Plant abiotic stress: a prospective strategy of exploiting promoters as alternative to overcome the escalating burden

ABSTRACT

Plants being sessile are shaped by evolution to adapt themselves and tolerate various stresses, be it salinity stress, drought, high/low temperature in nature. They have evolved with every alternate strategy to tackle serious abiotic stresses through considerable degree of developmental plasticity, including adaptation via cascades of molecular networks. Understanding the mechanism of genes responsible for plant adaptation to environment will help in predicting the scenarios, expanding the genetic aspect of abiotic stress-regulating genes to protect and extrapolate the level of tolerance or vulnerability conferred in natural ecosystems. Complementing the agronomic need for greater tolerance to abiotic stress, studying plant abiotic stress response can help in gaining insight into plant biology that can be practically applied to unlock the secrets in order to improve plant productivity to feed the ever increasing population of human beings.

KEYWORDS:

• Abiotic stress
• genetic engineering
• tolerance

Introduction

Abiotic stresses, notably extremes in temperature along with supply of water and inorganic solutes, frequently limit growth and productivity of major crop species. They reduce the average yield for major crop plants by more than 50% (Bray et al. 2000). If a single abiotic stress is to be identified as the most common in limiting the growth of crops worldwide, it most probably is low water supply (Boyer 1982; Araus et al. 2002). Water stress in its broadest sense includes both drought and salinity stress. Drought and salinity that are becoming increasingly significant in limiting plant growth are believed to cause serious salinization of more than 50% of all arable lands by the year 2050 (Ashraf 1994; Wang et al. 2001). Increasing salinization of arable land is expected to have devastating global effects, resulting in 30% land losses within the next 25 years and up to 50% by the year 2050 (Wang et al. 2003). One abiotic stress decreases the ability of plant to resist a second stress. For example, low water supply makes plants more susceptible to damage from high irradiance due to reduction in its ability to reoxidize NADPH, involved in dissipating energy delivered to photosynthetic light-harvesting reaction centers.

Abiotic stresses such as drought, salt and low temperature, which adversely affect plant growth and productivity, lead to a series of morphological, physiological, biochemical and molecular changes in plants in order to adapt and as such survive under stress conditions (Wang et al. 2003). Plant abiotic stresses and response of plants to these stresses have been extensively studied. Improvement of crop plants with traits that confer tolerance to these stresses was practiced using traditional and modern breeding methods. Classical plant breeding methods involving inter-specific or inter-generic hybridization and in vitro-induced variation have been applied to improve the abiotic stress tolerance of various crop plants but have so far met with limited success. Conventional breeding strategies are limited by the complexity of quantitative traits, low genetic variance of yield components under stress conditions and lack of efficient selection criteria (Tester & Bacic 2005; Varshney et al. 2011). It is important, therefore, to look for alternative strategies to develop stress-tolerant crops. Genetic engineering that allows us to mobilize genes from virtually any source has given a strong landmark over plant breeding. Starting with 1.7 million hectares in 1996, a total of 134 million hectares of land was brought into cultivation of transgenic crops in 2009 (James 2009). Looking at the growth rate of approximately 80-fold between 1996 and 2009, transgenic technology represents the fastest adopted technology for the production of crops. This ability to manipulate genes could lead to rational and deliberate attempts to alter the crops for improvement of their agronomic performance. Molecular breeding together with genetic engineering contributed substantially to our understanding of the complexity of stress response. In order to understand the basis of stress tolerance, the diversity of the stress response and its utility for the survival of plants are needed to be investigated. This review focuses on recent advances in basic research of abiotic stress tolerance at the molecular level, with the main emphasis on genetic engineering approaches that are generally used to create economically important stress-resistant transgenic plants.

