Rubisco accumulation is important for the greening of the fln2-4 mutant in Arabidopsis.

The fructokinase-like protein2 (FLN2) is a component of the PEP complex. FLN2 knockout mutants displayed a delayed greening phenotype on sucrose-containing medium. Our previous work indicated that partial PEP activity is essential for its greening phenotype. In this study, we further report that sufficient Rubisco accumulation is critical for fln2-4 greening. Sugar serves many important functions, such as an energy source and signaling molecule. Through pharmacological experiments using a sugar analog and sugar signaling inhibitor, we demonstrate that sugar serves as energy to support the fln2-4 greening. Seed-reserve and photosynthetic CO2-fixation are the primary energy sources for early seedling growth. No obvious differences were observed in the seed-reserve of the wild-type and fln2-4 by comparing their seed size and dark-germination, indicating that the defective carbon fixation may account for the energy deficit in fln2-4 during its early seedling growth. The Rubisco content was low in fln2-4, but it rapidly accumulated during the greening of fln2-4. Expression of a nuclear-encoded rbcL gene facilitates Rubisco accumulation and partially complements the mutant defects. These results suggest that the Rubisco accumulation is critical for fln2-4 greening. In summary, the rapid Rubisco accumulation that depends on sufficient PEP activity is important for normal seedling growth.