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Physiological mechanisms conferring ameliorative effects of nitric oxide and cytokinin on salinity stress tolerance in pea and barley

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Sidana, S ORCID: 0000-0002-0650-5398 2017 , 'Physiological mechanisms conferring ameliorative effects of nitric oxide and cytokinin on salinity stress tolerance in pea and barley', PhD thesis, University of Tasmania.

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Abstract

In the current time, soil salinity is posing significant constraints in satisfying the needs of the regularly growing population by lessening the agricultural yield. Salinity is affecting nearly 7% of the world’s land area leading to higher level economic loss due to decline in crop yield across the world. The approaching situation of food shortfall puts extreme pressure on devising new techniques to make agriculture productive in saline areas. One of the methods to counteract this problem is by developing salt tolerant plants either by genetic manipulation or by selective breeding. However, exogenous application of various organic products including phytohormones and inorganic chemicals to plants has shown the ameliorating effect on growth and productivity under different types of stress. Nitric oxide (NO) is one of the most important signalling molecules investigated in the last two decades, which played a significant role in plant growth under biotic and abiotic stress. Application of NO donor -sodium nitroprusside (SNP) significantly raised Na\(^+\) efflux and compartmentalization in vacuoles maintaining intracellular ionic homeostasis in the leaves, whereas use of NO scavenger reversed the effects of SNP on the Na\(^+\) / K\(^+\) ratio. Taking into account, previous studies reporting the positive influence of NO on salt stressed plants, a comprehensive study of monitoring effect of NO foliar spray on the glasshouse grown barley and pea was undertaken. Pea (cv Onslow) and barley (cv Gairdner and CM 72) differed in their tolerance to salinity. After optimization, NO concentration (100 μM) was applied as a foliar spray on the plants of barley and pea. Results showed that NO application was unaffected in improving pea and barley growth under salt stress. It's reflected in the growth, CO\(_2\) assimilation, chlorophyll content and photochemical efficiency of these plants under salt stress. The exogenous application of NO is mainly challenged by its 1) photo-instability 2) uptake 3) dose-time dependency factors.
Plant hormones have also been reported to elicit an adaptive response in plants under stress. The application of cytokinin (CYT) with different concentrations (20, 50 μM) has been investigated to see its effect on the salinity tolerance in this study. The amelioration effect of CYT was found to be dose dependent. The pre-treated pea plants with CYT were able to maintain high K\(^+\)/ Na\(^+\) ratio which is vital for survival under salinity. CYT pre-treatment enhanced H\(^+\)-ATPase activity under salt stress which helped in reducing membrane depolarisation and reduced the K\(^+\) efflux via depolarization-activated potassium outward rectifying (KOR) channels. Na\(^+\) exclusion ability of pea mesophyll increased significantly (P<0.05) in CYT (20 μM) pre-treated samples compared with that of non-treated inducing SOS1 in pea mesophyll leading to the extrusion of Na\(^+\) from the cytosol. The management of oxidative stress by CYT was investigated by using non-invasive microelectrode ion flux measuring (MIFE) system. The results indicated the positive influence of CYT on the biomass yield, photosynthetic mechanism; ions flux response (P<0.05) though the results of mitigation of oxidative stress were not consistent. The experiments with optimized dose of CYT (5 μM) were repeated with barley (Gairdner) roots. CYT was able to elicit positive effect on root biomass and maintaining high K\(^+\) Na\(^+\) ratio by preventing NaCl-induced K\(^+\) loss from the roots and Na\(^+\) accumulation. CYT treatment enhanced the SOS1-like activity and shifted the membrane potential of salt-treated root cells to more negative under saline conditions. CYT makes the plant salt tolerant by retaining K\(^+\) which can be facilitated by better control of GORK (activated by depolarization) and good control of ROS-activated channels (such as NSCC). Pharmacological experiments showed that TEA\(^+\) (voltage-gated K\(^+\) channel blocker) effect was major while Gd\(^{3+}\) (NSCC blocker) effect was minor. CYT improves salinity stress tolerance via two concurrent mechanisms: (1) enhanced Na\(^+\) efflux capacity originating form activation of SOS1-like Na\(^+\)/H\(^+\) exchangers and (2) improved K\(^+\) retention resulting from CYT activation of H\(^+\)- ATPase and better voltage control, preventing GORK-mediated stress-induced K\(^+\) loss. There is also a potential evidence for CYT being able to desensitize GORK responses to ROS.

Item Type: Thesis - PhD
Authors/Creators:Sidana, S
Keywords: nitric oxide, cytokinin, salinity, pea, barley
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Copyright 2017 the author

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