The catabolism of branched-chain amino acids and tyrosine has a low contribution to the mitochondrial metabolism during senescence in Brassica napus L.

Abstract

Winter oilseed rape (WOSR) is characterized by a low nitrogen remobilisation efficiency during developmental leaf senescence (DLS) compared to cereal crops. The catabolism of branched-chain amino acids (BCAA) and tyrosine plays a significant role in the recycling of protein-bound nitrogen and the fuelling of mitochondrial respiration during stress-induced senescence. In this study, we elucidated the role of BCAA and tyrosine catabolism during DLS in WOSR. To this end, we employed a multi-faceted approach, integrating transcriptional fingerprints, mitochondrial respiration measurements and 13C-labelling experiments during both DLS and dark-induced senescence (DIS). In general, the transcriptional regulation of BCAA and tyrosine catabolism was weakly correlated with apparent catabolic fluxes during DLS and DIS in WOSR. The absolute quantification and 13C-analysis of organic and amino acids by GC-MS showed that the catabolism of BCAA and tyrosine had a comparable contribution to the functioning of the tricarboxylic acid (TCA) cycle during DLS and DIS in WOSR. Isotopologue analysis of TCA cycle intermediates provided evidence that BCAA and tyrosine catabolism contribute minimally to the functioning of the TCA cycle during senescence. The role of BCAA and tyrosine catabolism could be essentially devoted to the recycling of nitrogen during senescence.