Drought Frontiers in Rice

The following sections are included:

• Contents

• Foreword

Drought is a major constraint to rice production in Asia. Drought occurs frequently and is one of the major reasons for wide fluctuations in rainfed production. The economic cost of drought estimated in this study was found to be substantial in rainfed areas of eastern India. The economic cost of drought depends largely on the frequency and coverage of drought, and the importance of rice in total farm income. Farmers deploy various coping strategies but these strategies were found to be largely unable to prevent a reduction in income and consumption in rainfed areas of eastern India. As a result, a large number of people fall back into poverty during drought years. The overall implications of these results for research, technology design, and policy interventions for a long-term mitigation of drought are discussed.

We present a preliminary crop growth simulation model-based characterization of the spatial and temporal distribution of drought stress in rice production. The main objectives of this approach are to assist in estimating the potential benefits of drought-tolerant rice varieties, and to help select target areas for evaluation and dissemination of these varieties. The simulation model results provide a simple way to reduce daily weather data to a single or a few indices, to be used as predictors in characterization or data-mining modeling methods. We emphasize the need to refine the simulation modeling methods and to integrate the simulation results with census data and those obtained from studies of farmer behavior in response to drought, and the effect of drought on rice yield in farmers' fields.

Drought is the most important constraint affecting yield in rainfed rice. Drought effects are most severe in unbunded upland fields and upper-toposequence bunded fields that infrequently accumulate standing water. Drought reduces productivity both through direct effects on biomass production and grain set and through delaying or rendering impossible crop management operations such as transplanting, fertilizer application, and weeding. There is substantial genetic variation for tolerance of drought stress, direct seeding in dry soil, and delayed transplanting. In areas where transplanting is likely to remain the major establishment system, varieties with tolerance of both drought and delayed transplanting are needed to reduce risk and increase productivity. In light-textured soils, direct seeding of drought-tolerant varieties in dry, unpuddled fields has the potential to eliminate the risk of transplanting failure and to advance maturity sufficiently to permit the production of a postrice crop. However, varieties for use in this establishment system must be highly weed competitive and have a high degree of tolerance of drought at the reproductive stage. The IRRI breeding program now routinely screens lines targeted at dry direct-seeding systems for rapid early biomass accumulation under aerobic conditions, a trait that has been shown to be closely associated with weed competitiveness. Lines targeted at transplanted systems are screened for yield under transplanting as 60-day-old seedlings. For both systems, advanced breeding lines are screened both for yield potential and for yield under continuous recurring drought stress after maximum tillering. IRRI research has confirmed that yield under drought stress has both a moderate heritability and a moderate positive correlation with yield potential, permitting the development of varieties combining high yield potential with stress tolerance. IRRI research has shown that direct selection for yield under drought stress, combined with selection for yield potential under favorable conditions, is an effective way to develop such cultivars. It has also been shown that hybrids are higher yielding than pure lines, on average, under both moderate lowland stress and delayed transplanting. Lines and hybrids combining high yield potential with yield of more than 2 t ha^-1 under severe lowland stress, and more than 1.5 t ha^-1 under severe upland stress, have been identified. Such varieties have the potential to reduce risk and increase overall productivity in drought-prone environments. Recent research indicates that, in many crosses, approximately 20–50% of the genetic variation for yield under severe stress is controlled by factors that also affect yield potential. The remainder appears to be affected by relatively few genes with large effects that are detectable only in drought-stressed environments. Such genes may be used to increase the drought tolerance of widely grown varieties via marker-assisted backcrossing.

