PhD ThesisHigh 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.
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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
