Suppressed ABA signal transduction in the spike promotes sucrose use in the stem and reduces grain number in wheat under water stress

Water stress is a primary trigger for reducing grain number per spike in wheat during the reproductive period. However, under stress conditions, the responses of plant organs and the interactions between them at the molecular and physiological levels remain unclear. In this study, when water stress occurred at the young microspore stage, RNA-seq data indicated that the spike had 970 differentially expressed genes, while the stem, comprising the two internodes below the spike (TIS), had 382. Abscisic acid (ABA) signal transduction genes were down-regulated by water stress in both these tissues, although to a greater extent in the TIS than in the spike. A reduction in sucrose was observed, and was accompanied by increases in cell wall invertase (CWIN) and sucrose:sucrose 1-fructosyl-transferase (1-SST) activities. Hexose and fructan were increased in the TIS but decreased in the spike. ABA was increased in the spike and TIS, and showed significant positive correlation with CWIN and 1-SST activities in the TIS. Overall, our results suggest that water stress induces the conversion of sucrose to hexose by CWIN, and to fructan by 1-SST, due to increased down-regulation of ABA signal transduction related-genes in the TIS; this leads to deficient sucrose supply to the spike and a decrease in grain number.
Deficient sucrose and weak expression of ABA signal transduction genes in the spike may be the major reasons for the decrease in grain number under water stress.