Drought is one of the major constraints to rice production in the rainfed ecology in sub-Saharan Africa (SSA). It occurs not only in uplands but also in lowlands; for example, 70% of lowland rice farmers experience a drought problem at the reproductive stage, which can reduce yield more severely than at the vegetative stage. At WARDA, therefore, drought is one priority area to be addressed. In WARDA's interspecific breeding between Oryza sativa and O. glaberrima for the rainfed ecology, one of the most important characteristics is short duration to evade drought that often occurs in the latter days of the cropping season. Several interspecific lines (NERICA: New Rice for Africa) with growth duration of 90–100 days have been developed. Apart from this escape type, NERICAs showing better drought resistance than local O. sativa checks have also been identified. Several O. glaberrima landraces showing high resistance were identified after collection from the flood plains of the Niger River and crossing of these landraces with O. sativa and existing NERICAs with high agronomic performance has already been done. Several O. sativa varieties were also screened for drought resistance at both vegetative and reproductive phases. Promising O. sativa varieties with good drought resistance have been used in crosses with susceptible genotypes for creating breeding and mapping populations. The populations being created are expected to segregate for root characteristics and/or osmotic adjustment, both of which are important drought-resistance traits in rice. A WARDA-JIRCAS joint project also aims to develop drought-resistant varieties. The project narrowed target characters down to root penetration into the deeper soil layers, which is very effective for growth maintenance in certain drought situations. Highly promising O. sativa lines have already been identified, and the next step is to identify genes/QTLs for this trait for use in marker-assisted selection. Past drought research at WARDA mostly concentrated on varietal improvement. However, an agronomic approach is also within our purview. We inventory farmers' existing cultural practices to minimize the risks of yield reduction by drought and test their true usefulness. Integrated drought management options for rainfed rice combining resistant varieties and cultivation practices will also be developed and evaluated.

Drought is a major problem in rainfed lowland rice in Thailand, Laos, and Cambodia. Together with national agricultural research institutes, the University of Queensland has been involved in research on breeding strategies to minimize the adverse effects of drought for the past 15 years. Several putative drought resistance characters were examined in an attempt to identify plant characters that confer drought resistance in rainfed lowland rice in the region. Our approach was to grow a population under irrigated and drained conditions to identify genotypes that yielded well under drained conditions relative to irrigated conditions, and then examine the variation in relative yield in relation to putative drought resistance characters. The common pattern that emerged from this study was that rainfed lowland rice was a drought avoider, and those genotypes that were able to maintain higher internal water potential tended to produce higher grain yield under drought that developed around flowering and thereafter. A shorter delay in flowering was an indication of a genotype's drought avoidance when drought developed just prior to flowering. However, the effectiveness of the drought resistance characters in minimizing yield losses depended on the timing of drought development; for example, a shorter delay in flowering was not effective when drought developed well before flowering. The drought environment in rainfed lowlands was characterized in these countries by determining a free water level above and below the soil surface throughout growth at several locations for several years. The most common type of drought was late-season drought, but water deficit developed also at other times and affected yield. Yield reduction due to drought was related to the free water level in the field at around anthesis. Lateral and downward water movement contributed to a large variation in available water during crop growth, and top toposequence positions lost water faster, resulting in earlier development of plant water stress. A water balance model has been developed to accommodate various water flows in rainfed lowlands.

We have screened a large number of genotypes under drained conditions in the field in the wet season. The selection was mostly based on grain yield adjusted for flowering time and potential yield, but spikelet sterility, leaf water potential, and delay in flowering were also considered for the selection of drought-tolerant genotypes. A large number of putative drought-tolerant lines were selected and crossed with elite lines with high potential yield and grain quality. The populations thus developed are available for further testing for advancing the yield levels of rainfed lowlands in these countries. The contribution of drought tolerance to overall grain yield in rainfed lowlands in these countries was not always high when drought was mild, and potential yield that was obtained under irrigated conditions contributed most to the variation in yield under drained conditions. Thus, it is important to combine the ability to produce high yield under no water limitation and the ability to tolerate drought in one variety.

Rice production consumes disproportionally large amounts of water worldwide and there is an urgent need to develop drought-tolerant or water-saving rice cultivars because of the increasing threats of water shortage for arable lands worldwide. Few drought-tolerant rice varieties have been commercially released in the past decades, which could largely be attributed to the lack of breeding efforts specifically targeting improving drought tolerance (DT) and water-use efficiency (WUE), and partially to the complexity of genetics and physiology associated with DT/WUE in rice. Efforts have been limited in applying marker-assisted selection to improve DT in rice despite the numerous studies in genetically dissecting DT in rice using the QTL mapping approach. Progress was made recently in developing drought-tolerant rice cultivars at IRRI, which indicates that the conventional breeding approach is effective for breeding DT for higher-elevation rainfed ecosystems of Asia. However, developing hybrid rice cultivars should be an effective strategy to combine high yield potential with a good level of WUE/DT for most shallow rainfed lowlands of Asia. Backcross breeding and designed QTL pyramiding appear to be a new and promising breeding strategy for the development of large numbers of DT introgression lines in elite genetic backgrounds by large-scale backcross breeding; deep exploitation of useful genetic diversity for DT from the primary gene pool of rice; effective selection, discovery, allelic mining, and characterization of QTL networks for DT; and directed trait improvement by designed QTL pyramiding based on accurate genetic information of QTL networks. Many promising drought-tolerant lines have been developed in the program, even though the theoretical aspects underlying the genetic networks of the target traits and QTL pyramiding by design remain to be fully established.

