TaExpA6, VRT-A2 and TaGW2 genes differentially affect grain weight, grain number and yield of wheat through their physiological determinants

Physiological basis of the trade-off between grain number (GN) and thousand grain weight (TGW) is key to improve wheat yield. To that end, three wheat line groups were assessed at conventional (300-350 pl m-2) and low (44 pl m-2) plant rates in the field: TaExpA6 (ectopic expression), TaGW2 (TaGW2 triple mutant), and TaP1xGW2A (VRT-A2 ectopic expression and TaGW2-A knock-out), together with their wild-types (WT). Except TaP1xGW2A, the manipulated lines increased TGW over their WTs. However, only the transgenic TaExpA6 line reached higher grain yield (GY) than its WT, whereas increased TGW recorded by the triple knockout of TaGW2 was fully compensated by reduced spike number (SpN) and grain number per spike (GNS). Contrasting effects of TaExpA6 and TaGW2 lines were found on the ovary weight and floret dynamics, associated with the trade-off between GW and GN. Interestingly, the overexpression of TaExpA6 gene acts in grain tissues at post-anthesis avoiding the overlap with GN determination, while TaGW2 disruption constrains tillering, likely as a pleiotropic effect, and shifts intra-spike resource allocation promoting early ovary growth at the expense of distal floret development and GNS. Low plant rate increased resource allocation to the spike, mitigating the GW-GNS trade-off and increasing both traits simultaneously.