A comparative physiological, proteomic and biochemical analysis of sorghum seedlings under salt stress

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Moloi, Sellwane Jeannette
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University of the Free State
Soil salinity negatively affects plant growth and development, causing crop losses, worldwide. Understanding plant response mechanisms towards salt stress is thus an important step in ensuring food security. Sorghum [Sorghum bicolor (L.) Moench] is a drought and moderately salt tolerant crop and thus provides a potentially good model system for stress response studies. In this study, a comparative physiological, biochemical and proteomic analysis of two sorghum varieties was conducted under salt stress. SA 1441 is drought tolerant, while ICSB 338 is both drought and salt susceptible. The sorghum seedlings were grown in soil until the V3 growth stage before salt-stress treatment with 0-400 mM NaCl for seven days was applied. Physiological parameters such as relative water content, leaf water loss, stomatal conductance, chlorophyll content, shoot and root length and weight were measured following the salt-stress treatment. The results indicated a statistically significant decrease in the measured parameters with an increase in salt stress in ICSB 338, compared to SA 1441. Furthermore, the leaf relative water content of ICSB 338 was significantly lower than that of SA 1441. Proline and glycine betaine accumulation was also analysed in both sorghum varieties across all NaCl treatments. The results showed that proline played a major role in osmotic adjustment in both sorghum varieties when compared to glycine betaine. However, SA 1441 accumulated more proline levels at higher salt treatment levels than ICSB 338. Based on the physiological and biochemical results, the 100 mM NaCl treatment was selected for proteomic analysis. The isobaric tags for relative and absolute quantitation (iTRAQ) proteomic method was used to identify root and leaf salt-stress responsive proteins in both sorghum varieties. In the root proteome, 522 and 544 proteins were positively identified in SA 1441 and ICSB 338 sorghum varieties, respectively. From these root proteins, 26 (SA 1441) and 31 (ICSB 338) were responsive to 100 mM NaCl-induced salt stress. Most of the differentially expressed root proteins were involved in disease/defence functions in both sorghum varieties. In the leaf proteome, 829 (SA 1441) and 591 (ICSB 338) proteins were positively identified. From these leaf proteins, 75 (SA 1441) and 24 (ICSB 338) were responsive to salt-stress. Most of the differentially expressed leaf proteins were associated with disease/defence functions in SA 1441, followed by energy and metabolism in both sorghum varieties. Collectively, the physiological and proteomic results suggest that SA 1441, the drought tolerant variety was more protected against the salt stress better than ICSB 338, the drought and salt susceptible variety. Therefore, the results also highlight the differences in salt tolerance between the two sorghum varieties and possibly reinforce the cross-link between drought and salt-stress response mechanisms. The results of this study can be used as reference tools in studies focusing on the differences in salt stress tolerance between varieties.
Dissertation (M.Sc. (Plant Sciences))--University of the Free State (Qwaqwa Campus), 2018, Sorghum, Salinity stress, Physiological and growth parameters, Chlorophyll content, Proline, Glycine betaine, Proteomics, iTRAQ