Plant Signal Transduction/Pooja, Ashwani Kumar, Jogendra Singh, Anshuman Singh and Vijayata Singh

7/29/2019 Plant Signal Transduction/Pooja, Ashwani Kumar, Jogendra Singh, Anshuman Singh and Vijayata Singh

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Signal transduction during Drought and Salt stresses

Pooja1, Ashwani Kumar2, Jogendra Singh2, Anshuman Singh2 and Vijayata Singh2

1Sugarcane Breeding Institute, Regional Center, Karnal (Haryana)-132001, India

2Central Soil Salinity Research Institute, Karnal (Haryana)-132001, India

Plants are subjected to various abiotic stresses because of unavoidable environmental conditions

which adversely affect their growth and development. Abiotic stress in fact is the principal cause

of crop failure worldwide, dipping average yields for most major crops by more than 50%.

Abiotic stresses cause losses worth hundreds of million dollars each year due to reduction in crop

productivity and crop failure. Drought, or more generally inadequate availability of water, and salt

stress due to soil or quality of irrigated water, are the main abiotic stresses to which crops are

exposed in India. Depending upon the extent of stress, the plants try to adapt to the changing

environmental conditions. For example, under osmotic and ionic stresses, the plants must getadequate amount of water for their growth and development of reproductive structures. The

closure of stomata limits water loss and the integrity of the photosynthetic and carbon fixation

apparatus is maintained by the initiation of a series of physiological processes.

A simplified presentation of the effect of abiotic stresses at molecular level (Adopted from Kaur and Gupta, 2005).

The stress is first perceived by the receptors present on the membrane of the plant cells

(Fig. 1A), the signal is then transduced downstream and this results in the generation of second

messengers including calcium, reactive oxygen species (ROS) and inositol phosphates. These

second messengers, such as inositol phosphates, further modulate the intracellular calcium level.


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This perturbation in cytosolic Ca2+ level is sensed by calcium binding proteins, also known as Ca2+

sensors. These sensors apparently lack any enzymatic activity and change their conformation in a

calcium dependent manner. These sensory proteins then interact with their respective interacting

partners often initiating a phosphorylation cascade and target the major stress responsive genes or

the transcription factors controlling these genes. The products of these stress genes ultimately lead

to plant adaptation and help the plant to survive and surpass the unfavorable conditions.

Fig. 1A (adopted from Mahajan and Tuteja, 2005): Generic signal transduction pathway as well as the expression of

early and late genes in response to abiotic stress signaling. (A) Represents the overview of signaling pathway under

stress condition. Stress signal is first perceived by the membrane receptor, which activates PLC and hydrolyses PIP2

to generate IP3 as well as DAG. Following stress, cytoplasmic calcium levels are up-regulated via movements of

Ca2+ ions from apoplast or from its release from intracellular sources mediated by IP3. This change in cytoplasmic

Ca2+ level is sensed by calcium sensors which interact with their downstream signaling components which may be

kinases and/or phosphatases. These proteins affect the expression of major stress responsive genes leading to

physiological responses.

Plant responds to stresses as individual cells and synergistically as a whole organism.

Stress induced changes in gene expression in turn may participate in the generation of hormones

like ABA, salicylic acid and ethylene. These molecules may amplify the initial signal and initiate


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a second round of signaling that may follow the same pathway or use altogether different

components of signaling pathway. Certain molecules also known as accessory molecules may not

directly participate in signaling but participate in the modification or assembly of signaling

components. These proteins include the protein modifiers, which may be added cotranslationally

to the signaling proteins like enzymes for myristoylation, glycosylation, methylation and

ubiquitination.

Stress responsive genes can be broadly categorized as early and late induced genes (Fig.

1B). Early genes are induced within minutes of stress signal perception and often express

transiently. These genes include the major stress responsive genes such as RD (responsive to

dehydration)/KIN (cold induced)/COR (cold responsive), which encodes and modulate the

proteins needed for synthesis, for example LEA-like proteins (late embryogenesis abundant),

antioxidants, membrane stabilizing proteins and synthesis of osmolytes.