Abiotic stress and plant productivity

Plant productivity is severely affected by abiotic stresses. Abiotic stresses negatively influence survival, biomass production, accumulation and grain yield of most crops (Grover et al. 2001). Drought and salinity are two major abiotic stresses that adversely affect at least 20% of world arable land and approximately 40% of irrigated land to various degrees (Boyer 1982; Chinnusamy & Zhu 2002; Xiong et al. 2002). About one-third of irrigated land is considered to be affected by salinity. As a consequence, physiological and biochemical responses in plants vary as cellular aqueous and ionic equilibriums are disturbed. Low precipitation, high surface evaporation, weathering of native rocks, deposition of oceanic salts carried in wind and rain, irrigation with saline water and poor agricultural practices are major phenomena taking place that lead to salinization of agricultural land. The United Nations Environment Program estimates that approximately 20% of agricultural land and 50% of crop land in the world is salt stressed and salinized areas are increased at a rate of 10% annually (Flowers 2004).

Plants have developed several strategies to overcome these challenges either adopting a mechanism which will allow them to survive the adverse conditions or specific growth habits to avoid stress conditions. Also, hundreds of genes and their products respond to abiotic stresses at transcriptional and translational levels (Cushman & Bohnert 2000). Genes responding to a particular stress vary between species and even genotypes. Genes that lead to successful adaptations and eventual tolerance have broadly been categorized into four groups: (i) genes encoding enzymes involved in osmolyte (proline, mannitol, glycine, betaine, trehalose) biosynthesis; (ii) genes encoding antioxidants (superoxide dismutase, ascorbate peroxidase and catalase) protectants; (iii) genes encoding stress-induced proteins such as late embryogenesis abundant (LEA) proteins, antifreeze proteins, chaperons and heat shock proteins and so on involved in maintaining cell integrity and (iv) genes encoding for protein kinases and trans-acting factors such as DREB1/CBF, AP2/ERF, DREB2, NAC, MYB/MYC, basic leucine-Zipper proteins and Zinc-finger (Tayal et al. 2004; Sangam et al. 2005). On the basis of function, genes associated with abiotic stress tolerance are divided into three groups: signaling factors, functional proteins and transcriptional factors. Compared to signaling factors that include proteins involved in regulation of signal transduction, functional proteins include genes that control synthesis of abscisic acid (ABA), reactive oxygen species scavenging proteins, antioxidant protectants, LEA proteins, chaperons and heat shock proteins (HSPs) involved in conferring protection. Transcriptional factors (DREB1/CBF, AP2/ERF, DREB2, NAC, MYB/MYC, basic leucine-Zipper proteins and Zinc-finger proteins) are associated with integrity of cell and ion homeostasis.

Biotechnological strategies for improved tolerance to abiotic stress

Plants distinguish abiotic stress and elicit an appropriate response with altered metabolism, growth and development (Bartels & Salamini 2001). At the molecular level, abiotic stress tolerance can be achieved through gene transfer by altering the accumulation of osmoprotectants, production of chaperones, superoxide radical scavenging mechanisms, exclusion or compartmentation of ions by efficient transporter and symporter systems (Ingram & Bartels 1996; Hasegawa et al. 2000; Zhu 2002; Chinnusamy et al. 2004; Valliyodan & Nguyen 2006). Keeping in view the fact that understanding the function of stress-inducible genes would help to unravel the possible mechanisms of stress tolerance, there has been a spurt in understanding the molecular mechanisms of genes associated with different cellular pathways that control the complex trait of abiotic stress tolerance (Shinozaki et al. 2003). Over the past 10 years, various strategies have been employed to understand the basis of stress tolerance and isolate the genes that are involved in the stress response (Vij & Tyagi 2007). They include the following:

• Generation of unique transcript-specific short sequence of 9–17 bp by serial analysis of gene seems essential for analysing the global gene expression (Saha et al. 2002).

• Massively parallel signature sequencing (MPSS) is another powerful technique for transcription profiling on a genome-wide scale. The MPSS resource is available for three plant species (Arabidopsis, rice and grapes) in a public database http://mpps.udel.edu (Nakano et al. 2006).

• Microarray has revolutionized global gene expression profiling by allowing the entire gene complement of the genome to be studied in a single experiment (Duggan et al. 1999).

