Abstract
The sugar transporter (STP) gene family encodes monosaccharide transporters that contain 12 transmembrane domains and belong to the major facilitator superfamily. STP genes play critical roles in monosaccharide distribution and participate in diverse plant metabolic processes. To investigate the potential roles of STPs in cassava (Manihot esculenta) tuber root growth, genome-wide identification and expression and functional analyses of the STP gene family were performed in this study. A total of 20 MeSTP genes (MeSTP1–20) containing the Sugar_tr conserved motifs were identified from the cassava genome, which could be further classified into four distinct groups in the phylogenetic tree. The expression profiles of the MeSTP genes explored using RNA-seq data showed that most of the MeSTP genes exhibited tissue-specific expression, and 15 out of 20 MeSTP genes were mainly expressed in the early storage root of cassava. qRT-PCR analysis further confirmed that most of the MeSTPs displayed higher expression in roots after 30 and 40 days of growth, suggesting that these genes may be involved in the early growth of tuber roots. Although all the MeSTP proteins exhibited plasma membrane localization, variations in monosaccharide transport activity were found through a complementation analysis in a yeast (Saccharomyces cerevisiae) mutant, defective in monosaccharide uptake. Among them, MeSTP2, MeSTP15, and MeSTP19 were able to efficiently complement the uptake of five monosaccharides in the yeast mutant, while MeSTP3 and MeSTP16 only grew on medium containing galactose, suggesting that these two MeSTP proteins are transporters specific for galactose. This study provides significant insights into the potential functions of MeSTPs in early tuber root growth, which possibly involves the regulation of monosaccharide distribution.
Highlights
Sugar, regarded as an essential substrate in carbon and energy metabolism, plays diverse roles in plant growth and development
The uptake of hexose hydrolyzed from sucrose in the apoplast is regulated by a group of transporters, such as sugar transporter proteins (STPs) in Arabidopsis, monosaccharide transporters (MSTs) in rice (Oryza sativa), and hexose transporters (HTs) in grape (Vitis vinifera), which are involved in sugar unloading and contribute to carbon partitioning, crop yield, and environmental adaptation [3]
The results showed that the signal of GFP in cells transfected with the empty vector was mainly detected in the cytoplasm and plasma membrane, whereas the GFP signals in cells expressing the 16 MeSTP fusion proteins were confined to the plasma membrane (Figure 8), suggesting that these MeSTPs might be membrane proteins
Summary
Sugar, regarded as an essential substrate in carbon and energy metabolism, plays diverse roles in plant growth and development. A variety of strategies have been evolved by plants for the release of sucrose from the phloem to the sink tissues. At the release phloem, sucrose can be transported to sink cells via a symplastic pathway through plasmodesmata or via an apoplastic pathway mediated by sucrose transporters, cell wall invertases, and monosaccharide transporters [2]. The uptake of hexose (e.g., glucose and fructose) hydrolyzed from sucrose in the apoplast is regulated by a group of transporters, such as sugar transporter proteins (STPs) in Arabidopsis, monosaccharide transporters (MSTs) in rice (Oryza sativa), and hexose transporters (HTs) in grape (Vitis vinifera), which are involved in sugar unloading and contribute to carbon partitioning, crop yield, and environmental adaptation [3]
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