Low sucrose availability reduces basal spikelet fertility by inducing ABA and JA synthesis in wheat.

Within a spike of wheat, central spikelet usually generates 3-4 fertile florets, while basal spikelet hardly achieves it; in fact, the physiological and transcriptional mechanism behind the difference in fertility between basal and central spikelet is unclear. This study reports a high temporal-resolution investigation of transcriptomes, number and morphology of floret primordia and physiological traits. The W6.5-W7.5 stage was regarded as a boundary domain to distinguish between fertile and abortive potential of floret primordia; those floret primordia reaching the W6.5-7.5 stage during differentiation phase (3-9 days after terminal spikelet stage, DAT) usually developed into fertile florets in the next dimorphism phase (12-27 DAT), whereas the others aborted. Central spikelet had a greater number of fertile florets than basal spikelet, which was associated with more floret primordia reaching the W6.5-7.5 stage. Physiological and transcriptional results demonstrated that central spikelet had a higher sucrose content, lower abscisic acid (ABA) and jasmonic acid (JA) accumulation than basal spikelet due to down-regulation of genes involved in ABA and JA synthesis. Collectively, we proposed a model in which ABA and JA accumulation was induced under limiting sucrose availability (basal spikelet) through up-regulating genes involved in ABA and JA synthesis; this led to floret primordia in basal spikelet hardly to reach fertile potential (W6.5-7.5 stage) during differentiation phase and then aborted. This fertility repression module may also regulate spikelet fertility in other cereal crops and potentially provide genetic resources to improve spikelet fertility.