The Role of Hydrogen Sulfide (H2S) in Drought Tolerance in Wheat

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The Role of Hydrogen Sulfide (H2S) in Drought Tolerance in Wheat, Triticum aestivum L, and Spinach, Spinacia oleracea.
Introduction
Land plants such as wheat, Triticum aestivum L, and spinach, Spinacia oleracea are increasingly subjected to several abiotic stress factors, including heat, drought, and excessive salinity. Among the well-studied signaling molecules, which act as priming agents include hydrogen sulfide (H2S), and nitric oxide (NO) which regulate the response of plants during stress conditions. Different chemical donors provide H2S and NO separately. However, NOSH is a novel chemical donor of both H2S and NO to land plants while NOSH-aspirin is critical in providing the molecule acetylsalicylic acid that is used for pharmaceutical purposes (Antoniou et al., 1). Cell signaling, which is triggered by hydrogen sulfide and NO activates several biochemical processes that lead to plant tolerance to biotic and abiotic stresses (Da-Silva & Modolo 150). This current paper aims at investigating the role of hydrogen sulfide (H2S) in drought-stressed Triticum aestivum L, and Spinacia oleracea. The paper proposes the novel role of the hydrogen sulfide molecule as an efficient plant priming agent against drought through the synchronized regulation of several defense mechanisms, thus providing an insight into the field of chemical priming research in the use of target-specific chemicals for enhancing stress tolerance in land plants such as wheat and spinach.
H2S and Drought Resistance in Wheat
Different land plants are increasingly subjected to a range of abiotic stresses, including drought, heavy metal toxicity, salinity, and stresses of high temperatures at various stages of their life cycle beginning from germination, seedling, and reproduction. As a way to protect themselves from different stresses, plants often release different signaling molecules, which initiates a cascade of events for stress-signaling, thus resulting in either plant acclimation or programmed cell death. The widely recognized gasotransmitter molecules play a critical role in gene expression regulation, cross-talking with other hormones, and posttranslational modification (PTM) and hydrogen sulfide (H2S) and NO (Paul & Roychoudhury 374). While the exact roles of H2S and NO in plants have for long remained unclear and that these molecules are species depended, several studies have found a positive link between the accumulation of the molecules and environmental stressors such as drought among land plants. These molecules also participate in various stress responses as well as acting antagonistically or synergistically as signaling chemicals, depending on their concentration (Paul & Roychoudhury 374). In general, according to da-Silva and Modolo (150), H2S triggers cell signaling that triggers several biochemical reactions which result in plant tolerance to both biotic and abiotic stressors.
There is little information available to describe the impact of exogenous H2S on the abscisic acid (ABA) pathway that results in the achievement of drought resistance in wheat plants. Ma et al (1) have investigated the physiological parameters, the ABA and H2S contents, and the transcription levels of different genes that participate in the ABA pathway in wheat leaves and roots under drought stresses in response to the treatment with sodium hydrogen sulfide (NaHS). Results from Ma et al (1) study show that NaHS treatment potentially increases the plant height and relative water content (RWC) of leaves and seedlings in wheat experiencing drought. Apart from drought resistance, H2S has been shown to increase antioxidant enzyme activities and reduces malondialdehyde (MDA) and H2O2 contents in the roots and leaves of wheat. The treatment with NaHS also increases the expression levels of ABA biosynthesis as well as ABA reactivation genes in leaves and the upregulation of ABA catabolism genes and the expression levels of ABA biosynthesis in roots. Results obtained from Ma et al (1) study show that ABA plays a role in drought resistance, which is as a result of the exogenous H2S, and that there are varying levels of drought resistance in roots and leaves. Besides, the levels of transcription genes that encode ABA receptors are also upregulated with NaHS pretreatment during drought conditions in roots and leaves. At the same time, the H2S content in roots and leaves is increased with NaHS pretreatment which the ABA contents in roots and leaves are decreased. According to Ma et al(1), this means that there is sophisticated crosstalk between the two signaling chemicals and that drought resistance through H2S involves the ABA signaling pathway.
Method to Study the Role of H2S in Wheat
Ma et al (3) used the common wheat (Tricum aestivul L.) cultivar “Yumai49-198 in their experiment. The investigators used NaHS, Sigma as an H2S donor, and 70 percent alcohol was used to surface sterilize the wheat seeds for 5 minutes. The seeds were then spayed with 0.1 percent HgCl for 15 minutes and then washed six times using distilled water. Next, the seeds were germinated in Petri dishes that were set in a temperature-regulated setting, and the germinated seeds were transferred to a temperature-regulated setting with a 16hr/8hr light and dark cycle with 250 μmol m−2s −1), 25/15°C (light/dark), and 60/75% relative humidity. The seedlings were then watered daily with the suitable volumes of Hoagland’s solution until the leafing phase. The researchers selected the 500 μM as the treatment concentration for their experiment and at the leafing stage, wheat seedlings were divided into three categories: the well-water control (CK) group, the PEG treatment (PEG), and the combination of PEF with NaHS pretreatment (NaHS + PEG) groups. The concentration of H2S was measured by absorbance at 412 nm, which were expressed as μmol/g fresh weight (FW) and the measurement of ABA was quantitatively done through an enzyme-linked immune-sorbent assay (ELISA). ABA concentration was expressed as ng/g FW and the samples to measure leaf RWC were weighed immediately as FW then cut into 6 cm sections before immersing them in distilled water for 24 hours at 40oC in darkness. The investigators then removed the leaves from the water and the surface water was then blotted off the wheat leaves and recorded the turgid weights (TW). The samples were then dried in an oven at about 70oC to constant weight and recorded the dry weight (DW) of each seedling. The leaf RWC was then calculated through the formula below:
RWC%=FW-DWTW-DWX100
Fig 1 Formular of calculating leaf RWC (Ma et al 3)
H2S and Drought Resistance in Spinach
The biochemical and physiological regulation of H2S in plants is an important role that plant scientists have acknowledged. Chen et al (1) have examined the role of H2S in the regulation response to drought in spinach, Spinacia oleracea seedlings. In their study, Chen et al (1) have shown that drought stress significantly decreases the RWC of leaves, the effectiveness of photosystem II(PSII), and photosynthesis in general. Besides, drought causes the accumulation of reactive oxygen species (ROS) and increased the levels of MDA. Nevertheless, the treatment with NaHS counteracts the drought-induced variations in the parameters. Secondly, the treatment with NaHS increases the osmotic potential and water content in the leaves of spinach. Besides, osmoprotectants like glycinebetaine (B) and proline content are altered following the application of NaHS under drought, thus suggesting that the ...