Results and discussionBiomass production, yield and NUE. In the experimental conditions tested when more than 10.0 g N m −2 N fertilizer was supplied, the dry matter production and brown rice yield in RBCS-sense plants were increased by up to 23% (Fisher's test, P = 0.000-0.006) and up to 28% (Fisher's test, P = 0.000-0.016), respectively, compared to wild-type plants (Tables 1 and 2). The greatest dry matter production and brown rice yield from the RBCS-sense plants were observed in the 15.0 g N m −2 plot in 2019, at 1,657 g m −2 and 706 g m −2 , respectively, and the highest ratios of increase for both parameters were found in the 14.1 g N m −2 plot in 2018. In the plot with no N application from 2017 to 2019, no difference in dry matter production (Fisher's test, P = 0.506 in 2017; P = 0.208 in 2018; P = 0.208 in 2019) or brown rice yield (Fisher's test, P = 0.842 in 2017; P = 0.335 in 2018; P = 0.911 in 2019) was observed between RBCS-sense and wild-type plants, and brown rice yield of RBCSsense plants in the 7.1 g N m −2 plot in 2018 was lower than that of wild-type plants (Fisher's test, P = 0.000) (Tables 1 and 2). In RBCSantisense plants, the brown rice yield was 14-34% lower than in wild-type plants (Fisher's test, P = 0.000-0.030)-this was regardless of N application except in the plot with no N application in 2019 (Fisher's test, P = 0.934) (Tables 1 and 2). The lack of difference in 2019 might have been the effect of 2 yr without N fertilization. In the 17.0 g N m −2 plot in 2017, all plants were blown down by two typhoons and then almost lodged during their ripening stagestheir dry matter production and brown rice yield were, therefore, The green revolution's breeding of semi-dwarf rice cultivars in the 1960s improved crop yields, with large increases in the use of nitrogen (N) fertilizer. However, excess N application has caused serious environmental problems, including acid rain and the eutrophication of rivers and oceans. To use N to improve crop yields, while minimizing the associated environmental costs, there is a need to produce crops with higher N-use efficiency and higher yield components. Here we show that transgenic rice overproducing ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco)-the key enzyme of photosynthesis-exhibits increased yields with improved N-use efficiency for increasing biomass production when receiving sufficient N fertilization in an experimental paddy field. This field experiment demonstrates an improvement in photosynthesis linked to yield increase due to a higher N-use efficiency in a major crop.

The availability of inorganic phosphate (Pi) for ATP synthesis is thought to limit photosynthesis at elevated [CO2] when Pi regeneration via sucrose or starch synthesis is limited. We report here another mechanism for the occurrence of Pi-limited photosynthesis caused by insufficient capacity of chloroplast triosephosphate isomerase (cpTPI). In cpTPI-antisense transgenic rice (Oryza sativa) plants with 55% to 86% reductions in cpTPI content, CO2 sensitivity of the rate of CO2 assimilation (A) decreased and even reversed at elevated [CO2]. The pool sizes of the Calvin-Benson cycle metabolites from pentose phosphates to 3-phosphoglycerate increased at elevated [CO2], whereas those of ATP decreased. These phenomena are similar to the typical symptoms of Pi-limited photosynthesis, suggesting sufficient capacity of cpTPI is necessary to prevent the occurrence of Pi-limited photosynthesis and that cpTPI content moderately affects photosynthetic capacity at elevated [CO2]. As there tended to be slight variations in the amounts of total leaf-N depending on the genotypes, relationships between A and the amounts of cpTPI were examined after these parameters were expressed per unit amount of total leaf-N (A/N and cpTPI/N, respectively). A/N at elevated [CO2] decreased linearly as cpTPI/N decreased before A/N sharply decreased, owing to further decreases in cpTPI/N. Within this linear range, decreases in cpTPI/N by 80% led to decreases up to 27% in A/N at elevated [CO2]. Thus, cpTPI function is crucial for photosynthesis at elevated [CO2].

