Transgenic Strategies for Improved Drought Tolerance in Legumes of Semi-Arid Tropics
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Journal of Crop Improvement, 24:92111, 2010Copyright Taylor & Francis Group, LLC ISSN: 1542-7528 print/1542-7535 onlineDOI: 10.1080/15427520903337095
WCIM1542-75281542-7535Journal of Crop Improvement, Vol. 24, No. 1, October 2009: pp. 00Journal of Crop Improvement
Transgenic Strategies for Improved Drought Tolerance in Legumes of Semi-Arid Tropics
Transgenic Strategies for Improved Drought ToleranceP. Bhatnagar-Mathur et al.
POOJA BHATNAGAR-MATHUR, J. SHRIDHAR RAO, VINCENT VADEZ, and KIRAN K. SHARMA
Genetic Transformation Laboratory, International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru, Andhra Pradesh, India
Water deficit is the most prominent abiotic stress that severely limitscrop yields, thereby reducing opportunities to improve livelihoodsof poor farmers in the semi-arid tropics (SAT) where most of thelegumes, including groundnut and chickpea, are grown. Sustainedlong-term efforts in developing these legume crops with betterdrought tolerance through conventional breeding have been metwith only limited success mainly because of an insufficient under-standing of the underlying physiological mechanisms and lack ofsufficient polymorphism for drought tolerance-related traits. Exhaus-tive efforts are being made at the International Crop ResearchInstitute for Semi-Arid Tropics (ICRISAT) to improve crop produc-tivity of the SAT crops by comprehensively addressing the constraintscaused by water limitations. The transgenic approach has beenused to speed up the process of molecular introgression of puta-tively beneficial genes for rapidly developing stress-tolerant legumes.Nevertheless, the task of generating transgenic cultivars requiressuccess in the transformation process and proper incorporation ofstress tolerance into plants. Hence, evaluation of the transgenicplants under stress conditions and understanding the physiologicaleffect of the inserted genes at the whole plant level is critical. Thisreview focuses on the recent progress achieved in using transgenictechnology to improve drought tolerance, which includes evalua-tion of drought-stress response and protocols developed for testingtransgenic plants under near-field conditions. A trait-based approachwas considered, in which yield was dissected into components.
Address correspondence to Pooja Bhatnagar-Mathur at Genetic Transformation Laboratory,International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru,Andhra Pradesh 502 324, India. E-mail: firstname.lastname@example.org
Transgenic Strategies for Improved Drought Tolerance 93
Yield (Y) is defined as transpiration (T) x transpiration efficiency(TE) x harvest index (HI).
KEYWORDS drought, transgenic, chickpea, groundnut, transpi-ration efficiency, water-use efficiency
Climate change is a major global concern that can make dryland agricultureeven more risk-prone, especially in the developing world. Abiotic stressesare a primary cause of crop loss worldwide, reducing average yields by>50% in most major crops (Boyer 1982; Bray, Bailey-Serres, & Weretilnyk 2000).Crops are often exposed to multiple stresses in many regions. Approximately,19% of the worlds agricultural land is subject to salt stress and 5% todrought stress (FAO, 1996).
The semi-arid tropics (SAT) cover parts of 55 developing countries andaccount for about 1.4 billion people of the world. The SAT has very shortgrowing seasons, separated by very hot and dry periods. In addition, rapidand unforeseen disturbances in these environments have resulted in stressfulconditions, which include paucity of water for long periods because of lackof irrigation, infrequent rains, or lowering of water table. These conditionscause drought stress. On the other hand, excess water from rain, cyclones,or frequent irrigation results in flooding, submergence or anaerobic stress.Climatologists believe that the changing global climate might produce evenmore severe and widespread dry conditions in these regions, with poten-tially serious consequences for agriculture and food availability (Wenzel &Wayne 2008). In addition, water limitation may prove to be a critical con-straint to crop productivity under future scenarios (Fischer et al. 2001). Sincerainfed agriculture, particularly in the developing countries of the SAT, con-tributes significantly to total food production, food security will be unsustain-able in the absence of dramatic yield increases in the marginal environments,especially in the drought prone areas.
