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Exogenous Salicylic Acid Ameliorates Short-term Drought Stress in Mustard (brassica Juncea L.) Seedlings by Up-regulating the Antioxidant Defense and Glyoxalase System

Md. Mahabub Alam
Laboratory of Plant Stress Responses, Department of Applied Biological Science, Faculty of Agriculture, Kagawa University, 2393 Ikenobe, Miki-cho, Kita-gun, Kagawa 761-0795, Japan

Mirza Hasanuzzaman
Department of Agronomy, Faculty of Agriculture, Sher-e-Bangla Agricultural University, Sher-e-Bangla Nagar, Dhaka-1207, Bangladesh

Kamrun Nahar
Department of Agricultural Botany, Faculty of Agriculture, Sher-e-Bangla Agricultural University, Sher-e-Bangla Nagar, Dhaka-1207, Bangladesh

Masayuki Fujita*
Laboratory of Plant Stress Responses, Department of Applied Biological Science, Faculty of Agriculture, Kagawa University, 2393 Ikenobe, Miki-cho, Kita-gun, Kagawa 761-0795, Japan

Abstract/ Executive Summary:

In this study, the protective role of salicylic acid (SA) in relation to the water status, chlorophyll content, antioxidant defense and glyoxalase system was investigated in drought stressed mustard (Brassica juncea L. cv. BARI Sharisha 11) seedlings. Two sets of 10-d-old seedlings were subjected to two different levels of drought (10% and 20% PEG, 48h), where one set of seedlings was supplemented with 50 μM SA. The relative water content (RWC), Chl b and Chl (a+b) decreased at any level of drought, while Chl a content decreased only at severe drought (20% PEG). Drought stress caused a sharp increase in proline (Pro) content. A sharp increase in malondialdehyde (MDA) and H₂O₂ was observed in both levels of stresses. Decline in AsA content and increase in GSH and GSSG content was observed at both levels of drought. Compared to control the activities of catalase (CAT) and monodehydroascorbate reductase (MDHAR) did not change due to drought stress. The activity of glutathione reductase (GR) slightly increased only at 10% PEG, while ascorbate peroxidase (APX) and glutathione S-transferase (GST) activity increased at any level of stress. The activities of glutathione peroxidase (GPX) and glyoxalase II (Gly II) decreased only at severe stress (20% PEG), while dehydroascorbate reductase (DHAR) and glyoxalase I (Gly I) activities decreased at any level of stress. Spraying with SA alone had little influence on the non-enzymatic antioxidants and the activities of antioxidant enzymes. However, supplementation of SA in drought stressed seedlings increased the RWC and Chl content, increased the AsA and GSH, decreased the GSSG content and maintained a higher ration of GSH/GSSG. Salicylic acid supplemented drought stressed seedlings also enhanced the activities of MDHAR, DHAR, GR, GPX, CAT, Gly I, and Gly II as compared to the drought-stressed plants without SA supplementation, with a concomitant decrease in H₂O₂ , and lipid peroxidation level. These results suggest that the exogenous application of SA assisted the plants to become more tolerant to drought stress-induced oxidative damage by enhancing their antioxidant defense and glyoxalase systems.

Keywords: Abiotic stress tolerance; AsA-GSH cycle; Methylglyoxal; Oxidative stress; Polyethylene glycol.

Location:

Start Date:

End Date:

Program Area: Risk Management in Agriculture

Commodity: Mustard, Drought

Objectives:

The objective of our study was to observe the beneficial role of SA in enhancing the activities of antioxidant enzymes and glyoxalase pathway enzymes as well as relative water content (RWC), chlorophyll (Chl) and proline (Pro) content under drought stress condition in mustard seedlings.

