Nitrogen remobilization and protein degradation during senescence

Feller, Urs (1990). Nitrogen remobilization and protein degradation during senescence. In: Abrol, Y. P. (ed.) Nitrogen in Higher Plants. Research Studies in Botany and Related Applied Fields: Vol. 6 (pp. 195-222). Chichester: John Wiley and Sons Inc

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Plants are able to utilize nitrogen very economically. The high mobility of this macronutrient in xylem and phloem allows redistribution throughout the life cycle. The nitrogen demand of expanding leaves and maturing fruits is often covered to a great extent by remobilization from senescing parts of the same plant. Therefore, senescence is important for an efficient use of the nitrogen available in the soil and for crop yields.
Leaf senescence is accompanied by a loss of assimilatory capacity. Thus, a study of the regulation of this destructive process is very important. Different aspects of senescence control have been discussed in recent reviews. Senescence proceeds in a highly organized manner. In addition to interactions within the plant, environmental factors may also affect the geometry and velocity of nitrogen redistribution. Remarkable differences in the senescence behaviour have been observed between species and even varieties of the same species may differ considerably. Caution is recommended for the generalization of results obtained with a particular species.
Nitrogen metabolism changes at the onset of senescence from assimilation to remobilization, embracing enzyme patterns and structural change. In non-senescing cells, nitrogen is mainly incorporated into macromolecules (proteins, nucleic acids). The catabolism of cell constituents and the formation of transport compounds are important steps in the redistribution of nitrogen. The physiological function associated with a macromolecule is terminated by its degradation. The metabolic change from nitrogen assimilation to remobilization is based on a sequential loss of macromolecules with anabolic or catabolic functions; e.g. some enzymes are inactivated early, some remain active longer and others reach highest activities during senescence. Enzyme and membrane proteins serve after hydrolysis as nitrogen sources, the amino acids produced being trans1ocated to nitrogen sinks within the same plant.
The steps contributing to the degradation of cell constituents are not yet completely resolved. Regulatory mechanisms that operate at whole plant level, at cellular level and at the level of individual macromolecules are still hypothetical. This chapter reviews knowledge concerning nitrogen export during senescence and the control mechanisms involved. It also discusses the speculative ideas arising from this work and their importance in stimulating future research. The exploration of plant senescence is still at its beginning and constitutes a challenge for plant physiologists to investigate the processes involved and their regulation.

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