Abstract
Hydrogen gas (H2) and carbon monoxide (CO) are involved in plant growth and developmental processes and may induce plant tolerance to several stresses. However, the independent roles and interaction effect of H2 and CO in adventitious root development under drought conditions have still not received the needed research attention. We hypothesize that there exists crosstalk between H2 and CO during adventitious root development under drought stress. The results of our current study revealed that 50% (v/v) hydrogen-rich water (HRW), 500 μM Hemin (the CO donor) and 30% (w/v) CO aqueous solution apparently promoted the development of adventitious roots in cucumber explants (Cucumis Sativus L.) under drought stress. H2 and CO increased relative water content (RWC), leaf chlorophyll content (chlorophyll a, b, and a+b), and chlorophyll fluorescence parameters [photochemical efficiency of photosystem II (PSII), PSII actual photochemical efficiency and photochemical quench coefficient] under drought condition. When the CO scavenger hemoglobin (Hb) or zinc protoporphyrin IX (ZnPPIX) was added to HRW/CO aqueous solution, the positive effect of HRW/CO aqueous solution on RWC, leaf chlorophyll content, and chlorophyll fluorescence parameters were reversed. Additionally, superoxide dismutases, peroxidase, catalase, and ascorbate peroxidase was significantly increased in the explants treated with HRW and CO aqueous solution under drought stress, thus alleviating oxidative damage, as indicated by decreases in thiobarbituric acid reactive substances (TBARS), hydrogen peroxide (H2O2), and superoxide radical (O2-) levels. H2 and CO also improved the levels of water soluble carbohydrate, total soluble protein, and proline content. However, the above CO/H2-mediated effects were reversed by CO scavenger Hb or CO specific synthetic inhibitor ZnPPIX. Therefore, CO may be involved in H2-induced adventitious rooting under drought stress and alleviate oxidative damage by enhancing RWC, leaf chlorophyll content, chlorophyll fluorescence parameters, metabolic constituent content, activating anti-oxidant enzymes and reducing TBARS, O2-, and H2O2 levels.
Highlights
Drought stress which is among the important abiotic stresses that adversely affects plant growth and development has become a global problem (Farooq et al, 2009)
Explants pretreated with 30, 50, and 100% hydrogen-rich water (HRW) had a significant increase in root number compared with polyethylene glycol 6000 (PEG)-treated explants, indicating that PEG-induced drought stress was alleviated by HRW
Our results indicated that the applications of both low concentration of the carbon monoxide (CO) donor Hemin (10, 100, and 500 μM, w/v) and CO aqueous solution (10, 30, and 50%, v/v) promoted adventitious rooting in cucumber explants under drought stress
Summary
Drought stress which is among the important abiotic stresses that adversely affects plant growth and development has become a global problem (Farooq et al, 2009). Many studies have focused on understanding adventitious rooting response signaling pathways under drought stress. Adventitious root formation is a very complex process Enzymes such as peroxidase (POD), polyphenol oxidase (PPO), and indoleacetic acid oxidase (IAAO) are known to be intimately involved in the process of adventitious root formation (Smart et al, 2003). This process is affected by factors such as temperature, light conditions, water, and nutrient supply (Takahashi et al, 2003). The intricate signaling network that participates in adventitious root development remains unresolved
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