Carbohydrate Composition and Structure Changes as Phloem Sap is Converted to Nectar in Borago officinalis L. and Select Brassica spp. L.

Abstract

Nectar is a carbohydrate-rich solution produced by nectary organs as a reward to pollinators and animal mutualists. Nectar production involves the upload of carbohydrate (i.e., sucrose) from the phloem sap, intracellular (symplastic) and/or intercellular (apoplastic) transport of carbohydrates into the nectary, and secretion of carbohydrates to the nectary exterior as nectar. To investigate carbohydrate composition and structure changes during nectar production, the carbohydrate composition of phloem sap, nectary fluid, and nectar of Borago officinalis L. and two Brassica spp. L. (Brassica napus L. var. AC Excel, B. napus L. transgenic var. AV 225 R. R., and B. rapa L. var. AC Parkland) were determined employing high performance anion exchange-pulsed amperometric detection (HPAE-PAD) and capillary gas chromatography-flame ionization detection (CGC-FID) chromatographic methods. To elucidate the mechanism(s) of carbohydrate transformation during nectar production, substrate hydrolysis experiments were conducted on both nectaries and nectar, and nectary proteomics analysis was also employed. Carbohydrate composition results showed that: a) sucrose (S; >95% w:v) was present in the phloem sap of both genera; b) fructose (F; >50%), glucose (G; ~45%), and non-sucrose oligosaccharides were present in the nectary fluids of B. officinalis and Brassica spp., indicating that sucrose hydrolysis and carbohydrate synthesis occurred; c) F, G, S, and non-sucrose oligosaccharides were detected in the nectars of both genera with significant concentration differences; d) B. officinalis nectar was sucrose-dominant (S; 61%), whereas Brassica spp. nectars were hexose-dominant (average, F + G; 99%) indicating that sucrose was resynthesized in B. officinalis; and e) common non-sucrose oligosaccharides were detected in B. officinalis and Brassica spp. nectars and unique non-sucrose oligosaccharides were detected in both genera. The observed hydrolysis of sucrose and the synthesis of non-sucrose oligosaccharides in the nectaries and nectars of B. officinalis and Brassica spp. can be explained by the presence of carbohydrases (α-glucosidase, β-fructosidase β-glucosidase) and synthases (sucrose synthase, sucrose phosphate synthase) as confirmed by select substrate and proteomics experiments. The significant difference in the sucrose concentration of the floral nectar of B. officinalis is attributed to sucrose phosphate synthase activity in B. officinalis when compared to Brassica spp., and by the type of carbohydrate transport pathway (symplast vs apoplast) followed.