• A positional cloning strategy allows the use of a phenotype to determine the position of the mutant gene or natural allele by examine the linkage to markers whose physical location in the genome is already known.

• Targeting -induced local lesions in a genome are another high throughput technology to identify mutations in a selected gene or variant alleles (Henikoff & Comai 2003).

• Activation tagging makes use of enhancer elements in the construct that activates transcription of genes near the site of insertion (Springer 2000).

• Target gene inactivation by homologous recombination followed by overexpression of target gene using transgenic approach (Iida & Tareda 2005).

With the increasing applications of genetically engineered plants, the need of promoters for transgene expression in a variety of ways or in response to different stimuli has greatly increased. Indeed a variety of promoters is necessary at all levels of genetic engineering in plants, from basic research discoveries, concepts and questions, to development of economically viable crops and plant commodities, to addressing legitimate concerns raised about the safety and containment of transgenic plants in an environment. Over the years, numerous promoters have been isolated from a wide variety of organisms and applied to plant genetic engineering. These include constitutive, tissue-specific, inducible, viral and synthetic promoters (Potenza et al. 2004). An abiotic stress-inducible promoter selection has become increasingly important for successful gene transfer and expression of transgenes in plants (Dale et al. 2002). The availability of a broad spectrum of stress-inducible promoters that differ in their ability to regulate the expression patterns of the transgene can dramatically increase the successful application of transgenic technology for improved tolerance to abiotic stress in crop plants. Recently, the use of stress-inducible promoters that have low background expression under normal growth condition in conjunction with the transgenes was accomplished to achieve increased stress tolerance without retarded growth (Bhatnagar-Mathur et al. 2008). As strong abiotic stress-inducible promoters are required for transgene expression responsible for different abiotic-stress tolerance at different stages of growth, selection of a suitable promoter is critical for devising intelligent strategies for genetic engineering.

Role of abiotic-stress-inducible promoters in a transgenic plant

Plant genetic transformation is the most powerful application used to study gene expression in plants using tools of plant genetic engineering to improve the crops. It has contributed a lot in understanding the process of gene regulation and plant development for manipulation and analysis of biochemical processes. As it is important to control the expression of the transgene in order to minimize the adverse effects of an engineered plant on other beneficial and non-targeted organisms, regulated expression of transgenes through diverse promoters especially developmental-stage or tissue-specific promoters is essential. Promoters are the DNA sequence upstream to a gene's coding region that contains specific sequences recognized by proteins for initiation of transcription (Buchanan et al. 2001). Regulation occurs at different stages of gene expression, predominantly during transcription. For the majority of genes encoding proteins, initiation of transcription includes the binding and activation of RNA polymerase II. Variability in gene expression comes about when other diverse, semi-conserved sequence elements are present upstream of the RNA polymerase binding site that offers a binding site for protein factors involved in controlling the level and pattern of expression of a gene. Work on inducible promoters in plant genetic engineering research has been pursued to a great extent and a lot of information has been brought forward on promoters that get induced during abiotic stresses such as high/low temperature and salt stress.

Constitutive promoters are directly expressed virtually in all tissue and are largely, if not entirely, independent of environmental and developmental factors. As their expression is not conditioned by endogenous factors, constitutive promoters are usually active across species and even across kingdoms. The most commonly used constitutive overexpressed promoters are those derived from plant viruses such as cauliflower mosaic (CaMV) 35S promoter (Odell et al. 1985). The CaMV 35S promoter drives high levels of transgene expression in both dicots and monocots (Battraw & Hall 1990; Benfey et al. 1990). A number of endogenous plant promoters that are used to regulate or drive high constitutive levels of transgene expression are those derived from actin and ubiquitin genes (Gupta et al. 2001; Dhankher et al. 2002). Actin is a fundamental cytoskeletal component that is expressed in nearly every plant cell. Act2 and actin 1 promoters were developed from actin gene family in Arabidopsis and rice, respectively, while a ubiquitin 1 promoter (pUbi) is a popular promoter used for transformation is maize (An et al. 1996). Constitutive expression can be problematic as a specific transgene is overexpressed at the wrong time in developmental stages or in tissues where it is not normally expressed, which can have unexpected consequences on plant growth and development. On the other hand, constitutive expression of signal-transduction ‘master-switches’ for pathogen resistance can lead to decreased growth or enhanced susceptibility to other pathogens (Bowling et al. 1997; Berrocal-Lobo et al. 2002).

