Rubisco overexpression in rice hampers photosynthetic performance under P deficiency

Although Rubisco overexpression enhances productivity of some plant species, wild-type plants of these species do not accumulate Rubisco to such high levels, possibly due to associated disadvantages. As the utilization of inorganic phosphate in photosynthesis appeared to be disturbed in Rubisco-overexpressing rice (Oryza sativa L.) plants, we examined the effects of P deficiency on their photosynthetic performance. The severity of P deficiency was adjusted by varying the duration of the treatment in hydroponic culture. Rubisco-overexpressing plants exhibited a 1.2-1.4-fold overproduction of Rubisco protein under P deficiency. P deficiency generally reduced the rate of CO₂ assimilation (A) at various CO₂ levels and the Rubisco activation state at normal CO₂ levels, consistent with previous reports. Rubisco overexpression enhanced these reductions under severe P deficiency by up to approximately 20%, whereas such effects were limited under mild P deficiency. The ratio of A to total leaf-N was also lower in Rubisco-overexpressing plants than in wild-type plants by up to approximately 15% and 20% under mild and severe P deficiency, respectively, indicating compromised photosynthesis relative to that expected based on total leaf-N level, even under mild P deficiency. These differences were not attributable to leaf P status. When P deficiency treatment was further prolonged, total biomass in Rubisco-overexpressing plants was approximately 10% lower than that in wild-type plants. These results clearly indicate that Rubisco overexpression in rice hampers photosynthetic performance and thereby plant productivity, particularly under severe P deficiency. The underlying mechanisms of reduced photosynthetic performance are discussed in relation to photosynthetic metabolism.