HLS1 promotes apical hook formation by regulating YUCCA8 and GH3.17 expression differently in the inner and outer side of the hook in cotton

HOOKLESS1 (HLS1) plays an indispensable role in apical hook development by promoting asymmetric auxin distribution in the hook. However, the precise mechanisms by which HLS1 regulates the differential gene expression in the inner and outer sides of the apical hook to promote this asymmetric auxin distribution remains unknown. The inner and outer sides of the cotton apical hook were separated for gene sequencing to search for differently expressed auxin-related genes. Then, the GhYUCCA8 and GhGH3.17 genes, which are highly expressed in the inner and outer side, respectively, as well as GhHLS1, were silenced by the SSA-VIGS method to study their interaction and function on auxin asymmetric distribution and hook formation. The hook formation was inhibited in the VIGS-GhHLS1, VIGS-GhYUCCA8 and VIGS-GhGH3.17 seedlings. Silencing GhHLS1 decreased the expression of GhYUCCA8 on the inner side but increased the expression of GhGH3.17 on the outer side of the apical hook, which weakened the auxin asymmetric distribution in the hook. Our results indicated that the high expression of GhHLS1 in the inner side caused high expression of GhYUCCA8 and GhGh3.17 in the inner and outer side of the apical hook, respectively, which caused the gradient of free auxin between the inner and outer side and ultimately promoted apical hook formation.