Strong and constitutive promoters are beneficial for a high-level expression of selectable marker genes, necessary for efficient selection and generation of transgenic plants. However, constitutively active promoters are not always desirable for plant genetic engineering because the constitutive overexpression of a transgene may compete for energy and building blocks for synthesis of proteins, RNA and so on that are required for plant growth under normal conditions. In these cases, use of a stress-inducible promoter of moderate strength may be more desirable. In plants, various types of abiotic stresses induce a large number of well-characterized and useful promoters. However, it has been found that most of the stress-induced promoters have poor strength of expression when compared to constitutive promoters. The early responses of plants to stress include sensing and subsequent signal transduction that lead to stress-responsive gene expression. In response to osmotic stress elicited by water deficit or high salt, the expression of a set of genes is altered, some of which are also induced by low temperature stress (Zhu 2000; Thomashow 2001). These stress-induced genes have been systematically termed rd (responsive to dehydration), erd (early responsive to dehydration), cor (cold-regulated), lti (low-temperature induced) and kin (cold-inducible).

Till now, it has been revealed from the research that a constitutive expression of a transgene causes either stunted/abnormal growth under normal conditions or mild/severe growth retardation in the aerial parts and reduced sugar content, compared to non-transformed plants (Table 1). High levels of constitutive expression of Adc gene cause inability of callus tissue to develop into a differentiated plant (Capell et al. 1998). otsA and otsB/TPS1 genes in transgenic potato and tobacco resulted in improved drought tolerance but exhibited stunted growth lancet-shaped leaves and reduced sugar content (Goddijn et al. 1997; Romero et al. 1997). Similarly, constitutive overexpression of mltD and IMT genes in transgenic N. tabacum had abnormal flower development, 20–25% smaller in size and possessed reduced sugar content compared with the wild type (Karakas et al. 1997; Sheveleva et al. 2000). The Arabidopsis CBF1 gene that on expression in the transgenic tomato exhibited tolerance against chilling and oxidative stress also exhibited a pattern of growth retardation, but however resumed growth after GA3 treatment (Hsieh et al. 2002a). Constitutively expressed ABF3/ABF4 in transgenic Arabidopsis exhibited mild/severe growth retardation in the arial parts, petioles were shorter and leaves were rounder and smaller and flowering was delayed (Kang et al. 2002). Constitutive overexpression of rice DREB1 (OsDREB1) transgenic Arabidopsis revealed salt, cold and drought tolerance (Dubouzet et al. 2003; Ito et al. 2006). However, the constitutive ectopic overexpression of the DREB1 gene in a transgenic plant showed growth retardation under normal growth conditions. Shen et al. (2003) found that rice plants expressing DREB1 (TaDREB1) under the control of the CaMV35S promoter were more stress tolerant but under an unstressed condition caused a dwarf phenotype. Recently it has been revealed that constitutive overexpression of the XTH gene from hot pepper in transgenic Arabidopsis exhibited improved tolerance to severe water deficit and lesser extent to high salinity in comparison to the wild type, but on other hand had abnormal leaf morphology such as twisted and bending edge, severely wrinkled leaf shape etc. (Cho et al. 2006). To overcome these problems, stress-inducible promoters that have low background expression under a normal growth condition have been used in conjunction with the transgenes to achieve increased stress tolerance without the retarded growth. Stress-inducible overexpression of transcription factor AtDREB1A under the control of AtRD29A in transgenic Arabidopsis and rice showed enhanced tolerance to chilling, drought and salt stress with overcoming the problem of growth retardation (Kasuga et al. 2004). Similar results were observed in Potato (Solanum tuberosum), Dendranthema grandiflorum and Peanut (Arachis hypogea) by Behnam et al. (2007); Hong et al. (2006) and Bhatnagar-Mathur et al. (2008).

