Asymmetric bending of lateral petals, a novel trait originated within Delphinium (Ranunculaceae), reinforces their mirrored helical rotation during flower opening
During the process of flower opening, most petals move downward in the direction of pedicel (i.e., epinastic movement). In most Delphinium flowers, however, their two lateral petals display a very peculiar movement, the mirrored helical rotation. Such a distinctive petal movement requires the twist of the 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, the petal asymmetric bending is a novel trait that has not been noticed yet so that its morphological nature, developmental process and molecular mechanisms remain largely unknown. Here, by using D. anthriscifolium as a model, we determined that the petal asymmetric bending was caused by localized expansion of cell width, accompanied with specialized arrangement of surface ornamentation, 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 the 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 surface ornamentation, through regulating the auxin distribution, 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 insight into the mechanisms of flower opening and helical movement in plants. Overall design: The dorsal spurred and lateral non-spurred petals at four developmental stages (S9, S10, S12 and S16) were sampled for RNA-sequencing. For comparison, the three sub-types of sepals (i.e., dorsal spurred, lateral and ventrally non-spurred), stamens and carpels at corresponding stages, as well as leaves and bracts that are just underneath the S9 floral buds were also included. For the VIGS-treated plants, the mock and ans-lmi1 strongly silenced petals at stage 12 of floral development, were separately subjected to toRNA-sequencing.