Plant Salt Tolerance Mechanisms

This assignment delves into the mechanisms employed by plants to tolerate salt stress. It examines various physiological adaptations, including root architecture modifications, osmotic adjustments, ion compartmentalization, and aquaporin function. The discussion highlights the role of membrane transporters, silicon application, and halo tolerant rhizomicrobes in enhancing salt tolerance. Understanding these mechanisms is crucial for developing salt-tolerant crops to address the challenges posed by increasing salinity in agricultural lands.

Aquaporins as a novel role to
enhance salinity tolerance in wheat

Table of Contents
Summary..................................................................................................................................................3
Literature review......................................................................................................................................4
Aims and Objectives of research.............................................................................................................6
Research Plan...........................................................................................................................................7
Outputs of the projects...........................................................................................................................10
Outcomes from this project ...................................................................................................................11
Project feasibility and risk management:...............................................................................................11
References..............................................................................................................................................12
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Aquaporins as a novel role to enhance salinity tolerance in wheat
Summary
Slainity tolerence level impacts over the growth of crop and times it leads to the spoil
of crop. Aquaporin is considered as integral membrane of protien that works as channel in
order to transfer water. It can be said that aquaporin have ability to reduce affect of salt which
is affecting growth of the crop. In this report, it has been analysed that their are mainly three
limitation of salt which affects over growth of aggriculture that is Osmotic Tolerance, Na+
exclusion and Na+ Tissue tolerance. The report is mainly based on two objectives which are
formulated in order to identify actual output of whole research.
Literature review
Salinity as an issue in agriculture:
It has been estimated that nearly 950 million hectares on earth were affected due to
salinity and over half of the arable irrigated land in the world suffers from secondary-
induced salinity. Due to globalization and rapid climatic changes the annual rainfall is
expected to decrease in subtropical regions. The shortage of rainfall and water must be
replaced with brackish and saltwater for better agriculture production. By the time 2050
global food production need to be increased to 50% to reach the demands of a growing
population(Shabala 2013). In Australia, by the year 2020, an expectation of 25% of land
may be out of production due to Secondary salinization. The threats imposed by salinity
were causing billions of dollars lost to the agriculture industry. In the past few years at
Murray-Darling Basin, an estimated range of 260 million dollars of the economy got
affected.
Major Constraints impose by salinity: Osmotic and ionic stresses were the major
consequences of salinity. The extra osmotic gradient generates by excess levels of Na+
that drives water out of the cell. In order to undergo normal cellular metabolism water
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from vacuole moves to the cytosol and compensates the water depletion. As a result, the
cell turgor get reduces and elongation of roots takes place.
The second limitation due to salt stress was Na+ toxicity. In general, the physiochemical
property of Na+ and K+ were similar in nature, due to their similarity, Na+ compensate
with K+ ions in the key metabolic processes like ribosome functions, protein synthesis
and enzymatic reactions(Marschner 1995).
The third important constraint induced by salinity is the K+ deficiency and other
nutritional disorders (Shabala 2013).Na+ competes with K+ at specific sites of a plasma
membrane. When Na+ crosses along the plasma membrane, a momentous membrane
depolarization is observed and such kind of depolarization disturbs the K+ uptake. The
Final result will be an impaired metabolism in the cell.
The other constraint imposed by Salinity is reducing of Hydraulic conductance in roots
and minimize the absorption of H2O2 (Aroca et al. 2005; Ye & Steudle 2006).
Few adaptive mechanisms of the plant under Salinity: Salinity Tolerance is a
combination of three important basic mechanisms:
Osmotic Tolerance: It refers to the capability of the plant to resist toxic salinity
components under saline conditions by maintaining both stomatal conductance and
normal shoot growth.