Methodology:

Relative water content Leaf relative water content (RWC) of mustard seedlings was decreased significantly upon exposure to drought stress. In the media containing 10% and 20% of PEG, RWC decreased by 46% and 53%, respectively compared to control. Non-stressed seedlings which were sprayed with SA did not show any differences in RWC compared to control. The drought-stressed seedlings sprayed with SA showed 40 and 50% increase in RWC at 10 and 20% of PEG, respectively compared to the drought imposed seedlings which were not sprayed with SA. Chlorophyll and proline content Chlorophyll a content of the leaves did not changed under 10% PEG, however, it decreased significantly under 20% PEG (39% lower than control). Salicylic acid treated control seedlings also showed decrease in Chl a content. Compared to drought stress alone, the seedlings exposed to 20% PEG supplemented with SA spray showed 31% higher Chl a content. The Chl b content also significantly decreased under any level of drought stress (47 and 59% lower at 10 and 20% of PEG, respectively). However, the seedlings sprayed with SA showed significantly higher amount of Chl b content (79% higher) compared to stress (20% PEG) alone. Consequently, Chl (a+b) content also markedly decreased under drought stress which was measured as 30 and 48% lower at 10 and 20% PEG, respectively compared to control. However, the drought stress seedlings when supplemented with SA, showed significantly higher amount of Chl (a+b) compared to stress alone. Drought stress caused a profound increase in Pro content in mustard seedlings. Upon exposure to 10 and 20% PEG, Pro contents increased by 501 and 896%, respectively compared to control. On the other hand, SA supplemented drought stressed (20% PEG only) seedlings maintained the Pro content significantly lower than the seedlings treated with PEG only. Levels of MDA and H₂O₂ A sharp increase in MDA content (the product of lipid peroxidation) was observed under drought stress. The seedlings sprayed with 10 and 20% PEG caused 58 and 179% increase in MDA content compared to control. On the other hand, SA supplemented drought-stressed seedlings showed significantly lower MDA content compared to drought exposed seedlings without SA (22 and 32% lower compared to the seedlings exposed to 10 and 20% PEG only). Compared to control, the levels of H₂O₂ increased by 41 and 95% with 10 and 20% PEG, respectively. However, SA sprayed seedlings maintained same level of H₂O₂ as control. Importantly, a significant reduction of H₂O₂ was observed in SA supplemented drought-stressed seedlings compared to the seedlings expose to drought only. Contents of ascorbate and glutathione A significantly decline in AsA content was observed in drought stressed mustard seedlings (14 and 34% lower at 10 and 20% PEG, respectively) compared to untreated control (Fig. 4A). The SA supplemented drought-stressed seedlings significantly restored the AsA content (17 and 28% at 10 and 20% PEG, respectively) compared to the drought stressed seedlings without SA. Compared to control GSH contents were increased by 32 and 25% under 10 and 20% PEG, respectively. Moreover, SA supplemented drought-stressed seedling showed further increase in GSH content compared to drought stress alone and control as well. GSSG content markedly increase with increasing levels of drought and it was observed that compared to control the GSSG contents were 48 and 101% higher at 10 and 20% PEG, respectively. In contrary, the seedlings which were sprayed with SA showed lower GSSG levels similar to control. GSH/GSSG ratio showed no change under mild stress (10% PEG), while it significantly decreased (34% lower than control) upon exposure to severe drought stress (20% PEG). However, SA supplemented drought stressed seedlings showed significantly improved GSH/GSSG ratio (68 and 90% higher with 10 and 20% PEG) compared to drought stress alone.

Source of Information: AJCS 7(7):1053-1063 (2013)

Article Link (Online Journal):

Funding Source:

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Total Budget (Tk.) :

Achievement/Findings:

Based on the above results, together with results found in the available literature, we therefore conclude that exogenous SA spray is an effective way to improve short-term drought tolerance, which could be partially attributed to the increase in non-enzymatic and enzymatic antioxidants as well as the activities of glyoxalase enzymes. Although a number of reports indicated the protective role of SA under abiotic stress conditions, to date the research conducted on the physiological roles of SA under stressful conditions is scarce. Therefore, further studies are required to elucidate the molecular mechanism and signaling pathways underlying the role of SA in the stress tolerance of plants. Complete elucidation of the physiological roles of SA with its detailed protective mechanisms would be helpful for developing stress tolerance in plants.

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