Global Reprogramming of Transcription and Metabolism in Response to Drought, and Genome Wide Association Studies of Drought Related Traits in Medicago

Periodic drought is a primary limitation on plant growth and yield of crops in many agricultural systems.  We have taken two complementary genome-based approaches to identify genes and biological processes that may underpin drought tolerance in the model legume, Medicago truncatula, with a view to improving drought tolerance in the important pasture legume, Medicago sativa (alfalfa).  Transcriptome analysis of roots and shoots from control, mildly, moderately and severely drought-stressed, and re-watered M. truncatula and M. sativa plants, identified thousands of genes that were altered in expression in response to drought. Expression of many of these genes was tightly coupled to plant water potential (i.e. drought intensity) changes suggesting involvement in drought adaptation. Combining metabolomic data with the transcriptomic data yielded insights into the regulation of metabolic pathways during drought stress.  Among the metabolites detected in drought-stressed Medicago plants, proline, myo-inositol and pinitol had striking regulatory profiles indicating involvement in Medicago drought tolerance (Kang et al. 2011, Plant J. 68, 871–889; Zhang et al. 2014, Plant Cell Environ. 37, 2553-76).

In complementary work, we carried out genome wide association studies (GWAS) to identify genes associated with drought adaptation and biomass production in 220 M. truncatula ecotypes from the so-called HapMap population (Kang et al. 2015, Plant Cell Environ. 38, 1997–2011).  Characterized traits included shoot biomass, maximum leaf size, specific leaf weight, stomatal density, trichome density, and shoot carbon-13 isotope discrimination (δ13C) of well-watered M. truncatula plants, and leaf performance under in vitro dehydration stress. Reverse-genetics, using a Tnt1-insertion mutant population (Cheng et al. 2014, New Phytol. 201, 1065-76) is being used to test predicted roles of specific genes in abiotic stress tolerance.