Na+ exclusion: It refers to the plant ability to minimize the amount of Na+ entry into
shoots was the major toxicity occurs. This is achieved by controlling Na+ entry into roots
from the soil and developing of Na+ efflux mechanism that excludes sodium ions back to
the external environment. HKT family genes were more highlighted in the breeding
world, whereas these genes were responsible for root adaptive mechanism seen in
Arabidopsis, barley, eucalyptus, wheat etc.HKT family genes were usually divided into
two subclasses based on their transport selectivity. Transporters of one family are
selective to Na+ ions rather than K+, whereas the other family is selective to both Na+
and K+ absorption. K+ selectivity is strictly due to the deposition of glycine in specific
positions of HKT subfamilies. (Shabala 2017)In recent times an important concept of
water and solute transporters came into existence and role played by Aquaporins in this
relation. So far More than 30 MIPs were identified in plants like Rice, Arabidopsis and
maize (Maurel 2007) There was no clear information about the mechanism behind the
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role of Aquaporins played in the increase of salinity tolerance. Still, more research is
needed to be done.
Na+ Tissue tolerance: It refers to plant's ability to compartmentalise the Na+ ions into
vacuoles that minimize the toxic effect in cellular metabolisms that occurs in the
cytoplasm of the cells. Plants that are capable of intracellular compare.
International progress in developing Saline Tolerant Plants: Development of Saline
tolerant transgenic crops were not fruitful approach because the fundamental mechanism
of Ion fluxes in plants was not completely known. Although many attempts were made to
develop transgenic crops the complete success was not achieved yet. Failure of these
experiments was due to the trails of experiments was carried mostly in greenhouse basis,
when the scenario applied in on-field the problems arises again. This failure is due to the
availability of different salt levels in the field (Richards 1983). Shoot salt exclusion was
considered as a special character to control adverse effects of salinity. Plants membranes-
localised with high K transporters called as (HKT) genes had given a limited success
(Golldack et al. 2003). Whereas in conventional approach of breeding the expression and
elevation of SOS1 and SOS4 genes responsible for salinity tolerance in halophytes were
failed to perform in transgenic agricultural crops (Talaat 2018) Genes like NHX ( anti-
transporters’) of Na and protons have been failed whereas the enzymes responsible for
detoxification of ROS has differing levels of success(Roy, Negrão & Tester 2014).
Emerging role of Silica in regulating Aquaporins under Salinity
Silicon is the 2nd most abundant mineral that is present in the earth crust, and it is reported
that it played a key role in elevating salinity stress in barley and rice by minimising the
ion accumulation in leaves.(Gong, Randall & Flowers 2006) The underlying mechanism
behind the concept is Si elevates plant roots is by minimising the absorption of sodium
uptake and this is achieved by accumulating the Si content in roots. In a general view,
salinity reduces plant growth in two phases: ion toxicity and making difficulty in water
uptake by reducing osmotic potential (Munns & Tester 2008). One of the immediate
effects on plant due to osmotic stress is reduced in root hydraulic conductivity. Water
uptake in plants occurs in three ways: Transcellular, Apoplastic and Symplastic. Among
them, Symplastic and Apoplastic moments were elaborated as cell-to-cell pathways that
are further regulated by Aquaporins (Sutka et al. 2011) Aquaporins are major intrinsic
proteins that covey water and solutes to pass over them (Maurel 2007). These integral
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membrane proteins are divided into subclasses like Tonoplast intrinsic proteins, plasma
membrane intrinsic proteins, Nodulin 26-like proteins, x intrinsic proteins, SIPs (small
intrinsic proteins). In addition to the transportation of water few reports suggest that
MIPs transport some non-charged molecules along with water and solutes (Ma et al.
2008). MIPs were observed to transport many important substrates like urea, boron, lactic
acid, arsenate, Formamide and glycerol(Hove & Bhave 2011).
Gene Cloning: It refers to the cloning of those genes of wheat which owns the ability of
tolerating salinity maximum. It can be said that by using gene cloning the researcher can
enhance level crop production as wheat is a mandaory crop which is required by human
beings. The procedure will directly contribute in developing growth level of crop
production. In this procedure, farmers can take help of scientific methodologies in order
to make their crop successful by increasing their level of tolerence for salinity.
Phenotyping glasshouse experiements: Phenotyping refers to the quantification of
plant's quality, productivity level, development, architecture and photosynthesis. In this
sensors are placed in the laboratory where crops are irrigated. These sensors specify
actual feature of plant which will be seen in future after its completion, sensors also sets
the level of salinity tolerence of wheat in order to protect the plant for longer duration.
