Physiological, biochemical and molecular characterization of Zea mays proteins in response to salt stress

Abstract

Maize (Zea mays) is an important cereal crop positioned third world-wide after wheat and rice due to its multiple uses such as in human diets. However, it is highly sensitive to salt stress and widely cultivated under a range of soil types and climatic conditions. Salt stress is a major abiotic stress that severely affects the morphological, biochemical, physiological, and molecular processes of plants. Thus, dissecting the molecular pathways utilized by plants when exposed to salt stress is significant for ensuring food security in the world’s narrowing agricultural land and increasing population. To evaluate the effects of salt stress in plants, this study examined the morphological, physiological, biochemical and molecular responses of maize exposed to varying concentrations of NaCl (0 - 400 mM) for 14 days. Salt stress inhibited the general morphology of maize plants, which resulted in reduced plant height, wilted leaves, and reduction in leaf number. Various physiological parameters such as shoot fresh and dry weights, root and shoot lengths, relative water content and chlorophyll content were assessed. The results of the study indicated that salt stress negatively impacted the growth and development of maize. A reduction in the shoot fresh and dry weights, root and shoot lengths, relative water content, and chlorophyll content was observed in plants for all stressed treatments. A slight increase in the relative water content was noted at 300 mM salt stress treatment. Moreover, biochemical parameters such as the antioxidant enzyme activities (catalase, glutathione reductase and superoxide dismutase) were also evaluated, and the results revealed that salt stress inhibited the catalase and superoxide dismutase activities of maize compared to the control. Notably, glutathione reductase activity significantly increased under salt stress. Protein separation demonstrated similar banding patterns for both the control and salt-treated maize with an overexpression at 300 and 400 mM NaCl treatment suggesting upregulation/downregulation of various proteins. Liquid chromatography tandem mass-spectrometry proteomic analysis was used to identify the changes in salt-stressed responsive leaf proteins in maize, where high salt concentration treatments, decreased the number of differentially expressed proteins (DEPs) compared to the control. Gene ontology was used to determine the biological process, molecular function and cellular component of the differentially expressed

proteins. Majority of the differentially expressed leaf proteins were involved in metabolic processes, followed by those with stimulus response. Various molecular functions of the salt responsive proteins were involved in catalytic, transport and binding activities. For the cellular component category, proteins were only associated with cellular anatomy. The common proteins identified for all treatments, were mainly involved in 38.7% metabolic process; followed by 38.7% involved in cellular process; 16.1% localization and 6.5% response to stimulus. Molecular functions of the commonly identified proteins were classified into three categories: whereby 57.9% of the proteins were involved in catalytic activity, while 31.6% were involved in transporter activity and 10.5% in binding activity. In combination, the physiological, biochemical and proteome profiling findings have proposed for possible response pathways underlying salt stress tolerance in maize. Additionally, this study has demonstrated the negative effects of salt stress on maize while on the other hand it has also shown some tolerance/adaptation abilities at the physiological, biochemical and proteomic levels. The data obtained here can provide insight in understanding the molecular basis of salt tolerance differences in maize as an essential agricultural crop and other related cereals.