Nectary photosynthesis contributes to the production of mānuka (Leptospermum scoparium) floral nectar

Abstract

Current models of floral nectar production do not include a contribution from photosynthesis by green nectary tissue, even though many species have green nectaries. M anuka (Leptospermum scoparium) floral nectaries are green, and in addition to sugars, their nectar contains dihydroxyacetone (DHA), the precursor of the antimicrobial agent in the honey. We investigated causes of variation in m anuka floral nectar production, particularly the effect of light incident on the nectary.Flower gas exchange, chlorophyll fluorescence, and the effects on nectar of age, temperature, light, sucrose, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), pyridoxal phosphate, and 13 CO 2 , were measured for attached and excised flowers.Flower age affected all nectar traits, whilst temperature affected total nectar sugar only. Increased light reduced floral CO 2 efflux, increased nectar sugar production, and affected the ratio of DHA to other nectar sugars. DCMU, an inhibitor of photosystem II, reduced nectar sugar production. Pyridoxal phosphate, an inhibitor of the chloroplast envelope triose phosphate transporter, reduced nectar DHA content. Incubation of excised flowers with 13 CO 2 in the light resulted in enrichment of nectar sugars, including DHA.Photosynthesis within green nectaries contributes to nectar sugars and influences nectar composition. M anuka nectar DHA arises from pools of triose phosphate that are modulated by nectary photosynthesis.

A phosphatase gene is linked to nectar dihydroxyacetone accumulation in mānuka (Leptospermum scoparium)

“…Three known nectary‐associated genes, SWEET9 (g16350.t1; Table S3; Lin et al ., 2014), CRABS CLAW (gene model g7327.1; Bowman & Smyth, 1999) and Pin6 (gene model g2930.1; Bender et al ., 2013), had relatively high (transcript base mean 147 282), medium (3190) and low (14) transcript abundance, respectively, indicating sufficient nectary tissue had been captured. SWEET9 is a sugar transporter required for nectar secretion in multiple species (Lin et al ., 2014; Roy et al ., 2017) and was included because of the presumed eccrine model of nectar secretion (Roy et al ., 2017) in mānuka (Clearwater et al ., 2021).…”

“…AtSGPP has dephosphorylating activity on a range of pentose and hexose phosphor‐sugars as well as on the triose glycerol 3‐phosphate (although DHAP was not tested) (Caparrós‐Martín et al ., 2013). As a phosphatase has been predicted to be required for releasing DHA from DHAP, thus allowing DHA to move into the nectar (Clearwater et al ., 2021), if mānuka SGPP2 could use DHAP as a substrate it would directly affect DHA concentrations in the cell. Thus, we considered this gene to be the lead candidate from this data set for the nectar trait.…”

“…The economic importance of mānuka honey has resulted in it being not only one of the only woody perennial species for which nectary biology and nectar traits have been studied in detail but also one of the few species with green, photosynthetic nectaries to have been characterised. Such studies have found that mānuka nectar traits, including DHA content, can be highly variable (Smallfield et al ., 2018; Noe et al ., 2019; Clearwater et al ., 2021), making it challenging in previous work to separate the relative influence of plant genetics from environment on nectar composition (Noe et al ., 2019).…”

“…The first was to identify genes with nectary‐associated expression that were differentially expressed between relatively high or low nectar DHA genotypes (H‐DHA and L‐DHA, respectively), and that encoded proteins that could have a predicted role in regulating DHA amounts from a primary metabolism or biochemical perspective. Of particular interest were transcripts encoding phosphatases, as it has been postulated that the first step in the process for DHA to accumulate in nectar would be dephosphorylation of DHAP (Clearwater et al ., 2021).…”

“…However, there are also some plant species, including m anuka, that have green, chloroplast-containing nectaries (L€ uttge, 2013). Clearwater et al (2021) recently demonstrated that the nectaries of m anuka photosynthesise and are a significantalthough not the onlysource of the fructose, glucose and trace amounts of sucrose that are characteristic of m anuka nectar. Furthermore, the study also found that nectary photosynthesis was important for the accumulation of DHA in m anuka nectar.…”