Abstract
BackgroundThe SWEET proteins are a group of sugar transporters that play a role in sugar efflux during a range of biological processes, including stress responses. However, there has been no comprehensive analysis of the SWEET family genes in Brassica oleracea (BoSWEET), and the evolutionary pattern, phylogenetic relationship, gene characteristics of BoSWEET genes and their expression patterns under biotic and abiotic stresses remain largely unexplored.ResultsA total of 30 BoSWEET genes were identified and divided into four clades in B. oleracea. Phylogenetic analysis of the BoSWEET proteins indicated that clade II formed first, followed by clade I, clade IV and clade III, successively. Clade III, the newest clade, shows signs of rapid expansion. The Ks values of the orthologous SWEET gene pairs between B. oleracea and Arabidopsis thaliana ranged from 0.30 to 0.45, which estimated that B. oleracea diverged from A. thaliana approximately 10 to 15 million years ago. Prediction of transmembrane regions showed that eight BoSWEET proteins contain one characteristic MtN3_slv domain, twenty-one contain two, and one has four. Quantitative reverse transcription-PCR (qRT-PCR) analysis revealed that five BoSWEET genes from clades III and IV exhibited reduced expression levels under chilling stress. Additionally, the expression levels of six BoSWEET genes were up-regulated in roots of a clubroot-susceptible cabbage cultivar (CS-JF1) at 7 days after inoculation with Plasmodiophora brassicae compared with uninoculated plants, indicating that these genes may play important roles in transporting sugars into sink roots associated with P. brassicae colonization in CS-JF1. Subcellular localization analysis of a subset of BoSWEET proteins indicated that they are localized in the plasma membrane.ConclusionsThis study provides important insights into the evolution of the SWEET gene family in B. oleracea and other species, and represents the first study to characterize phylogenetic relationship, gene structures and expression patterns of the BoSWEET genes. These findings provide new insights into the complex transcriptional regulation of BoSWEET genes, as well as potential candidate BoSWEET genes that promote sugar transport to enhance chilling tolerance and clubroot disease resistance in cabbage.
Highlights
The SWEET proteins are a group of sugar transporters that play a role in sugar efflux during a range of biological processes, including stress responses
Examples of genes in Clade II were first observed in the four algal species, which contained fewer SWEET members (1–3), and clade I and clade IV were predominantly observed in P. patens and S. moellendorffii, respectively
In this study, 30 BoSWEET genes were identified in the B. oleracea genome and further clustered into four clades based on a phylogenetic tree of 205 SWEET homologs from fourteen representative plant species
Summary
The SWEET proteins are a group of sugar transporters that play a role in sugar efflux during a range of biological processes, including stress responses. Sugar transport is mediated by proteins in the sucrose transporter (SUT) and monosaccharide transporter (MST) and SWEET families [3, 4]. SWEET, a novel class of sugar transporters, is a distinct transporter family which mediates influx or efflux of sugars from phloem parenchyma into the phloem apoplast [5,6,7]. Phylogenetic analysis of SWEET proteins has shown that they can be divided into four clades [3], with clades I and II preferentially transporting hexoses, clade III transporting sucrose, and clade IV being responsible for the flux of fructose across the tonoplast [9,10,11,12]
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