<scp>MicroRNA156</scp> and its targeted <i>SPL</i> genes interact with the photoperiod, vernalization, and gibberellin pathways to regulate wheat heading time

Heading time has a large impact on adaptation to different environments and crop productivity. In this study, we characterized the effect of the endogenous age pathway on heading time and its interactions with the photoperiod and vernalization pathways in the leaves of tetraploid wheat (Triticum turgidum ssp. durum). Plants with reduced levels of microRNA156 or increased expression of its downstream targets, the SQUAMOSA PROMOTER BINDING PROTEIN-LIKE genes SPL3, SPL4, and SPL13 exhibited accelerated heading time, with stronger effects under suboptimal inductive conditions. Earlier heading was associated with the upregulation of miR172 and flowering-promoting genes VRN1, FUL2, and FT1 and the downregulation of flowering-repressing genes AP2L1 and VRN2. Additionally, we uncovered complex interactions among SPL, SQUAMOSA (VRN1 and FUL2), and DELLA proteins that modulate wheat heading time. We showed that DELLA proteins, which are negative regulators in the gibberellic acid pathway, can interact with SPL proteins reducing their ability to induce flowering. We also discovered previously unknown interactions between SQUAMOSA and DELLA proteins in wheat that compete with the DELLA-SPL interactions, likely reducing DELLA's ability to repress SPL3 and SPL4 activity. Since SPL3 and SPL4 directly promote VRN1 and FUL2 transcription, these interactions generate a positive regulatory feedback loop that accelerates wheat heading time. Finally, we developed dominant miR156-resistant alleles rSPL3, rSPL4, and rSPL13 that accelerate wheat heading time under both optimal and suboptimal inductive conditions. These publicly available genetic resources can be used to fine-tune heading time and improve wheat adaptation to changing environments.