Roles of the Ortholog in the Petal Identity Specification and Morphological Differentiation in Flowers

The genus (Ranunculaceae) with its unique and highly complex floral structure is an ideal system to address some key questions in terms of morphological and evolutionary studies in flowers. In , for example, the original eight petal primordia differentiate into three types at maturity (i.e., two dorsal spurred, two lateral flat, and four ventral reduced petals). The mechanisms underlying their identity determination and morphological differentiation remain unclear. Here, through a comprehensive approach combining digital gene expression (DGE) profiles, hybridization, and virus-induced gene silencing (VIGS), we explore the role of the () ortholog in . Our findings reveal that the not only functions as a traditionally known petal identity gene but also plays a critical role in petal morphological differentiation. The gene is expressed in all the petal primordia before their morphological differentiation at earlier stages, but shows a gradient expression level difference along the dorsventral floral axis, with higher expression level in the dorsal spurred petals, intermediate level in the lateral flat petals and lower level in the ventral reduced petals. VIGS experiments revealed that flowers with strong phenotypic changes showed a complete transformation of all the three types of petals into non-spurred sepals. However, in the flowers with moderate phenotypic changes, the transformation of spurred petals into flat petals is associated with moderate silencing of the gene, suggesting a significant impact of expression level on petal morphological differentiation. This research also shed some insights into the role of changes in gene expression levels on morphological differentiation in plants.