Drought and Heat Stress: Constraints and Opportunities
Authors: Prashant Raghunath Shingote, Yashoda Bhausaheb Etther and Gokul Uttamgir Gosavi
Abiotic stresses like drought, Heat, cold, toxic metals are amongst most serious problems out-coming in the latest phase of climate change. Concurrently, food security issue for the endlessly growing population is also a major constraint for agriculture production. Researchers and environmentalists are trying to understand the molecular basis of plant tolerance to stress situations using a variety of approaches; however, success has been limited. Therefore, bioprospecting of accessible genotypes for newer and competent genes holds great potential in near future. Studies associated with epigenetic inheritance of stress tolerance are also in the progress and are placed under transgenerational plasticity. Usual breeding approaches in combination with genomics and new age phenomics have the ability to symbolise better combinations to face abiotic stress conditions. Current progress in the crop improvement for abiotic stress tolerance involves; integrated use of genomics, transcriptomics, metabolomics, bioinformatics and molecular breeding (MAS). Selection and development of abiotic stress tolerant cultivars will ultimately help in enhancing food security and sustainability at elevated and adverse abiotic stress conditions.
Drought and heat stress:
Drought and Heat stress conditions occur due to insufficient rainfall and are the most devastating abiotic stress affecting agricultural productivity. The seriousness of drought stress depends on its timing, duration and intensity (Serraj et al., 2005). Altered rainfall and desertification are considered to drive climate change or emissions of greenhouse gases; this leads to change in recommendation for agricultural inputs. Among the major world food grain crops wheat, rice, maize etc. are most sensitive to drought at their critical developmental stages. Drought and Heat stress causes increased evapotranspiration and affects photosynthetic abilities which further reduces yield. Drought and Heat tolerance is a complex trait controlled by several QTLs. Because of puzzling complexities of plant responses to drought and Heat, it is crucial to identify the physiological and molecular basis for their interactions with plants. Complex traits also come up with risk for utilising conventional and modern breeding approaches in improvement studies (Passioura, 2010; Sinclair, 2011).
Integrated solution for drought tolerance:
The significant versatile approaches including genetic engineering, physiological and genomics have been demonstrated earlier in several crop species. In crop improvement, QTLs can be deployed through molecular breeding and candidate genes possibly can be used for the transgenic purpose. However, the biosafety regulatory process and negative public opinion cause limitation to the application of genetic engineering, while there is a wider acceptance of products generated through molecular breeding (Varshney et al., 2011). Hence, integrated uses of modern breeding approaches for drought and Heat tolerance have better applicability and acceptance. In recent past, molecular plant breeding has provided a significant scientific framework for modern crop improvement era. Here, three key areas of molecular plant breeding are illustrated: Identification of QTLs, Screening of candidate genes and MAS based utilisation.
Identified genes or markers for drought and Heat tolerance can be further used in breeding program. Modern and applicable methods of marker-assisted selection (MAS) are: Marker assisted backcross (MABC), Marker-assisted recurrent selection (MARS) and Genome-wide selection (GWS).
Quantitative trait loci (QTL) identification:
QTLs for drought tolerance have been identified in several major and agriculturally important crop species. Molecular markers are generally used for identification of the trait. Three groups of markers are Hybridization based DNA markers which include restriction fragment length polymorphisms (RFLP) and oligonucleotide fingerprinting. Secondly, PCR-based markers such as random amplified polymorphic DNAs (RAPD), sequence characterized amplified regions (SCAR), simple sequence repeats (SSR), sequence tagged sites (STS), amplified fragment length polymorphisms (AFLP), inter-simple sequence repeat (ISSR), cleaved amplified polymorphic sequences (CAPS) and amplicon length polymorphisms (ALPs) and third one is DNA chip and sequencing-based DNA markers such as single nucleotide polymorphisms (SNPs). To date, QTLs were identified for a variety of important traits including: yield and yield contributing traits under water-deficit conditions, physiological responses (water-soluble carbohydrates, osmotic potential, chlorophyll content, relative water content, leaf osmotic potential, osmotic adjustment etc.), flowering time, root traits, stay green, nitrogen fixation etc.