Rainfed upland rice is a major crop in Brazil, contributing approximately 30% of the total rice production. Although upland rice is planted in areas with favorable rainfall distribution, much could be gained by increasing its resistance to drought. The total amount of rain in the season is normally sufficient; however, rainfall distribution is irregular in some years and the soil has low water retention capacity, and thus is unable to buffer those irregularities. Consequently, upland rice is regarded as a high-risk crop, which receives low inputs, resulting in low mean yield. Research on drought resistance in rice at Embrapa had three periods: more intense in the 1980s, with the release of varieties with reasonably high drought resistance; low emphasis in the 1990s, with a change in focus to plant type and grain quality; a rebirth in the 2000s, with projects focusing especially on the root system. Roots are being evaluated in soil columns or in the field during the dry season. Research projects are being implemented and breeding populations are being prepared, although no resistant varieties have been released yet as a result from those recent projects. Embrapa recognizes drought in rice as an important research theme, which will require medium-term multidisciplinary projects to deliver a significant impact on upland rice production.

Classical quantitative genetics aids crop improvement by providing the means to estimate heritability, genetic correlations, and predicted responses to various selection schemes. Genomics has the potential to aid quantitative genetics and applied crop improvement programs via large-scale high-throughput DNA sequencing and fingerprinting, gene expression analyses, and reverse genetics methods. To date, these techniques have mainly been useful in the identification of genes with discrete or at least moderate effects on high-value traits. A practical result of this research is the development of allele-specific markers that tend to be useful across many breeding populations. For example, knowledge of the fatty acid biosynthesis pathway in plants and the sequencing of genes in that pathway is being exploited to produce DNA markers to aid selection for specific modified fatty acid traits in soybean. The application of large-scale gene mapping techniques to the improvement of highly quantitative traits (controlled by many genes of small effects) is not yet proven, however, and, even if useful, may not be highly cost-effective unless large-scale genomics infrastructure is already in place to aid breeding programs. An example of the application of genomics to large-scale genetic diversity studies is the large-scale maize QTL mapping study under way based on the development of 26 related RIL populations that capture a large portion of the genetic variation available worldwide among public lines. For genomics to be useful for the improvement of drought resistance in rice, the identification of component traits with relatively simpler architecture and/or a very large-scale investment in genomics-assisted breeding may be required.

The unpredictability of drought patterns and the complexity of the physiological responses involved have made it difficult to characterize component traits required for improved performance, thus limiting crop improvement to enhance drought resistance. The various stress response mechanisms and options to enhance plant survival under severe stress do not usually translate into yield stability under water deficit. Increased crop yield and water productivity require the optimization of the physiological processes involved in the initial critical stages of plant response to soil drying, water-use efficiency, and dehydration avoidance mechanisms. New high-throughput phenotyping methodologies allow fast and detailed evaluation of potential drought-resistant donors and the large number of lines developed by drought breeding programs. Similarly, large collections of rice germplasm, including minicore sets, wild relatives, and mutant lines, are screened for drought-resistance traits. Genetic sources of drought resistance have now been identified for all major rice agroecosystems and some of the associated traits have been characterized. The identification and genetic mapping of QTLs for yield and related physiological traits under drought stress across environments are currently a major focus. This approach provides a powerful tool to dissect the genetic and physiological bases of drought resistance. If validated with accurate phenotyping and properly integrated in marker-assisted breeding programs, this will accelerate the development of drought-resistant genotypes. This paper reviews IRRI's recent progress and achievements in understanding the physiology of drought resistance in rice and presents future perspectives on the genetic enhancement of drought adaptation.

