Senescence of plant organs or whole plants is a natural phase of development. While it can be brought on by adverse environmental conditions, disease, or pathogen attack, it occurs at the genetically determined time in otherwise healthy plants despite under the most optimal growth conditions. Senescence-related changes progress in an orderly manner. Organelles, such as chloroplasts, with their vast reservoirs of cell proteins and lipids, are degraded first, releasing amino acids, fatty acids, and minerals for further metabolism and/or transport out of the senescing organs. These nutrients are transported to other parts of the plant, especially developing seeds and, in deciduous perennials, to storage parenchyma cells in stems and roots. Many genes encoding proteases, RNases, enzymes involved in lipid metabolism, generation of sucrose from fatty acids, and generation of phloem-transport-specific amino acids are expressed in senescing leaves.

In addition, because senescent leaves are susceptible to pathogen attack and senescence causes the production of reactive oxygen species, various pathogenesis-related proteins, as well as antioxidants, are synthesized. The developmental factors that cause senescence-related changes to begin are not known. Also, mutants specific to discrete steps in senescence are rare. Considerable data support the postulate that a high cytokinin content in the leaf delays senescence, whereas production of or exposure to ethylene accelerates senescence. However, data from transgenic plants with altered levels of cytokinins or ethylene indicate that these hormones retard or hasten senescence, respectively, but do not alter the normal progression of senescence once it starts. These two hormones may not be the only hormones regulating senescence. There is evidence that ABA may hasten the senescence of flowers in some plants. Senescing organs are abscised and/or die; senescing whole plants also die, but senescence seems to differ from programmed cell death.

Regards

Joseph Mareddy

Assistant Managing Editor

Journal of Plant Biochemistry and physiology