Objectives The overexpression of the sugar transporter protein OsSTP1 affects rice yield by participating in the loading and unloading processes of carbohydrates in the phloem and regulating source-sink allocation. We investigated the carbohydrate allocation and yield in OsSTP1 overexpression materials under different nitrogen application levels, to deepen our understanding of the molecular biological mechanisms of nitrogen regulation of rice yield.

Methods A field trial was conducted at Hunan Agricultural University using the wild type (WT) from the genetic background of Taipei 309 and two OsSTP1 overexpression lines, OsSTP1-OE1 and OsSTP1-OE2, as test materials. The experiment was conducted in a split-plot design, and five nitrogen application rates were set under each genetic material, namely: 0 (N0), 60 (N60), 90 (N90), 120 (N120), and 180 (N180) kg/hm². Plant samples were taken to determine amylase (AMY), adenosine diphosphate glucose pyrophosphorylase (AGP), sucrose synthase (SUS), and sucrose phosphate phosphorylase (SPS) in stem sheaths and kernels at 12 days post-pollination. Samples were taken at tasseling stage and 24 days after pollination to determine the non-structural carbohydrate (NSC) content and to calculate the NSC translocation rate. Rice yield and its components were investigated at maturity.

Results Compared with WT, the activities of AMY and SPS in the stem sheath of the two OsSTP1 overexpression lines were significantly increased by 23.3%−34.3% and 11.9%−18.3% under the five N application treatments, with the increase being the highest in N120, and the activities of AGP and SUS in the spike were increased by 8.20%−14.4% and 24.1%−33.3%, and the highest increase was observed in N90 and N60 treatments. Sucrose and soluble sugar contents in the stem sheaths of the two OsSTP1 overexpression lines were not significantly different from those of the WT, but were significantly higher than those of WT in the kernels; starch contents in the stem sheaths were lower than those of the WT but higher than those of WT in the kernels in parallel. The NSC translocation rate in the stem sheaths of OsSTP1 overexpression lines was significantly higher than that of the WT by 24.2%−40.2% under the same nitrogen treatment, with the greatest increase in N120. The yields and harvest index of the OsSTP1 overexpression lines were significantly higher than those of WT under nitrogen application treatments, with increases ranging from 5.50%−14.5%, and 6.20% − 13.2%, respectively, and the highest increase was observed in N90. The nitrogen fertilizer utilization rate of the OsSTP1 overexpression lines were increased by 11.9%−17.9% compared with those of the WT. The fitting analysis showed that the yield of OsSTP1 overexpression lines at N 70 kg/hm² was the same as that of the WT at 120 kg/hm², which indicated that planting OsSTP1 overexpression strain had some potential for reducing nitrogen fertilizer.

Conclusions OsSTP1 overexpression increased amylase and sucrose phosphate synthase activities in rice stem sheaths and sucrose synthase and adenosine diphosphate glucose pyrophosphorylase activities in spikes, promoting the reactivation of carbohydrates in stem sheaths, increasing the rate of non-structural carbohydrate transport in stem sheaths, and facilitating the accumulation of sucrose, starch, and soluble sugars in seeds. The high allocation of carbohydrates in grains of the rice mateirals overexpressing the sugar transporter protein OsSTP1 acquired high nitrogen use efficiency, and produced the same yield at low nitrogen rate as the wild type did at high nitrogen rate, indicating the sugar transporter protein OsSTP1 being an important gene for high-N-efficient rice breeding.