In rice, a deep and thick root system is generally considered to be a useful trait for maintaining yield under water stress in a broad range of conditions, notably in rainfed ecosystems. A large natural variation in root architecture is observed in rice due to the specific adaptation of groups of rice cultivars to contrasting hydrological conditions (irrigated, rainfed, or upland). Understanding the genetic and molecular mechanisms controlling the development of the root system and its adaptive plasticity in response to water availability and soil environment would help in improving yield stability of rice crops confronting drought situations. This paper reviews current knowledge on QTLs detected for constitutive and adaptive root development, the present status of QTL integration on a consensus QTL map linked to the rice physical map through sequenced markers, and QTL validation in near-isogenic or substitution lines. A key step is now to link QTLs to genes, which can be done through several approaches. We are presently exploring some of them: looking for QTLs for physiological parameters derived from models, fine-mapping QTLs for root depth using meta-analyses and recombinant genetics, searching for known root architecture and stress-response genes by direct genetics in an insertion mutant collection, and searching for orthologs in rice of genes involved in root development in other species. We will combine these approaches with association studies between polymorphism within validated candidate genes and phenotypic variation that will confirm the interest of the genes in the target plant material, and, in addition, will give access to a range of alleles at the genes. We think that these combined approaches could hasten the discovery of important genes and alleles involved in root traits, provided that a high-throughput reliable phenotyping technique linked to field performance, which is presently missing, is developed. This will provide breeding programs with markers allowing the combination of favorable alleles at key loci for root traits. Although genotype building involving a few loci is now well under control, we are still lacking experience in how to accumulate alleles at many loci in the most efficient way. A marker-aided recurrent selection strategy could help achieve this goal.

Progress in crop improvement is limited by the ability to identify favorable combinations of genotypes (G) and management practices (M) given the resources available to search among possible combinations in the target population of environments (E). Crop improvement can be viewed as a search strategy on a complex G × M × E adaptation or fitness landscape. Here we consider the design of an integrated systems approach to crop improvement that incorporates advanced technologies in molecular markers, statistics, bioinformatics, and crop physiology and modeling. We suggest that such an approach can enhance the efficiency of crop improvement relative to conventional phenotypic selection by changing the focus from the paradigm of identifying superior varieties to a focus on identifying superior combinations of genetic regions and management systems. A comprehensive information system to support decisions on identifying target combinations is the critical core of the approach. We discuss the role of ecophysiology and modeling in this integrated systems approach by reviewing (1) applications in environmental characterization to underpin weighted selection, (2) complex trait physiology and genetics to enhance the stability of QTL models by linking the vector of coefficients defining the dynamic model to the genetic regions generating variability, and (3) phenotypic prediction in the target population of environments to assess the value of putative combinations of traits and management systems and enhance the utility of QTL models in selection. We examine in silico evidence of the value of ecophysiology and modeling to crop improvement for complex traits and note that, although there is no definitive position, it seems clear that there is sufficient promise to warrant continued effort. We discuss criteria determining the nature of models required and argue that a greater degree of biological robustness is required for modeling the physiology and genetics of complex traits. We conclude that, although an integrated systems approach to crop improvement is in its infancy, we expect that the potential benefits and further technology developments will likely enhance its rate of development and that this approach will be particularly relevant in breeding for adaptation to water limitation.

The rice-based rainfed lowland system in Asia covers about 45 million hectares, almost 30% of the total rice area worldwide. Rice (Oryza sativa L.) is the main crop in this system and it grows in bunded fields that are flooded for at least part of the season. Overall, drought stress is considered the most important limitation to production in rainfed lowlands and is estimated to frequently affect about 19 to 23 million hectares. Severe and regular droughts affect mainly rainfed lowlands in South and Southeast Asia, but regional weather patterns, topography, and soil characteristics cause considerable drought-risk variations within and beyond these regions. In addition, drought-prone environments are often simultaneously affected by other abiotic stresses such as submergence, adverse soil conditions, pests, and weeds. Two main management strategies for drought-stress alleviation in rice can be distinguished. The first strategy is based on management options that allow escaping drought by either avoiding dry periods or by providing access to additional water resources. The second strategy is to moderate drought by reducing unproductive water losses and thereby "saving" water for productive transpiration. Both strategies contain several management options that offer considerable scope for improving drought-prone rainfed lowlands; however, direct seeding and improved nutrient management are probably the most widely applicable options. "Aerobic rice" as a new system of rice cultivation is still under development but promises considerable opportunities in specific target environments within the rainfed lowlands of Asia.