Research gap:
1. Plant aquaporins are the membrane intrinsic proteins that usually regulate water and
solutes in plant roots. As we know aquaporins are different types, some channels
transport water and some transport, metalloids, ROS, solutes etc (Yool & Campbell
2012), but there is no clear evidence that the aquaporins transports ions like sodium in
plant roots (Holm et al. 2005). Then there may be an important role of aquaporins in
plant osmotic adjustment and nutrient transport.
2. NIP Subclass of Aquaporins directly influences plant responses by influencing the Si
transport and SI uptake under saline conditions. For rice varieties, the OSLSIL gene
(NIP2 HOMOLOG) expression was elevated in the salt tolerant varieties by inducing
greater Silicon intake. The expression of NIP2 homolog in addition to Si application
only was the opposite which was observed in salinity only. So the combinational
study of these two Principles may open the gates for understanding a clear principle
behind the role of Aquaporins in controlling salinity (Senadheera, Singh & Maathuis
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2009; Shi et al. 2016).NIP subclass Aquaporins enhance the Si entrance into the cell
that further promotes the expression of PIP subfamily that enhances the effect of root
hydraulic conductance, Optimal water moment, Reduced levels of Na+. Hence this
concept lack in clear evidence that states the right mechanism of Aquaporins and the
direct involvement of silicon in the regulation of NIPs function. This concept needs
further experiments and validations.
Aims and Objectives of research
1. My principle aim was to investigate and conform that Aquaporins regulate sodium
entry in plant roots.
2. Investigating the role of silicon in Osmotic adjustment, Na accumulation, and root
water uptake via aquaporins.
Research Plan
Methodology: 10 Seedlings of wheat variety will be imbibed in de-ionised water for
at least two days and transferred to an incubator at a temperature of 30C in dark environment.
The seeds are then transferred to trays that are filled with vermiculite. After 5 days the seed
will be transferred to a container that contains Hoagland nutrient solution. The relative
humidity will be maintained at 60% during days and 80% at nights. After 15 days of plant
growth the plantlets will be transferred to a new container where plants treated with 50mM of
NaCl and five plantlets will be treated with both NaCl and 10Mm of CaCl2.Here CaCl2 acts
as an aquaporin blocker. (Martínez-Ballesta et al. 2008)
Achieving the 1st objective:
Measurement of Sodium Content in Roots: The oven dried roots must be digested
in HNO3 solution. In the next step the digested root material is filtered and subjected to dilute
with distilled water and Na+ analysis will be done by using atomic absorption
spectrophotometer. An analysis was performed in all the treatments and the results will be
compared between both aquaporin blocked and unblocked plants.
Measuring Root Hydraulic Conductivity: Plant stem will be sliced just above the
root collar and the root plugs are transferred to a pressure bomb in a way that protrudes the
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excised stem from the chamber. In the next step, the pressure inside the chamber should be
increased from 0.7 Mpa to 0.69 Mpa and flow should measure after thirty minutes
equilibration to constant flow. The same procedure will be repeated for having 5 readings by
introducing a pre-weighed capsule that is attached to a sponge in contact with the stem
portion and weight is determined by using a digital balance. The same procedure should be
followed by manipulating different pressure levels. In the final step root, hydraulic
conductance should be calculated by using the slope of linear regression between the
variables.(Trubat, Cortina & Vilagrosa 2012) The results will be compared between
aquaporin blocked and normal saline treated plants.
Measurement of Root Xylem Osmotic Potential: Roots from all the treatments
must be thoroughly rinsed in deionised water and should be placed on a filter paper and
subjected to frozen in nitrogen solution. In the next step with the help of syringe is used to
extract the root sap. Osmotic potential of the extracted sap is determined by using Cryoscopic
Osmometer at 25 C. The osmotic potential can be calculated by the equation: ᵠs = -RTC,
where T is a thermodynamic temperature, R is molar gas constant and C is recorded by using
micro voltmeter(Yin et al. 2013). Same process of comparison should be done between
CaCl2 treated and normal saline treated plants.
