The mechanism underlying asymmetric bending of lateral petals in Delphinium (Ranunculaceae).

During the process of flower opening, most petals move downward in the direction of the pedicel (i.e., epinastic movement). In most Delphinium flowers, however, their two lateral petals display a very peculiar movement, the mirrored helical rotation, which requires the twist of the petal stalk. However, in some lineages, their lateral petals also exhibit asymmetric bending that increases the degree of mirrored helical rotation, facilitating the formation of a 3D final shape. Notably, petal asymmetric bending is a novel trait that has not been noticed yet, so its morphological nature, developmental process, and molecular mechanisms remain largely unknown. Here, by using D. anthriscifolium as a model, we determined that petal asymmetric bending was caused by the localized expansion of cell width, accompanied by the specialized array of cell wall nano-structure, on the adaxial epidermis. Digital gene analyses, gene expression, and functional studies revealed that a class I homeodomain-leucine zipper family transcription factor gene, DeanLATE MERISTEM IDENTITY1 (DeanLMI1), contributes to petal asymmetric bending; knockdown of it led to the formation of explanate 2D petals. Specifically, DeanLMI1 promotes cell expansion in width and influences the arrangement of cell wall nano-structure on the localized adaxial epidermis. These results not only provide a comprehensive portrait of petal asymmetric bending for the first time but also shed some new insights into the mechanisms of flower opening and helical movement in plants.