Abstract
During anthesis, there is an increased demand for carbohydrates due to pollen maturation and nectary secretion that warrants a systematic phloem unloading strategy for sugar partitioning. Sugar transporters are key components of the apoplasmic phloem unloading strategy and control the sugar flux needed for plant development. Currently, the phloem unloading strategy during anthesis has not been explored in cucumber, and the question of which sugar transporters are active during flower anthesis is poorly understood. In this study, a study utilizing the phloem-mobile symplasmic tracer carboxyfluorescein (CF) suggested that the phloem unloading was symplasmically isolated in the receptacle and nectary of cucumber flowers at anthesis. We also identified a hexose transporter that is highly expressed in cucumber flower, Sugar Will Eventually be Exported Transporter 7a (SWEET7a). CsSWEET7a was mainly expressed in receptacle and nectary tissues in both male and female flowers, where its expression level increased rapidly right before anthesis. At anthesis, the CsSWEET7a protein was specifically localized to the phloem region of the receptacle and nectary, indicating that CsSWEET7a may function in the apoplasmic phloem unloading during flower anthesis. Although cucumber mainly transports raffinose family oligosaccharides (RFOs) in the phloem, sucrose, glucose, and fructose are the major sugars in the flower receptacle and the nectary as well as in nectar at anthesis. In addition, the transcript levels of genes encoding soluble sugar hydrolases (α-galactosidase, sucrose synthase, cytoplasmic invertase, and cell wall invertase) were correlated with that of CsSWEET7a. These results indicated that CsSWEET7a may be involved in sugar partitioning as an exporter in the phloem of the receptacle and nectary to supply carbohydrates for flower anthesis and nectar secretion in cucumber.
Highlights
In flowering plants, the reproductive tissues require large amounts of carbohydrates for pollen maturation, nectar secretion, pollen tube generation, and seed initiation (Borghi and Fernie, 2017)
Our results showed that the expression level of CsSWEET7a slowly increased from stage 9 to stage 11 before rapidly increasing from stage 11 and peaking at anthesis (Supplementary Figure 1B), when flower tissues matured and prepared for anthesis, e.g., uninuclear pollen developed to mature pollen, and nectary tissues are fully developed in male flowers (Bai et al, 2004)
Compared with symplasmic unloading, which depends on both the sugar gradient and the density of plasmodesmata, during apoplasmic unloading, sugars were exported from SECCs to apoplasmic space via an exporter, before taking up by an energy-dependent importer into phloem parenchyma cells against the concentration gradient
Summary
The reproductive tissues require large amounts of carbohydrates for pollen maturation, nectar secretion, pollen tube generation, and seed initiation (Borghi and Fernie, 2017). In Arabidopsis, carbohydrate movement into the anther via the filament uses a symplasmic unloading strategy (Imlau et al, 1999). Phloem unloading in the Arabidopsis petal is symplasmic (Imlau et al, 1999). It is not clear which phloem unloading strategies are utilized in the nectary and receptacle. As non-photosynthetic organs, most of the nectary, as well as the secreted nectar, are dependent on phloem-derived sugars from sources (Roy et al, 2017). In Arabidopsis (Lin et al, 2014) and squash (Solhaug et al, 2019a), several steps are important for nectar secretion, including starch accumulation at the early developmental stage of nectary and subsequent starch degradation pre-anthesis, sucrose synthesis, and sucrose export. As a model plant of unisexual floral development (Gu et al, 2011), cucumber offers a great opportunity to streamline the study of carbohydrate partitioning in flowers
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