Background Improvement in photosynthesis is one of the most promising approaches to increase grain yields. Transgenic rice plants overproducing Rubisco by 30% (RBCS-sense rice plants) showed up to 28% increase in grain yields under sufficient nitrogen (N) fertilization using an isolated experimental paddy field (Yoon et al. in Nat Food 1:134–139, 2020). The plant N contents above-ground sections and Rubisco contents of the flag leaves were higher in the RBCS-sense plants than in the wild-type rice plants during the ripening period, which may be reasons for the increased yields. However, some imprecise points were left in the previous research, such as contributions of photosynthesis of leaves below the flag leaves to the yield, and maintenance duration of high photosynthesis of RBCS-sense rice plants during ripening periods. Result In this research, the photosynthetic capacity and canopy architecture were analyzed to explore factors for the increased yields of RBCS-sense rice plants. It was found that N had already been preferentially distributed into the flag leaves at the early ripening stage, contributing to maintaining higher Rubisco content levels in the enlarged flag leaves and extending the lifespan of the flag leaves of RBCS-sense rice plants throughout ripening periods under sufficient N fertilization. The higher amounts of Rubisco also improved the photosynthetic activity in the flag leaves throughout the ripening period. Although the enlarged flag leaves of the RBCS-sense rice plants occupied large spatial areas of the uppermost layer in the canopy, no significant prevention of light penetration to leaves below the flag leaves was observed. Additionally, since the CO2 assimilation rates of lower leaves between wild-type and RBCS-sense rice plants were the same at the early ripening stage, the lower leaves did not contribute to an increase in yields of the RBCS-sense rice plants. Conclusion We concluded that improvements in the photosynthetic capacity by higher leaf N and Rubisco contents, enlarged leaf area and extended lifespan of flag leaves led to an increase in grain yields of RBCS-sense rice plants grown under sufficient N fertilization.

The Green Revolution allowed a large amount of nitrogen (N) fertilization to increase crop yield but has led to severe environmental pollution. Therefore, increasing the crop grain yield must be achieved without such considerable input of N fertilization. A large‐grain japonica rice cultivar, Akita 63, significantly increased grain yield and improved N‐use efficiency (NUE) for yield per amount of N absorbed by plants. This study found that the nonsense mutated GS3 gene, the gs3 allele of Akita 63, has a superior yield production with enlarged grain size. The gs3 allele increased the yield with improvements in harvest index and NUE for yields per plant N content by analyzing the near‐isogenic line of rice plants with a large grain (LG‐Notohikari), which was developed by introducing the gs3 allele of Akita 63 into normal‐grain japonica cultivar, Notohikari. Thus, the gs3 allele would be promising for further yield increase without additional large input of N fertilization in non‐ gs3 ‐allele rice varieties.

Background: Improvement in photosynthesis is one of the most promising approaches to increase grain yields in crop plants. In our previous research using an isolated experimental paddy field, transgenic rice plants overproducing Rubisco by 30% (RBCS-sense rice plants) showed up to 28% increase in grain yields under sufficient nitrogen (N) fertilization. Furthermore, the plant N contents above-ground sections and Rubisco contents of the flag leaves were higher in the RBCS-sense rice plants than the wild-type rice plants during the ripening period, which may be reasons for the increased yields.Result: In this research, the photosynthetic capacity and canopy architecture were analyzed to explore factors for the increased yields of RBCS-sense rice plants. It was found that N had already been preferentially distributed into the flag leaves at the early ripening stage, contributing to maintaining higher Rubisco content levels in the enlarged flag leaves and extending the lifespan of the flag leaves of RBCS-sense rice plants throughout ripening periods under sufficient N fertilization. The higher amounts of Rubisco also improved the photosynthetic activity in the flag leaves throughout the ripening period. Although the enlarged flag leaves of the RBCS-sense rice plants occupied large spatial areas of the uppermost layer in the canopy, no significant prevention of light penetration to leaves below the flag leaves was observed. Additionally, since the CO2 assimilation rates of lower leaves between wild-type and RBCS-sense rice plants were the same at the early ripening stage, the lower leaves did not contribute to an increase in yields between the two genotypes.Conclusion: It was concluded that improvements in the photosynthetic capacity by higher leaf N and Rubisco contents, enlarged the leaf area, and extended the lifespan of flag leaves, causing an increase in grain yields of RBCS-sense rice plants grown under sufficient N fertilization.