Nitrogen storage and remobilization in Brassica napus L. during the growth cycle:: effects of methyl jasmonate on nitrate uptake, senescence, growth, and VSP accumulation

The role of methyl jasmonate (MeJa) in promoting senescence has been described previously in many species, but it has been questioned in monocarpic species whether induced senescence is a result of a potential death hormone like MeJa, or a consequence of an increased metabolic drain resulting from the growth of reproductive tissue. In oilseed rape (Brassica napus L.), a polypeptide of 23 kDa has been recently identified as a putative vegetative storage protein (VSP). This polypeptide could be used as a storage buffer between N losses from senescing leaves putatively promoted by methyl jasmonate that might be produced by flowers, and grain filling which occurs later on, while N uptake is strongly reduced. In order to describe causal relationships during Brassica napus L. plant responses to MeJa treatment, a kinetic experiment was performed to determine the order and the amplitude with which general processes such as growth, photosynthesis, chlorophyll content, N uptake, and N storage under the form of the 23 kDa VSP are affected. One of the most immediate consequences of MeJa treatment was the strong reduction of nitrate uptake within 6 h, relative to control plants. However, this was not a specific effect as K+ uptake was similarly affected. Photosynthesis was reduced later (after 24 h), while chlorophyll content as well as leaf growth also decreased in a similar way. Moreover, this was concomitant with a remobilization of endogenous unlabelled N from senescing leaves to roots. Accumulation of the 23 kDa VSP was induced in the taproot after 24 h of MeJa treatment and was increased 10-fold within 8 d. On the other hand, the reversible effect of a MeJa pretreatment was tested in the long term (i.e. along the growth cycle) using plants previously grown in field conditions induced for flowering. Results show that a MeJa pulse induced a reversible effect on N uptake inhibition. In parallel, protein immunologically related to the 23 kDa VSP was detected in stems with a similar molecular weight (23 kDa), and in flowers and leaves with a molecular weight of 24 kDa. This accumulation was concomitant with the remobilization of both subunits of Rubisco. During stem and pod development, this protein induced by MeJa is fully hydrolysed. The external and intermittent supply of MeJa mimic some of the plant physiological processes previously reported under natural conditions. This suggests that in oilseed rape, methyl jasmonate could be considered as a possible monocarpic senescence factor while accumulation/mobilization of the 23 kDa VSP in taproot could be a marker for the cessation of N uptake and the initiation of a massive leaf senescence.