Girondé Alexandra, Etienne Philippe, Trouverie Jacques, Bouchereau Alain, Le Cahérec Françoise, Leport Laurent, Orsel Mathilde, Niogret Marie-Françoise, Nesi Nathalie, Carole Deleu, Soulay Fabienne, Masclaux-Daubresse Céline, Avice Jean-Christophe
BMC Plant Biol. 2015 Feb 21;15:59. doi: 10.1186/s12870-015-0437-1.
Oilseed rape is the third largest oleaginous crop in the world but requires high levels of N fertilizer of which only 50% is recovered in seeds. This weak N use efficiency is associated with a low foliar N remobilization, leading to a significant return of N to the soil and a risk of pollution. Contrary to what is observed during senescence in the vegetative stages, N remobilization from stems and leaves is considered efficient during monocarpic senescence. However, the contribution of stems towards N management and the cellular mechanisms involved in foliar remobilization remain largely unknown. To reach this goal, the N fluxes at the whole plant level from bolting to mature seeds and the processes involved in leaf N remobilization and proteolysis were investigated in two contrasting genotypes (Aviso and Oase) cultivated under ample or restricted nitrate supply.
During seed filling in both N conditions, Oase efficiently allocated the N from uptake to seeds while Aviso favoured a better N remobilization from stems and leaves towards seeds. Nitrate restriction decreased seed yield and oil quality for both genotypes but Aviso had the best seed N filling. Under N limitation, Aviso had a better N remobilization from leaves to stems before the onset of seed filling. Afterwards, the higher N remobilization from stems and leaves of Aviso led to a higher final N amount in seeds. This high leaf N remobilization is associated with a better degradation/export of insoluble proteins, oligopeptides, nitrate and/or ammonia. By using an original method based on the determination of Rubisco degradation in the presence of inhibitors of proteases, efficient proteolysis associated with cysteine proteases and proteasome activities was identified as the mechanism of N remobilization.
The results confirm the importance of foliar N remobilization after bolting to satisfy seed filling and highlight that an efficient proteolysis is mainly associated with (i) cysteine proteases and proteasome activities and (ii) a fine coordination between proteolysis and export mechanisms. In addition, the stem may act as transient storage organs in the case of an asynchronism between leaf N remobilization and N demand for seed filling.
油菜是世界上第三大油料作物，但需要大量氮肥，而种子中仅回收50%的氮肥。这种较低的氮利用效率与叶片氮素再转运率低有关，导致大量氮素回归土壤并存在污染风险。与营养生长阶段衰老过程中观察到的情况相反，在一次结实衰老过程中，茎和叶中的氮素再转运被认为是有效的。然而，茎对氮素管理的贡献以及叶片再转运所涉及的细胞机制在很大程度上仍不清楚。为了实现这一目标，在充足或受限硝酸盐供应条件下种植的两种对比基因型（阿维索和奥塞）中，研究了从抽薹到成熟种子整个植株水平的氮通量以及叶片氮素再转运和蛋白水解过程。
在两种氮条件下的种子灌浆期，奥塞有效地将吸收的氮分配到种子中，而阿维索则更有利于茎和叶中的氮素更好地再转运到种子中。硝酸盐限制降低了两种基因型的种子产量和油质，但阿维索的种子氮填充效果最佳。在氮限制条件下，阿维索在种子灌浆开始前从叶片到茎的氮素再转运更好。之后，阿维索茎和叶中更高的氮素再转运导致种子中的最终氮含量更高。这种高叶片氮素再转运与不溶性蛋白质、寡肽、硝酸盐和/或氨的更好降解/输出有关。通过使用一种基于在蛋白酶抑制剂存在下测定Rubisco降解的原始方法，与半胱氨酸蛋白酶和蛋白酶体活性相关的有效蛋白水解被确定为氮素再转运的机制。
结果证实了抽薹后叶片氮素再转运对满足种子灌浆的重要性，并强调有效蛋白水解主要与（i）半胱氨酸蛋白酶和蛋白酶体活性以及（ii）蛋白水解与输出机制之间的精细协调有关。此外，在叶片氮素再转运与种子灌浆的氮需求不同步的情况下，茎可能作为临时储存器官。
