Role Of Phytohormones In Plant Abiotic Stress Tolerance

INTRODUCTION

The human population is rapidly increasing and needs a substantial increase in agricultural productivity worldwide. However, various biotic and abiotic stresses are majorfactors limiting crop productivity [1]. To feed the world population, productivity must be increased by 70% for an additional 2.3 billion people by 2050.The mechanism underlying environmental stress response and tolerance in plants are different and more complex than improve crops nutritionally and economically. So here lets discuss the phytohormones and their roles in abiotic stress response, in addition to their improvement to confer abiotic stress tolerance in crop plants to increase food quantity and quality.

Phytohormones are molecules produced in very low concentrations but able to regulate a variety of cellular processes in plants. They work as chemical messengers to communicate cellular activities in higher plants [2]. Phyto- hormones play key roles and coordinate various signal transduction pathways during abiotic-stress response.They regulate external as well as internal stimuli [3]. Some phytohormones, such as ABA, have been identified as stress hormones. ABA plays critical roles in plant development: maintenance of seed dormancy, inhibition of germination, growth regulation, stomatal closure, fruit abscission besides mediating abiotic and biotic stress responses .

Environmental factors imposing stress on plants, including drought, salinity, heat, chilling, freezing, ozone, pathogens, and UV radiation are the major environmental cuses that limit crop productivity. The period and development of stress, stages of the plant, and biotic and abiotic factors may influence the stress response [4].Some crops may be affected at an early stage,but recover and finally survive. Susceptibility or resistance to stresses may differ markedly among species or genotypes of crops. Among these stress conditions, drought is the most severe stress, responsible for decreased agricultural production worldwide. It affects plants in many ways: plant growth, membrane integrity, pigment content, osmotic adjustments, water relations,and photo synthetic activity[5,6].As rightly pointed out by Postel [7], water will be the oil of the 21st century. Many rivers around the globes are drying daily; most have no water to discharge to the sea [7]. After drought, salinity is the second major stress reducing crop productivity.

PHYTOHORMONES

It is a diverse group of signaling molecules found in small quantities in cells, mediate these responses. They have pivotal roles in promoting plant acclimatization to ever-changing environments by mediating growth, development, source/sink transitions, and nutrient allocation have been well established [7]. Although plant response to abiotic stresses depends on various factors, phytohormones are considered the most important endogenous substances to modulate physiological and molecular responses, a critical requirement for plant survival as sessile organisms [8]. Phytohormones act either at their site of synthesis or elsewhere in plants following their transport [9]. Phytohormones are of key importance in plant development and plastic growth. They include auxin (IAA), cytokinins (CKs), abscisic acid (ABA), ethylene (ET), gibberellins (GAs), salicylic acid (SA), brassinosteroids(BRs) and jasmonates(JAs).The strigolactone(SL)are relatively new.

Abscisic acid (ABA)

Abscisic acid owes its name for its role in abscission of plant leaves and perhaps the most studied phytohormone for its response and distinct role in plant adaptation to abiotic stresses, and is accordingly termed a “stress hormone.” It is an isoprenoid plant hormone produced in the plastidal 2-C methyl-D-erythritol-4-phosphate pathway. ABA tolerance received much attention .In response to environmental stresses, endogenous ABA levels increase rapidly, activating specific signaling pathways and modifying gene expression levels[10]. .Nemhauser etal.[11] have reported that ABA transcriptionally regulates up to 10% of protein-encoding genes. ABA also acts as an internal signal enabling plants to survive under adverse environmental conditions[12].

Under water-deficit conditions, ABA plays a much vital role in providing plants the ability to signal to their shoots that they are experiencing stressful conditions around the roots,eventually resulting in water-saving antitranspirant activity, notably stomatal closure and reduced leaf expansion [13]. ABA is also involved in robust root growth and other architectural modification sunder drought stress [14] and nitrogen deficiency[15]. ABA regulates the expression of numerous stress-responsive genes and the synthesis of LEA proteins, dehydrins, and other protective proteins ABA upregulates the processes involved in cell turgor maintenance and synthesis of osmoprotectants and antioxidant enzymes conferring desiccation tolerance[16] .Zhangetal.[17] reported a proportional increase in ABA concentration upon exposure of plants to salinity.

