A homolog of the ALOG family controls corolla tube differentiation in Torenia foumieri

Flowers of honey plants (Torenia) face various abiotic stressors, including rain, that can damage pollens and dilute nectar. Many Torenia species are thought to have evolved a modified corolla base termed the corolla neck to prevent raindrops from contacting the nectar. Although this hypothesis was postulated long ago, direct validation is lacking. Here, we have evaluated Torenia foumieri, the corolla tube of which differentiates into distinct regions: a conical tube above that connects to an inflated base through a constriction. This constriction and inflated base are collectively referred to as the corolla neck. Using transcriptomic sequencing and genome-editing approaches, we have characterized an ALOG gene, TfALOG3, that is involved in formation of the corolla neck. TfALOG3 was found expressed in the epidermis of the corolla neck. Cells in the corolla bottom differentiated and expanded in wild-type T. foumieri, whereas such cells in TfALOG3 loss-of-function mutants failed to develop into a corolla neck. Water easily contacted the nectary in the absence of the corolla neck. Taken together, our study unveils a novel gene that controls corolla tube differentiation and demonstrates a hypothetical property of the corolla neck.