Utilisation of Candidate gene approach:
Genome sequences for model plants and major crop species have become available recently. Genome annotation and functional genomics helpful to disclose data of the candidate genes responsible for various important traits including abiotic stress responses. These genes can directly involve in the expression of proteins for the trait of importance or may be involved in regulatory pathways i.e. for expression of MYB like transcription factors. Studies related to this will be useful for understanding the molecular basis of abiotic stress tolerance and in their breeding applications through MAS. More than 30 genes related to drought and Heat tolerance are known and before their application in the breeding programme their validation is essential. Different approaches for validation of such important candidate genes are QTL mapping, association mapping, expression studies by qRT-PCR and TILLING. However, no expected results have found with respect to breeding for drought tolerance.
Use of marker assisted selection:
RAPD, AFLP and RFLP are not preferred for MAS because of the poor reproducibility. PCR-based SSR and SNP markers are likely to be preferred because of easier applicability for large breeding populations. SSR markers are highly reproducible, co-dominant in inheritance, relatively simple and cheap to use and highly polymorphic. Other approaches of MAS for drought screening are: Sequence tagged site (STS), sequence characterised amplified region (SCAR) or single nucleotide polymorphism (SNP) markers that are linked to a gene or quantitative trait locus (QTL) i.e. yield, grain quality, drought and Heat tolerance. SNPs can act as high-throughput molecular markers and can be examined and exploited. The increase in available expressed sequence tags (ESTs) has recommended SNP-as marker of choice for plant breeders for improving stress tolerance.
The conventional breeding needs the assistance of new technologies; marker assisted selection (MAS) is one of them to maximise the probability of success. This allows a breeder to achieve early selection of individuals in breeding programs, and it is particularly useful when the trait is under complex genetic control, or when phenotypic trials are unreliable, destructive or expensive. MAS can be used to pyramid several different QTL's to the target genotype. The success of a marker-assisted breeding depends on following factors: availability of genetic map with an adequate number of uniformly-spaced polymorphic markers to accurately locate desired QTLs or major gene(s); Close linkage between the QTL or a major gene of interest and adjacent markets; adequate recombination between the markers and rest of the genome. Hence, the desirable properties of molecular markers for use in marker-assisted selection are: it should be tightly linked to target loci, should have high reliability to predict phenotype, should be highly polymorphic and should able to discriminate between different genotypes and technique should be simple, cost effective and rapid. QTLs independent of epigenetic effector showing consistent result should be involved in MAS and unstable QTL i.e. spotted in only one environment should not be used for MAS program.
With the advancement of molecular studies; identification, cloning and introgression of target loci become faster. Candidate genes which are directly and indirectly involved in abiotic stress tolerance are having great future. MAS supports breeder to improve effective selection and preciseness of positive combinations of genes in early generation. The accessibility of large numbers of publicly available markers and new marker technologies like SNP, EST can potentially reduce the cost of MAS. Marker-assisted breeding does not suffer from any public opposition which limits the application and acceptability of transgenic technologies. There are many challenges for breeders to fully exploit molecular marker in actual breeding programs in context to epigenetic effects. The objective of these approaches is to get direct and indirect benefits from biotechnology based research to answer the current global food security issues.
1. Mir R.R., Zaman-Allah M., Sreenivasulu N., Trethowan R. and Varshney R.K. (2012) Integrated genomics, physiology and breeding approaches for improving drought tolerance in crops. Theor. Appl. Genet.
2. Passioura J.B. (2010) Scaling up: the essence of effective agricultural research. Funct. Plant Biol., 37:585-591.
3. Serraj R., Hash C.T. and Rivzi S.M.H. (2005) Recent advances in marker assisted selection for drought tolerance in pearl millet. Plant Prod Sci 8:334-337.
4. Sinclair T.R. (2011) Challenges in breeding for yield increase for drought. Trends Plant Sci. 16:289-293.
5. Varshney R.K., Bansal K.C., Aggarwal P.K., Datta S.K. and Craufurd P.Q. (2011) Agricultural biotechnology for crop improvement in a variable climate: hope or hype? Trends Plant Sci. 16:363-371.
About Author / Additional Info:
I have completed my graduation and post graduation in Agricultural biotechnology. From last 4-5 years I have been working on different aspects of plant molecular genetics and functional genomics.