Effect of individual suppression of RBCS multigene family on Rubisco contents in rice leaves

In higher plants, a small subunit of Rubisco is encoded for by an RBCS multigene family in the nuclear genome. However, it is unknown how each multigene member contributes to the accumulation of Rubisco holoenzyme. Here, four RBCS genes that are highly expressed in leaf blaedes of rice (Oryza sativa L.) were individually suppressed by RNAi, and the effects on leaf Rubisco content were examined at seedling, vegetative and reproductive stages. Rubisco contents in each transgenic line declined irrespective of growth stage, and the ratios of Rubisco-N to total N were 66-96% of wild-type levels. The mRNA levels of the suppressed RBCS genes declined significantly, whereas those of the unsuppressed ones did not change drastically. These results indicate that four RBCS genes all contribute to accumulation of Rubisco holoenzyme irrespective of growth stage and that suppression of one RBCS gene is not fully compensated by other RBCS genes. Additionally, the mRNA levels of the large subunit of Rubisco showed a change similar to that of total RBCS mRNA level irrespective of genotype and growth stage. These results suggest that gene expression of RBCS and RBCL is regulated in a coordinated manner at the transcript level in rice.