The worldwide water shortage and uneven distribution of rainfall, made more erratic by global climate change, make the improvement of drought resistance especially important for rice, which is the major cereal crop of monsoon-based agriculture of Asia. The intensity, duration, and occurrence of water stress in relation to various phenological phases differ in the diverse ecosystems in which rice is cultivated. Therefore, the development of specific rice genotypes with enhanced resistance to soil moisture deficit that occurs in a target environment is necessary to enhance water productivity in rice. To maximize production under these conditions, there is a need to have a combination of options that include drought-resistant varieties as well as effective crop management strategies. Important mechanisms of drought resistance include drought avoidance via enhanced water uptake, reduced water loss, and enhanced water-use efficiency (WUE), and drought tolerance via osmotic adjustment, antioxidant capacity, and desiccation tolerance. Depending upon the nature of drought stress, plants may employ some or all of these mechanisms. Most research efforts in rice have been directed toward physiological dissection of complex drought resistance mechanisms into component traits, and mapping of the quantitative trait loci (QTLs) for these physiological traits. QTLs have been shown to contribute from 5% to 50% of the phenotypic variation in a single component trait. This suggests that pyramiding of QTLs by marker-assisted selection (MAS) is necessary for significant improvement of drought resistance. The lack of consistency of QTLs across population limits their immediate application in MAS. The identification of common QTLs across populations under near realistic field stress conditions with standard-stress-assays will enhance the pace of their use in MAS. Genetic engineering efforts are also being made to enhance the drought resistance of rice by overexpressing effector genes or transcriptional regulators. Transcriptome engineering is emerging as an important tool to combat abiotic stresses. Combination of metabolomics with expression QTLs (eQTLs) will increase the pace of our understanding of the molecular basis of drought resistance.

Interspecific hybrid rice cultivars Xeiyou 9308 and Liangyoupeijiu were used as test materials and three levels of soil water content were applied during grain filling in irrigated fields to observe their effects on translocation and allocation of carbohydrates. The results showed that, in conventional flooding or nonflooding cultivation, the export rates of stored carbohydrates from stems and carbon assimilates from leaves were 60% and 90%, respectively. The export rate of carbohydrates decreased significantly in nonflooding cultivation. The filling grains were the major sink for carbohydrate storage during the grain-filling stage. Grains received nearly 50% of leaf-sheath-stored carbohydrates and 80% of leaf-stored carbon assimilates. In nonflooding conditions, the C assimilates import rates of grains decreased significantly by 10% and 20% from leaf sheath and leaf blades, respectively. Drought stress caused a large decrease in absorbing ability of inferior grains. A two-time water saturation during the grain-filling stage slowed the decline in root respiration and root exudates. The effects caused by drought stress during the grain-filling stage were mostly associated with inferior spikelets, resulting in a lower seed-setting rate and lower grain weight. This may be the main cause for lower yield under water stress during grain filling. We showed in this experiment that there was less effect of water stress during grain filling in Xieyou 9308 than in Liangyoupeijiu. Therefore, it seemed that Xieyou 9308 showed higher drought resistance than Liangyoupeijiu.

To elucidate global responses to drought stress in rice, we used a 60-mer oligomer microarray covering 22K of unique genes based on the sequences of full-length cDNA clones to profile gene expression changes in shoots at the seedling stage and in peduncles at the heading stage. Cluster analysis of genes up- and down-regulated by drought stress at these two different growth stages revealed stage-specific and stress treatment–specific gene expression profiles. Among 503 differentially expressed transcription factor–encoding genes, all the paralogous members of PHD and SNF2 were up-regulated, and those of Jumonji and TCP were down-regulated, by four drought-stress treatments. AP2-EREBP, AUX/IAA, bZIP, C2C2-GATA, C3H, CPP, HB, HMG, HSF, MYB-related, NAC, SBP, SNF2, and Trihelix were commonly up-regulated, and Alfin-like, AUX/IAA, BES1, bHLH, bZIP, MYB, NAC, WRKY, and ZIM were commonly down-regulated, by the four drought-stress treatments. The promoter regions (1 kb upstream from the start site of transcription) of the genes clustered after microarray experiments were examined by using information from the PLACE database. The cis-elements known to be localized in the promoter regions of drought stress–responsive genes in Arabidopsis were also found to be localized in the promoter regions of the rice genes. Data mining using gene annotation data (e.g., Rice Annotation Project database, Osa1 from TIGR Gene Ontology term), pathway data, and genome-mapping data suggested the existence of transcription networks of drought stress–responsive genes.

