Vegetative Storage Proteins in Poplar

Abstract

Bark, wood, and root tissues of several Populus species contain a 32- and a 36-kilodalton polypeptide which undergo seasonal fluctuations and are considered to be storage proteins. These two proteins are abundant in winter and not detectable in summer as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunodetection. An antibody raised against the 32-kilodalton storage protein of Populus trichocarpa (T. & G.) cross-reacts with the 36-kilodalton protein of this species. The synthesis of the 32- and 36-kilodalton proteins can be induced in micropropagated plants by short-day conditions in the growth chamber. These proteins are highly abundant in structural roots, bark, and wood and combined represent >25% of the total soluble proteins in these tissues. Nitrate concentration in the leaves and nitrate uptake rate decreased dramatically when LD plants were transferred to short-day conditions; the protein content in leaves was unaffected. A decrease of the 32- and 36-kilodalton polypeptides occurs after transferring induced plants back to LD conditions. Both polypeptides are glycosylated and can be efficiently purified by affinity chromatography using concanavalin A-Sepharose 4B. The 32- and the 36-kilodalton polypeptides have identical basic isoelectric points and both consist of at least three isoforms. The storage proteins show a loss in apparent molecular mass after deglycosylation with trifluoromethanesulfonic acid. It is concluded that the 32- and 36-kilodalton polypeptides are glycoforms differing only in the extent of glycosylation. The relative molecular mass of the native storage protein was estimated to be 58 kilodalton, using gel filtration. From the molecular mass and the elution pattern it is supposed that the storage protein occurs as a heterodimer composed of one 32- and one 36-kilodalton subunit. Preliminary data suggest the involvement of the phytochrome system in the induction process of the 32- and 36-kilodalton polypeptides.