A study of seed storage protein accumulation by ectopic expression in Arabidopsis

Understanding the mechanisms plants utilize for seed storage protein (SSP) synthesis, transport and deposition have the potential rewards of enabling high yields of modified or foreign proteins. Hayashi et al. (1999) indicated that the machinery devoted to the synthesis of protein storage vacuoles in cotyledon cells can be induced in vegetative tissue by the constitutive expression of a pumpkin 2S albumin phosphinothricin-acetyl-transferase gene fusion (pumpkin 2S-PAT) resulting in the biogenesis of precursor-accumulating (PAC) vesicles in Arabidopsis leaves. This discovery was the impetus behind the work described which sought to examine this phenomenon further by ectopically evoking SSP trafficking and vesicle biogenesis machinery in leaves. With the aim of elucidating the mechanisms necessary to evoke PAC vesicle biogenesis, a suite of constructs including the pumpkin 2S-PAT and analogous napin-PAT and napin-GFP variants were synthesized. Analysis of these transgenes in Arabidopsis revealed that the pumpkin 2S albumin has a capacity unique from napin peptides to result in fusion protein accumulation. Further, the truncated pumpkin 2S albumin peptide and the pumpkin 2S albumin C-terminus were found to direct deposition to vesicles; however, the C-terminus alone was not enough to direct deposition to vesicles unless combined with a significantly shortened napin peptide. An increased ER protein throughput was correlated to trafficking of the fusion protein by Golgi-independent mechanisms resulting in stable accumulation of the unprocessed protein whereas less ER throughput indicated passage through the Golgi-dependent pathway resulting in accumulation of a processed variant. At the level of gene expression, as examined by a microarray study, both inducible and constitutive ectopic expression of pumpkin 2S-PAT resulted in substantial perturbations of the endomembrane system affecting protein folding, flowering time and ER-associated biosynthetic functions which indicated that modulation of flowering time and photoperiodism are highly dependent on protein trafficking and vacuolar biogenesis mechanisms and that high ER protein throughput occurs at the expense of biosynthesis and cessation of ER functioning