Strigolactones coordinate barley tillering and grain size

The strigolactone (SL) plant hormone pathway inhibits tiller bud outgrowth. SLs also influence plant height, grain number, and grain size, but it is unclear how these traits are connected. To separate the effects of SLs on plant architecture, grain size, and yield, we tested SL barley mutant plants in a range of conditions and utilized exogenous hormone-related treatments. SL mutants consistently showed smaller average grain mass, even when tiller number was similar to that of the wild type. However, direct hormone treatments of developing grains caused a reduction in grain size, while inhibition of the pathway had an opposite effect. A direct effect of SL in the grain is consistent with the expression of specific SL-related genes across key stages of grain development. These findings suggest that SLs have a dual action by regulating vegetative stages to indirectly promote barley grain size, while directly repressing grain size later during reproduction. This raises important implications for increasing crop yield through manipulation of the SL pathway.