Achieving the 2nd objective:
RNA extraction and identification of wheat NIP Aquaporin Genes: Total RNA
from wheat roots can be extracted by using TRIZOL reagent. In the next step to find out the
RNA concentration, the Spectrophotometrical measurement will be performed and the
integrity can be checked by using Agarose gels. The first stand of wheat DNA sequence can
be extracted from total RNA and amplified. Multiple DNA sequences were then deducted by
using Clustalx.
Heterogeneous expression of wheat NIPs and SI transport essay in Xenopus
Oocytes. To find the heterogeneous expression DNA NIPs should be amplified with the help
of PCR by using isoform-specific primers that contains uracil-specific excision reagent and
cloned into PacL- containing the PSP64T vector that carries the non-translated b-globin gene
from Xenopus Laevis. The exact orientation of clones is then identified by a sequencing
process and restriction mapping technique. The oocyte isolation, In vitro RNA synthesis and
microinjection of RNA, can be performed as described by (Fetter et al. 2004). In the process,
the pool oocytes will be treated with Si and intracellular Si measurements were taken.
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Dosing of Silicon in Oocytes: The concentrated nitric acid solution will be added to
oocytes and further dried for 2 hours. In the next step, an ultra-trace elemental analyser
(plasma- grade water) will be added and incubated for 2 hours at normal room temperature.
These samples will be Vortexed and subjected to centrifugation for 10 minutes and a Zeeman
atomic spectrometer was used to trace the concentrations of Si. In the final step, a standard
curve can be plotted by using Ammonium Hexafluorosilicate solution and data will be
analysed by using Spectra software.
Water permeability: After 24 hours of injecting CRNA oocytes will be transferred to
the Hypo-Osmotic solution. The Osmolality can be estimated by using Osmometer. The
changes in the volume of oocytes can be captured by using a victim colour camera. The
osmotic adjustment can be calculated by using an equation POS=VO (d (V/Vo) / dt) / S ×Vw
(Osmin=Osmout).
Electrophysiology: Electrode voltage clamps will be performed in oocytes after 48
hours of water injection. The voltage electrodes were removed to find the resistance in
anND96 solution that is filled with KCl. The both solution will be continually perfused in the
experimental process. TEVC experiments will be performed on oocytes with high membrane
potential and the hyperpolarised Oocytes were eliminated. In the solutions with low levels of
Na concentration Ethane Sulfonic Acid is used as anions. The voltage readings will be
recorded using gene CLAMPS software and Gene clamp Amplifier.
Quantitative real-time PCR analyses:
After the treatment of plants with NACL, the roots must be subjected to frozen in a liquid
nitrogen solution. And must be stored -80ᵒ C. In the next stem a prime script RT reagent kit
was used to extract the First-strand DNA sequence which is further used for QPCR analyses.
Quantitative PCR experiments should be conducted on a CFX 96 PCR analysing system
using SYBR green master mix. To perform the real-time PCR the temperatures should be
maintained at 95 C for nearly 10 min and followed for 40 cycles at 94 C for a period of 15 s
and 60 C for 1 min. All the above-mentioned reaction should be performed on a reaction
plate using the iQ5 machine. In the final step quantification analysis will be carried by using
comparative method a standard curve transform the threshold cycle to express the level of
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genes. Standard deviations and prime efficiencies are then calculated by using q-base
software(Hellemans et al. 2007). The standard curve should be plotted by using a 5-fold
dilution series for 6 dilution points. The expression levels of these genes are next transformed
to analysis tools, Normfinder(Andersen, Jensen & Ørntoft 2004), Bestkeeper (Pfaffl et al.
2004), which are needed to be performed according to the guidelines given in the respective
manuals.
• Gantts Chart and timeline
Starting Date End Date Description Duration days
1st July 10th July Selection of research topic 10 days
10th July 20th august
Constructing research
plan and research
methodology selection 40 days
20th August 30th Jan Growing the plants 150 days
30th Jan 15th Feb.
