Abstract
Oilseed rape is characterized by a low nitrogen remobilization efficiency during leaf senescence, mainly due to a lack of proteolysis. Because cotyledons are subjected to senescence, it was hypothesized that contrasting protease activities between genotypes may be distinguishable early in the senescence of cotyledons. To verify this assumption, our goals were to (i) characterize protease activities in cotyledons between two genotypes with contrasting nitrogen remobilization efficiency (Ténor and Samouraï) under limiting or ample nitrate supply; and (ii) test the role of salicylic acid (SA) and abscisic acid (ABA) in proteolysis regulation. Protease activities were measured and identified by a proteomics approach combining activity-based protein profiling with LC-MS/MS. As in senescing leaves, chlorophyll and protein contents decrease in senescing cotyledons and are correlated with an increase in serine and cysteine protease activities. Two RD21-like and SAG-12 proteases previously associated with an efficient proteolysis in senescing leaves of Ténor are also detected in senescing cotyledons. The infiltration of ABA and SA provokes the induction of senescence and several cysteine and serine protease activities. The study of protease activities during the senescence of cotyledons seems to be a promising experimental model to investigate the regulation and genotypic variability of proteolysis associated with efficient N remobilization.
Highlights
Oilseed rape (Brassica napus L.) is the second largest oleaginous crop worldwide behind soybean with a world production attaining about 64 million tonnes of grain [1] and the production of oilseed rape is experiencing a resurgence of interest thanks to the development of many food processing, green chemistry, and industrial markets based on biosourced products
Regardless of the genotype, this study demonstrated that the senescence of cotyledons in oilseed rape was characterized by a decrease in chlorophyll and soluble protein contents correlated with an increase in SH and cysteine proteases (CPs) activities, in response to N limitation
These modifications and many identified CPs/serine proteases (SPs) were observed during the leaf senescence of oilseed rape [9,29,62], meaning that cotyledons may provide a promising experimental system to study the proteolysis machinery associated with senescence processes, as well as the regulation of senescence
Summary
Oilseed rape (Brassica napus L.) is the second largest oleaginous crop worldwide behind soybean with a world production attaining about 64 million tonnes of grain [1] and the production of oilseed rape is experiencing a resurgence of interest thanks to the development of many food processing, green chemistry, and industrial markets based on biosourced products. Of N from fertilizers being recovered in seeds [3] This low N use efficiency is mainly related to a low N remobilization efficiency during the ‘sequential’ leaf senescence that occurs during the vegetative stages and the transition between vegetative and reproductive phases of development [4,5,6,7,8,9]. ‘Sequential’ senescence, affecting older leaves along the axis of the plant, leads to nutrient remobilization from source leaves to the sink organs such as young leaves [7]. This process is a degenerative process, but is an essential process for the survival of the rest of the plant and the survival of the species via seed production [13]. Senescence is a process linked to seed yield and plant productivity [14,15,16]
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