CRABS CLAW-independent floral nectary development in Penstemon barbatus

Plants secrete carbohydrate-rich nectar from specialized floral nectary glands to attract pollinators. Arabidopsis and Petunia (rosid and asterid core eudicots, respectively) share genetic control of nectary development requiring activation of CRABS CLAW by euAGAMOUS and PLENA. This module is conserved despite striking differences in nectary morphology between the two species. We tested whether the euAGAMOUS/PLENA-CRABS CLAW module likely extends to an asterid species with novel nectary morphology, Penstemon barbatus.

To test conservation of the euAGAMOUS/PLENA-CRABS CLAW module and nectar sugar metabolism in P. barbatus, we carried out comparative transcriptomics, leveraging Penstemon's unique nectary morphology. Specifically, lateral and ventral stamen filaments develop with and without nectaries at their base, respectively, allowing straightforward differential expression analyses to uncover nectary-specific gene expression.

Many genes were differentially expressed between nectary and non-nectary tissues, and pre- and post-nectar-removal. Differentially expressed genes pointed to key phloem, abaxial specification, and epidermal cell shape functions during nectary development. Differentially expressed genes pointed to regulation of sugar processing and secretion after nectary maturation.

Our results suggest that P. barbatus nectaries develop independently of the canonical core-eudicot CRABS CLAW genetic module, despite being positioned within reproductive floral organ whorls, and that P. barbatus nectar production follows conserved sugar metabolic pathways.