From the archives: BETL, gamma, alpha—an unexpected transporter mediating sucrose transfer in maize, innovation by whole-genome duplication, and cooperative DNA binding by Opaque2 (original) (raw)

An unexpected transporter mediating sucrose transfer in maize The yield, quality, and nutritional value of seeds are complex traits whose development depends on the transport of sugars and other nutrients from maternal to filial tissues. The identification of nutrient transporters and their mechanisms of action in seeds can lay a foundation for breeding varieties with improved grain yield and quality. Last year, Bo Yang and colleagues (Yang et al. 2022) identified an unexpected sugar transporter in maize. While screening an ethyl methanesulfonate-mutagenized library for homozygous mutants of maize, the authors identified a line with altered kernel size named poorly filled kernel2109 (pfk2109). Using map-based cloning and sequencing, they identified the mutant gene, previously annotated as a nitrate transporter 1/peptide transporter (NRT1/PTR). NRT1/PTR-type transporters are known to transport nitrate, peptides, and ions (Léran et al. 2014). Due to the defects in sugar loading during the mutant grain development, Yang and coauthors (2022) renamed this maize gene as Sucrose and Glucose Carrier 1 (ZmSUGCAR1). The authors found that the ZmSUGCAR1 mRNA expression peak coincides with the most active grain-filling (storage) stage, and the proteins were specifically expressed in the basal endosperm transfer layer (BETL), which mediates nutrient transfer from maternal to seed tissue. The authors then generated complementation and CRISPR-Cas9-mediated knockout lines and showed that ZmSUGCAR1 acts as a sugar transporter able to import sucrose and glucose into the seed endosperm. Furthermore, the authors showed that ZmSUGCAR1 contributes to potassium loading but does not affect nitrate accumulation in the kernels. This study identified an unexpected sugar transporter crucial for maize grain development that complements the actions of other well-known sugar transporters, such as SWEETs (Fig. 1). November 2018: Innovation by whole-genome duplication Gene duplication is commonly observed in plant genomes and often leads to evolutionary novelty such as new gene functions. About 75% of flowering plants originated after consecutive whole-genome duplications commonly referred to as the gamma (γ) triplication event (Jiao et al. 2012). Zhicheng Zhang and coauthors (Zhang et al. 2018) decided to investigate the effect of the γ event on the MADS-box transcription factors. They used statistical methods to reconstruct protein interaction networks (PINs) between representatives of different MADS-box gene subfamilies before and after the γ triplication and from the origin of Arabidopsis and tomato. They reconstructed 26 ancestral proteins with high confidence, and by synthesizing their DNA sequences and performing a high-throughput yeast 2-hybrid (Y2H) assay, they genetically validated their proteinprotein interactions. Zhang et al. (2018) also looked at the gene dosages of the reconstructed ancestral proteins that In Brief