A cell wall invertase controls nectar volume and sugar composition

Abstract

Flowering plants produce nectar to attract pollinators. The main nectar sugars are sucrose, glucose, and fructose, which can vary widely in ratio and concentration across species. Brassica spp. produce a hexose-dominant nectar (high in the monosaccharides glucose and fructose) with very low levels of the disaccharide sucrose. Cell wall invertases (CWINVs) catalyze the irreversible hydrolysis of sucrose into glucose and fructose in the apoplast. We found that BrCWINV4A is highly expressed in the nectaries of Brassica rapa. Moreover, a brcwinv4a null mutant has (1) greatly reduced cell wall invertase activity in the nectaries, and (2) produces a sucrose-rich nectar with little hexose content, but (3) with significantly less volume. These results were recapitulated via exogenous application of an invertase inhibitor to wild-type flowers. Honeybees prefer nectars with some sucrose, but wild-type B. rapa flowers were much more heavily visited than those of brcwinv4a, suggesting that the potentially attractive sucrose-rich nectar of brcwinv4a could not compensate for its low volume. These results cumulatively indicate that BrCWINV4A is not only essential for producing a hexose-rich nectar, but also support a model of nectar secretion in which its hydrolase activity is required for maintaining a high intracellular-to-extracellular sucrose ratio that facilitates the continuous export of sucrose into the apoplast via SWEET9. Extracellular hydrolysis of each sucrose into two hexoses by BrCWINV4A also likely creates the osmotic potential required for nectar droplet formation. In summary, modulation of CWINV activity can at least partially account for naturally occurring differences in nectar volume and sugar composition.