Table 1. Constitutive overexpression of stress-responsive genes in transgenic results in growth abnormalities.

Gene	Source	Host	Promoter	Comments	Reference
CBF1	Arabidopsis	cv. umatilla	CaMV35S	Negative phenotypes, including smaller leaves, stunted growth, delayed flower and reduction or lack of tuber production	Pino et al. (2007)
CBF2
CBF3
GUS		Tobacco	AAP2 under influence of CaMV35S	Exhibited twofold to fivefold increase in AAP2 promoter activity and becomes active in all tissue types whereas the AAP2 promoter is active only in the vascular tissue	Zheng et al. (2007)
iaaM			AGL5 under influence of CaMV35S	AGL5:iaa5 showed a wild-type phenotype except seedless fruit whereas CaMV35S:iaa5 showed drastic morphological alternation in non-ovary organs including roots, stem and flowers
Barnase			PAB5under influence of CaMV35S	Drastically reduced transformation efficiency, expression of barnase gene in non-reproductive organ such as calli, shoot primordia whereas PAB5 is the pollen-ovule and early embryo specific promoter
CaXTH	Hot pepper	Arabidopsis	CaMV35S	Exhibited abnormal leaf morphology, twisted and bending edge. Improved tolerance to severe water deficit and lesser extent to high salinity in comparison to wild type	Cho et al. (2006)
DREB1A	Arabidopsis	D. Grandiflorum	CaMV35S	Compared with wild type severe growth retardation was observed	Hong et al. (2006)
DREB1A	O. sativa	Rice	CaMV35S	Improved tolerance to drought, high salt and low temperature. Growth retardation under normal growth conditions	Ito et al. (2006)
DREB1A	Arabidopsis	Tobacco	CaMV35S	Severe growth retardation, tolerance to drought, freezing than control plant	Kasuga et al. (2004)
DREB1A	Oryza sativa	Arabidopsis	CaMV35S	Tolerance to drought, high salt and cold stress, showed mild growth retardation, especially in bolting timing	Dubouzet et al. (2003)
DREB1	T. astivum	Rice	CaMV35S	Suggest that TaDREB1 functions as a DRE-binding transcription factor in wheat and TaDREB1 gene under an unstressed condition causing a dwarf phenotype	Shen et al. (2003)
PIP1b	Arabidopsis	Tobacco	CaMV35S	Significantly increased plant growth under normal conditions, but the plants were more sensitive to drought stress	Aharon et al. (2003)
mltD	E. coli	Wheat	Ubiquitin	had severe abnormalities including sterility, stunted growth, twisted heads and curled leaves	Abebe et al. (2003)
CBF4	Arabidopsis	Arabidopsis	CaMV35S	Plants are more tolerant to freezing and drought stress but CBF4 expression caused the most severe growth phenotype	Haake et al. (2002)
CBF1	Arabidopsis	Tomato	CaMV35S	Exhibited pattern of growth retardation, but resumed the growth after GA3 treatment, also exhibited tolerance against chilling and oxidative stress	Hsieh et al. (2002b)
ABF3		Arabidopsis	CaMV35S	Exhibited mild growth retardation in the arial parts, petioles were slightly shorter and leaves were rounder	Kang et al. (2002)
ABF4				Severe growth retardation, petioles were shorter, leaves were smaller and flowering was delayed
mltD and IMT		N. tabacum	CaMV35S	Had abnormal flower development and reduced sugar content	Sheveleva et al. (2000)
CBF3	Arabidopsis	Arabidopsis	CaMV35S	Increases the freezing tolerance of cold-acclimated plants, had a ‘dwarf’ phenotype	Gilmour et al. (2000)
Adc		Oryza sativa	CaMV35S	High levels of Adc gene expression and inability of callus tissue to develop normally into differentiated plant	Capell et al. (1998)
Stpd1	Apple	Tobacco	CaMV35S	Resulted in necrosis and sterility	Sheveleva et al. (1998)
TPS1	S. cereviaseae	N. tabacum	CaMV35S	Improved drought stress, exhibited stunted growth lancet-shaped leaves, reduced sugar content	Romero et al. (1997)
mtlD		Tobacco	CaMV35S	Transgenics were 20–25% smaller in size and had reduced sucrose compared with wild type	Karakas et al. (1997)
TPS1	S. cereviaseae	Tobacco	CaMV35S	Stunted growth, lancet-shaped leaves, reduced sucrose content and altered stem morphology	Pilon-Smits et al. (1998)
otsA and otsB	E. coli	N. tabacum S. tuberosum	CaMV35S	Results in severely stunted growth	Goddijn et al. (1997)

