The contrasting N management of two oilseed rape genotypes reveals the mechanisms of proteolysis associated with leaf N remobilization and the respective contributions of leaves and stems to N storage and remobilization during seed filling

Abstract

BackgroundOilseed 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.ResultsDuring 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.ConclusionThe 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0437-1) contains supplementary material, which is available to authorized users.

Living to Die and Dying to Live: The Survival Strategy behind Leaf Senescence

“…Unlike cereals, which have a long history of domestication, oilseed rape (Brassica napus) and Arabidopsis still show considerable variation in N remobilization efficiency across related populations (Chardon et al, 2014;Girondé et al, 2015). Moreover, in many Brassica spp.…”

“…The salvaging of the chloroplasts allows a major portion of leaf lipids and proteins to be recycled (Ischebeck et al, 2006). As chloroplasts contain the majority of leaf proteins, they represent a rich source of nitrogen (N), and their salvaging provides up to 80% of the final N content of grains (Girondé et al, 2015).…”

Nitrogen use efficiency in rapeseed. A review

“…In rapeseed, the stems have been described as N-storage buffer organs that could improve this synchronization (Hocking et al 1997;Rossato et al 2001;Malagoli et al 2005a). According to Girondé et al (2015), a high amount of N in the stems at the beginning of the reproductive stage, and better N remobilization from the stems to the seeds, are characteristic features of high nitrogen remobilization efficiency. There is evidence for genetic variation in N stem remobilization in rapeseed (Berry et al 2010;Girondé et al 2015), but that variation is not always correlated with nitrogen utilization efficiency under low N supply.…”

“…According to Girondé et al (2015), a high amount of N in the stems at the beginning of the reproductive stage, and better N remobilization from the stems to the seeds, are characteristic features of high nitrogen remobilization efficiency. There is evidence for genetic variation in N stem remobilization in rapeseed (Berry et al 2010;Girondé et al 2015), but that variation is not always correlated with nitrogen utilization efficiency under low N supply. This finding was confirmed by data from diverse winter rapeseed accessions grown under low and high N fertilization, where the N content in the stems at flowering was not significantly related to nitrogen utilization efficiency (Fig.…”

Nitrogen Recovery and Transport Efficiency of Winter Rapeseed and Residual Nitrogen Effect on Subsequent Sesame using 15N Labelling Technique

“…Cai et al (1995) investigated the rapeseed nitrogen recovery efficiency by 15 N labelling technique and reported that rapeseed plant recovery efficiency of nitrogen as basal and topdressing at the flowering stage was 44.0% and 33.4% of the applied N, respectively. Meanwhile, many researchers maintained that the apparent N transport efficiency of rapeseed grain was about 41.0~65.0% from vegetative tissues, and the transport capacity was obviously higher under moderate nitrogen stress conditions than under normal or severe nitrogen stress conditions, with the apparent N transport efficiency of rapeseed grain being roughly 70-94% (Hocking et al, 1997;Rossato et al, 2001;Malagoli et al, 2005;Gombert et al, 2010;Franzaring et al, 2012;Gironde et al, 2015a). However, there have been few reports available on the use efficiency of nitrogen fertilizer as basal and topdressing in different organs of rapeseed and the residual N recovery efficiency by the subsequent crop.…”