The SWEET (sugars will eventually be exported transporter) family was identified as a new class of sugar transporters that function as bidirectional uniporters/facilitators and facilitate the diffusion of sugars across cell membranes along a concentration gradient. SWEETs are found widely in plants and play central roles in many biochemical processes, including the phloem loading of sugar for long-distance transport, pollen nutrition, nectar secretion, seed filling, fruit development, plant–pathogen interactions and responses to abiotic stress. This review focuses on advances of the plant SWEETs, including details about their discovery, characteristics of protein structure, evolution and physiological functions. In addition, we discuss the applications of SWEET in plant breeding. This review provides more in-depth and comprehensive information to help elucidate the molecular basis of the function of SWEETs in plants.

Brassica oleracea, globally cultivated for its vegetable crops, consists of very diverse morphotypes, characterized by specialized enlarged organs as harvested products. This makes B. oleracea an ideal model for studying rapid evolution and domestication. We constructed a B. oleracea pan-genome from 27 high-quality genomes representing all morphotypes and their wild relatives. We identified structural variations (SVs) among these genomes and characterized these in 704 B. oleracea accessions using graph-based genome tools. We show that SVs exert bidirectional effects on the expression of numerous genes, either suppressing through DNA methylation or promoting probably by harboring transcription factor-binding elements. The following examples illustrate the role of SVs modulating gene expression: SVs promoting BoPNY and suppressing BoCKX3 in cauliflower/broccoli, suppressing BoKAN1 and BoACS4 in cabbage and promoting BoMYBtf in ornamental kale. These results provide solid evidence for the role of SVs as dosage regulators of gene expression, driving B. oleracea domestication and diversification.

Cabbage Fusarium wilt is a major disease worldwide that can cause severe yield loss in cabbage (Brassica olerecea). Although markers linked to the resistance gene FOC1 have been identified, no candidate gene for it has been determined so far. In this study, we report the fine mapping and analysis of a candidate gene for FOC1 using a double haploid (DH) population with 160 lines and a F₂ population of 4000 individuals derived from the same parental lines. We confirmed that the resistance to Fusarium wilt was controlled by a single dominant gene based on the resistance segregation ratio of the two populations. Using InDel primers designed from whole-genome re-sequencing data for the two parental lines (the resistant inbred-line 99–77 and the highly susceptible line 99–91) and the DH population, we mapped the resistance gene to a 382-kb genomic region on chromosome C06. Using the F₂ population, we narrowed the region to an 84-kb interval that harbored ten genes, including four probable resistance genes (R genes): Bol037156, Bol037157, Bol037158 and Bol037161 according to the gene annotations from BRAD, the genomic database for B. oleracea. After correcting the model of the these genes, we re-predicted two R genes in the target region: re-Bol037156 and re-Bol0371578. The latter was excluded after we compared the two genes’ sequences between ten resistant materials and ten susceptible materials. For re-Bol037156, we found high identity among the sequences of the resistant lines, while among the susceptible lines, there were two types of InDels (a 1-bp insertion and a 10-bp deletion), each of which caused a frameshift and terminating mutation in the cDNA sequences. Further sequence analysis of the two InDel loci from 80 lines (40 resistant and 40 susceptible) also showed that all 40 R lines had no InDel mutation while 39 out of 40 S lines matched the two types of loci. Thus re-Bol037156 was identified as a likely candidate gene for FOC1 in cabbage. This work may lay the foundation for marker-assisted selection as well as for further function analysis of the FOC1 gene.

Due to its variegated and colorful leaves, ornamental kale (Brassica oleracea L. var. acephala) has become a popular ornamental plant. In this study, we report the fine mapping and analysis of a candidate purple leaf gene using a backcross population and an F2 population derived from two parental lines: W1827 (with white leaves) and P1835 (with purple leaves). Genetic analysis indicated that the purple leaf trait is controlled by a single dominant gene, which we named BoPr. Using markers developed based on the reference genome ‘02–12’, the BoPr gene was preliminarily mapped to a 280-kb interval of chromosome C09, with flanking markers M17 and BoID4714 at genetic distances of 4.3 cM and 1.5 cM, respectively. The recombination rate within this interval is almost 12 times higher than the usual level, which could be caused by assembly error for reference genome ‘02–12’ at this interval. Primers were designed based on ‘TO1000’, another B. oleracea reference genome. Among the newly designed InDel markers, BRID485 and BRID490 were found to be the closest to BoPr, flanking the gene at genetic distances of 0.1 cM and 0.2 cM, respectively; the interval between the two markers is 44.8 kb (reference genome ‘TO1000’). Seven annotated genes are located within the 44.8 kb genomic region, of which only Bo9g058630 shows high homology to AT5G42800 (dihydroflavonol reductase), which was identified as a candidate gene for BoPr. Blast analysis revealed that this 44.8 kb interval is located on an unanchored scaffold (Scaffold000035_P2) of ‘02–12’, confirming the existence of assembly error at the interval between M17 and BoID4714 for reference genome ‘02–12’. This study identified a candidate gene for BoPr and lays a foundation for the cloning and functional analysis of this gene.

Cabbage (Brassica oleracea var. capitata) is an important vegetable crop widely grown throughout the world, providing plentiful nutrients and health-promoting substances. To facilitate further genetics and genomic studies and crop improvement, we present here a high-quality reference genome for cabbage. We report a de novo genome assembly of the cabbage double-haploid line D134. A combined strategy of single-molecule real-time (SMRT) sequencing, 10× Genomics and chromosome conformation capture (Hi-C) produced a high quality cabbage draft genome. The chromosome-level D134 assembly is 529.92 Mb in size, 135 Mb longer than the current 02-12 reference genome, with scaffold N50 length being raised as high as 38 times. We annotated 44,701 high-quality protein-coding genes, and provided full-length transcripts for 45.59% of the total predicted gene models. Moreover, we identified novel genomic features like underrated TEs, as well as gene families and gene family expansions and contractions during B. oleracea evolution. The D134 draft genome is a cabbage reference genome assembled by SMRT long-read sequencing combined with the 10× Genomics and Hi-C technologies for scaffolding. This high-quality cabbage reference genome provides a valuable tool for improvement of Brassica crops.