Fig. 1B (adopted from Mahajan and Tuteja, 2005): Early and delayed gene expression in response to abiotic stress

signaling. Various genes are triggered in response to stress and can be grouped under early and late responsive

genes. Early genes are induced within minutes of stress perception and often express transiently. In contrast, various

stress genes are activated slowly, within hours of stress expression and often exhibit a sustained expression level.

Early genes encode for the transcription factors that activate the major stress responsive genes (delayed genes). The

expression of major stress genes like RD/KIN/COR/RAB18/RAB29B result in the production of various osmolytes,

antioxidants, molecular chaperones and LEA-like proteins, which function in stress tolerance.

Water stress may arise as a result of two conditions, either due to excess of water or waterdeficit. Flooding is an example of excess of water, which primarily results in reduced oxygen

supply to the roots. Reduced O2 results in the malfunctioning of critical root functions including

limited nutrient uptake and respiration. The more common water stress encountered is the water

deficit stress known as the drought stress. Removal of water from the membrane disrupts the

normal bilayer structure and results in the membrane becoming exceptionally porous when

desiccated. Stress within the lipid bilayer may also result in displacement of membrane proteins


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and this contributes to loss of membrane integrity, selectivity, disruption of cellular

compartmentalization and a loss of activity of enzymes, which are primarily membrane based.

Drought signaling encompasses three important parameters:

(1) Reinstating osmotic as well as ionic equilibrium of the cell to maintain cellular homeostasis

under the condition of stress.(2) Control as well as repair of stress damage by detoxification signaling.

(3) Signaling to coordinate cell division to meet the requirements of the plant under stress.

Figure 2: Pathways for activation of LEA-like class of stress-responsive genes (adopted from Kaur and Gupta, 2005).

As a consequence of drought stress many changes occur in the cell and these include

change in the expression level ofLEA/dehydrin-type genes, synthesis of molecular chaperones,

which help in protecting the partner protein from degradation and proteinases that function to

remove denatured and damaged proteins. This stress also leads to activation of enzymes involved

in the production and removal of ROS.

Dehydrins, also known as group 2 LEA proteins accumulate in response to both

dehydration as well as low temperature. Various stress signals and ABA share common elements

in their signaling pathways and these common elements cross talk with each other, to maintain

cellular homeostasis. ABA also prevents the precocious germination of premature embryos.

Stomatal closure under drought conditions prevents the intracellular water loss and thus ABA is

aptly called as a stress hormone. The main function of ABA seems to be the regulation of plant


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water balance and osmotic stress tolerance. Earlier studies suggest that osmotic stress imposed by

high salt or drought is transmitted through at least two pathways; one is ABA-dependent and the

other ABA independent. ABA induced expression often relies on the presence of cis acting

element called ABRE (Figure 3). ABA-dependent and ABA-independent pathways involved cross

talk or even converge in the signaling pathway. Calcium, which serves as a second messenger for

various stresses, which can mediate such cross talk (Figure 2). Drought stress include response of

stomata, effect of drought on photosynthetic machinery, role of sugars and other osmolytes, and

the role of MAP Kinases in mediating osmotic stress tolerance.

Figure 3: Cell signaling of the water deficit-induced ABA accumulation in relation to the whole signaling cascades in

response to water deficit. ABA as an intracellular signal mediates the expressions of numerous water deficit

responsive genes, and also as an intercellular signal regulates the water relation for whole plant. The ABA

accumulation was a prerequisite for ABA as a stress signal. ZEP, NCED, SDR1 and AAO are genes encoding keyenzymes in ABA biosynthesis pathway. Breakdown of ABAvia 80-hydroxylation is also indicated. Ca2+ ions, protein

tyrosine phosphatases (PTP) and mitogen-activated protein kinases (MAPKs) are possible signaling components in

the early perception of dehydration (modified from Jia et al., 2002b).

