Differential regulation of salt tolerance mechanisms in Arabidopsis thaliana and Thellungiella halophila (salsuginea)

Abstract

High salt concentrations in soil are the leading cause of salt stress restraining crop production in different parts of the globe. It is anticipated that stresses from abiotic factors including salinity will result in over 50% decrease in average yield of major crops under current agricultural practices by 2050. Therefore, extensive work has been conducted during the last 20 years to understand the basic mechanisms for stresstolerance to develop plants that can survive under extreme environmental conditions including salinity. The key mechanisms for salt-tolerance are now well known and they involve osmoregulation via increased production of compatible solutes (e.g. proline, glycine betaine), sequestration of salts in the vacuole, exclusion of salts by the roots and extrusion of salts from the roots and/or leaves as well as alleviation of the negative effects of salt-stress. It is becoming clear that these mechanisms are expressed in most plants, with differential and spatiotemporal regulation of the expression of these mechanisms being the key to the salt-tolerance trait. It is, however, not clear as to what is behind the differential expression of these mechanisms and the research already conducted in this field lacks detail in terms of the responses to salt-stress. This project aimed at exploring in depth the differences in salt-responses shown by two close relatives, Arabidopsis thaliana (salt-sensitive) and Thellungiella halophila (salt-tolerant). It also aimed at understanding the regulatory processes behind the observed differential responses by exploring the regulation of genes playing key roles under salt-stress in the two plant species. Detailed analysis of the kinetics of responses to salt-stress were conducted in the two plant species including physiological responses (growth, photosynthesis), metabolic responses (production of osmoregulators, accumulation of sugars, uptake of salts), gene responses (P5CS1 and SOS1) and role of regulatory components in A. thaliana null mutants (signalling elements and transcription factors). T. halophila showed faster and stronger responses to salttreatment in the regulation of the accumulation of key compatible metabolites such as sucrose, fructose, inositol and proline compared to A. thaliana. The difference in proline accumulation between the two species was mirrored by P5CS1 transcript abundance. Along with P5CS1 gene the SUS3, UGP2, FBA1 and PPC1genes showed higher transcript levels under saline conditions in T. halophila. Analysis of the P5CS1 gene suggests the possibility of the presence of two isogenes in T. halophila as suggested by the promoter regions as well as the numbers of introns. Moreover differential splicing of the P5CS1 transcripts under salt-treatment occurred between T. halophila and A. iii thaliana. Finally targeted screening for potential key signalling elements (protein kinases: NPK15, CPK11 and ORG1) and transcription factors (Rp2.4f) using A. thaliana null-mutants for these genes suggested these components had an indirect role in the regulation of the responses to salt-treatment, probably via the regulation of the metabolic background of the plant. The results suggest that along with differential gene regulation between glycophytes and halophytes, salt tolerance also depends upon the level of metabolic plasticity of the plant to mount rapidly appropriate responses to salt stress and the capacity of the plant to modulate the response.