Overexpression of DREB1A from Zea mays under the control of the RD29A promoter in Arabidopsis played a dual functional role in mediating the expression of genes responsive to both water and heat stress without phenotypic alteration (Qin et al. 2007). The Arabidopsis DREB1A/CBF3 gene was placed under control of the RD29A promoter and transferred via biolistic transformation into bread wheat (Pellegrineschi et al. 2004). Plants expressing the DREB1A/CBF3 gene demonstrated substantial resistance to water stress in comparison through checks under experimental greenhouse conditions, manifested by a 10-day delay in wilting when water was withheld. These results indicate that a combination of the RD29A promoter and DREB1A is useful for improvement of various kinds of transgenic plants that are tolerant to environmental stress. Besides these, a sunflower HD-Zip protein gene promoter Hahb4 was found to express in specific tissues like root, leaves and cotyledons when induced by water stress, high salt and ABA (Dezar et al. 2005). Stress-induced expression of AtDREB1A/CBF1 under the control of HVA1/RD29A promoters in transgenic Bahiagrass (Paspalum notatum), tomato and S. tuberosum exhibited enhanced survival and biomass production under severe dehydration and salt stress and do not compromise plant productivity, performance and fertility under non-stress conditions (Lee et al. 2003; Pino et al. 2007; James et al. 2008; Xiao et al. 2009). The performance of the transgenic China Rose (Rosa chinensis) overexpressed with DREB1C from Medicago truncatula under freezing stress was superior to untransformed controls. Transgenic plants continued to grow, flowered under unstressed conditions and were phenotypically normal (Chen et al. 2010).

Transgenic plants overexpressing ERA antisense driven by the RD29A promoter were more resistant to water deficit-induced seed abortion during flowering and with adequate water, under moderate drought stress at flowering, the seeds yield of transgenic canola was significantly higher than the control (Wang et al. 2005). Overexpression of otsA and otsB, P5CS and TPS1 genes under the control of stress-inducible promoters in transgenic plants revealed significantly higher tolerance to stress produced by high levels of NaCl or water deficiency as judged by faster growth of shoots and roots in comparison with non-transformed plants (Garg et al. 2002; Su & Wu 2004; Karim et al. 2007). Dalal et al. (2009) reported that stress-induced overexpression of the Bnlea4-1 gene exhibited enhanced drought tolerance. Xiao et al. (2007); RoyChoudhury et al. (2007); Ahmad et al. (2008); Wu et al. (2009) and Kobayashi et al. (2010) observed similar results in rice and potato transgenics by expressing the transgene driven by abiotic stress-inducible plant promoters. These are some excellent examples that showed modification of transcription factor levels can successfully produce stress tolerance by using stress-inducible plant promoters (Table 2). It is, therefore, realistic to hold an optimistic view that these and other stress-related transcription factors would also be able to confer drought tolerance in diverse crop species ranging from dicots to monocots. Promoters that target transgene expression can be used to avoid potential transgene genetic drift through pollen or address the hypothetical risk of viral recombination events between host species DNA and viral promoters heavily utilized in plant genetic engineering (Vaden & Melcher 1990; Morel & Tepfer 2000; Daniell 2002). The promoters that drive transgene expression can be a part of the answers to questions about the safety of transgenic plants. Therefore, isolation and characterization of promoters suitable for plant genetic engineering are highly desirable.

