Field trials reveal trade-offs between grain size and grain number in wheat ectopically expressing a barley sucrose transporter.

Optimizing assimilate partitioning to wheat grains is important to improve grain size and filling. Manipulation of gene expression of assimilate transporters is promising to improve uptake capacity and partitioning. The barley sucrose transporter HvSUT1 has been ectopically expressed in winter wheat controlled by the barley Hordein B1 promoter (HOSUT lines). In greenhouse experiments, HOSUT lines produced lager grains and higher grain yields than the untransformed control cultivar 'Certo'. This study aimed to evaluate and verify the potential of three HOSUT lines regarding grain yield and quality under field conditions. A three-year field trial in Switzerland using up to three N fertilization levels was performed. In addition to the three HOSUT lines and 'Certo', three Swiss reference lines were tested. Yield parameters, grain composition, phenology and disease scores were recorded. Grain weight was increased for HOSUT lines by 3–8 % compared to wildtype 'Certo', whereas grain number per spike was reduced by 4–12 %. Thus overall, grain yield per area remained largely unchanged. However, higher levels of iron and zinc found in HOSUT grains could be health-promoting. No negative unintended effects on chemical composition, disease susceptibility, and plant phenology were detected, with the exception of complete anther retention in one of the three HOSUT lines. While stimulation of sucrose uptake and/or partitioning in HOSUT lines resulted in higher grain size, an increase of total grain yield may only be achieved if trade-offs between grain size and grain number are overcome. The study contributes to the understanding of sucrose transporter modifications in wheat. It demonstrates the importance of field-testing crop plants that exhibit promising traits under controlled environments. More research is needed to identify the bottlenecks that appear under field conditions to unlock the potential of the HOSUT strategy for agronomic application. • Transgenic HOSUT wheat expresses a barley sucrose transporter to improve grain assimilate uptake. • 3 HOSUT lines were field tested in a 3-year yield trial in Switzerland with different N-levels. • HOSUT grains were heavier compared to wildtype (Certo), but spikes contained fewer grains. • To obtain higher yields, trade-offs between grain size and grain number need to be overcome. [ABSTRACT FROM AUTHOR]