Suppression of chloroplast triose phosphate isomerase evokes inorganic phosphate-limited photosynthesis in rice

Suppression of chloroplast triose phosphate isomerase suppresses photosynthesis at elevated [CO2] with the typical symptoms of photosynthesis limited by Pi availability for ATP synthesis. The availability of inorganic phosphate (P-i) for ATP synthesis is thought to limit photosynthesis at elevated [CO2] when P-i regeneration via sucrose or starch synthesis is limited. We report here another mechanism for the occurrence of P-i-limited photosynthesis caused by insufficient capacity of chloroplast triose phosphate isomerase (cpTPI). In cpTPI-antisense transgenic rice (Oryza sativa) plants with 55%-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 P-i-limited photosynthesis, suggesting sufficient capacity of cpTPI is necessary to prevent the occurrence of P-i-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].

Automated summary

Suppression of chloroplast triose phosphate isomerase leads to photosynthesis limitation at elevated [CO2], mainly caused by Pi availability for ATP synthesis. Decreased cpTPI content affects the CO2 sensitivity of CO2 assimilation and pool sizes of metabolites, underscoring the crucial role of cpTPI in preventing Pi-limited photosynthesis. The linear relationship between cpTPI content and photosynthetic capacity at elevated [CO2] highlights the necessity of sufficient cpTPI functioning for optimal photosynthesis.