Table 2. Stress-inducible over expression of stress-responsive genes in transgenic results in stress tolerant with overcoming the problem of growth retardation/abnormalities.

Gene	Source	Host	Promoter	Comments	Reference
Osmotin	Tobacco	Mulberry	RD29A	More responsive to drought	Das et al. (2010)
DREB1C	M. truncatula	China Rose (R. chinensis)	RD29A	Performance under freezing stress was superior. Transgenic plant phenotypically normal and flowered	Chen et al. (2010)
IDEF1	Rice	Rice	IDS2	Exhibited tolerance to iron deficiency in both hydroponic culture and calcareous soil	Kobayashi et al. (2010)
CBF3		Rice Arabidopsis	HVA22	Significantly higher yield per plant and had higher spikelet fertility than WT, and showed relatively better effect in improving drought resistance of transgenic rice under field conditions	Xiao et al. (2009)
SOS2
NPK1
LOS5
ZAT10
WRKY11	Rice	Rice cultivar Sasanishiki	HSP101	Showed significant heat and drought tolerance, as indicated by the slower leaf-wilting and less-impaired survival rate of green parts of plants	Wu et al. (2009)
DREB1	soybean (Glycine max)f	Alfalfa (Medicago sativa)	RD29A	Significantly higher salt tolerance as measured by ion leakage, chlorophyll fluorescence value, contents of free proline and total soluble sugars; additionally, transcript level of P5CS was up-regulated	Jin et al. (2010)
Bnlea4-1	B. napus	Arabidopsis	RD29A	Enhanced drought tolerance	Dalal et al (2009)
OsLEA3-1	Rice	Rice	OsLEA3-1	Significantly higher relative yield	Xiao et al. (2007)
Rab16A	Rice	Rice	Rab16A	Conferring salt tolerance without affecting growth and yield	RoyChoudhury et al. (2007)
codA	A. globiformis	S. tuberosum	SWPA2	Maintained higher water contents and accumulated higher levels of vegetative biomass	Ahmad et al. (2008)
CBF1	Arabidopsis	cv. Umatilla	RD29A	Restore tuber production to levels similar to wild-type plants and imparting same level of freezing tolerance as the 35S version	Pino et al. (2007)
CBF2
CBF3
DREB1A	Arabidopsis	Peanut (hypogea)	RD29A	No growth retardation or visible phenotypic alternation and achieved higher transpiration efficiency (TE) under water-limiting conditions than the untransformed control	Bhatnagar-Mathur et al. (2008)
DREB1A	Arabidopsis	Bahiagrass (P. notatum)	HVA1	No compromised plant productivity, performance and fertility under non-stress conditions. Enhanced survival and biomass production under severe dehydration and salt stress compared to wild-type plants	James et al. (2008)
OsNAC6	Rice	Rice	OsNAC6	Stress-inducible over expression of OsNAC6 in rice can improve stress tolerance by suppressing the negative effects of OsNAC6 on growth under normal growth conditions	Nakashima et al. (2007)
TPS1	S. cerevisiae	Tobacco	RBCS1A	Enhanced drought tolerance but caused growth aberrations of the green plant and root abnormalities. Shunted growth, slowly growing plants with lancent-shaped leaves	Karim et al. (2007)
DREB1A	Zea mays	A. thaliana	RD29A	Play a dual functional role in mediating the expression of genes responsive to both water and heat stress	Qin et al. (2007)
DREB1A	Arabidopsis	D. Grandiflorum	RD29A	No growth retardation on RD29A:DREB1A plants, exhibited significant stronger tolerance to drought and salt stress than 35S:DREB1A plants. Proline content and SOD activity were increased inductively in RD29A; DREB1A plants	Hong et al. (2006)
DREB1A	Arabidopsis	Potato	RD29A	Improved the dehydration salt and freezing tolerance with overcoming the problem of growth retardation	Behnam et al. (2007)
ERA1 antisense			RD29A	More resistance to water deficit-induced seed abortion during flowering and with adequate water. Transgenic canola produce same amount of seeds as the parental control. Under moderate drought stress at flowering, the seeds yield of transgenic canola was significant higher than the control	Wang et al. (2005)
DREB1A	Arabidopsis	Bread wheat	RD29A	Sustainable resistance to H2O stress in comparison with checks under experimental green house conditions, manifested by a 10-day delay in wilting when water was withheld	Pellegrineschi et al. (2004)
otsA and otsB	E.Coli	Rice	ABRC1	After prolonged exposure to salt stress, almost all of transgenic plants survived and displayed vigorous root and shoot growth	Garg et al. (2002)
P5CS	V. conitifolia	Rice	APIC	Showed significantly higher tolerance to stress produced by high levels of NaCl or water deficiency as judged by faster growth of shoots and roots in comparison with non-transformed plants	Su and Wu (2004)
DREB1A	Arabidopsis	Tobacco	RD29A	Exhibited slightly growth retardation, showed higher tolerance to drought and freezing than the wt plants	Kasuga et al. (2004)
CBF1	Arabidopsis	Tomato	ABRC	Exhibited enhanced tolerance to chilling water deficit and salt stress, minimizing the negative effects in transgenic tomato plants pertaining to growth and yield compared with the use of CaMV35S	Lee et al. (2003)
DREB1A	Arabidopsis	Arabidopsis	RD29A	Exhibited enhanced tolerance to chilling, drought and salt stress. Also solved the problem of growth retardation	Kasuga et al. (1999)