Drought resistance is a very complex trait with distinct molecular and physiological mechanisms in different plant species. Irrigated rice has been domesticated in an ecosystem with full irrigation and it is extremely sensitive to drought. With a long-term goal of improving drought resistance in rice, we have adopted a strategy of integrating approaches, including germplasm exploitation and using genetics and functional genomics to identify loci/genes effective for improving drought resistance in rice. In this paper, we describe the approaches and the major progress made to discover genes for improving drought resistance. On the basis of a genetic dissection of drought resistance of rice, more than 30 QTLs have been targeted for the construction of near-isogenic lines and marker-assisted molecular breeding. Several drought resistance–associated genes were identified through drought screening of T-DNA insertion mutants of rice. Hundreds of candidate genes were identified for drought resistance through comparative expression profiling analysis. More than 50 drought-responsive candidate genes were transferred into rice for drought resistance testing, and four genes (SNAC1, OsCIPK12, OsLEA3-1, and OCPI1) showed a significant effect in improving drought resistance in transgenic rice. Finally, the problems and perspectives of improving drought resistance in rice are discussed.

Characterization of single nucleotide polymorphisms (SNP) in candidate genes for drought tolerance is a promising approach for identifying alleles that are associated with drought phenotypes. For SNP discovery, we have used the technique of EcoTILLING contrasting diverse varieties to both japonica variety Nipponbare and indica variety IR64. Our germplasm panel of 1,536 Oryza sativa varieties covers the variety groups and eco-cultural types and includes parents of mapping populations used for drought QTL analyses. A set of candidate genes for drought tolerance was identified through convergent evidence taking into account genome annotation, function, expression, and localization with a yield-component QTL under water stress. These genes include DREB2A, ERF3, trehalose-6-phosphate phosphatase, and actin depolymerizing factor among others. EcoTILLING of a set of 900 of the Oryza sativa lines for 1,800 bp of coding and regulatory region of ERF3 identified a range of putative SNPs. Sequence confirmation from selected lines for each of the observed patterns identified 31 SNPs and short indels that grouped into nine haplotypes corresponding to variety types. Within-group association tests for drought-related traits were performed with one of the indica subgroups found to have a significant association with yield stability during water stress. Putative mismatches have been identified at 10 other candidate gene loci. Sequencing of these mismatches is under way to verify the SNP setting the stage for additional association tests of the SNPs with drought phenotypes. In addition to SNP analysis of individual candidate genes, we will take a genome-wide approach to relate SNP haplotypic variation with expression polymorphism and phenotypes. Through a multiple-partner SNP discovery project, we expect to obtain genome-wide SNP haplotype data for 20 diverse rice genotypes, some of which are known to have contrasting phenotypic responses to drought stress. The combination of SNP and drought stress-transcriptome data may reveal causal relationships among SNP haplotypic blocks, expression polymorphisms, and adaptive responses to drought.

This article summarizes recent research activities aiming at developing drought-tolerant rice at the Japan International Research Center for Agricultural Sciences (JIRCAS). We are using two approaches, molecular and conventional, to reach the targeted goal. In the molecular approach, characterizations of transcriptional factors such as DREB, AREB, and stress tolerance of the resultant transformants will be introduced. In the conventional approach, the current status of screening for drought-tolerant germplasm will be presented.

