Hexose transporter CsSWEET7a in cucumber mediates phloem unloading in companion cells for fruit development

Abstract

In the fleshy fruit of cucumbers (Cucumis sativus L.), the phloem flow is unloaded via an apoplasmic pathway, which requires protein carriers to export sugars derived from stachyose and raffinose into the apoplasm. However, transporter(s) involved in this process remain unidentified. Here, we report that a hexose transporter, CsSWEET7a (Sugar Will Eventually be Exported Transporter 7a), was highly expressed in cucumber sink tissues and localized to the plasma membrane in companion cells of the phloem. Its expression level increased gradually during fruit development. Down-regulation of CsSWEET7a by RNA interference (RNAi) resulted in smaller fruit size along with reduced soluble sugar levels and reduced allocation of 14C-labelled carbon to sink tissues. CsSWEET7a overexpression lines showed an opposite phenotype. Interestingly, genes encoding alkaline α-galactosidase (AGA) and sucrose synthase (SUS) were also differentially regulated in CsSWEET7a transgenic lines. Immunohistochemical analysis demonstrated that CsAGA2 co-localized with CsSWEET7a in companion cells, indicating cooperation between AGA and CsSWEET7a in fruit phloem unloading. Our findings indicated that CsSWEET7a is involved in sugar phloem unloading in cucumber fruit by removing hexoses from companion cells to the apoplasmic space to stimulate the raffinose family of oligosaccharides (RFOs) metabolism so that additional sugars can be unloaded to promote fruit growth. This study also provides a possible avenue towards improving fruit production in cucumber.

Plasmodesmata and their role in the regulation of phloem unloading during fruit development

“…The presence of PD in the SE-CC connection has been reported as evidence of symplasmic unloading in tomatoes [ 53 ], kiwi [ 54 ], grape berries [ 55 ] and jujube [ 25 ] ( Table 1 ). In other fruits, such as apples [ 49 ], strawberries [ 56 ], cucumbers [ 57 , 58 , 59 ] and watermelons [ 60 ], apoplastic phloem delivery predominates at all developmental stages ( Table 1 ). Phloem unloading can switch from the symplasmic to the apoplastic pathway during fruit development according to physiological demands and environmental conditions [ 11 , 48 ] ( Figure 1 ).…”

The hexose transporter SWEET5 confers galactose sensitivity to Arabidopsis pollen germination via the galactokinase GALK

“…So far, all plant SWEETs with a measured Km, e.g. AtSWEET1 (Chen et al, 2010), AtSWEET12 (Chen et al,2012), AtSWEET17 (Guo et al, 2014), SlSWEET1a (Ho et al, 2019) and CsSWEET7a (Li et al, 2021), have been found to function as low-affinity glucose or sucrose transporters, ranging from ~10 to ~120 mM. It is not unusual to observe that a SWEET transports different sugars but with different efficiency and Kms (Chen et al, 2010, Kuanyshev et al, 2021.…”

“…So far, all plant SWEETs with measured Km, e.g. AtSWEET1 (Chen et al, 2010), AtSWEET12 (Chen et al,2012), AtSWEET17 (Guo et al, 2014), SlSWEET1a (Ho et al, 2019) and CsSWEET7a (Li et al, 2021), have been found to function as low-affinity sugar transporters, ranging from ~10 to ~120 mM. AtSWEET5/VEX, belonging to SWEET family Clade II that primarily transports hexose (Chen et al, 2010;Eom et al, 2015), is strongly expressed in the pollen vegetative cell (Engel et al, 2005), but no other details have been reported.…”

“…Facilitator SWEETs (Sugar Will Eventually be Exported Transporter) have been shown to transport different sugars including galactose. OsSWEET5 (Zhou et al, 2014) and CsSWEET7a (Li et al, 2021) have been demonstrated to transport galactose. So far, all plant SWEETs with a measured Km, e.g.…”

The Role of Sugar Transporter CsSWEET7a in Apoplasmic Phloem Unloading in Receptacle and Nectary During Cucumber Anthesis

“…Sucrose transporter (SUT) and monosaccharide transporters [e.g., sugar transport protein (STP)] were reported to function at the latter step ( Büttner, 2010 ; Borghi and Fernie, 2017 ), while a sugar exporter (functioning at the first step) has not been reported in flowers. Cucumber Sugar Will Eventually be Exported Transporter 7a (SWEET7a) is known to localize the companion cells in fruit vascular bundles and to export hexose to the apoplasmic space to stimulate sugar unloading in fruit ( Li et al, 2021 ). In Arabidopsis pollen, AtSWEET8 and AtSWEET13 are involved in pollen maturation, and an aborted silique phenotype was observed in sweet8 single mutant ( Guan et al, 2008 ), while this phenotype was more severe in sweet8;13 double mutant ( Sun et al, 2013 ).…”