus excess-S increased antioxidant enzyme activity, photosynthetic-S use efficiency (p-SUE), Rubisco activity, photosynthesis, and growth under heat stress. Further, plants receiving melatonin and excess-S in the presence of norbornadiene (NBD; an ethylene action inhibitor) under heat stress showed an inhibited STI and lower photosynthesis and growth. This suggested that ethylene was involved in the melatonin-mediated heat stress reversal effects on photosynthesis in plants. The interaction mechanism between melatonin and ethylene is still elusive. This study provides avenues to explore the melatonin-ethylene-S interaction for heat stress tolerance in plants.

Indexing Resilience to Heat and Drought Stress in the Wild Relatives of Rapeseed-Mustard

Wild species are weedy relatives and progenitors of cultivated crops, usually maintained in their centres of origin. They are rich sources of diversity as they possess many agriculturally important traits. In this study, we analysed 25 wild species and 5 U triangle species of Brassica for their potential tolerance against heat and drought stress during germination and in order to examine the early seedling stage. We identified the germplasms based on the mean membership function value (MFV), which was calculated from the tolerance index of shoot length, root length, and biochemical analysis. The study revealed that B. napus (GSC-6) could withstand high temperatures and drought. Other genotypes that were tolerant to the impact of heat stress were B. tournefortii (RBT 2002), D. gomez-campoi, B. tournefortii (Rawa), L. sativum, and B. carinata (PC-6). C. sativa resisted drought but did not perform well when subjected to high temperatures. Tolerance to drought was observed in B. fruticulosa (Spain), B. tournefortii (RBT 2003), C. bursa-pastoris (late), D. muralis, C. abyssinica (EC694145), C. abyssinica (EC400058) and B. juncea (Pusa Jaikisan). This investigation contributes to germplasm characterization and the identification of the potential source of abiotic stress tolerance in the Brassica breeding programme. These identified genotypes can be potential sources for transferring the gene(s)/genomic regions that determine tolerance to the elite cultivars.

Effect of Heat Stress on Some Physiological and Anatomical Characteristics of Rice (Oryza sativa L.) cv. KDML105 Callus and Seedling

Simple Summary

Climate change is currently threatening agriculture all around the world, resulting in a lack of water and restricting the growth of plants, especially rice. Rice production decreases with the increase in temperature. An improvement in fundamental knowledge is necessary to comprehend plant adaptation mechanisms as responses to heat stress. Physiological and anatomical responses of Khao Dawk Mali 105 (KDML105) rice to artificial heat stress were studied. Our findings offer useful data for projects aimed at improving heat stress tolerance in rice to enhance long-term global food security.

Abstract

Global warming is a serious problem, with significant negative impacts on agricultural productivity. To better understand plant anatomical adaptation mechanisms as responses to heat stress, improved basic knowledge is required. This research studied the physiological and anatomical responses of Khao Dawk Mali 105 (KDML105) to artificial heat stress. Dehusked seeds were sterilized and cultured on Murashige and Skoog (MS) medium, supplemented with 3 mg/L 2,4-Dichlorophenoxyacetic acid (2,4-D) for callus induction. The cultures were maintained at 25 °C and 35 °C for 4 weeks, while the other culture was treated with heat shock at 42 °C for 1 week before further incubation at 25 °C for 3 weeks. Results revealed that elevated temperatures (35 °C and 42 °C) adversely impacted seedling growth. Plant height, root length, leaf number per plant, fresh and dry weight, chlorophyll a, chlorophyll b and total chlorophyll content decreased after heat stress treatment, while malondialdehyde (MDA) and electrolyte leakage percentage significantly increased, compared to the control. Heat stress induced ROS accumulation, leading to lipid peroxidation and membrane instability. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) results also confirmed negative correlations between MDA, electrolyte leakage and other parameters. MDA content and electrolyte leakage are effective indicators of heat stress in rice. Surface anatomical responses of rice seedlings to heat stress were studied but significant alterations were not observed, and heat stress had no significant negative effects on KDML105 calli. Size and mass of calli increased because heat stress stimulated gene expression that induced thermotolerance. Our results provide useful information for rice breeding and heat stress tolerance programs to benefit long-term global food security.

