Looking at Halophytic Adaptation to High Salinity Through Genomics Landscape

Abstract

Soil salinity is an important stress factor that limits plant growth and productivity. For a given plant species, it is critical to sense and respond to salt stimuli followed by activation of multitude of mechanisms for plants to survive. Halophytes, the wonders of saline soils, have demonstrated ability to withstand and reproduce in at least 200 mM NaCl concentration, which makes them an ideal system to study mechanism of salt adaptation for imparting salt tolerance in glycophytes. Halophytes and salt sensitive glycophytes adapt different defense strategies towards salinity stress. These responses in halophytes are modulated by a well orchestrated network of signaling pathways, including calcium signaling, reactive oxygen species and phytohormones. Moreover, constitutive expression of salt stress response related genes, which is only salt inducible in glycophytes, maintains salt tolerance traits in halophytes. The focus of this review is on the adaptive considerations of halophytes through the genomics approaches from the point of view of sensing and signaling components involved in mediating plant responses to salinity.

Keywords: Genomics; Halophytes; Hormonal regulation; Redox homeostasis; Salinity; miRNAs.

Fig. (1)

General overview of salt induced effects and plats response: Plants face osmotic and ionic stress under salt stress which enhances ROS production and subsequent oxidation of bio molecules, causing irreversible damage to plants. ROS also acts as stress indicator and its enhanced concentration activates downstream processes, like increased antioxidant activity, osmolytes which scavenges ROS and plants show normal growth. In ROS signaling, hormones and calcium play an important role. Some of the halophytes switch their carbon assimilation mode from C3 to C4 or CAM and vice versa.

Fig. (2)

Mechanism of salinity tolerance in halophytes: The excess entry of toxic ions disturbs osmotic balance of plants by causing excess ROS production and oxidative damage. Increased concentration of ions in soil imposes water stress which leads to stomatal closure and low CO₂ availability to photosynthetic machinery which reduces ETC. This decreases NADP/NADPH ratio and generates ROS in Chloroplast, mitochondria, peroxisomes and apoplastic space. In defense, halophytes increase enzymatic (SOD, CAT, APX, GR, etc.) and non enzymatic (Ascorbate, Glutathione) antioxidants and osmolytes. Also, enhanced expression of salt responsive genes and transcription factors (TFs), is observed. The excessive toxic ions are sequestered in vacuoles. This is an energy dependent process and utilizes energy in the form of ATP which reduces the ATP/ADP ratio. Effective balance between oxidative stress, antioxidant generation and cellular energetics makes halophytes more salt tolerant.

Fig. (3)

Schematic representation of common genetic pathways involved in salt tolerance mechanism of plants. The membrane bound sensors senses increasing salt concentration and activates the signaling molecules like Ca²⁺, ABA, and ROS. These signaling molecules activate downstream salt responsive genes like CBLs, CIPKs, CDPKs, different transcription factors, membrane bound ATPases, synthesis of osmolytes, antioxidants, activation of SOS and MAPK pathways and finally sequestration of sodium either in to the vacuole or excluded out of the cell.