An integrated transcriptomics and metabolomics analysis of theCucurbita peponectary implicates key modules of primary metabolism involved in nectar synthesis and secretion

Abstract

Nectar is the main reward that flowers offer to pollinators to entice repeated visitation. Cucurbita pepo (squash) is an excellent model for studying nectar biology, as it has large nectaries that produce large volumes of nectar relative to most other species. Squash is also monoecious, having both female and male flowers on the same plant, which allows comparative analyses of nectary function in one individual. Here we report the nectary transcriptomes from both female and male nectaries at four stages of floral maturation. Analysis of these transcriptomes and subsequent confirmatory experiments revealed a metabolic progression in nectaries leading from starch synthesis to starch degradation and to sucrose biosynthesis. These results are consistent with previously published models of nectar secretion and also suggest how a sucrose-rich nectar can be synthesized and secreted in the absence of active transport across the plasma membrane. Non-targeted metabolomic analyses of nectars also confidently identified 40 metabolites in both female and male nectars, with some displaying preferential accumulation in nectar of either male or female flowers. Cumulatively, this study identified gene targets for reverse genetics approaches to study nectary function, as well as previously unreported nectar metabolites that may function in plant-biotic interactions.

Carbohydrate Metabolism and Signaling in Squash Nectaries and Nectar Throughout Floral Maturation

“…Previous studies have shown that degradation of nectary starch occurs simultaneously with the production of nectar sugar in many species (Nepi et al, 1996;Peng et al, 2004;Ren et al, 2007;Lin et al, 2014;Solhaug et al, 2019). In order to further examine starch metabolism in C. pepo nectaries, we first asked whether the starch was degraded during the dark period immediately preceding nectar secretion.…”

“…Some species, such as Arabidopsis (Arabidopsis thaliana), produce nectar rich in hexoses (Davis et al, 1998). Other species, such as tobacco (Nicotiana tabacum; Ren et al, 2007) and squash (Nepi et al, 2001;Solhaug et al, 2019), produce nectar rich in Suc. Previous research has shown that squash (Cucurbita pepo) cultivars with a higher Suc/hexose ratio in their nectar have increased pollinator visits compared with cultivars with lower Suc relative to hexoses (Roldán-Serrano and Guerra-Sanz, 2005), suggesting that composition of nectar sugar can influence pollinator visitation, which has also been shown to have a direct impact on fruit yield (Motzke et al, 2015;Pereira et al, 2015;Zou et al, 2017).…”

“…Early stages of nectary maturation are typically associated with build-up of starch in the nectary. Indeed, presecretory accumulation of starch in nectaries has been reported for many flowering species (Nepi et al, 1996;Peng et al, 2004;Ren et al, 2007;Lin et al, 2014;Solhaug et al, 2019). As secretion proceeds, nectary starch is degraded and nectar sugar is produced concurrently (Solhaug et al, 2019).…”

“…Although the connection between starch degradation and nectar secretion has been well established in a number of species (Peng et al, 2004;Ren et al, 2007;Lin et al, 2014;Solhaug et al, 2019), the proportion of nectar sugar coming from starch versus sugar derived directly from the phloem is not well understood. In some species, such as tobacco (Ren et al, 2007) and Anigozanthos flavidus (Wenzler et al, 2008), phloem sugar can be directly transported into nectar (bypassing starch).…”

“…Transcriptomic studies have suggested that the mechanism of nectar secretion in C. pepo is similar to that in Arabidopsis (Solhaug et al, 2019). However, additional experiments are warranted to test whether this genetic model is reflected by the enzymatic patterns and flux of metabolites through the nectary system.…”