Drought Tolerance in Soybeans

Drought is the major abiotic stress factor limiting crop productivity worldwide. Water is an increasingly limited resource, and water availability limits crop productivity in many parts of the U.S. and the world. The genetic basis of drought tolerance is not well understood, and understanding how plant growth and developmental responses to drought are regulated is vital for efforts to modify the impact of water supply on soybean production. In soybean, drought is the greatest threat to profitability and too often a crop with great promise ends up with only fair or poor yields because of dry weather. Drought genomics is one of the major research focuses at the NCSB. The long-term goal of the NCSB scientists is to solve this problem through the delivery of high-yielding drought-tolerant varieties to U.S. farmers. We utilize both traditional breeding and marker-assisted selection programs, and functional genomics tools to dissect the molecular genetic mechanisms and to improve stress tolerance in soybean.

Plants possess several adaptive traits to endure period of drought. In soybean, rooting depth, water use efficiency, nitrogen fixation, leaf wilting is the most important traits to evaluate for drought tolerance. An important feature of root system responses to soil drying is the ability of the roots to continue elongation at low water potentials that completely inhibit the shoot growth. The NCSB scientists are studying the spatial and temporal pattern of cell expansion within the root growth zone to understand the physiological mechanisms which further support the functional genomic analysis. Additional research component in this area is the comparative legume root biology and associated drought responses. Another collaborative effort from the NCSB scientists (Drs. Henry Nguyen, Robert Sharp, David Sleper and Grover Shannon) is the investigation of genetic variation in root architecture and plasticity under drought. This work includes field screening of soybean plant introductions in collaboration with Dr. Jim Specht (University of Nebraska-Lincoln).

Development of RIL populations from contrasting parental lines for mapping root and other drought tolerance traits is in progress. Parents and RILs will be evaluated in the controlled conditions and field for different traits related to drought tolerance. Genotyping (parental polymorphism survey and molecular mapping), genetic map construction and QTL analysis will be done following standard lab protocols. Currently we have more than 1,000 SSR and 1,000 SNP markers available for QTL mapping. We have constructed cDNA libraries for root hairs and drought stressed root tips in collaboration with. More than 17,000 ESTs associated with root hair and drought responses in the roots of soybean plants were deposited to GenBank and are utilized for gene transcript profiling. Also, we have constructed normalized cDNA libraries from the drought stressed root tissue at V3 stage and the sequencing of the ESTs and FLcDNA in collaboration with DOE-JGI help to identify more stress related and root specific transcripts. In collaboration with Dr. Guo-Liang Wang at Ohio State University and DOE-JGI we have constructed soybean root related and drought responsive 5' and 3' RATE (Rapid Amplification of Transcript Ends) libraries and sequenced using the 454 platform. All these resources along with the gene expression profiling studies help predict and construct the soybean root transcriptome map.

We are conducting gene expression profiling studies of physiologically defined soybean seedling root tip regions and, soybean leaves and roots from more mature stages (V3) under drought stress to elucidate the pattern of gene expression and signaling cascades. Laser capture micro-dissection (LCM) technique is being used to target specific cell types. This will help discover candidate genes, specific promoters and transcription factors which will be used further for the genetic engineering for stress tolerance in soybean. Moreover, the LCM-driven microgenomics will be useful to dissect the early events of signal perception and root to shoot communications. The soybean transcription factor resource developed by the NCSB scientists were actively used to screen and discover the specific molecular switches regulating the stress tolerance mechanisms. We are utilizing deep sequencing technology platforms such as 454 capture maximum and rare transcript expression pattern under water deficit conditions. We are also conducting comparative genomics analysis between soybean and other legumes such as Medicago truncatula and Lotus japonicus and, model plants such as Arabidopsis and rice.

As part of the umbrella of drought research program, NCSB scientists are also elucidating gene regulatory networks of soybean seed development under stress using transcriptome, proteome and metabolome tools. Water availability plays a major role in the regulation of seed filling and development. Drought stress imposed during pod lengthening and seed filling stages had the greatest effect on the number of pods produced per unit of dry matter, and decreased the seed weight and led to some seed abortion in the pod.

Overall goal of these approaches is to discover novel genes, molecular switches and stress specific promoters associated with drought stress responses and to characterize them. The Nguyen lab has identified several drought related genes and transcription factors and the characterization of these candidates and the engineering of selected candidates through translational genomics pipeline are in progress.