Transcriptomic and metabolomic analysis of grain weight potential induced by water stress in wheat

The sink strength of developing ovary in wheat determines the grain weight potential. The period from booting to grain setting stage is critical for the growth and development of ovary and potential sink capacity determination. However, the underlying regulation mechanism during this period by which wheat plant balance and coordinate the florets number and ovary/grain weight under water stress has not been clarified. Therefore, we designed two irrigation treatments W0 (no seasonal irrigation) and W1 (additional 75 mm irrigation at jointing stage), and analyzed the responses of the ovary/grain weight to water stress covering the phenotypic, metabolomic, and transcriptomic levels. Results showed that W0 irrigation treatment decreased soil water content, plant height, and green area of flag leaf, thus reduced grain number, especially the inferior grain, but improved the grain weight of superior grain and inferior grain as well as average grain weight at maturity, meanwhile the average ovary/grain weight and volume during -3 to 10 days after anthesis (DAA) also increased. Transcriptomic analysis indicated that the genes involved in both the sucrose metabolism and phytohormone signal transduction were prominently accelerated by W0 treatment, accompanied by increased enzyme activities of soluble acid invertase (SAI) and sucrose synthase (Sus) and elevated abscisic acid (ABA) and indole-3-acetic acid (IAA) levels. The sucrose content was thus decreased, while glucose and fructose content were increased. In addition, several TaTPP genes (especially TaTPP-6) were downregulated and the IAA biosynthesis genes TaTAR1 and TaTAR2 genes were upregulated under W0 treatment before anthesis, which improved IAA level further. Collectively, water stress reduced vegetative organs growth and eliminated most of the inferior grains, but improved the ABA and IAA levels of the surviving ovaries/grains, promoting enzyme activity of Sus and degrading sucrose into glucose and fructose; as a result, the strong sucrose utilization ability, the enhanced enzyme activity of SAI and the ABA- and IAA-mediated signaling jointly improved the weight and volume of surviving ovaries/grains, and finally achieved the trade-off between ovary/grain weight and number in wheat under water stress.