Role of MAP kinases in osmotic stress

In plants several MAPKs (mitogen activated protein kinase) are activated in response to

hyperosmotic stress. The MAP kinase pathways are intracellular signal modules that mediate

signal transduction from the cell surface to the nucleus. MAPKs are signalling modules that

phosphorylate specific serine/threonine residues on the target protein substrate and regulate a

variety of cellular activities. Activated MAPK is imported into the nucleus, where it

phosphorylates and activates specific downstream signaling components, such as transcription

factors to induce cellular responses. Nine MAPK genes have been identified from rice. Each

MAPK encodes a distinct protein kinase that plays a role in mediating drought tolerance. MAPK

increases in response to osmotic stress. This ultimately results in the accumulation of osmolytes

that helps reestablish the osmotic balance, protection from stress damage or repair mechanisms by

induction ofLEA/dehydrin-type stress genes.


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The two component system may couple with a downstream MAPK or directly

phosphorylate specific targets to initiate cellular responses. There is accumulating evidence

indicating that plants rapidly activate MAPK when exposed to multiple abiotic stress stimuli. The

best characterized two component histidine kinase is the yeast osmosensor SLNI. Together with

XPDISSK1 response regulator, this two component signal unit regulates the high osmolarity

glycerol (HOG) MAPK cascade, resulting in the production of glycerol to survive osmotic stress.

The diverse and multiple stress responses of MAPKs suggest that there is a fundamental

difference in functional specificity of MAPKs with respect to drought/salt response.

Understanding of the MAPK cascade can provide insight to understanding and solving the

problem of drought/salt stress in agricultural crops.

Osmotic stress activates phospholipids signaling

Membrane phospholipids constitute a dynamic system that generates a multitude of

signaling molecules like inositol 1, 4, 5-triphosphate (IP 3), diacylglycerol (DAG), phosphatidic

acid (PA), etc. Phospholipase C (PLC) catalyzes the hydrolysis of phosphatidylinositol 4, 5-

bisphosphate (PIP2) into IP3 and DAG, which acts as second messengers. IP3 releases Ca2+ from

internal stores. PLD was rapidly activated in response to drought stress in two plant species.

During osmotic stress, plant cells may increase the production of PIP2 by upregulating the

expression ofPI5K, a gene that encodes a phosphatidyl inositol 4-phosphate 5-kinase functioning

in the production of PIP2 (Figure 4). PIP2 levels were found to be increased in ATH cells cultured

under osmotic stress.

Figure 4: Role of PI 4P5 kinase in signal transduction(adopted from Kaur and Gupta, 2005).


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Phospholipase D (PLD) can also be involved in transduction of stress signals. PLD

hydrolyses phospholipids to generate phosphatidic acid (PA), another second messenger in animal

cells that can activate PI-PLC and protein kinase C. PA may also serve as a second messenger in

plants. Drought and hyper-osmolarity activated PLD and increased PA level in plants. PLD

appeared to be activated by osmotic stress through a G protein independent of ABA. Drought

stress-induced PLD activities were found to be higher in drought-sensitive than in drought-

tolerant cultivars of cowpea, suggesting that a high PLD activity may jeopardize membrane

integrity as PA is a non-bilayer lipid favouring hexagonal phase formation and may destabilize

membranes at high concentrations.

Conclusion and future prospects

Each stress is a multigenic trait and therefore their manipulation may result in alteration of

a large number of genes as well as their products. A deeper understanding of the transcription

factors regulating these genes, the products of the major stress responsive genes and cross talk

between different signaling components should remain an area of intense research activity in

future.

References

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Jonak, C, Kiegerl, S, Ligterink, W, Barkert, PJ, Huskissont, N S and Hirt, H. (1996). Stress signaling in

plants: A mitogen-activated protein kinase pathway is activated by cold and drought. Proc. Natl.

Acad. Sci. 93, 11274-11279.

Kaur, N and Gupta, A K. (2005). Signal transduction pathways under abiotic stresses in plants. Current

science. 88(11): 1771-1780.

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Plant Signal Transduction/Pooja, Ashwani Kumar, Jogendra Singh, Anshuman Singh and Vijayata Singh

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