Flux in the field: genome-scale modelling reveals changes in soybean (Glycine max) seed reserve metabolism under...

Soybean (Glyine max) is particularly sensitive to drought stress during reproductive growth, which causes decreased productivity. Rates of reserve accumulation are altered, indicating perturbations in source-sink relations between different organs and in the metabolism of the developing seeds. Here we performed field experiments using controlled irrigation and in silico flux balance analysis to determine how drought alters the mobilisation of reserves in vegetative organs and their accumulation in the seed. Reduced irrigation resulted in stomatal closure and lower rates of photosynthesis and transpiration, together with earlier senescence in leaves and stems, characterized by decreases in starch, nitrogen and fatty acids. On the other hand, seed reserve accumulation, demonstrated by decreasing starch and increasing nitrogen, fatty acids and raffinose series oligosaccharides occurred earlier under stress. This shift in development, with a shortened period of grain filling, ultimately resulted in decreased productivity. The experimental data was used to constrain a genome-scale metabolic model of soybean, and flux balance analysis of seed metabolism predicted that whilst stress affected metabolism the relative distribution of fluxes showed a degree of robustness. Simulations performed with different rates of photon usage support multiple possible roles for seed photosynthesis in reserve accumulation. Overall drought leads to earlier senescence and a shorter window for seed filling that together with alterations in seed metabolism results in reduced productivity.