Stress responses and adaptation in crops involve complex mechanisms since the plants must respond to variable stress occurrence parameters and interacting environmental factors. However, experiments describing the enhancement of stress resistance by a transgenic approach are usually restricted to the modulation of the plant response to one stress factor. In this note, we review examples from the recent literature and focus, in more detail, on the methodology and the results of seven published studies that describe the enhancement of drought resistance in rice using various strategies (transcription factors, cell metabolism, water fluxes, and reactive oxygen species scavenging). Although the transformation methodology is somehow similar, the protocols for plant evaluation and the parameters used to assess plant resistance are diverse and difficult to compare. The low number of independent transgenic lines and the poor assessment of a gene effect versus an overall effect due to gene insertion and somaclonal variation could be major drawbacks. The relevance of some screening methods for drought is questionable from a breeding perspective and only a few studies provide actual data about enhanced drought resistance under field conditions. In addition, it is noteworthy to observe that most of the experiments are based on the overexpression of a single gene. If the use of transgenic crops could potentially enhance drought responses of plants by promoting changes in the genome, there is clearly an important need to define or redefine the major steps and criteria to obtain better crop performance in the field. We summarize in this paper some of the major steps and key criteria to identify better rice cultivars with enhanced drought resistance using GM technology.

Transgenic methods offer a complementary approach for classical breeding to improve the tolerance of plants toward biotic and abiotic stresses. The objective of our study at RC Biotechnology LIPI in Indonesia, in collaboration with the Institute of Biology of Leiden University (The Netherlands), is to explore the use of HD-Zip transcription factors in improving the performance of rice under dry conditions. Enhancing drought resistance entails transgenic expression of HD-Zip transcription factors involved in drought response of rice. Earlier, seven HD-Zip I and II genes were identified in rice that were analyzed for drought-responsiveness. One of the two dehydration-repressed HD-Zip genes was induced by 4 hours of flooding, as a treatment opposite to drought. This Oshox4 gene from the HD-Zip gene family was selected for study in transgenic plants. Transgenic Nipponbare lines overexpressing this particular gene developed smaller leaves and exhibited a reduction in senescence compared with the controls. Transformation of Indonesian rice with Oshox4 and characterization of transgenic lines, including drought phenotyping, are now being carried out. Experiments carried out in Arabidopsis showed that overexpression of this particular HD-Zip gene can confer resistance to drought. Our second approach aims at identifying novel drought-resistance genes and is based on insertional mutagenesis in rice using gene trap and activation tag constructs.

This paper starts with a general discussion about genomics and bioinformatics research,¹ then proceeds to highlight the status of IRRI-hosted and externally available rice bioinformatics resources for crop genomics research, with a special emphasis on some of the special kinds of data we have pertinent to drought research. The paper will be a bit of a brainstorming exercise on where we could go in the future with bioinformatics to help answer questions on drought resistance. We conclude with a brief discussion about what community-level coordination to drought research can be achieved using available informatics-based collaboration.

Drought is the major constraint to rice production in rainfed areas across Asia and sub-Saharan Africa. Frequent droughts result in enormous economic losses and have long-term destabilizing socioeconomic effects on resource-poor farmers and communities. In the context of current and predicted water scarcity scenarios, irrigation is generally not a viable option to alleviate drought problems in the rainfed rice-growing systems. It is therefore critical that genetic management strategies of drought focus on maximum extraction of available soil moisture and its efficient use in crop establishment, growth, and maximum biomass and seed yield. Drought mitigation, through improved drought-resistant rice varieties and complementary management practices, represents an important exit pathway from poverty. Recent advances in drought physiology and genetics together with progress in cereal functional genomics have set the stage for an initiative focusing on the genetic enhancement of drought resistance in rice. Extensive genetic variation for drought resistance exists in the rice germplasm. However, the current challenge is to decipher the complexities of drought resistance in rice and exploit all available genetic resources to produce rice varieties combining drought adaptation with high yield potential, quality, and tolerance of biotic stresses. The aim will be to develop a pipeline for elite "prebred" varieties or hybrids in which drought-resistance genes can be effectively delivered to rice farmers. The Frontier Project on Drought-Resistant Rice will scale up gene detection and delivery for use in marker-aided breeding. The development of high-throughput, high-precision phenotyping systems will allow genes for component traits to be efficiently mapped, and their effects assessed on a range of drought-related traits, moving the most promising into widely grown rice mega-varieties.