Abstract
Halophytes are plants which naturally survive in saline environment. They account for ∼1% of the total flora of the world. They include both dicots and monocots and are distributed mainly in arid, semi-arid inlands and saline wet lands along the tropical and sub-tropical coasts. Salinity tolerance in halophytes depends on a set of ecological and physiological characteristics that allow them to grow and flourish in high saline conditions. The ability of halophytes to tolerate high salt is determined by the effective coordination between various physiological processes, metabolic pathways and protein or gene networks responsible for delivering salinity tolerance. The salinity responsive proteins belong to diverse functional classes such as photosynthesis, redox homeostasis; stress/defense, carbohydrate and energy metabolism, protein metabolism, signal transduction and membrane transport. The important metabolites which are involved in salt tolerance of halophytes are proline and proline analog (4-hydroxy-N-methyl proline), glycine betaine, pinitol, myo-inositol, mannitol, sorbitol, O-methylmucoinositol, and polyamines. In halophytes, the synthesis of specific proteins and osmotically active metabolites control ion and water flux and support scavenging of oxygen radicals under salt stress condition. The present review summarizes the salt tolerance mechanisms of halophytes by elucidating the recent studies that have focused on proteomic, metabolomic, and ionomic aspects of various halophytes in response to salinity. By integrating the information from halophytes and its comparison with glycophytes could give an overview of salt tolerance mechanisms in halophytes, thus laying down the pavement for development of salt tolerant crop plants through genetic modification and effective breeding strategies.
Highlights
Soil salinity is the major abiotic stress affecting plant productivity worldwide
Study of responses to salinity at proteome, metabolome, and ionic level would endorse for better understanding of physiological mechanisms underlying salt tolerance of halophytes
This review provides the recent knowledge that offers some ways for increasing salt tolerance
Summary
Soil salinity is the major abiotic stress affecting plant productivity worldwide. Many crop species, which countless people rely for survival, are negatively affected by high salinity (Wang et al, 2008a; Golldack et al, 2014). Halophytes are plants of great importance for proteomic, metabolomic, and ionomic studies to unravel their salt tolerance mechanisms with the longer term aim of transferring the tissue tolerance trait to commercial crops which have much lower salt tolerance efficacy. In this context many researchers have reported differential regulation of proteins, metabolites and ions related to salinity stress. Many earlier studies have indicated up-regulation and downregulation of certain metabolites in halophytes, viz, Brosché et al (2005) have reported a study on the salt-tolerant tree Populus euphratica which revealed increase in amino acid levels, proline, valine, and β-alanine along with changes in sugar and polyol metabolism. The proteomic, metabolomics, and ionomic studies in different halophytes in response to high salinity by many other researchers have been discussed later in this review
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