Conventional breeding and enhanced management practices haveaddressed several constraints that limit crop productivity or quality, but thereare situations where the existing germplasm lacks the required traits. Whileplant breeding in the past has relied heavily on empirical approaches fordrought tolerance in crop plants, there is a broad consensus that strategicapproaches based on sound physiological and genetic understanding ofyield will also be required if further yield gains are to be achieved (Jacksonet al. 1996; Miflin 2000; Slafer 2003; Snape et al. 2001).
Broadly, the drought response of plants can be divided into threemechanisms: 1) drought escape, 2) drought avoidance, and 3) drought tol-erance. To date, the most important contribution to drought tolerance has
94 P. Bhatnagar-Mathur et al.
come from breeding for altered phonological traits like early flowering andmaturity (Araus et al. 2002) that essentially involves escape from droughtconditions rather then tolerance per se. Besides, plants undergo droughtpostponement either by conserving water or by enhanced water uptake,which is usually achieved by structural modifications such as increased cuticlethickness, deeper roots, reduced leaf area, or reduced stomatal conductance(Jones 2004). Limiting stomatal conductance can result in high water-useefficiency (WUE), which is a trait for postponement of stress. Under extremestress, tolerance is about survival and not productivity. The drought toler-ance is achieved through changes in the biochemical composition that pro-tect the macromolecules and membranes or maintain cell turgor throughosmotic adjustment.
Yield losses due to constraints like drought are highly variable innature depending on the stress timing, intensity, and duration, which isrelated to location-specific environmental stress factors such as high irradi-ance and temperature that make breeding for drought tolerance throughconventional approaches difficult. Since most crop plants have not beenselected for meeting exigencies caused by these stress factors, their capacityto adjust to such conditions is usually limited. Nevertheless, efforts to breedcrop species for high WUE and stomatal conductance have met with limitedsuccess. This is in part because the physiological data and informationabout the molecular events underlying the abiotic stress responses arescarce, besides the non-availability of molecular tools that could help inassisting the breeding activities for such complex traits. Besides, a main con-straint in the advancement efforts of crop improvement for drought toleranceincludes insufficient know-how of tolerance mechanisms, a poor under-standing of its low inheritance, and a scarcity of efficient techniques forscreening of germplasm and breeding material.
TRANSGENIC INTERVENTIONS FOR DROUGHT TOLERANCE
Nevertheless, agricultural biotechnology has the potential to address thischallenge and can play a role in developing crops that use water more effi-ciently, thus reducing the negative consequences of drought. The use ofgenetic engineering technology potentially offers a more targeted gene-basedapproach for improving plants adaptation to water-limiting conditions.Transgenic approaches offer a powerful means of gaining valuable informa-tion to understand the mechanisms governing stress tolerance, providing acomplementary means for the genetic betterment of the genome of fieldcrops and thus promising the alleviation of some of the major constraints tocrop productivity in the developing countries (Sharma & Ortiz 2000). Whena plant is subjected to abiotic stress, a number of genes are turned on, resultingin increased levels of several osmolytes and proteins, some of which may
Transgenic Strategies for Improved Drought Tolerance 95
be responsible for conferring a certain degree of protection from thesestresses. Therefore, it will likely be necessary to transfer several potentiallyuseful genes into the same plant in order to obtain a high degree of toler-ance to drought or salt stress. Novel genes accessed from exotic sourcesplants, animals, bacteria, even virusescan be introduced into the cropthrough biolistics or by using Agrobacteriummediated genetic transforma-tion (Sharma & Kavanya 2002). Further, it is possible to control the timing,tissue-specificity, and expression level of transferred genes for their optimalfunction.
Association of several traits with tolerance has been tested in transgenicplants. The results of transgenic modifications of biosynthetic and metabolicpathways indicate that higher stress tolerance can be achieved by engineering.However, the results of simulation modeling also suggest that changes in agiven metabolic process, relatively apart from the yield architecture, mayend up with little benefit for actual yield under stress (Sinclair, Purcell, &Sneller 2004). Various transgenic technologies have been used to improvestress tolerance in plants (Allen 1995). In recent years, the genes responsiblefor low-molecular-weight metabolites have been shown to confer increasedtolerance to salt or drought stress in transgenic dicot plants (mainly tobacco).Metabolic traits, especially pathways with few enzymes, have been geneti-cally characterized and are more amenable to manipulations than structuraland d