Background
Excessive salt accumulation in the soil affects the productivity of about
one-third of the world's limited arable land . Nowadays more arable land is
lost due to salinity than the amount gained by clearing valuable rainforests.
Understanding the phytohormone mechanisms of plant salt tolerance will enable
humans to improve the performance of crop plants in saline soils especially
in arid parts of the world. Salinity has been an important historical factor
for the decline of ancient agrarian societies and has recently been the cause
of the decline of agricultural production in large parts of India . There
is currently a lot of research working towards genetically modifying crops
to grow in conditions of drought and in arid areas.
Salinity is one of the stress factors that are known to cause changes in physiological
processes in plants. One such process affected by environmental conditions
is photosynthesis. A plant's tolerance toward unfavorable environmental factors
depends on the adaptive ability of the photosynthetic apparatus; therefore
the tolerance to salt stress varies from plant to plant. An increased salt
concentration in the soil destroys osmotic relations within the plant. So
the study of salt stress induced phytohormonal responses provides valuable
knowledge about plant osmotic stress responses and cellular ion homeostasis.
Plant hormones (or phytohormones) are the key mediators of environmental signaling (such as salt stress) and plant growth and development. The main types of hormones found in autotrophs are abscisic acid, auxins, cytokinins, gibberallins, ethylene and brassinosteroids. Phytohormones have an important role in the regulation of plant metabolism, coordinating the functional activities of the whole plant as well as their reaction to a wide variety of stresses. Ethylene stimulates dormancy, stimulates shoot and root growth and differentiation, may also have functions with response to root formation, stimulates leaf and fruit abscission (the shedding of leaves, flowers, or fruits following the formation of scar tissue in plants ), induction of femaleness in dioecious fruits, stimulates flower opening, flower and leaf senescence or growing old, and fruit ripening. Due to the many important functions of ethylene in plant growth, the study of the effect salt stress on this hormone would enable one to further the understanding towards producing a genetically modified salt resistant plant.
Other important plant hormones that are prominently affected by salt stress
and the ones used in this study are abscisic acid, gibberellins and brassinosteroids.
Abscisic acid is a plant hormone that promotes dormancy in buds and seeds,
retardation of growth, and shedding of leaves, flowers, and fruits. Gibberallins
are "a class of related plant hormones that stimulate growth in the stem
and leaves, trigger the germination of the seed and breaking of bud dormancy,
and stimulate fruit development with auxin" . Brassinosteroids "induce
a broad spectrum of developmental responses, such as stem elongation, pollen
tube growth, leaf bending and epinasty, root growth inhibition, and xylem
differentiation ".
Ethylene is a key gaseous hormone that plays a major role in the growth of
a plant and is also a key stress hormone. The main objective of this study
was to investigate ethylene's interactions in response to salt stress. This
study also examined ethylene's interactions with other phytohormone mutants
in relation to salt stress. The phytohormone mutants examined in the study
were Wild type (Columbia), Ethylene insensitive, Ethylene overproducer, ABA
deficient, Auxin resistant, BR insensitive and BR overproducer of A. thaliana.
The knowledge gained from this study could possibly allow the development
of salt stress tolerant plants by genetically modifying the expression and
accumulation of phytohormones in the plants.