Fig. 8: Working model demonstrating how the drought tolerance difference between Hap4 and other haplotypes is regulated at the molecular level.

All haplotypes except Hap4 are collectively referred to as BnaA9.NF-YA7^Hap. Arrows indicate upregulation, and lines ending with bars represent downregulation. The thickness of the lines represents the strength of the regulation. The BnaABF3/4 proteins promote the upregulation of BnaA9.NF-YA7 expression by direct DNA-binding at its promoter region under drought stress. To prevent excessive amplification of the ABA signal, the upregulated BnaA9.NF-YA7 regulates the cascade strength of the ABA signal by reducing the H3K36me3 level to inhibit the expression of BnaABF3/4s. On the other hand, BnaA9.NF-YA7 negatively regulates drought tolerance by promoting BnaA4.DOR activity via direct DNA binding at its promoter region. Hap4 containing a two-base sequence variant (CCAAT-box deletion) at the promoter is associated with a lower binding affinity for NF-Y than the other haplotypes. Low BnaA9.NF-YA7 expression in Hap4 and differences in transcriptional activation produced by the M63I substitution result in high BnaABF3/4s levels and low BnaA4.DOR level, consequently decreasing the transpiration rate and water loss rate and leading to stronger drought resistance.