Source ability is regulated by THOUSAND-GRAIN WEIGHT 6 in rice.

An indole-3-acetic acid (IAA)-glucose hydrolase, THOUSAND-GRAIN WEIGHT 6 (TGW6), negatively regulates rice grain weight and starch accumulation before heading. A 1-bp deletion in tgw6 results in loss of function and enhances grain size and yield. Thus, TGW6 has been a target for breeding strains with increased rice yield. Although the effect of loss of TGW6 function on sink size has been well understood, its impact on source ability (the ability to produce carbohydrates from leaves and supply to sink organs, referred to as shoot carbohydrate accumulation here) has been unclear. Here, we investigated the starch content of leaves, gene expression and carbohydrate translocation using cv. Koshihikari and a near-isogenic line carrying tgw6 (NIL(TGW6)). We found that NIL(TGW6) accumulated more starch in lower leaf sheaths than cv. Koshihikari. Gene analysis of lower leaf sheaths from both lines indicated that the expression of starch synthesis-related genes was up-regulated, and those involved with starch degradation were down-regulated in the NIL(TGW6) line. Measurements of changes in carbohydrate accumulation indicated that the loss of TGW6 function activated carbohydrate translocation and that starch accumulation in the leaf sheath contributed directly to the increase in starch uploaded to the panicles. These results provide new insights into TGW6 function and how it affects the source ability of rice.