Recycling and Remobilization of Nitrogen during Senescence

Abstract

Senescence is a process of programmed cell death that is highly dynamic and coordinated in nature, involving the breakdown of chloroplasts, reduction of photosynthesis, and degradation of biological macromolecules such as proteins, lipids, and nucleic acids. Nitrogen being central to the majority of biomolecules constitutes a major portion of the cargo that is re-mobilized from senescing leaves to the developing tissues or reserve organs, including seeds. Nitrogen remobilization and recycling during senescence redirect nutrients to the plant throughout its lifecycle and therefore contributes significantly to its survival and developmental program. Nitrogen breakdown during leaf senescence involves many nuclear and plastidial proteases, nucleases, regulators, subcellular organelles, and dynamic protein movements to radically transform proteins/peptides of high molecular weight into portable and reusable hydrolyzed ingredients. Three main pathways have been found to be responsible for the breakdown and the recycling of nitrogen: the autophagic and vacuolar pathway, the chloroplast degradation pathway, and the ubiquitin/proteasome system. Leaf senescence and nitrogen recycling studies have offered new avenues for improving the efficiency of nitrogen remobilization. As a result, nitrogen remobilization efficiency may be exploited as a hotspot to improve crop productivity in plants grown in low-nitrogen environments by reducing the nitrogen uptake from the environment. The focus of the present chapter is to provide an outline of the recycling and remobilization of nitrogen during senescence, mechanisms involved, recent advances to understand nitrogen remobilization, and targets to improve nitrogen remobilization efficiency.