Salinity Stress Responses and Adaptive Mechanisms in Major Glycophytic Crops: The Story So Far

Abstract

In many areas of the world, salinity is a major abiotic stress-limiting growth and productivity of plants due to increasing use of poor quality of water for irrigation and soil salinisation. Various physiological traits, metabolic pathways and molecular or gene networks are involved in plant adaptation or tolerance to salinity stress. This chapter deals with the adaptive mechanisms that plants can employ to cope with the challenge of salt stress and provide updated overview of salt-tolerant mechanisms in major glycophytic crops with a particular interest in rice (Oryza sativa), soybean (Glycine max), wheat (Triticum aestivum) and Arabidopsis plants. Salt stress usually inhibits seed germination, seedling growth and vigour, biomass accumulation, flowering and fruit set in major glycophytic crops. In addition, elevated Na + levels in agricultural lands are increasingly becoming a serious threat to the world agriculture. Plants suffer osmotic and ionic stress under high salinity due to the salts accumulated at the outside of roots and those accumulated at the inside of the plant cells, respectively. Salinity stress significantly reduces growth and productivity of glycophytes, which are the majority of agricultural products. Plants tolerant to NaCl implement a series of adaptations to acclimate to salinity, including morphological, physiological, biochemical and molecular changes regulating plant adaptation and tolerance to salinity stress. These changes affect plant growth and development at different levels of plant organisation, e.g. they may reduce photosynthetic carbon gain and leaf growth rate and increase in the root/canopy ratio and in the chlorophyll content in addition to changes in the leaf anatomy that ultimately lead to preventing leaf ion toxicity, thus maintaining the water status in order to limit water loss and protect the photosynthesis process. Finally, we also provide an updated discussion on salt-induced oxidative stress at the subcellular level and its effect on the antioxidant machinery in major glycophyte crops plants. In response to salinity stress, the productions of ROS, such as singlet oxygen, superoxide, hydroxyl radical and hydrogen peroxide, are enhanced, and overexpression of genes leading to increased amounts and activities of antioxidant enzymes like superoxide dismutase (SOD), catalase (CAT) and glutathione-S-transferase (GST)/glutathione peroxidase (GPX) increases the performance of plants under stress. The molecular mechanism of stress tolerance is complex and requires information at the miRNA/omics level to understand it effectively. During abiotic stress conditions, the advancement of “omics” is providing a detailed fingerprint of proteins, transcripts or all metabolites upregulated or downregulated in plant cells. However, the regulatory mechanisms of these protein-coding genes are largely unknown; in this regard, the microRNAs (miRNAs) may prove extremely important in deciphering these gene regulatory mechanisms and the stress responses. Some miRNAs are functionally conserved across plant species and are regulated by salt stress. In major crops through transgenic technologies, miRNAs represent themselves as potent targets to engineer abiotic stress tolerance, due to the critical roles in post-transcriptional regulation of gene expression in response to salinity and resultant growth attenuation.