Conclusion

Rapid population growth, economic development and international economic integration have intensified resource use in every region of the world. Considering that the human population is expected to total 9 billion by 2050, producing more food from the same area of land while reducing the environmental impacts requires what has been called sustainable intensification, in order to feed, clothe and provide energy to such a large population (Hussain 2006; Godfray et al. 2010). In exactly the same way that yields can be increased with the use of existing technologies, many options currently exist to reduce negative externalities. Transgenic approaches have been shown to be powerful tools to help understand and manipulate the responses of plants to stress. Integrating global analyses from transcription to proteins to metabolites during abiotic stress will advance our understanding of major metabolic pathways and provide direction for achieving abiotic stress-tolerant plants. The viable evaluation of transgenes that enhance crop performance under both stress and optimal conditions is a prolonged, tedious and expensive process. It is proposed that current stance of plant stress tolerance can be significantly polished by thorough characterization of individual genes and evaluating their contribution to stress tolerance. A plethora of regulatory proteins like transcription factors associated with salt, drought and temperature tolerance have helped our understanding of the molecular mechanisms associated with them and also the plant survival and crop yields to some extent under these conditions. To facilitate accurate evaluation in the field of the genotypes for abiotic stress resistance, a deeper understanding of the transcription factors regulating major stress-responsive genes and cross-talk between divergent signaling components still remains an area of research activity in future. There is dire need to understand traits that are associated with root architecture and plasticity especially in agronomically superior genotypes under abiotic conditions because their manipulation might help to advance our knowledge of crop tolerance to various abiotic stresses. The stress biotechnology research in the near future will emphasize on strength and stress-induced expression of the transgenes, combined with the regulatory machinery involving transcription factors as a new genetic manipulation tool for controlling the expression of many different stress-responsive genes.

Acknowledgement

The authors would like to acknowledge the Council of Scientific and Industrial Research (CSIR), India, for financial assistance in terms of SRF to one of the fellow (Mudsser Azam).

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The authors are thankful to the University Grant Commission (UGC), New Delhi, India, for providing funds (39-109/2010SR) to carry out this work.