Auxins(IAA)

Some interconnecting pathways have been proposed for biosynthesis of auxin in plants, including four tryptophan (Trp)-dependent and one Trp-independent pathway [18]. IAA (indole-3-acetic acid) is one of the most multi-functional phytohormones and is vital not only for plant growth and development but also governing and/orcoordinating plant growth under stress conditions[19].The presence of an auxin biosynthesis, signaling, and transport apparatus in single-celled green algae show clear evidence of the evolutionary role played by auxin during the adaptation of plants to diverse land environments [20]. Though there has been a recent upsurge in our understanding of auxin regulation of plant growth and development, its role as a regulator of stress response is still little understood[29].

Interestingly, there is growing evidence that IAA plays an integral part in plant adaptation to salinity stress[21].It increases root and shoot growth of plants growing under salinity or heavy metal stresses[22,23].Salinity reduced IAA levels in maize plant ,but salicylic acid application effectively increased them .

Cytokinins(CKs)

CKs play important roles in many plant growth and developmental processes and are considered as master regulators during plant growth and development [24,25]. Alteration of endogenous levels of CKs in response to stress indicates their involvement in abiotic stress including drought [24] and salinity[25]..

Although plant responses for CKs have been evaluated most often via their external application, stressful conditions are also known to enhance their endogenous levels via uptake and enhanced biosynthesis [26]. In contrast to the ABA inhibition of seed germination, they also release seeds from dormancy. CKs are often considered ABA antagonists[27] .In water-stressed plants, decreased CK content and accumulation of ABA lead to an increased ABA/CK ratio. The reduced CK levels enhance apical dominance, which, together with the ABA regulation of stomatal help to adapt drought stress.

Ethylene(ET):

ET, a gaseous phytohormone, is involved in many phases of plant growth and development, notably fruit ripening, flower senescence, and leaf and petal abscission, besides being an essential regulator of stress responses[28,29].

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Abiotic stresses including low temperature and salinity change endogenous ET levels in plants. Enhanced tolerance was accordingly achieved with higher ET concentrations [30]. ET also plays a important role in the defense response of plants to heat stress [31]. Environmental stress induces ET accumulation which increases plant survival chances under these adverse conditions .

Gibberellins(GAs)

The GAs show positive effects on seed germination ,leaf expansion, stem elongation,flower and trichome initiation ,and flower and fruit development[32]. They are essential for plants through out their life cycle for growth-stimulatory functions..Interestingly ,there is increasing evidence for their vital role in abiotic stress response and adaptation[33]. Recently, experiments have been performed to investigate role of GAs in osmotic stress response in Arabidopsis thaliana seedlings. GAs are known to interact with all other phytohormones in many developmental and stimulus-response processes[34]

Brassinosteroids(BRs)

Brassinosteroids(BRs) are relatively new group of polyhydroxy steroidal plant hormones with strong growth and development promoting potential.They were firs tisolated and characterized in pollen of the rape plant( Brassicanapus). More than 70BRs have been isolated from plants. However, recent findings suggest the stress-impact mitigating roles of BRs and associated compounds in various plants subjected to various abiotic stresses. These abiotic stresses are high temperature, chilling, soil salinity, light, drought, flooding, metals/metalloids, and organic pollutants. Recent research has shown great potential of BRs and associated compounds in the modulation of components of antioxidant defense system in-response-to and to counteract the abiotic stress-induced oxidative burst, reviewed by Vardhini and Anjum. However, there is tremendous scope for further research focused on the sites, pathways, and enzymology of their biosynthesis, source–sink relationships, developmental and stress physiology, their interactions with microorganisms, fungi, and animals, and the realization of their powerful applications.