Measuring the root
sodium content and
hydraulic conductivity 15 days
15th Feb 5th March
RNA extraction and
Quantitative PCR
technique 20 days
5th March 5th April
Heterogeneous expression
on oocytes, Testing water
permeability and
electrophysiology 30 days
5th April 15st April
Statistical analysis of
results 10days
15th April 1st May
Delivery of research
project 15 days
Activity J
ul
Au
gus
Sept
emb
Oct
obe
Nov
emb
Dece
mbe
J
a
F
e
M
ar
A
pr
M
a
10

y t er r er r n b ch il y
Selection of research topic × × × × ×
×
×
× ×
×
× × ×
Construction of research plan
and research methodology
selection × × × × × × ×
×
× ×
Growing plant × × × × ×
Measuring root Na+ content
and hydraulic conductivity × × × × × × × × ×
RNA extraction and qPCR
analysis × × × × × × × × ×
Heterogeneous expression on
xenopous oocytes × × × × × × × × ×
Statistical analysis × × × × × × × × ×
Delivery of the project × × × × × × × × × ×
Outputs of the projects
The first objective of the project delivers evidences about how aquaporins transport
sodium and how their function takes place under saline conditions. The second objective of
the project highlights the osmotic adjustment under salinity and studies the role of Silicon on
water uptake and sodium uptake and delivers the expression of aquaporins. Moreover the
project delivers the role of silicon in salinity tolerance, enhanced water permeability under
saline conditions and the underlying mechanisms in wheat roots. Once the mechanism was
clearly understood then it will be easy to exploit the breeding processors that control the
aquaporin expression and new saline varieties can be generated. In conventional agronomic
practice silicon usage can be increased and can be recommended as a supplement to control
sodium threat under saline conditions. The expected outputs of the project are expected to be
published in six high quality publications at various national and international research
conferences on salinity stress in plants results will be available to wider community and
industry via ACROSS annual reports as well as through media reports.
Outcomes from this project
With a most important experiment in the direction of world agriculture that requires
production of 70 percent additional food yield for an added 2.3 billion individuals by the year
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2050. Salinity is a most important stress preventive the upsurge in the ultimatum for the food.
The national land and water resources audit estimates that approximately 5.8 million hectares
of land has a potential to convert into salinity this figure may be estimated to increase up to
17 million hectares by the year 2050 if the effective control is not implemented this is
expected to cost 1 billion a year. My project is going to contribute to overcome these
problems. Salinity tolerance includes a compound of rejoinder at molecular, metabolic,
biological and entire plant level. With the aid of most recent knowledge and the techniques
described above can help the healthy growth of the plant that enhances salinity tolerance in
crop production. Also the Genetic manufacturing has been demonstrated to be a methodology
to the expansion of Salinity Tolerant plant and this attitude will turn out to be more
commanding as additional applicant genes related to aquaporin over expression can have a
clear solution that can enhance salinity tolerant. Also the time to time checking of the plant
growth after supplementing the needed silicon fertilizers to the plant shall help it to grow
healthy and tolerate the salinity. As the root is the important part of the plant methodologies
that avoid the sodium absorption can restrict the accumulation of sodium concentrations in
roots. Prevention is better than cure so the above concept of aquaporins can prevent the
consequences of salinity stress in plants.
Academic outcomes: The new approach and methodologies in studying the functions
and Sodium transport by aquaporins are highly original and there is only limited instances
used before. It allows answering large number of questions related to aquaporins and salinity
tolerance and can result in a series of presentations and publications at major international
conferences in the area, promoting the image of Australian science worldwide.
Social outcomes: understanding the molecular and ionic transport through aquaporins and
effect of Si in salinity tolerance is more vital for sustainable agricultural practices, with long-
term benefits for Australian rural communities.
Project feasibility and risk management:
The feasibility of this project will be high and there will be no expected risks. But a
partial risk from the project is, as we get the results of the project according to the aims then
further there will be a chance to develop many transgenic wheat varieties that have capability
of over expressing the aquaporin genes. These transgenic plants can be saline tolerant but in
the process of genetic modification there is a chance of losing several desired characters that
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may further reduce the productivity. So it is better to opt for conventional agronomic
practices by applying silicon fertilizers that increase the activity of aquaporins and controls
sodium accumulation in plants.
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