Alterations of Oxidative Stress Indicators, Antioxidant Enzymes, Soluble Sugars, and Amino Acids in Mustard [Brassica juncea (L.) Czern and Coss.] in Response to Varying Sowing Time, and Field Temperature

The impact of elevated temperature at the reproductive stage of a crop is one of the critical limitations that influence crop growth and productivity globally. This study was aimed to reveal how sowing time and changing field temperature influence on the regulation of oxidative stress indicators, antioxidant enzymes activity, soluble sugars (SS), and amino acids (AA) in Indian Mustard. The current study was carried out during the rabi 2017-2018 and 2018-2019 where, five varieties of mustard viz. Pusa Mustard 25 (PM-25) (V1), PM-26 (V2), BPR-541-4 (V3), RH-406 (V4), and Urvashi (V5) were grown under the field conditions on October 30 (normal sowing; S1), November 18 (late sowing; S2) and November 30 (very late sowing; S3) situations. The S1 and S3 plants, at mid-flowering stage, showed a significant variation in accumulation of SS (8.5 and 17.3%), free AA (235.4 and 224.6%), and proline content (118.1 and 133%), respectively, and played a crucial role in the osmotic adjustment under stress. The results showed that S3 sowing, exhibited a significant induction of the hydrogen peroxide (H₂O₂) (110.2 and 86.6%) and malondialdehyde (23.5 and 47.5%) concentrations, respectively, which indicated the sign of oxidative stress in plants. Interestingly, the polyphenol oxidase, peroxidase, superoxide dismutase, and catalase enzyme activities were also significantly increased in S3 plants compared to S1 plants, indicating their significant roles in ameliorating the oxidative stress. Furthermore, the concentration of fatty acid levels such as palmitic, stearic, oleic, and linoleic acids level also significantly increased in S3 plants, which influenced the seed and oil quality. The study suggests that the late sowing significantly impaired the biochemical mechanisms in Indian mustard. Further, the mustard variety V4 (RH-406) was found to be effective for cultivation as well as environmental stress adoption in Indian soils, and it could be highly useful in breeding for developing heat-tolerant genotypes for ensuring the food security.

Molecular insights into sensing, regulation and improving of heat tolerance in plants

Climate-change-mediated increase in temperature extremes has become a threat to plant productivity. Heat stress-induced changes in growth pattern, sensitivity to pests, plant phonologies, flowering, shrinkage of maturity period, grain filling, and increased senescence result in significant yield losses. Heat stress triggers multitude of cellular, physiological and molecular responses in plants beginning from the early sensing followed by signal transduction, osmolyte synthesis, antioxidant defense, and heat stress-associated gene expression. Several genes and metabolites involved in heat perception and in the adaptation response have been isolated and characterized in plants. Heat stress responses are also regulated by the heat stress transcription factors (HSFs), miRNAs and transcriptional factors which together form another layer of regulatory circuit. With the availability of functionally validated candidate genes, transgenic approaches have been applied for developing heat-tolerant transgenic maize, tobacco and sweet potato. In this review, we present an account of molecular mechanisms of heat tolerance and discuss the current developments in genetic manipulation for heat tolerant crops for future sustainable agriculture.

Triacontanol attenuates drought-induced oxidative stress in Brassica juncea L. by regulating lignification genes, calcium metabolism and the antioxidant system

Effect of triacontanol on drought-induced stress was studied in Brassica juncea L. Foliage of sixteen-days-old plants was sprayed with concentrations (0, 10, 20 and 30 μM) of triacontanol (TRIA) for 7 days. Subsequently, plants were subjected to drought stress (10% polyethylene glycol, PEG6000) for 7 days. Drought stress increased oxidative stress (TBARS, O₂^●- and H₂O₂), however, their contents were reduced by TRIA. Total soluble sugars, reduced glutathione, and proline content in stressed plants were increased by TRIA. Activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR), catalase (CAT), and phenylalanine ammonia-lyase (PAL) activity were significantly increased in a dose-dependent manner with TRIA. Potassium (K⁺) level declined, while magnesium (Mg²⁺) and calcium (Ca²⁺) contents increased. The elevated level of lignin under drought with TRIA was significantly associated with MYB46 and PAL gene expression patterns. Altogether, our results suggest that foliar spray of 20 μM TRIA was more operative in reducing the negative impact of drought stress in B. juncea by regulating the antioxidant system, calcium, and lignification.