Jasmonates(JAs)

The cyclo pentanone phytohormones derived from the metabolism of membrane fatty acids including primarily methyl jasmonate (MeJA) and its free acid jasmonic acid (JA) are collectively known as jasmonates (JAs) and are widespread in the plant kingdom .These multi functional compounds are involved in crucial processes associated with plant development and survival including reproductive processes, flowering, fruiting, senescence, secondary metabolism, and direct and indirect defense responses[35,36]. JA activates plant defense responses to pathogenic attack as well as environmental stresses including drought, salinity, and low temperature. JAs are vital signaling molecule induced by various environmental stresses including alinity,droughtand UV irradiation. They have great potential to mitigate an area of threatening environmental stresses. The exogenous application of MeJA effectively reduced salinity stress symptoms in soybean seedlings. Endogenous levels of JA increased in rice roots under salinity stress and reported to counteract the deleterious effects of salinity stress [37]. JAs applications alleviate heavy metal stress in plants by activating the antioxidant machinery

Salicylic acid(SA)

Salicylicacid(SA) is a naturally occurring phenolic compound involved in the regulation of pathogenesis associated in protein expression[38] .In addition to defense responses,it plays a vital role in the regulation of plant growth ,ripening and develop ment ,as well as abiotic stresses.

SA is involved in plant response to abiotic stresses such as drought,salinity,chilling ,and heat. SA along with ABA is involved in the regulation of drought response .The SA content in barley roots was increased approximately two fold by water deficit[39].

RECENT ATTEMPTS AT DECIPHERING THE ROLE OF PHYTOHORMONES IN ABIOTIC STRESS TOLERANCE

• Hormones help in pollen development under cold stress-

Abiotic stress can affect any developmental stages, but the reproductive stage is the most critical. If stresses are applied at the reproductive stage, it may damage the plant and ultimately reduce the crop yield .Further,among the various reproductive stages, pollen development is crucial and is affected by abiotic stresses such as drought, cold, and heat, leading to reduced crop yield [40]. In pollen development ,pollen meiosisis the stage most sensitive to cold, which causes pollen sterility and also reduces anther dehiscence, pollen load to the stigma, pollen germination, and pollen tube growth. Hormones such as GA and ABA are considered major signals for cold-induced pollen sterility and understanding of molecular mechanism of pollen development under stress and non-stress conditions can be beneficial for hybrid seed production by producing novel sterile plants, which will be a new development.

• Hormonal balance under cold stress-

Kolaksazov et al. [41]reported that stress phytohormones such as ABA, JA, and SA trigger phosphoprotein cascade pathways, leading to the expression of genes associated with cold stress tolerance. They reported high contents of JA in the three different populations at a controlled temperature of 22 °C, with a 10-fold reduction in sensitive plants but no change intolerant plants at 4°C.

• Salicylic acid increases germination, seedling growth,and enzymatic activity-

Gharib and Hegazi [42]showed that SA stimulated various growth aspects of the bean seedlings ,was responsible for biosynthesis of growth-promoting and -inhibiting substances, and reduced the adverse effect of cold stress in common bean.They conducted an experiment on six common bean varieties with optimal temperature 25 °C and chilling stress at 15 °C. They found that germination and seedling growth of the six varieties were slowed under chilling stress. Seeds treated with SA showed increased germination germination rate compared to the untreated(control) seeds under control as well as chilling stress.

• Phytohormones alleviate high-temperature stress-

Owing to high temperatures, crop productivity is decreasing in many parts of the world. Chhabra et al. [43]performed an experiment to test the effect of various hormonal concentrations on heat-stress effects and observed that both the growth-promoting and growth-retarding hormones mitigated heat-stress effects. They also reported that the most effective concentration of kinetin was at 100 μmol L−1followed by 50 μmol L−1and that soaking seeds in ABA delayed 50% seedling mortality by 1 h and 50 min at 0.5 and 1.0 μmolL−1concentrations.