Nitrogen storage and remobilization in Brassica napus L. during the growth cycle: identification, characterization and immunolocalization of a putative taproot storage glycoprotein

In taproot of oilseed rape (Brassica napus L.), a 23 kDa polypeptide has been recently identified as a putative vegetative storage protein (VSP) because of its accumulation during flowering and its specific mobilization to sustain grain filling when N uptake is strongly reduced. The objectives were to characterize this protein more precisely and to study the effect of environmental factors (N availability, daylength, temperature, water deficit, wounding) or endogenous signals (methyl jasmonate, abscisic acid) that might change the N source/sink relationships within the plant, and may therefore trigger its accumulation. The 23 kDa putative VSP has two isoforms, is glycosylated and both isoforms share the same N-terminal sequence which had been used to produce specific polyclonal antibodies. Low levels of an immunoreactive protein of 24 kDa were found in leaves and flowers. In taproot, the 23 kDa putative VSP seems to accumulate only in the vacuoles of peripheral cortical parenchyma cells, around the phloem vessels. Among all treatments tested, the accumulation of this protein could only be induced by abscisic acid and methyl jasmonate. When compared to control plants, application of methyl jasmonate reduced N uptake by 89% after 15 d, induced a strong remobilization of N from senescing leaves and a concomitant accumulation of the 23 kDa putative VSP. These results suggested that, in rape, the 23 kDa protein is used as a storage buffer between N losses from senescing leaves promoted by methyl jasmonate and grain filling.